spring-framework/spring-framework-reference/src/aop-api.xml

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<chapter id="aop-api">
  <title>Spring AOP APIs</title>

  <section id="aop-api-introduction">
    <title>Introduction</title>

    <para>The previous chapter described the Spring 2.0 and later version's
    support for AOP using @AspectJ and schema-based aspect definitions. In
    this chapter we discuss the lower-level Spring AOP APIs and the AOP
    support used in Spring 1.2 applications. For new applications, we
    recommend the use of the Spring 2.0 and later AOP support described in the
    previous chapter, but when working with existing applications, or when
    reading books and articles, you may come across Spring 1.2 style examples.
    Spring 3.0 is backwards compatible with Spring 1.2 and everything
    described in this chapter is fully supported in Spring 3.0.</para>
  </section>

  <section id="aop-api-pointcuts">
    <title>Pointcut API in Spring</title>

    <para>Let's look at how Spring handles the crucial pointcut
    concept.</para>

    <section id="aop-api-concepts">
      <title>Concepts</title>

      <para>Spring's pointcut model enables pointcut reuse independent of
      advice types. It's possible to target different advice using the same
      pointcut.</para>

      <para>The <literal>org.springframework.aop.Pointcut</literal> interface
      is the central interface, used to target advices to particular classes
      and methods. The complete interface is shown below:</para>

      <programlisting language="java">public interface Pointcut {

    ClassFilter getClassFilter();

    MethodMatcher getMethodMatcher();

}</programlisting>

      <para>Splitting the <interfacename>Pointcut</interfacename> interface
      into two parts allows reuse of class and method matching parts, and
      fine-grained composition operations (such as performing a "union" with
      another method matcher).</para>

      <para>The <interfacename>ClassFilter</interfacename> interface is used
      to restrict the pointcut to a given set of target classes. If the
      <literal>matches()</literal> method always returns true, all target
      classes will be matched:</para>

      <programlisting language="java">public interface ClassFilter {

    boolean matches(Class clazz);
}</programlisting>

      <para>The <interfacename>MethodMatcher</interfacename> interface is
      normally more important. The complete interface is shown below:</para>

      <programlisting language="java">public interface MethodMatcher {

    boolean matches(Method m, Class targetClass);

    boolean isRuntime();

    boolean matches(Method m, Class targetClass, Object[] args);
}</programlisting>

      <para>The <literal>matches(Method, Class) </literal>method is used to
      test whether this pointcut will ever match a given method on a target
      class. This evaluation can be performed when an AOP proxy is created, to
      avoid the need for a test on every method invocation. If the 2-argument
      matches method returns true for a given method, and the
      <literal>isRuntime()</literal> method for the MethodMatcher returns
      true, the 3-argument matches method will be invoked on every method
      invocation. This enables a pointcut to look at the arguments passed to
      the method invocation immediately before the target advice is to
      execute.</para>

      <para>Most MethodMatchers are static, meaning that their
      <literal>isRuntime()</literal> method returns false. In this case, the
      3-argument matches method will never be invoked.</para>

      <tip>
        <para>If possible, try to make pointcuts static, allowing the AOP
        framework to cache the results of pointcut evaluation when an AOP
        proxy is created.</para>
      </tip>
    </section>

    <section id="aop-api-pointcut-ops">
      <title>Operations on pointcuts</title>

      <para>Spring supports operations on pointcuts: notably,
      <emphasis>union</emphasis> and <emphasis>intersection</emphasis>.</para>

      <itemizedlist>
        <listitem>
          <para>Union means the methods that either pointcut matches.</para>
        </listitem>

        <listitem>
          <para>Intersection means the methods that both pointcuts
          match.</para>
        </listitem>

        <listitem>
          <para>Union is usually more useful.</para>
        </listitem>

        <listitem>
          <para>Pointcuts can be composed using the static methods in the
          <emphasis>org.springframework.aop.support.Pointcuts</emphasis>
          class, or using the <emphasis>ComposablePointcut</emphasis> class in
          the same package. However, using AspectJ pointcut expressions is
          usually a simpler approach.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section id="aop-api-pointcuts-aspectj">
      <title>AspectJ expression pointcuts</title>

      <para>Since 2.0, the most important type of pointcut used by Spring is
      <literal>org.springframework.aop.aspectj.AspectJExpressionPointcut</literal>.
      This is a pointcut that uses an AspectJ supplied library to parse an
      AspectJ pointcut expression string.</para>

      <para>See the previous chapter for a discussion of supported AspectJ
      pointcut primitives.</para>
    </section>

    <section id="aop-api-pointcuts-impls">
      <title>Convenience pointcut implementations</title>

      <para>Spring provides several convenient pointcut implementations. Some
      can be used out of the box; others are intended to be subclassed in
      application-specific pointcuts.</para>

      <section id="aop-api-pointcuts-static">
        <title>Static pointcuts</title>

        <para>Static pointcuts are based on method and target class, and
        cannot take into account the method's arguments. Static pointcuts are
        sufficient - <emphasis>and best</emphasis> - for most usages. It's
        possible for Spring to evaluate a static pointcut only once, when a
        method is first invoked: after that, there is no need to evaluate the
        pointcut again with each method invocation.</para>

        <para>Let's consider some static pointcut implementations included
        with Spring.</para>

        <section id="aop-api-pointcuts-regex">
          <title>Regular expression pointcuts</title>

          <para>One obvious way to specify static pointcuts is regular
          expressions. Several AOP frameworks besides Spring make this
          possible.
          <literal>org.springframework.aop.support.Perl5RegexpMethodPointcut</literal>
          is a generic regular expression pointcut, using Perl 5 regular
          expression syntax. The <literal>Perl5RegexpMethodPointcut</literal>
          class depends on Jakarta ORO for regular expression matching. Spring
          also provides the <literal>JdkRegexpMethodPointcut</literal> class
          that uses the regular expression support in JDK 1.4+.</para>

          <para>Using the <literal>Perl5RegexpMethodPointcut</literal> class,
          you can provide a list of pattern Strings. If any of these is a
          match, the pointcut will evaluate to true. (So the result is
          effectively the union of these pointcuts.)</para>

          <para>The usage is shown below:</para>

          <para><programlisting language="xml">&lt;bean id="settersAndAbsquatulatePointcut" 
    class="org.springframework.aop.support.Perl5RegexpMethodPointcut"&gt;
    &lt;property name="patterns"&gt;
        &lt;list&gt;
            &lt;value&gt;.*set.*&lt;/value&gt;
            &lt;value&gt;.*absquatulate&lt;/value&gt;
        &lt;/list&gt;
    &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

          <para>Spring provides a convenience class,
          <literal>RegexpMethodPointcutAdvisor</literal>, that allows us to
          also reference an Advice (remember that an Advice can be an
          interceptor, before advice, throws advice etc.). Behind the scenes,
          Spring will use a <literal>JdkRegexpMethodPointcut</literal>. Using
          <literal>RegexpMethodPointcutAdvisor</literal> simplifies wiring, as
          the one bean encapsulates both pointcut and advice, as shown
          below:</para>

          <para><programlisting language="xml">&lt;bean id="settersAndAbsquatulateAdvisor" 
    class="org.springframework.aop.support.RegexpMethodPointcutAdvisor"&gt;
    &lt;property name="advice"&gt;
        &lt;ref local="beanNameOfAopAllianceInterceptor"/&gt;
    &lt;/property&gt;
    &lt;property name="patterns"&gt;
        &lt;list&gt;
            &lt;value&gt;.*set.*&lt;/value&gt;
            &lt;value&gt;.*absquatulate&lt;/value&gt;
        &lt;/list&gt;
    &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

          <para><emphasis>RegexpMethodPointcutAdvisor</emphasis> can be used
          with any Advice type.</para>
        </section>

        <section id="aop-api-pointcuts-attribute-driven">
          <title>Attribute-driven pointcuts</title>

          <para>An important type of static pointcut is a
          <emphasis>metadata-driven</emphasis> pointcut. This uses the values
          of metadata attributes: typically, source-level metadata.</para>
        </section>
      </section>

      <section id="aop-api-pointcuts-dynamic">
        <title>Dynamic pointcuts</title>

        <para>Dynamic pointcuts are costlier to evaluate than static
        pointcuts. They take into account method
        <emphasis>arguments</emphasis>, as well as static information. This
        means that they must be evaluated with every method invocation; the
        result cannot be cached, as arguments will vary.</para>

        <para>The main example is the <literal>control flow</literal>
        pointcut.</para>

        <section id="aop-api-pointcuts-cflow">
          <title>Control flow pointcuts</title>

          <para>Spring control flow pointcuts are conceptually similar to
          AspectJ <emphasis>cflow</emphasis> pointcuts, although less
          powerful. (There is currently no way to specify that a pointcut
          executes below a join point matched by another pointcut.) A control
          flow pointcut matches the current call stack. For example, it might
          fire if the join point was invoked by a method in the
          <literal>com.mycompany.web</literal> package, or by the
          <literal>SomeCaller</literal> class. Control flow pointcuts are
          specified using the
          <literal>org.springframework.aop.support.ControlFlowPointcut
          </literal>class.<note>
              <para>Control flow pointcuts are significantly more expensive to
              evaluate at runtime than even other dynamic pointcuts. In Java
              1.4, the cost is about 5 times that of other dynamic
              pointcuts.</para>
            </note></para>
        </section>
      </section>
    </section>

    <section id="aop-api-pointcuts-superclasses">
      <title>Pointcut superclasses</title>

      <para>Spring provides useful pointcut superclasses to help you to
      implement your own pointcuts.</para>

