spring-framework/spring-framework-reference/src/dynamic-languages.xml

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<!DOCTYPE chapter PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
 "http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">

<chapter id="dynamic-language">
  <title>Dynamic language support</title>

  <section id="dynamic-language-introduction">
    <title>Introduction</title>
    <sidebar>
      <title>Why only these languages?</title>
      <para>
        The supported languages were chosen because a) the languages
        have a lot of traction in the Java enterprise community, b) no requests were made
        for other languages within the Spring 2.0 development timeframe, and
        c) the Spring developers were most familiar with them.
      </para>
      <para>
        There is nothing stopping the inclusion of further languages though. If you want
        to see support for &lt;<emphasis>insert your favourite dynamic language here</emphasis>&gt;,
        you can always raise an issue on Spring's
        <ulink url="http://opensource.atlassian.com/projects/spring/secure/Dashboard.jspa">JIRA</ulink>
        page (or implement such support yourself).
      </para>
    </sidebar>
    <para>Spring 2.0 introduces comprehensive support for using classes and objects that have
    been defined using a dynamic language (such as JRuby) with Spring.
    This support allows you to write any number of classes in a supported dynamic language,
    and have the Spring container transparently instantiate, configure and dependency inject the
    resulting objects.</para>
    <para>The dynamic languages currently supported are:</para>
    <itemizedlist>
      <listitem>
        <para>JRuby 0.9 / 1.0</para>
      </listitem>
      <listitem>
        <para>Groovy 1.0 / 1.5</para>
      </listitem>
      <listitem>
        <para>BeanShell 2.0</para>
      </listitem>
    </itemizedlist>
    <para>
      Fully working examples of where this dynamic language support can be immediately useful
      are described in <xref linkend="dynamic-language-scenarios"/>.
    </para>
    <para>
      <emphasis>Note:</emphasis> Only the specific versions as listed above are supported
      in Spring 2.5. In particular, JRuby 1.1 (which introduced many incompatible API
      changes) is <emphasis>not</emphasis> supported at this point of time.
    </para>
  </section>

  <section id="dynamic-language-a-first-example">
    <title>A first example</title>
    <para>
      This bulk of this chapter is concerned with describing the dynamic language support
      in detail. Before diving into all of the ins and outs of the dynamic language support,
      let's look at a quick example of a bean defined in a dynamic language.
      The dynamic language for this first bean is Groovy (the basis of this example
      was taken from the Spring test suite, so if you want to see equivalent examples
      in any of the other supported languages, take a look at the source code).
    </para>
    <para>
      Find below the <interfacename>Messenger</interfacename> interface that the
      Groovy bean is going to be implementing, and note that this interface is defined
      in plain Java. Dependent objects that are injected with a reference to the
      <interfacename>Messenger</interfacename> won't know that the underlying
      implementation is a Groovy script.
    </para>
    <programlisting source="java"><![CDATA[package org.springframework.scripting;

public interface Messenger {

    String getMessage();
}]]></programlisting>
    <para>
      Here is the definition of a class that has a dependency on the
      <interfacename>Messenger</interfacename> interface.
    </para>
    <programlisting source="java"><![CDATA[package org.springframework.scripting;

public class DefaultBookingService implements BookingService {

    private Messenger messenger;

    public void setMessenger(Messenger messenger) {
        this.messenger = messenger;
    }

    public void processBooking() {
        // use the injected Messenger object...
    }
}]]></programlisting>
    <para>Here is an implementation of the <interfacename>Messenger</interfacename> interface
    in Groovy.</para>
    <programlisting source="java"><![CDATA[// from the file 'Messenger.groovy'
package org.springframework.scripting.groovy;

// import the Messenger interface (written in Java) that is to be implemented
import org.springframework.scripting.Messenger

// define the implementation in Groovy
class GroovyMessenger implements Messenger {

    String message
}]]></programlisting>
    <para>
      Finally, here are the bean definitions that will effect the injection of the
      Groovy-defined <interfacename>Messenger</interfacename> implementation into
      an instance of the <classname>DefaultBookingService</classname> class.
    </para>
    <note>
      <para>
        To use the custom dynamic language tags to define dynamic-language-backed beans,
        you need to have the XML Schema preamble at the top of your Spring XML
        configuration file. You also need to be using a Spring
        <interfacename>ApplicationContext</interfacename> implementation as your
        IoC container. Using the dynamic-language-backed beans with a plain
        <interfacename>BeanFactory</interfacename> implementation is supported,
        but you have to manage the plumbing of the Spring internals to do so.
      </para>
      <para>For more information on schema-based configuration, see <xref linkend="xsd-config"/>.</para>
    </note>
    <programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:lang="http://www.springframework.org/schema/lang"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang-3.0.xsd">

    ]]><lineannotation>&lt;!-- this is the bean definition for the Groovy-backed <interfacename>Messenger</interfacename> implementation --&gt;</lineannotation><![CDATA[
    <lang:groovy id="messenger" script-source="classpath:Messenger.groovy">
        <lang:property name="message" value="I Can Do The Frug" />
    </lang:groovy>