      <para>Because static pointcuts are most useful, you'll probably subclass
      StaticMethodMatcherPointcut, as shown below. This requires implementing
      just one abstract method (although it's possible to override other
      methods to customize behavior):</para>

      <para><programlisting language="java">class TestStaticPointcut extends StaticMethodMatcherPointcut {

    public boolean matches(Method m, Class targetClass) {
        // return true if custom criteria match
    }
}</programlisting>There are also superclasses for dynamic pointcuts.</para>

      <para>You can use custom pointcuts with any advice type in Spring 1.0
      RC2 and above.</para>
    </section>

    <section id="aop-api-pointcuts-custom">
      <title>Custom pointcuts</title>

      <para>Because pointcuts in Spring AOP are Java classes, rather than
      language features (as in AspectJ) it's possible to declare custom
      pointcuts, whether static or dynamic. Custom pointcuts in Spring can be
      arbitrarily complex. However, using the AspectJ pointcut expression
      language is recommended if possible.</para>

      <note>
        <para>Later versions of Spring may offer support for "semantic
        pointcuts" as offered by JAC: for example, "all methods that change
        instance variables in the target object."</para>
      </note>
    </section>
  </section>

  <section id="aop-api-advice">
    <title>Advice API in Spring</title>

    <para>Let's now look at how Spring AOP handles advice.</para>

    <section id="aop-api-advice-lifecycle">
      <title>Advice lifecycles</title>

      <para>Each advice is a Spring bean. An advice instance can be shared
      across all advised objects, or unique to each advised object. This
      corresponds to <emphasis>per-class</emphasis> or
      <emphasis>per-instance</emphasis> advice.</para>

      <para>Per-class advice is used most often. It is appropriate for generic
      advice such as transaction advisors. These do not depend on the state of
      the proxied object or add new state; they merely act on the method and
      arguments.</para>

      <para>Per-instance advice is appropriate for introductions, to support
      mixins. In this case, the advice adds state to the proxied
      object.</para>

      <para>It's possible to use a mix of shared and per-instance advice in
      the same AOP proxy.</para>
    </section>

    <section id="aop-api-advice-types">
      <title>Advice types in Spring</title>

      <para>Spring provides several advice types out of the box, and is
      extensible to support arbitrary advice types. Let us look at the basic
      concepts and standard advice types.</para>

      <section id="aop-api-advice-around">
        <title>Interception around advice</title>

        <para>The most fundamental advice type in Spring is
        <emphasis>interception around advice</emphasis>.</para>

        <para>Spring is compliant with the AOP Alliance interface for around
        advice using method interception. MethodInterceptors implementing
        around advice should implement the following interface:</para>

        <programlisting language="java">public interface MethodInterceptor extends Interceptor {
  
    Object invoke(MethodInvocation invocation) throws Throwable;
}</programlisting>

        <para>The <classname>MethodInvocation</classname> argument to the
        <methodname>invoke()</methodname> method exposes the method being
        invoked; the target join point; the AOP proxy; and the arguments to
        the method. The <methodname>invoke()</methodname> method should return
        the invocation's result: the return value of the join point.</para>

        <para>A simple <classname>MethodInterceptor</classname> implementation
        looks as follows:</para>

        <programlisting language="java">public class DebugInterceptor implements MethodInterceptor {

    public Object invoke(MethodInvocation invocation) throws Throwable {
        System.out.println("Before: invocation=[" + invocation + "]");
        Object rval = invocation.proceed();
        System.out.println("Invocation returned");
        return rval;
    }
}</programlisting>

        <para>Note the call to the MethodInvocation's
        <methodname>proceed()</methodname> method. This proceeds down the
        interceptor chain towards the join point. Most interceptors will
        invoke this method, and return its return value. However, a
        MethodInterceptor, like any around advice, can return a different
        value or throw an exception rather than invoke the proceed method.
        However, you don't want to do this without good reason!</para>

        <note>
          <para>MethodInterceptors offer interoperability with other AOP
          Alliance-compliant AOP implementations. The other advice types
          discussed in the remainder of this section implement common AOP
          concepts, but in a Spring-specific way. While there is an advantage
          in using the most specific advice type, stick with MethodInterceptor
          around advice if you are likely to want to run the aspect in another
          AOP framework. Note that pointcuts are not currently interoperable
          between frameworks, and the AOP Alliance does not currently define
          pointcut interfaces.</para>
        </note>
      </section>

      <section id="aop-api-advice-before">
        <title>Before advice</title>

        <para>A simpler advice type is a <emphasis role="bold">before
        advice</emphasis>. This does not need a
        <literal>MethodInvocation</literal> object, since it will only be
        called before entering the method.</para>

        <para>The main advantage of a before advice is that there is no need
        to invoke the <literal>proceed() </literal>method, and therefore no
        possibility of inadvertently failing to proceed down the interceptor
        chain.</para>

        <para>The <literal>MethodBeforeAdvice</literal> interface is shown
        below. (Spring's API design would allow for field before advice,
        although the usual objects apply to field interception and it's
        unlikely that Spring will ever implement it).</para>

        <programlisting language="java">public interface MethodBeforeAdvice extends BeforeAdvice {

    void before(Method m, Object[] args, Object target) throws Throwable;
}</programlisting>

        <para>Note the return type is <literal>void</literal>. Before advice
        can insert custom behavior before the join point executes, but cannot
        change the return value. If a before advice throws an exception, this
        will abort further execution of the interceptor chain. The exception
        will propagate back up the interceptor chain. If it is unchecked, or
        on the signature of the invoked method, it will be passed directly to
        the client; otherwise it will be wrapped in an unchecked exception by
        the AOP proxy.</para>

        <para>An example of a before advice in Spring, which counts all method
        invocations:</para>

        <programlisting language="java">public class CountingBeforeAdvice implements MethodBeforeAdvice {

    private int count;

    public void before(Method m, Object[] args, Object target) throws Throwable {
        ++count;
    }

    public int getCount() { 
        return count; 
    }
}</programlisting>

        <tip>
          <para>Before advice can be used with any pointcut.</para>
        </tip>
      </section>

      <section id="aop-api-advice-throws">
        <title>Throws advice</title>

        <para><emphasis role="bold">Throws advice</emphasis> is invoked after
        the return of the join point if the join point threw an exception.
        Spring offers typed throws advice. Note that this means that the
        <literal>org.springframework.aop.ThrowsAdvice</literal> interface does
        not contain any methods: It is a tag interface identifying that the
        given object implements one or more typed throws advice methods. These
        should be in the form of:</para>

        <programlisting language="java">afterThrowing([Method, args, target], subclassOfThrowable) </programlisting>

        <para>Only the last argument is required. The method signatures may
        have either one or four arguments, depending on whether the advice
        method is interested in the method and arguments. The following
        classes are examples of throws advice.</para>

        <para>The advice below is invoked if a
        <exceptionname>RemoteException</exceptionname> is thrown (including
        subclasses):</para>

        <programlisting language="java">public class RemoteThrowsAdvice implements ThrowsAdvice {

    public void afterThrowing(RemoteException ex) throws Throwable {
        <lineannotation>// Do something with remote exception</lineannotation>
    }
}</programlisting>

        <para>The following advice is invoked if a
        <exceptionname>ServletException</exceptionname> is thrown. Unlike the
        above advice, it declares 4 arguments, so that it has access to the
        invoked method, method arguments and target object:</para>

        <programlisting language="java">public class ServletThrowsAdviceWithArguments implements ThrowsAdvice {

    public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) {
        <lineannotation>// Do something with all arguments</lineannotation>
    }
}</programlisting>

        <para>The final example illustrates how these two methods could be
        used in a single class, which handles both
        <literal>RemoteException</literal> and
        <literal>ServletException</literal>. Any number of throws advice
        methods can be combined in a single class.</para>

        <programlisting language="java">public static class CombinedThrowsAdvice implements ThrowsAdvice {

    public void afterThrowing(RemoteException ex) throws Throwable {
        // Do something with remote exception
    }
 
    public void afterThrowing(Method m, Object[] args, Object target, ServletException ex) {
        // Do something with all arguments
    }
}</programlisting>

        <para><emphasis>Note:</emphasis> If a throws-advice method throws an
        exception itself, it will override the original exception (i.e. change
        the exception thrown to the user). The overriding exception will
        typically be a RuntimeException; this is compatible with any method
        signature. However, if a throws-advice method throws a checked
        exception, it will have to match the declared exceptions of the target
        method and is hence to some degree coupled to specific target method
        signatures. <emphasis>Do not throw an undeclared checked exception
        that is incompatible with the target method's
        signature!</emphasis></para>

        <tip>
          <para>Throws advice can be used with any pointcut.</para>
        </tip>
      </section>

      <section id="aop-api-advice-after-returning">
        <title>After Returning advice</title>

        <para>An after returning advice in Spring must implement the
        <emphasis>org.springframework.aop.AfterReturningAdvice</emphasis>
        interface, shown below:</para>

        <programlisting language="java">public interface AfterReturningAdvice extends Advice {

    void afterReturning(Object returnValue, Method m, Object[] args, Object target) 
            throws Throwable;
}</programlisting>

        <para>An after returning advice has access to the return value (which
        it cannot modify), invoked method, methods arguments and
        target.</para>

        <para>The following after returning advice counts all successful
        method invocations that have not thrown exceptions:</para>

        <programlisting language="java">public class CountingAfterReturningAdvice implements AfterReturningAdvice {

    private int count;

    public void afterReturning(Object returnValue, Method m, Object[] args, Object target)
            throws Throwable {
        ++count;
    }

    public int getCount() {
        return count;
    }
}</programlisting>

        <para>This advice doesn't change the execution path. If it throws an
        exception, this will be thrown up the interceptor chain instead of the
        return value.</para>

        <tip>
          <para>After returning advice can be used with any pointcut.</para>
        </tip>
      </section>

      <section id="aop-api-advice-introduction">
         

        <title>Introduction advice</title>

         

        <para>Spring treats introduction advice as a special kind of
        interception advice.</para>

         

        <para>Introduction requires an <literal>IntroductionAdvisor</literal>,
        and an <literal>IntroductionInterceptor</literal>, implementing the
        following interface:</para>

         

        <programlisting language="java">public interface IntroductionInterceptor extends MethodInterceptor {

    boolean implementsInterface(Class intf);
}</programlisting>

         

        <para>The <literal>invoke() </literal>method inherited from the AOP
        Alliance <literal>MethodInterceptor</literal> interface must implement
        the introduction: that is, if the invoked method is on an introduced
        interface, the introduction interceptor is responsible for handling
        the method call - it cannot invoke
        <literal>proceed()</literal>.</para>

         

        <para>Introduction advice cannot be used with any pointcut, as it
        applies only at class, rather than method, level. You can only use
        introduction advice with the <literal>IntroductionAdvisor</literal>,
        which has the following methods:</para>

         

        <programlisting language="java">public interface IntroductionAdvisor extends Advisor, IntroductionInfo {

	ClassFilter getClassFilter();

	void validateInterfaces() throws IllegalArgumentException;
}

public interface IntroductionInfo {

	Class[] getInterfaces();
}</programlisting>

         

        <para>There is no <interfacename>MethodMatcher</interfacename>, and
        hence no <interfacename>Pointcut</interfacename>, associated with
        introduction advice. Only class filtering is logical.</para>

         

        <para>The <literal>getInterfaces()</literal> method returns the
        interfaces introduced by this advisor.</para>

         The 

        <literal>validateInterfaces()</literal>

         method is used internally to see whether or not the introduced interfaces can be implemented by the configured 

        <literal>IntroductionInterceptor</literal>

         . 

        <para>Let's look at a simple example from the Spring test suite. Let's
        suppose we want to introduce the following interface to one or more
        objects:</para>

         

        <para>
          <programlisting language="java">public interface Lockable {
    void lock();
    void unlock();
    boolean locked();
}</programlisting>
        </para>

         

        <para>This illustrates a <emphasis role="bold">mixin</emphasis>. We
        want to be able to cast advised objects to Lockable, whatever their
        type, and call lock and unlock methods. If we call the lock() method,
        we want all setter methods to throw a
        <literal>LockedException</literal>. Thus we can add an aspect that
        provides the ability to make objects immutable, without them having
        any knowledge of it: a good example of AOP.</para>

         

        <para>Firstly, we'll need an
        <literal>IntroductionInterceptor</literal> that does the heavy
        lifting. In this case, we extend the
        <literal>org.springframework.aop.support.DelegatingIntroductionInterceptor</literal>
        convenience class. We could implement IntroductionInterceptor
        directly, but using
        <literal>DelegatingIntroductionInterceptor</literal> is best for most
        cases.</para>

         

        <para>The <literal>DelegatingIntroductionInterceptor</literal> is
        designed to delegate an introduction to an actual implementation of
        the introduced interface(s), concealing the use of interception to do
        so. The delegate can be set to any object using a constructor
        argument; the default delegate (when the no-arg constructor is used)
        is this. Thus in the example below, the delegate is the
        <literal>LockMixin</literal> subclass of
        <literal>DelegatingIntroductionInterceptor</literal>. Given a delegate
        (by default itself), a
        <literal>DelegatingIntroductionInterceptor</literal> instance looks
        for all interfaces implemented by the delegate (other than
        IntroductionInterceptor), and will support introductions against any
        of them. It's possible for subclasses such as
        <literal>LockMixin</literal> to call the
        <literal>suppressInterface(Class intf) </literal>method to suppress
        interfaces that should not be exposed. However, no matter how many
        interfaces an <literal>IntroductionInterceptor</literal> is prepared
        to support, the <literal>IntroductionAdvisor</literal> used will
        control which interfaces are actually exposed. An introduced interface
        will conceal any implementation of the same interface by the
        target.</para>

         

        <para>Thus LockMixin subclasses
        <literal>DelegatingIntroductionInterceptor</literal> and implements
        Lockable itself. The superclass automatically picks up that Lockable
        can be supported for introduction, so we don't need to specify that.
        We could introduce any number of interfaces in this way.</para>

         

        <para>Note the use of the <literal>locked</literal> instance variable.
        This effectively adds additional state to that held in the target
        object.</para>

         

        <para>
          <programlisting language="java">public class LockMixin extends DelegatingIntroductionInterceptor 
    implements Lockable {

    private boolean locked;

    public void lock() {
        this.locked = true;
    }

    public void unlock() {
        this.locked = false;
    }

    public boolean locked() {
        return this.locked;
    }

    public Object invoke(MethodInvocation invocation) throws Throwable {
        if (locked() &amp;&amp; invocation.getMethod().getName().indexOf("set") == 0)
            throw new LockedException();
        return super.invoke(invocation);
    }

}</programlisting>
        </para>

         

        <para>Often it isn't necessary to override the <literal>invoke()
        </literal>method: the
        <literal>DelegatingIntroductionInterceptor</literal> implementation -
        which calls the delegate method if the method is introduced, otherwise
        proceeds towards the join point - is usually sufficient. In the
        present case, we need to add a check: no setter method can be invoked
        if in locked mode.</para>

         

        <para>The introduction advisor required is simple. All it needs to do
        is hold a distinct <literal>LockMixin</literal> instance, and specify
        the introduced interfaces - in this case, just
        <literal>Lockable</literal>. A more complex example might take a
        reference to the introduction interceptor (which would be defined as a
        prototype): in this case, there's no configuration relevant for a
        <literal>LockMixin</literal>, so we simply create it using
        <literal>new</literal>.</para>

         

        <para>
          <programlisting language="java">public class LockMixinAdvisor extends DefaultIntroductionAdvisor {

    public LockMixinAdvisor() {
        super(new LockMixin(), Lockable.class);
    }
}</programlisting>
        </para>

         

        <para>We can apply this advisor very simply: it requires no
        configuration. (However, it <emphasis>is</emphasis> necessary: It's
        impossible to use an <literal>IntroductionInterceptor</literal>
        without an <emphasis>IntroductionAdvisor</emphasis>.) As usual with
        introductions, the advisor must be per-instance, as it is stateful. We
        need a different instance of <literal>LockMixinAdvisor</literal>, and
        hence <literal>LockMixin</literal>, for each advised object. The
        advisor comprises part of the advised object's state.</para>

         

        <para>We can apply this advisor programmatically, using the
        <literal>Advised.addAdvisor() </literal>method, or (the recommended
        way) in XML configuration, like any other advisor. All proxy creation
        choices discussed below, including "auto proxy creators," correctly
        handle introductions and stateful mixins.</para>

         
      </section>
    </section>
  </section>

  <section id="aop-api-advisor">
    <title>Advisor API in Spring</title>

    <para>In Spring, an Advisor is an aspect that contains just a single
    advice object associated with a pointcut expression.</para>

    <para>Apart from the special case of introductions, any advisor can be
    used with any advice.
    <literal>org.springframework.aop.support.DefaultPointcutAdvisor</literal>
    is the most commonly used advisor class. For example, it can be used with
    a <literal>MethodInterceptor</literal>, <literal>BeforeAdvice</literal> or
    <literal>ThrowsAdvice</literal>.</para>

    <para>It is possible to mix advisor and advice types in Spring in the same
    AOP proxy. For example, you could use a interception around advice, throws
    advice and before advice in one proxy configuration: Spring will
    automatically create the necessary interceptor chain.</para>
  </section>

  <section id="aop-pfb">
    <title>Using the ProxyFactoryBean to create AOP proxies</title>

    <para>If you're using the Spring IoC container (an ApplicationContext or
    BeanFactory) for your business objects - and you should be! - you will
    want to use one of Spring's AOP FactoryBeans. (Remember that a factory
    bean introduces a layer of indirection, enabling it to create objects of a
    different type.)</para>

    <note>
      <para>The Spring 2.0 AOP support also uses factory beans under the
      covers.</para>
    </note>

    <para>The basic way to create an AOP proxy in Spring is to use the
    <emphasis>org.springframework.aop.framework.ProxyFactoryBean</emphasis>.
    This gives complete control over the pointcuts and advice that will apply,
    and their ordering. However, there are simpler options that are preferable
    if you don't need such control.</para>

    <section id="aop-pfb-1">
      <title>Basics</title>

      <para>The <literal>ProxyFactoryBean</literal>, like other Spring
      <literal>FactoryBean</literal> implementations, introduces a level of
      indirection. If you define a <literal>ProxyFactoryBean</literal> with
      name <literal>foo</literal>, what objects referencing
      <literal>foo</literal> see is not the
      <literal>ProxyFactoryBean</literal> instance itself, but an object
      created by the <literal>ProxyFactoryBean</literal>'s implementation of
      the <literal>getObject() </literal>method. This method will create an
      AOP proxy wrapping a target object.</para>

      <para>One of the most important benefits of using a
      <literal>ProxyFactoryBean</literal> or another IoC-aware class to create
      AOP proxies, is that it means that advices and pointcuts can also be
      managed by IoC. This is a powerful feature, enabling certain approaches
      that are hard to achieve with other AOP frameworks. For example, an
      advice may itself reference application objects (besides the target,
      which should be available in any AOP framework), benefiting from all the
      pluggability provided by Dependency Injection.</para>
    </section>