    ]]><lineannotation>&lt;!-- an otherwise normal bean that will be injected by the Groovy-backed <interfacename>Messenger</interfacename> --&gt;</lineannotation><![CDATA[
    <bean id="bookingService" class="x.y.DefaultBookingService">
        <property name="messenger" ref="messenger" />
    </bean>

</beans>]]></programlisting>
    <para>
      The <literal>bookingService</literal> bean (a
      <classname>DefaultBookingService</classname>) can now use its private
      <literal>messenger</literal> member variable as normal because the
      <interfacename>Messenger</interfacename> instance that was injected
      into it <emphasis>is</emphasis> a <interfacename>Messenger</interfacename>
      instance. There is nothing special going on here, just plain Java and
      plain Groovy.
    </para>
    <para>
      Hopefully the above XML snippet is self-explanatory, but don't worry
      unduly if it isn't. Keep reading for the in-depth detail on the whys
      and wherefores of the above configuration.
    </para>
  </section>

  <section id="dynamic-language-beans">
    <title>Defining beans that are backed by dynamic languages</title>
    <para>
      This section describes exactly how you define Spring managed beans in
      any of the supported dynamic languages.
    </para>
    <para>
      Please note that this chapter does not attempt to explain the syntax and
      idioms of the supported dynamic languages. For example, if you want to
      use Groovy to write certain of the classes in your application, then the
      assumption is that you already know Groovy. If you need further details
      about the dynamic languages themselves, please consult
      <xref linkend="dynamic-language-resources" /> at the end of this chapter.
    </para>

    <section id="dynamic-language-beans-concepts">
      <title>Common concepts</title>
      <para>The steps involved in using dynamic-language-backed beans are as follows:</para>
      <orderedlist numeration="arabic">
        <listitem>
          <para>Write the test for the dynamic language source code (naturally)</para>
        </listitem>
        <listitem>
          <para><emphasis>Then</emphasis> write the dynamic language source code itself :)</para>
        </listitem>
        <listitem>
          <para>
            Define your dynamic-language-backed beans using the appropriate
            <literal>&lt;lang:language/&gt;</literal> element in the XML
            configuration (you can of course define such beans programmatically
            using the Spring API - although you will have to consult the source
            code for directions on how to do this as this type of advanced
            configuration is not covered in this chapter). Note this is an iterative
            step. You will need at least one bean definition per dynamic
            language source file (although the same dynamic language source
            file can of course be referenced by multiple bean definitions).
          </para>
        </listitem>
      </orderedlist>
      <para>
        The first two steps (testing and writing your dynamic language source files)
        are beyond the scope of this chapter. Refer to the language specification
        and / or reference manual for your chosen dynamic language and crack on with
        developing your dynamic language source files. You <emphasis>will</emphasis>
        first want to read the rest of this chapter though, as Spring's dynamic language
        support does make some (small) assumptions about the contents of your dynamic
        language source files.
      </para>

      <section id="dynamic-language-beans-concepts-xml-language-element">
        <title>The <literal>&lt;lang:language/&gt;</literal> element</title>
        <sidebar>
          <title>XML Schema</title>
          <para>
            All of the configuration examples in this chapter make use of the
            new XML Schema support that was added in Spring 2.0.
          </para>
          <para>It is possible to forego the use of XML Schema and stick with the old-style
          DTD based validation of your Spring XML files, but then you lose out
          on the convenience offered by the <literal>&lt;lang:language/&gt;</literal>
          element. See the Spring test suite for examples of the older style
          configuration that doesn't require XML Schema-based validation
          (it is quite verbose and doesn't hide any of the underlying Spring
          implementation from you).</para>
        </sidebar>
        <para>
          The final step involves defining dynamic-language-backed bean definitions,
          one for each bean that you want to configure (this is no different from
          normal JavaBean configuration). However, instead of specifying the
          fully qualified classname of the class that is to be instantiated and
          configured by the container, you use the <literal>&lt;lang:language/&gt;</literal>
          element to define the dynamic language-backed bean.
        </para>
        <para>
          Each of the supported languages has a corresponding
          <literal>&lt;lang:language/&gt;</literal> element:
        </para>
        <itemizedlist>
          <listitem>
            <para><literal>&lt;lang:jruby/&gt;</literal> (JRuby)</para>
          </listitem>
          <listitem>
            <para><literal>&lt;lang:groovy/&gt;</literal> (Groovy)</para>
          </listitem>
          <listitem>
            <para><literal>&lt;lang:bsh/&gt;</literal> (BeanShell)</para>
          </listitem>
        </itemizedlist>
        <para>
          The exact attributes and child elements that are available for
          configuration depends on exactly which language the bean has been
          defined in (the language-specific sections below provide the full
          lowdown on this).
        </para>
      </section>