    <section id="aop-pfb-2">
      <title>JavaBean properties</title>

      <para>In common with most <interfacename>FactoryBean</interfacename>
      implementations provided with Spring, the
      <classname>ProxyFactoryBean</classname> class is itself a JavaBean. Its
      properties are used to:</para>

      <itemizedlist>
        <listitem>
          <para>Specify the target you want to proxy.</para>
        </listitem>

        <listitem>
          <para>Specify whether to use CGLIB (see below and also
          <xref linkend="aop-pfb-proxy-types" />).</para>
        </listitem>
      </itemizedlist>

      <para>Some key properties are inherited from
      <classname>org.springframework.aop.framework.ProxyConfig</classname>
      (the superclass for all AOP proxy factories in Spring). These key
      properties include:</para>

      <itemizedlist>
        <listitem>
          <para><literal>proxyTargetClass</literal>: <literal>true</literal>
          if the target class is to be proxied, rather than the target class'
          interfaces. If this property value is set to
          <literal>true</literal>, then CGLIB proxies will be created (but see
          also <xref linkend="aop-pfb-proxy-types" />).</para>
        </listitem>

        <listitem>
          <para><literal>optimize</literal>: controls whether or not
          aggressive optimizations are applied to proxies <emphasis>created
          via CGLIB</emphasis>. One should not blithely use this setting
          unless one fully understands how the relevant AOP proxy handles
          optimization. This is currently used only for CGLIB proxies; it has
          no effect with JDK dynamic proxies.</para>
        </listitem>

        <listitem>
          <para><literal>frozen</literal>: if a proxy configuration is
          <literal>frozen</literal>, then changes to the configuration are no
          longer allowed. This is useful both as a slight optimization and for
          those cases when you don't want callers to be able to manipulate the
          proxy (via the <interfacename>Advised</interfacename> interface)
          after the proxy has been created. The default value of this property
          is <literal>false</literal>, so changes such as adding additional
          advice are allowed.</para>
        </listitem>

        <listitem>
          <para><literal>exposeProxy</literal>: determines whether or not the
          current proxy should be exposed in a
          <classname>ThreadLocal</classname> so that it can be accessed by the
          target. If a target needs to obtain the proxy and the
          <literal>exposeProxy</literal> property is set to
          <literal>true</literal>, the target can use the
          <methodname>AopContext.currentProxy()</methodname> method.</para>
        </listitem>

        <listitem>
          <para><literal>aopProxyFactory</literal>: the implementation of
          <interfacename>AopProxyFactory</interfacename> to use. Offers a way
          of customizing whether to use dynamic proxies, CGLIB or any other
          proxy strategy. The default implementation will choose dynamic
          proxies or CGLIB appropriately. There should be no need to use this
          property; it is intended to allow the addition of new proxy types in
          Spring 1.1.</para>
        </listitem>
      </itemizedlist>

      <para>Other properties specific to
      <classname>ProxyFactoryBean</classname> include:</para>

      <itemizedlist>
        <listitem>
          <para><literal>proxyInterfaces</literal>: array of String interface
          names. If this isn't supplied, a CGLIB proxy for the target class
          will be used (but see also <xref linkend="aop-pfb-proxy-types" />).</para>
        </listitem>

        <listitem>
          <para><literal>interceptorNames</literal>: String array of
          <interfacename>Advisor</interfacename>, interceptor or other advice
          names to apply. Ordering is significant, on a first come-first
          served basis. That is to say that the first interceptor in the list
          will be the first to be able to intercept the invocation.</para>

          <para>The names are bean names in the current factory, including
          bean names from ancestor factories. You can't mention bean
          references here since doing so would result in the
          <classname>ProxyFactoryBean</classname> ignoring the singleton
          setting of the advice.</para>

          <para>You can append an interceptor name with an asterisk
          (<literal>*</literal>). This will result in the application of all
          advisor beans with names starting with the part before the asterisk
          to be applied. An example of using this feature can be found in
          <xref linkend="aop-global-advisors" />.</para>
        </listitem>

        <listitem>
          <para>singleton: whether or not the factory should return a single
          object, no matter how often the <literal>getObject()</literal>
          method is called. Several <interfacename>FactoryBean</interfacename>
          implementations offer such a method. The default value is
          <literal>true</literal>. If you want to use stateful advice - for
          example, for stateful mixins - use prototype advices along with a
          singleton value of <literal>false</literal>.</para>
        </listitem>
      </itemizedlist>
    </section>

    <section id="aop-pfb-proxy-types">
      <title>JDK- and CGLIB-based proxies</title>

      <para>This section serves as the definitive documentation on how the
      <classname>ProxyFactoryBean</classname> chooses to create one of either
      a JDK- and CGLIB-based proxy for a particular target object (that is to
      be proxied).</para>

      <note>
        <para>The behavior of the <classname>ProxyFactoryBean</classname> with
        regard to creating JDK- or CGLIB-based proxies changed between
        versions 1.2.x and 2.0 of Spring. The
        <classname>ProxyFactoryBean</classname> now exhibits similar semantics
        with regard to auto-detecting interfaces as those of the
        <classname>TransactionProxyFactoryBean</classname> class.</para>
      </note>

      <para>If the class of a target object that is to be proxied (hereafter
      simply referred to as the target class) doesn't implement any
      interfaces, then a CGLIB-based proxy will be created. This is the
      easiest scenario, because JDK proxies are interface based, and no
      interfaces means JDK proxying isn't even possible. One simply plugs in
      the target bean, and specifies the list of interceptors via the
      <literal>interceptorNames</literal> property. Note that a CGLIB-based
      proxy will be created even if the <literal>proxyTargetClass</literal>
      property of the <classname>ProxyFactoryBean</classname> has been set to
      <literal>false</literal>. (Obviously this makes no sense, and is best
      removed from the bean definition because it is at best redundant, and at
      worst confusing.)</para>

      <para>If the target class implements one (or more) interfaces, then the
      type of proxy that is created depends on the configuration of the
      <classname>ProxyFactoryBean</classname>.</para>

      <para>If the <literal>proxyTargetClass</literal> property of the
      <classname>ProxyFactoryBean</classname> has been set to
      <literal>true</literal>, then a CGLIB-based proxy will be created. This
      makes sense, and is in keeping with the principle of least surprise.
      Even if the <literal>proxyInterfaces</literal> property of the
      <classname>ProxyFactoryBean</classname> has been set to one or more
      fully qualified interface names, the fact that the
      <literal>proxyTargetClass</literal> property is set to
      <literal>true</literal> <emphasis>will</emphasis> cause CGLIB-based
      proxying to be in effect.</para>

      <para>If the <literal>proxyInterfaces</literal> property of the
      <classname>ProxyFactoryBean</classname> has been set to one or more
      fully qualified interface names, then a JDK-based proxy will be created.
      The created proxy will implement all of the interfaces that were
      specified in the <literal>proxyInterfaces</literal> property; if the
      target class happens to implement a whole lot more interfaces than those
      specified in the <literal>proxyInterfaces</literal> property, that is
      all well and good but those additional interfaces will not be
      implemented by the returned proxy.</para>

      <para>If the <literal>proxyInterfaces</literal> property of the
      <classname>ProxyFactoryBean</classname> has <emphasis>not</emphasis>
      been set, but the target class <emphasis>does implement one (or
      more)</emphasis> interfaces, then the
      <classname>ProxyFactoryBean</classname> will auto-detect the fact that
      the target class does actually implement at least one interface, and a
      JDK-based proxy will be created. The interfaces that are actually
      proxied will be <emphasis>all</emphasis> of the interfaces that the
      target class implements; in effect, this is the same as simply supplying
      a list of each and every interface that the target class implements to
      the <literal>proxyInterfaces</literal> property. However, it is
      significantly less work, and less prone to typos.</para>
    </section>

    <section id="aop-api-proxying-intf">
      <title>Proxying interfaces</title>

      <para>Let's look at a simple example of
      <classname>ProxyFactoryBean</classname> in action. This example
      involves:</para>

      <itemizedlist>
        <listitem>
          <para>A <emphasis>target bean</emphasis> that will be proxied. This
          is the "personTarget" bean definition in the example below.</para>
        </listitem>

        <listitem>
          <para>An Advisor and an Interceptor used to provide advice.</para>
        </listitem>

        <listitem>
          <para>An AOP proxy bean definition specifying the target object (the
          personTarget bean) and the interfaces to proxy, along with the
          advices to apply.</para>
        </listitem>
      </itemizedlist>

      <para><programlisting language="xml">&lt;bean id="personTarget" class="com.mycompany.PersonImpl"&gt;
    &lt;property name="name" value="Tony"/&gt;
    &lt;property name="age" value="51"/&gt;
&lt;/bean&gt;

&lt;bean id="myAdvisor" class="com.mycompany.MyAdvisor"&gt;
    &lt;property name="someProperty" value="Custom string property value"/&gt;
&lt;/bean&gt;

&lt;bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"&gt;
&lt;/bean&gt;

&lt;bean id="person" 
    class="org.springframework.aop.framework.ProxyFactoryBean"&gt;
    &lt;property name="proxyInterfaces" value="com.mycompany.Person"/&gt;

    &lt;property name="target" ref="personTarget"/&gt;
    &lt;property name="interceptorNames"&gt;
        &lt;list&gt;
            &lt;value&gt;myAdvisor&lt;/value&gt;
            &lt;value&gt;debugInterceptor&lt;/value&gt;
        &lt;/list&gt;
    &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

      <para>Note that the <literal>interceptorNames</literal> property takes a
      list of String: the bean names of the interceptor or advisors in the
      current factory. Advisors, interceptors, before, after returning and
      throws advice objects can be used. The ordering of advisors is
      significant.</para>

      <note>
        <para>You might be wondering why the list doesn't hold bean
        references. The reason for this is that if the ProxyFactoryBean's
        singleton property is set to false, it must be able to return
        independent proxy instances. If any of the advisors is itself a
        prototype, an independent instance would need to be returned, so it's
        necessary to be able to obtain an instance of the prototype from the
        factory; holding a reference isn't sufficient.</para>
      </note>

      <para>The "person" bean definition above can be used in place of a
      Person implementation, as follows:</para>

      <programlisting language="java">Person person = (Person) factory.getBean("person");</programlisting>

      <para>Other beans in the same IoC context can express a strongly typed
      dependency on it, as with an ordinary Java object:</para>

      <para><programlisting language="xml">&lt;bean id="personUser" class="com.mycompany.PersonUser"&gt;
  &lt;property name="person"&gt;&lt;ref local="person"/&gt;&lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

      <para>The <literal>PersonUser</literal> class in this example would
      expose a property of type Person. As far as it's concerned, the AOP
      proxy can be used transparently in place of a "real" person
      implementation. However, its class would be a dynamic proxy class. It
      would be possible to cast it to the <literal>Advised</literal> interface
      (discussed below).</para>

      <para>It's possible to conceal the distinction between target and proxy
      using an anonymous <emphasis>inner bean</emphasis>, as follows. Only the
      <literal>ProxyFactoryBean</literal> definition is different; the advice
      is included only for completeness:</para>

      <para><programlisting language="xml">&lt;bean id="myAdvisor" class="com.mycompany.MyAdvisor"&gt;
  &lt;property name="someProperty" value="Custom string property value"/&gt;
&lt;/bean&gt;

&lt;bean id="debugInterceptor" class="org.springframework.aop.interceptor.DebugInterceptor"/&gt;

&lt;bean id="person" class="org.springframework.aop.framework.ProxyFactoryBean"&gt;
  &lt;property name="proxyInterfaces" value="com.mycompany.Person"/&gt;
  &lt;!-- Use inner bean, not local reference to target --&gt;
  &lt;property name="target"&gt;
    &lt;bean class="com.mycompany.PersonImpl"&gt;
      &lt;property name="name" value="Tony"/&gt;
      &lt;property name="age" value="51"/&gt;
    &lt;/bean&gt;
  &lt;/property&gt;
  &lt;property name="interceptorNames"&gt;
    &lt;list&gt;
      &lt;value&gt;myAdvisor&lt;/value&gt;
      &lt;value&gt;debugInterceptor&lt;/value&gt;
    &lt;/list&gt;
  &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

      <para>This has the advantage that there's only one object of type
      <literal>Person</literal>: useful if we want to prevent users of the
      application context from obtaining a reference to the un-advised object,
      or need to avoid any ambiguity with Spring IoC
      <emphasis>autowiring</emphasis>. There's also arguably an advantage in
      that the ProxyFactoryBean definition is self-contained. However, there
      are times when being able to obtain the un-advised target from the
      factory might actually be an <emphasis>advantage</emphasis>: for
      example, in certain test scenarios.</para>
    </section>

    <section id="aop-api-proxying-class">
      <title>Proxying classes</title>

      <para>What if you need to proxy a class, rather than one or more
      interfaces?</para>

      <para>Imagine that in our example above, there was no
      <literal>Person</literal> interface: we needed to advise a class called
      <literal>Person</literal> that didn't implement any business interface.
      In this case, you can configure Spring to use CGLIB proxying, rather
      than dynamic proxies. Simply set the <literal>proxyTargetClass</literal>
      property on the ProxyFactoryBean above to true. While it's best to
      program to interfaces, rather than classes, the ability to advise
      classes that don't implement interfaces can be useful when working with
      legacy code. (In general, Spring isn't prescriptive. While it makes it
      easy to apply good practices, it avoids forcing a particular
      approach.)</para>

      <para>If you want to, you can force the use of CGLIB in any case, even
      if you do have interfaces.</para>

      <para>CGLIB proxying works by generating a subclass of the target class
      at runtime. Spring configures this generated subclass to delegate method
      calls to the original target: the subclass is used to implement the
      <emphasis>Decorator</emphasis> pattern, weaving in the advice.</para>

      <para>CGLIB proxying should generally be transparent to users. However,
      there are some issues to consider:</para>

      <itemizedlist>
        <listitem>
          <para><literal>Final</literal> methods can't be advised, as they
          can't be overridden.</para>
        </listitem>

        <listitem>
          <para>You'll need the CGLIB 2 binaries on your classpath; dynamic
          proxies are available with the JDK.</para>
        </listitem>
      </itemizedlist>

      <para>There's little performance difference between CGLIB proxying and
      dynamic proxies. As of Spring 1.0, dynamic proxies are slightly faster.
      However, this may change in the future. Performance should not be a
      decisive consideration in this case.</para>
    </section>

    <section id="aop-global-advisors">
      <title>Using 'global' advisors</title>

      <para>By appending an asterisk to an interceptor name, all advisors with
      bean names matching the part before the asterisk, will be added to the
      advisor chain. This can come in handy if you need to add a standard set
      of 'global' advisors: <programlisting language="xml">
&lt;bean id="proxy" class="org.springframework.aop.framework.ProxyFactoryBean"&gt;
  &lt;property name="target" ref="service"/&gt;
  &lt;property name="interceptorNames"&gt;
    &lt;list&gt;
      &lt;value&gt;global*&lt;/value&gt;
    &lt;/list&gt;
  &lt;/property&gt;
&lt;/bean&gt;

&lt;bean id="global_debug" class="org.springframework.aop.interceptor.DebugInterceptor"/&gt;
&lt;bean id="global_performance" class="org.springframework.aop.interceptor.PerformanceMonitorInterceptor"/&gt;
</programlisting></para>
    </section>
  </section>

  <section id="aop-concise-proxy">
    <title>Concise proxy definitions</title>

    <para>Especially when defining transactional proxies, you may end up with
    many similar proxy definitions. The use of parent and child bean
    definitions, along with inner bean definitions, can result in much cleaner
    and more concise proxy definitions.</para>

    <para>First a parent, <emphasis>template</emphasis>, bean definition is
    created for the proxy:</para>

    <para><programlisting language="xml">&lt;bean id="txProxyTemplate" abstract="true"
        class="org.springframework.transaction.interceptor.TransactionProxyFactoryBean"&gt;
  &lt;property name="transactionManager" ref="transactionManager"/&gt;
  &lt;property name="transactionAttributes"&gt;
    &lt;props&gt;
      &lt;prop key="*"&gt;PROPAGATION_REQUIRED&lt;/prop&gt;
    &lt;/props&gt;
  &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

    <para>This will never be instantiated itself, so may actually be
    incomplete. Then each proxy which needs to be created is just a child bean
    definition, which wraps the target of the proxy as an inner bean
    definition, since the target will never be used on its own
    anyway.<programlisting language="xml">&lt;bean id="myService" parent="txProxyTemplate"&gt;
  &lt;property name="target"&gt;
    &lt;bean class="org.springframework.samples.MyServiceImpl"&gt;
    &lt;/bean&gt;
  &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

    <para>It is of course possible to override properties from the parent
    template, such as in this case, the transaction propagation
    settings:<programlisting language="xml">&lt;bean id="mySpecialService" parent="txProxyTemplate"&gt;
  &lt;property name="target"&gt;
    &lt;bean class="org.springframework.samples.MySpecialServiceImpl"&gt;
    &lt;/bean&gt;
  &lt;/property&gt;
  &lt;property name="transactionAttributes"&gt;
    &lt;props&gt;
      &lt;prop key="get*"&gt;PROPAGATION_REQUIRED,readOnly&lt;/prop&gt;
      &lt;prop key="find*"&gt;PROPAGATION_REQUIRED,readOnly&lt;/prop&gt;
      &lt;prop key="load*"&gt;PROPAGATION_REQUIRED,readOnly&lt;/prop&gt;
      &lt;prop key="store*"&gt;PROPAGATION_REQUIRED&lt;/prop&gt;
    &lt;/props&gt;
  &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

    <para>Note that in the example above, we have explicitly marked the parent
    bean definition as <emphasis>abstract</emphasis> by using the
    <emphasis>abstract</emphasis> attribute, as described <link
    linkend="beans-child-bean-definitions">previously</link>, so that it may
    not actually ever be instantiated. Application contexts (but not simple
    bean factories) will by default pre-instantiate all singletons. It is
    therefore important (at least for singleton beans) that if you have a
    (parent) bean definition which you intend to use only as a template, and
    this definition specifies a class, you must make sure to set the
    <emphasis>abstract</emphasis> attribute to <emphasis>true</emphasis>,
    otherwise the application context will actually try to pre-instantiate
    it.</para>
  </section>

  <section id="aop-prog">
    <title>Creating AOP proxies programmatically with the ProxyFactory</title>

    <para>It's easy to create AOP proxies programmatically using Spring. This
    enables you to use Spring AOP without dependency on Spring IoC.</para>

    <para>The following listing shows creation of a proxy for a target object,
    with one interceptor and one advisor. The interfaces implemented by the
    target object will automatically be proxied:</para>

    <para><programlisting language="java">ProxyFactory factory = new ProxyFactory(myBusinessInterfaceImpl);
factory.addInterceptor(myMethodInterceptor);
factory.addAdvisor(myAdvisor);
MyBusinessInterface tb = (MyBusinessInterface) factory.getProxy();</programlisting></para>

    <para>The first step is to construct an object of type
    <literal>org.springframework.aop.framework.ProxyFactory</literal>. You can
    create this with a target object, as in the above example, or specify the
    interfaces to be proxied in an alternate constructor.</para>

    <para>You can add interceptors or advisors, and manipulate them for the
    life of the ProxyFactory. If you add an
    IntroductionInterceptionAroundAdvisor you can cause the proxy to implement
    additional interfaces.</para>

    <para>There are also convenience methods on ProxyFactory (inherited from
    <classname>AdvisedSupport</classname>) which allow you to add other advice
    types such as before and throws advice. AdvisedSupport is the superclass
    of both ProxyFactory and ProxyFactoryBean.</para>

    <tip>
      <para>Integrating AOP proxy creation with the IoC framework is best
      practice in most applications. We recommend that you externalize
      configuration from Java code with AOP, as in general.</para>
    </tip>
  </section>

  <section id="aop-api-advised">
    <title>Manipulating advised objects</title>

    <para>However you create AOP proxies, you can manipulate them using the
    <literal>org.springframework.aop.framework.Advised</literal> interface.
    Any AOP proxy can be cast to this interface, whichever other interfaces it
    implements. This interface includes the following methods:</para>

    <programlisting language="java">Advisor[] getAdvisors();

void addAdvice(Advice advice) throws AopConfigException;

void addAdvice(int pos, Advice advice) 
        throws AopConfigException;

void addAdvisor(Advisor advisor) throws AopConfigException;

void addAdvisor(int pos, Advisor advisor) throws AopConfigException;

int indexOf(Advisor advisor);

boolean removeAdvisor(Advisor advisor) throws AopConfigException;

void removeAdvisor(int index) throws AopConfigException;

boolean replaceAdvisor(Advisor a, Advisor b) throws AopConfigException;

boolean isFrozen();</programlisting>

    <para>The <literal>getAdvisors()</literal> method will return an Advisor
    for every advisor, interceptor or other advice type that has been added to
    the factory. If you added an Advisor, the returned advisor at this index
    will be the object that you added. If you added an interceptor or other
    advice type, Spring will have wrapped this in an advisor with a pointcut
    that always returns true. Thus if you added a
    <literal>MethodInterceptor</literal>, the advisor returned for this index
    will be an <literal>DefaultPointcutAdvisor</literal> returning your
    <literal>MethodInterceptor</literal> and a pointcut that matches all
    classes and methods.</para>

    <para>The <literal>addAdvisor()</literal> methods can be used to add any
    Advisor. Usually the advisor holding pointcut and advice will be the
    generic <literal>DefaultPointcutAdvisor</literal>, which can be used with
    any advice or pointcut (but not for introductions).</para>

    <para>By default, it's possible to add or remove advisors or interceptors
    even once a proxy has been created. The only restriction is that it's
    impossible to add or remove an introduction advisor, as existing proxies
    from the factory will not show the interface change. (You can obtain a new
    proxy from the factory to avoid this problem.)</para>

    <para>A simple example of casting an AOP proxy to the
    <literal>Advised</literal> interface and examining and manipulating its
    advice:</para>

    <para><programlisting language="java">Advised advised = (Advised) myObject;
Advisor[] advisors = advised.getAdvisors();
int oldAdvisorCount = advisors.length;
System.out.println(oldAdvisorCount + " advisors");

// Add an advice like an interceptor without a pointcut
// Will match all proxied methods
// Can use for interceptors, before, after returning or throws advice
advised.addAdvice(new DebugInterceptor());

// Add selective advice using a pointcut
advised.addAdvisor(new DefaultPointcutAdvisor(mySpecialPointcut, myAdvice));

assertEquals("Added two advisors",
     oldAdvisorCount + 2, advised.getAdvisors().length);</programlisting></para>

    <note>
      <para>It's questionable whether it's advisable (no pun intended) to
      modify advice on a business object in production, although there are no
      doubt legitimate usage cases. However, it can be very useful in
      development: for example, in tests. I have sometimes found it very
      useful to be able to add test code in the form of an interceptor or
      other advice, getting inside a method invocation I want to test. (For
      example, the advice can get inside a transaction created for that
      method: for example, to run SQL to check that a database was correctly
      updated, before marking the transaction for roll back.)</para>
    </note>

    <para>Depending on how you created the proxy, you can usually set a
    <literal>frozen</literal> flag, in which case the
    <literal>Advised</literal> <literal>isFrozen()</literal> method will
    return true, and any attempts to modify advice through addition or removal
    will result in an <literal>AopConfigException</literal>. The ability to
    freeze the state of an advised object is useful in some cases, for
    example, to prevent calling code removing a security interceptor. It may
    also be used in Spring 1.1 to allow aggressive optimization if runtime
    advice modification is known not to be required.</para>
  </section>

  <section id="aop-autoproxy">
    <title>Using the "autoproxy" facility</title>

    <para>So far we've considered explicit creation of AOP proxies using a
    <literal>ProxyFactoryBean</literal> or similar factory bean.</para>

    <para>Spring also allows us to use "autoproxy" bean definitions, which can
    automatically proxy selected bean definitions. This is built on Spring
    "bean post processor" infrastructure, which enables modification of any
    bean definition as the container loads.</para>

    <para>In this model, you set up some special bean definitions in your XML
    bean definition file to configure the auto proxy infrastructure. This
    allows you just to declare the targets eligible for autoproxying: you
    don't need to use <literal>ProxyFactoryBean</literal>.</para>

    <para>There are two ways to do this:</para>

    <itemizedlist>
      <listitem>
        <para>Using an autoproxy creator that refers to specific beans in the
        current context.</para>
      </listitem>

      <listitem>
        <para>A special case of autoproxy creation that deserves to be
        considered separately; autoproxy creation driven by source-level
        metadata attributes.</para>
      </listitem>
    </itemizedlist>

    <section id="aop-autoproxy-choices">
      <title>Autoproxy bean definitions</title>

      <para>The <literal>org.springframework.aop.framework.autoproxy</literal>
      package provides the following standard autoproxy creators.</para>

      <section id="aop-api-autoproxy">
        <title>BeanNameAutoProxyCreator</title>

        <para>The <literal>BeanNameAutoProxyCreator</literal> class is a
        <literal>BeanPostProcessor</literal> that automatically creates AOP
        proxies for beans with names matching literal values or
        wildcards.</para>

        <para><programlisting language="xml">&lt;bean class="org.springframework.aop.framework.autoproxy.BeanNameAutoProxyCreator"&gt;
  &lt;property name="beanNames" value="jdk*,onlyJdk"/&gt;
  &lt;property name="interceptorNames"&gt;
    &lt;list&gt;
      &lt;value&gt;myInterceptor&lt;/value&gt;
    &lt;/list&gt;
  &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

        <para>As with <literal>ProxyFactoryBean</literal>, there is an
        <literal>interceptorNames</literal> property rather than a list of
        interceptors, to allow correct behavior for prototype advisors. Named
        "interceptors" can be advisors or any advice type.</para>

        <para>As with auto proxying in general, the main point of using
        <literal>BeanNameAutoProxyCreator</literal> is to apply the same
        configuration consistently to multiple objects, with minimal volume of
        configuration. It is a popular choice for applying declarative
        transactions to multiple objects.</para>

        <para>Bean definitions whose names match, such as "jdkMyBean" and
        "onlyJdk" in the above example, are plain old bean definitions with
        the target class. An AOP proxy will be created automatically by the
        <literal>BeanNameAutoProxyCreator</literal>. The same advice will be
        applied to all matching beans. Note that if advisors are used (rather
        than the interceptor in the above example), the pointcuts may apply
        differently to different beans.</para>
      </section>

      <section id="aop-api-autoproxy-default">
        <title>DefaultAdvisorAutoProxyCreator</title>

        <para>A more general and extremely powerful auto proxy creator is
        <literal>DefaultAdvisorAutoProxyCreator</literal>. This will
        automagically apply eligible advisors in the current context, without
        the need to include specific bean names in the autoproxy advisor's
        bean definition. It offers the same merit of consistent configuration
        and avoidance of duplication as
        <literal>BeanNameAutoProxyCreator</literal>.</para>

        <para>Using this mechanism involves:</para>

        <itemizedlist>
          <listitem>
            <para>Specifying a
            <literal>DefaultAdvisorAutoProxyCreator</literal> bean
            definition.</para>
          </listitem>

          <listitem>
            <para>Specifying any number of Advisors in the same or related
            contexts. Note that these <emphasis>must</emphasis> be Advisors,
            not just interceptors or other advices. This is necessary because
            there must be a pointcut to evaluate, to check the eligibility of
            each advice to candidate bean definitions.</para>
          </listitem>
        </itemizedlist>

        <para>The <literal>DefaultAdvisorAutoProxyCreator</literal> will
        automatically evaluate the pointcut contained in each advisor, to see
        what (if any) advice it should apply to each business object (such as
        "businessObject1" and "businessObject2" in the example).</para>

        <para>This means that any number of advisors can be applied
        automatically to each business object. If no pointcut in any of the
        advisors matches any method in a business object, the object will not
        be proxied. As bean definitions are added for new business objects,
        they will automatically be proxied if necessary.</para>

        <para>Autoproxying in general has the advantage of making it
        impossible for callers or dependencies to obtain an un-advised object.
        Calling getBean("businessObject1") on this ApplicationContext will
        return an AOP proxy, not the target business object. (The "inner bean"
        idiom shown earlier also offers this benefit.)</para>

        <para><programlisting language="xml">&lt;bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/&gt;

&lt;bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"&gt;
  &lt;property name="transactionInterceptor" ref="transactionInterceptor"/&gt;
&lt;/bean&gt;

&lt;bean id="customAdvisor" class="com.mycompany.MyAdvisor"/&gt;

&lt;bean id="businessObject1" class="com.mycompany.BusinessObject1"&gt;
  &lt;!-- Properties omitted --&gt;
&lt;/bean&gt;

&lt;bean id="businessObject2" class="com.mycompany.BusinessObject2"/&gt;
</programlisting></para>

        <para>The <literal>DefaultAdvisorAutoProxyCreator</literal> is very
        useful if you want to apply the same advice consistently to many
        business objects. Once the infrastructure definitions are in place,
        you can simply add new business objects without including specific
        proxy configuration. You can also drop in additional aspects very
        easily - for example, tracing or performance monitoring aspects - with
        minimal change to configuration.</para>

        <para>The DefaultAdvisorAutoProxyCreator offers support for filtering
        (using a naming convention so that only certain advisors are
        evaluated, allowing use of multiple, differently configured,
        AdvisorAutoProxyCreators in the same factory) and ordering. Advisors
        can implement the <literal>org.springframework.core.Ordered</literal>
        interface to ensure correct ordering if this is an issue. The
        TransactionAttributeSourceAdvisor used in the above example has a
        configurable order value; the default setting is unordered.</para>
      </section>

      <section id="aop-api-autoproxy-abstract">
        <title>AbstractAdvisorAutoProxyCreator</title>

        <para>This is the superclass of DefaultAdvisorAutoProxyCreator. You
        can create your own autoproxy creators by subclassing this class, in
        the unlikely event that advisor definitions offer insufficient
        customization to the behavior of the framework
        <literal>DefaultAdvisorAutoProxyCreator</literal>.</para>
      </section>
    </section>

    <section id="aop-autoproxy-metadata">
      <title>Using metadata-driven auto-proxying</title>

      <para>A particularly important type of autoproxying is driven by
      metadata. This produces a similar programming model to .NET
      <literal>ServicedComponents</literal>. Instead of using XML deployment
      descriptors as in EJB, configuration for transaction management and
      other enterprise services is held in source-level attributes.</para>

      <para>In this case, you use the
      <literal>DefaultAdvisorAutoProxyCreator</literal>, in combination with
      Advisors that understand metadata attributes. The metadata specifics are
      held in the pointcut part of the candidate advisors, rather than in the
      autoproxy creation class itself.</para>

      <para>This is really a special case of the
      <literal>DefaultAdvisorAutoProxyCreator</literal>, but deserves
      consideration on its own. (The metadata-aware code is in the pointcuts
      contained in the advisors, not the AOP framework itself.)</para>

      <para>The <literal>/attributes</literal> directory of the JPetStore
      sample application shows the use of attribute-driven autoproxying. In
      this case, there's no need to use the
      <literal>TransactionProxyFactoryBean</literal>. Simply defining
      transactional attributes on business objects is sufficient, because of
      the use of metadata-aware pointcuts. The bean definitions include the
      following code, in <literal>/WEB-INF/declarativeServices.xml</literal>.
      Note that this is generic, and can be used outside the JPetStore:</para>

      <para><programlisting language="xml">&lt;bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/&gt;

&lt;bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"&gt;
  &lt;property name="transactionInterceptor" ref="transactionInterceptor"/&gt;
&lt;/bean&gt;

&lt;bean id="transactionInterceptor"
    class="org.springframework.transaction.interceptor.TransactionInterceptor"&gt;
  &lt;property name="transactionManager" ref="transactionManager"/&gt;
  &lt;property name="transactionAttributeSource"&gt;
    &lt;bean class="org.springframework.transaction.interceptor.AttributesTransactionAttributeSource"&gt;
      &lt;property name="attributes" ref="attributes"/&gt;
    &lt;/bean&gt;
  &lt;/property&gt;
&lt;/bean&gt;

&lt;bean id="attributes" class="org.springframework.metadata.commons.CommonsAttributes"/&gt;</programlisting></para>

      <para>The <literal>DefaultAdvisorAutoProxyCreator</literal> bean
      definition (the name is not significant, hence it can even be omitted)
      will pick up all eligible pointcuts in the current application context.
      In this case, the "transactionAdvisor" bean definition, of type
      <literal>TransactionAttributeSourceAdvisor</literal>, will apply to
      classes or methods carrying a transaction attribute. The
      TransactionAttributeSourceAdvisor depends on a TransactionInterceptor,
      via constructor dependency. The example resolves this via autowiring.
      The <literal>AttributesTransactionAttributeSource</literal> depends on
      an implementation of the
      <literal>org.springframework.metadata.Attributes</literal> interface. In
      this fragment, the "attributes" bean satisfies this, using the Jakarta
      Commons Attributes API to obtain attribute information. (The application
      code must have been compiled using the Commons Attributes compilation
      task.)</para>

      <para>The <literal>/annotation</literal> directory of the JPetStore
      sample application contains an analogous example for auto-proxying
      driven by JDK 1.5+ annotations. The following configuration enables
      automatic detection of Spring's <literal>Transactional</literal>
      annotation, leading to implicit proxies for beans containing that
      annotation:</para>

      <para><programlisting language="xml">&lt;bean class="org.springframework.aop.framework.autoproxy.DefaultAdvisorAutoProxyCreator"/&gt;

&lt;bean class="org.springframework.transaction.interceptor.TransactionAttributeSourceAdvisor"&gt;
  &lt;property name="transactionInterceptor" ref="transactionInterceptor"/&gt;
&lt;/bean&gt;

&lt;bean id="transactionInterceptor"
    class="org.springframework.transaction.interceptor.TransactionInterceptor"&gt;
  &lt;property name="transactionManager" ref="transactionManager"/&gt;
  &lt;property name="transactionAttributeSource"&gt;
    &lt;bean class="org.springframework.transaction.annotation.AnnotationTransactionAttributeSource"/&gt;
  &lt;/property&gt;
&lt;/bean&gt;</programlisting></para>

      <para>The <literal>TransactionInterceptor</literal> defined here depends
      on a <literal>PlatformTransactionManager</literal> definition, which is
      not included in this generic file (although it could be) because it will
      be specific to the application's transaction requirements (typically
      JTA, as in this example, or Hibernate, JDO or JDBC):</para>

      <programlisting language="xml">&lt;bean id="transactionManager" 
    class="org.springframework.transaction.jta.JtaTransactionManager"/&gt;</programlisting>

      <tip>
        <para>If you require only declarative transaction management, using
        these generic XML definitions will result in Spring automatically
        proxying all classes or methods with transaction attributes. You won't
        need to work directly with AOP, and the programming model is similar
        to that of .NET ServicedComponents.</para>
      </tip>

      <para>This mechanism is extensible. It's possible to do autoproxying
      based on custom attributes. You need to:</para>

      <itemizedlist>
        <listitem>
          <para>Define your custom attribute.</para>
        </listitem>

        <listitem>
          <para>Specify an Advisor with the necessary advice, including a
          pointcut that is triggered by the presence of the custom attribute
          on a class or method. You may be able to use an existing advice,
          merely implementing a static pointcut that picks up the custom
          attribute.</para>
        </listitem>
      </itemizedlist>

      <para>It's possible for such advisors to be unique to each advised class
      (for example, mixins): they simply need to be defined as prototype,
      rather than singleton, bean definitions. For example, the
      <literal>LockMixin</literal> introduction interceptor from the Spring
      test suite, shown above, could be used in conjunction with an
      attribute-driven pointcut to target a mixin, as shown here. We use the
      generic <literal>DefaultPointcutAdvisor</literal>, configured using
      JavaBean properties:</para>

      <para><programlisting language="xml">&lt;bean id="lockMixin" class="org.springframework.aop.LockMixin"
    scope="prototype"/&gt;

&lt;bean id="lockableAdvisor" class="org.springframework.aop.support.DefaultPointcutAdvisor"
    scope="prototype"&gt;
  &lt;property name="pointcut" ref="myAttributeAwarePointcut"/&gt;
  &lt;property name="advice" ref="lockMixin"/&gt;
&lt;/bean&gt;

&lt;bean id="anyBean" class="anyclass" ...</programlisting></para>

      <para>If the attribute aware pointcut matches any methods in the
      <literal>anyBean</literal> or other bean definitions, the mixin will be
      applied. Note that both <literal>lockMixin</literal> and
      <literal>lockableAdvisor</literal> definitions are prototypes. The
      <literal>myAttributeAwarePointcut</literal> pointcut can be a singleton
      definition, as it doesn't hold state for individual advised
      objects.</para>
    </section>
  </section>

  <section id="aop-targetsource">
    <title>Using TargetSources</title>

    <para>Spring offers the concept of a <emphasis>TargetSource</emphasis>,
    expressed in the <literal>org.springframework.aop.TargetSource</literal>
    interface. This interface is responsible for returning the "target object"
    implementing the join point. The <literal>TargetSource</literal>
    implementation is asked for a target instance each time the AOP proxy
    handles a method invocation.</para>

    <para>Developers using Spring AOP don't normally need to work directly
    with TargetSources, but this provides a powerful means of supporting
    pooling, hot swappable and other sophisticated targets. For example, a
    pooling TargetSource can return a different target instance for each
    invocation, using a pool to manage instances.</para>

    <para>If you do not specify a TargetSource, a default implementation is
    used that wraps a local object. The same target is returned for each
    invocation (as you would expect).</para>

    <para>Let's look at the standard target sources provided with Spring, and
    how you can use them.</para>

    <tip>
      <para>When using a custom target source, your target will usually need
      to be a prototype rather than a singleton bean definition. This allows
      Spring to create a new target instance when required.</para>
    </tip>

    <section id="aop-ts-swap">
      <title>Hot swappable target sources</title>

      <para>The
      <literal>org.springframework.aop.target.HotSwappableTargetSource</literal>
      exists to allow the target of an AOP proxy to be switched while allowing
      callers to keep their references to it.</para>

      <para>Changing the target source's target takes effect immediately. The
      <literal>HotSwappableTargetSource</literal> is threadsafe.</para>

      <para>You can change the target via the <literal>swap()</literal> method
      on HotSwappableTargetSource as follows:</para>

      <para><programlisting language="java">HotSwappableTargetSource swapper = 
    (HotSwappableTargetSource) beanFactory.getBean("swapper");
Object oldTarget = swapper.swap(newTarget);</programlisting></para>

      <para>The XML definitions required look as follows:</para>

      <para><programlisting language="xml">&lt;bean id="initialTarget" class="mycompany.OldTarget"/&gt;

&lt;bean id="swapper" class="org.springframework.aop.target.HotSwappableTargetSource"&gt;
  &lt;constructor-arg ref="initialTarget"/&gt;
&lt;/bean&gt;

&lt;bean id="swappable" class="org.springframework.aop.framework.ProxyFactoryBean"&gt;
  &lt;property name="targetSource" ref="swapper"/&gt;
&lt;/bean&gt;</programlisting></para>

      <para>The above <literal>swap()</literal> call changes the target of the
      swappable bean. Clients who hold a reference to that bean will be
      unaware of the change, but will immediately start hitting the new
      target.</para>

      <para>Although this example doesn't add any advice - and it's not
      necessary to add advice to use a <literal>TargetSource</literal> - of
      course any <literal>TargetSource</literal> can be used in conjunction
      with arbitrary advice.</para>
    </section>

    <section id="aop-ts-pool">
      <title>Pooling target sources</title>

      <para>Using a pooling target source provides a similar programming model
      to stateless session EJBs, in which a pool of identical instances is
      maintained, with method invocations going to free objects in the
      pool.</para>

      <para>A crucial difference between Spring pooling and SLSB pooling is
      that Spring pooling can be applied to any POJO. As with Spring in
      general, this service can be applied in a non-invasive way.</para>

      <para>Spring provides out-of-the-box support for Jakarta Commons Pool
      1.3, which provides a fairly efficient pooling implementation. You'll
      need the commons-pool Jar on your application's classpath to use this
      feature. It's also possible to subclass
      <literal>org.springframework.aop.target.AbstractPoolingTargetSource</literal>
      to support any other pooling API.</para>

      <para>Sample configuration is shown below:</para>

      <para><programlisting language="xml">&lt;bean id="businessObjectTarget" class="com.mycompany.MyBusinessObject" 
    scope="prototype"&gt;
  ... properties omitted
&lt;/bean&gt;

&lt;bean id="poolTargetSource" class="org.springframework.aop.target.CommonsPoolTargetSource"&gt;
  &lt;property name="targetBeanName" value="businessObjectTarget"/&gt;
  &lt;property name="maxSize" value="25"/&gt;
&lt;/bean&gt;

&lt;bean id="businessObject" class="org.springframework.aop.framework.ProxyFactoryBean"&gt;
  &lt;property name="targetSource" ref="poolTargetSource"/&gt;
  &lt;property name="interceptorNames" value="myInterceptor"/&gt;
&lt;/bean&gt;</programlisting></para>

      <para>Note that the target object - "businessObjectTarget" in the
      example - <emphasis>must</emphasis> be a prototype. This allows the
      <literal>PoolingTargetSource</literal> implementation to create new
      instances of the target to grow the pool as necessary. See the havadoc
      for <literal>AbstractPoolingTargetSource</literal> and the concrete
      subclass you wish to use for information about its properties: "maxSize"
      is the most basic, and always guaranteed to be present.</para>

      <para>In this case, "myInterceptor" is the name of an interceptor that
      would need to be defined in the same IoC context. However, it isn't
      necessary to specify interceptors to use pooling. If you want only
      pooling, and no other advice, don't set the interceptorNames property at
      all.</para>

      <para>It's possible to configure Spring so as to be able to cast any
      pooled object to the
      <literal>org.springframework.aop.target.PoolingConfig</literal>
      interface, which exposes information about the configuration and current
      size of the pool through an introduction. You'll need to define an
      advisor like this:</para>

      <para><programlisting language="xml">&lt;bean id="poolConfigAdvisor" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean"&gt;
  &lt;property name="targetObject" ref="poolTargetSource"/&gt;
  &lt;property name="targetMethod" value="getPoolingConfigMixin"/&gt;
&lt;/bean&gt;</programlisting></para>

      <para>This advisor is obtained by calling a convenience method on the
      <literal>AbstractPoolingTargetSource</literal> class, hence the use of
      MethodInvokingFactoryBean. This advisor's name ("poolConfigAdvisor"
      here) must be in the list of interceptors names in the ProxyFactoryBean
      exposing the pooled object.</para>

      <para>The cast will look as follows:</para>

      <programlisting language="java">PoolingConfig conf = (PoolingConfig) beanFactory.getBean("businessObject");
System.out.println("Max pool size is " + conf.getMaxSize());</programlisting>

      <note>
        <para>Pooling stateless service objects is not usually necessary. We
        don't believe it should be the default choice, as most stateless
        objects are naturally thread safe, and instance pooling is problematic
        if resources are cached.</para>
      </note>

      <para>Simpler pooling is available using autoproxying. It's possible to
      set the TargetSources used by any autoproxy creator.</para>
    </section>

    <section id="aop-ts-prototype">
      <title>Prototype target sources</title>

      <para>Setting up a "prototype" target source is similar to a pooling
      TargetSource. In this case, a new instance of the target will be created
      on every method invocation. Although the cost of creating a new object
      isn't high in a modern JVM, the cost of wiring up the new object
      (satisfying its IoC dependencies) may be more expensive. Thus you
      shouldn't use this approach without very good reason.</para>

      <para>To do this, you could modify the
      <literal>poolTargetSource</literal> definition shown above as follows.
      (I've also changed the name, for clarity.)</para>

      <programlisting language="xml">&lt;bean id="prototypeTargetSource" class="org.springframework.aop.target.PrototypeTargetSource"&gt;
  &lt;property name="targetBeanName" ref="businessObjectTarget"/&gt;
&lt;/bean&gt;</programlisting>

      <para>There's only one property: the name of the target bean.
      Inheritance is used in the TargetSource implementations to ensure
      consistent naming. As with the pooling target source, the target bean
      must be a prototype bean definition.</para>
    </section>

    <section id="aop-ts-threadlocal">
      <title><classname>ThreadLocal</classname> target sources</title>

      <para><classname>ThreadLocal</classname> target sources are useful if
      you need an object to be created for each incoming request (per thread
      that is). The concept of a <classname>ThreadLocal</classname> provide a
      JDK-wide facility to transparently store resource alongside a thread.
      Setting up a <classname>ThreadLocalTargetSource</classname> is pretty
      much the same as was explained for the other types of target
      source:</para>

      <programlisting language="xml">&lt;bean id="threadlocalTargetSource" class="org.springframework.aop.target.ThreadLocalTargetSource"&gt;
  &lt;property name="targetBeanName" value="businessObjectTarget"/&gt;
&lt;/bean&gt;</programlisting>

      <note>
        <para>ThreadLocals come with serious issues (potentially resulting in
        memory leaks) when incorrectly using them in a multi-threaded and
        multi-classloader environments. One should always consider wrapping a
        threadlocal in some other class and never directly use the
        <classname>ThreadLocal</classname> itself (except of course in the
        wrapper class). Also, one should always remember to correctly set and
        unset (where the latter simply involved a call to
        <literal>ThreadLocal.set(null)</literal>) the resource local to the
        thread. Unsetting should be done in any case since not unsetting it
        might result in problematic behavior. Spring's ThreadLocal support
        does this for you and should always be considered in favor of using
        ThreadLocals without other proper handling code.</para>
      </note>
    </section>
  </section>

  <section id="aop-extensibility">
    <title>Defining new <interfacename>Advice</interfacename> types</title>

    <para>Spring AOP is designed to be extensible. While the interception
    implementation strategy is presently used internally, it is possible to
    support arbitrary advice types in addition to the out-of-the-box
    interception around advice, before, throws advice and after returning
    advice.</para>

    <para>The <literal>org.springframework.aop.framework.adapter</literal>
    package is an SPI package allowing support for new custom advice types to
    be added without changing the core framework. The only constraint on a
    custom <interfacename>Advice</interfacename> type is that it must
    implement the <interfacename>org.aopalliance.aop.Advice</interfacename>
    tag interface.</para>

    <para>Please refer to the
    <literal>org.springframework.aop.framework.adapter</literal> package's
    Javadocs for further information.</para>
  </section>

  <section id="aop-api-resources">
    <title>Further resources</title>

    <para>Please refer to the Spring sample applications for further examples
    of Spring AOP:</para>

    <itemizedlist>
      <listitem>
        <para>The JPetStore's default configuration illustrates the use of the
        <classname>TransactionProxyFactoryBean</classname> for declarative
        transaction management.</para>
      </listitem>

      <listitem>
        <para>The <literal>/attributes</literal> directory of the JPetStore
        illustrates the use of attribute-driven declarative transaction
        management.</para>
      </listitem>
    </itemizedlist>
  </section>
</chapter>