      <section id="dynamic-language-refreshable-beans">
        <title>Refreshable beans</title>
        <para>
          One of the (if not <emphasis>the</emphasis>) most compelling value adds
          of the dynamic language support in Spring is the
          <firstterm>'refreshable bean'</firstterm> feature.
        </para>
        <para>
          A refreshable bean is a dynamic-language-backed bean that with a small
          amount of configuration, a dynamic-language-backed bean can monitor
          changes in its underlying source file resource, and then reload itself
          when the dynamic language source file is changed (for example when a
          developer edits and saves changes to the file on the filesystem).
        </para>
        <para>
          This allows a developer to deploy any number of dynamic language source
          files as part of an application, configure the Spring container to create
          beans backed by dynamic language source files (using the mechanisms
          described in this chapter), and then later, as requirements change or
          some other external factor comes into play, simply edit a dynamic language
          source file and have any change they make reflected in the bean that is
          backed by the changed dynamic language source file. There is no need to
          shut down a running application (or redeploy in the case of a web application).
          The dynamic-language-backed bean so amended will pick up the new state
          and logic from the changed dynamic language source file.
        </para>
        <note>
          <para>Please note that this feature is <emphasis>off</emphasis> by default.</para>
        </note>
        <para>
          Let's take a look at an example to see just how easy it is to start using
          refreshable beans. To <emphasis>turn on</emphasis> the refreshable beans
          feature, you simply have to specify exactly <emphasis>one</emphasis>
          additional attribute on the <literal>&lt;lang:language/&gt;</literal> element
          of your bean definition. So if we stick with
          <link linkend="dynamic-language-a-first-example">the example</link> from earlier
          in this chapter, here's what we would change in the Spring XML configuration
          to effect refreshable beans:
        </para>
        <programlisting language="xml"><![CDATA[<beans>

    ]]><lineannotation>&lt;!-- this bean is now 'refreshable' due to the presence of the 'refresh-check-delay' attribute --&gt;</lineannotation><![CDATA[
    <lang:groovy id="messenger"
          refresh-check-delay="5000" ]]><lineannotation>&lt;!-- switches refreshing on with 5 seconds between checks --&gt;</lineannotation><![CDATA[
          script-source="classpath:Messenger.groovy">
        <lang:property name="message" value="I Can Do The Frug" />
    </lang:groovy>

    <bean id="bookingService" class="x.y.DefaultBookingService">
        <property name="messenger" ref="messenger" />
    </bean>

</beans>]]></programlisting>
        <para>
          That really is all you have to do. The <literal>'refresh-check-delay'</literal>
          attribute defined on the <literal>'messenger'</literal> bean definition
          is the number of milliseconds after which the bean will be refreshed with
          any changes made to the underlying dynamic language source file.
          You can turn off the refresh behavior by assigning a negative value
          to the <literal>'refresh-check-delay'</literal> attribute.
          Remember that, by default, the refresh behavior is disabled. If you don't
          want the refresh behavior, then simply don't define the attribute.
        </para>
        <para>
          If we then run the following application we can exercise the refreshable feature;
          please do excuse the <emphasis>'jumping-through-hoops-to-pause-the-execution'</emphasis>
          shenanigans in this next slice of code. The <literal>System.in.read()</literal>
          call is only there so that the execution of the program pauses while I (the author)
          go off and edit the underlying dynamic language source file so that the refresh will
          trigger on the dynamic-language-backed bean when the program resumes execution.</para>
        <programlisting source="java"><![CDATA[import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scripting.Messenger;

public final class Boot {

    public static void main(final String[] args) throws Exception {

        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        Messenger messenger = (Messenger) ctx.getBean("messenger");
        System.out.println(messenger.getMessage());
        // pause execution while I go off and make changes to the source file...
        System.in.read();
        System.out.println(messenger.getMessage());
    }
}]]></programlisting>
        <para>
          Let's assume then, for the purposes of this example, that all
          calls to the <literal>getMessage()</literal> method of
          <interfacename>Messenger</interfacename> implementations have to be
          changed such that the message is surrounded by quotes.
          Below are the changes that I (the author) make to the
          <filename>Messenger.groovy</filename> source file when the execution of
          the program is paused.
        </para>
        <programlisting source="java"><![CDATA[package org.springframework.scripting

class GroovyMessenger implements Messenger {

    private String message = "Bingo"

    public String getMessage() {
        // change the implementation to surround the message in quotes
        return "'" + this.message + "'"
    }

    public void setMessage(String message) {
        this.message = message
    }
}]]></programlisting>
        <para>
          When the program executes, the output before the input pause will be
          <computeroutput>I Can Do The Frug</computeroutput>. After the change
          to the source file is made and saved, and the program resumes execution,
          the result of calling the <literal>getMessage()</literal> method on the
          dynamic-language-backed <interfacename>Messenger</interfacename> implementation
          will be <computeroutput>'I Can Do The Frug'</computeroutput> (notice
          the inclusion of the additional quotes).
        </para>
        <para>
          It is important to understand that changes to a script will
          <emphasis>not</emphasis> trigger a refresh if the changes occur
          within the window of the <literal>'refresh-check-delay'</literal> value.
          It is equally important to understand that changes to the script are
          <emphasis>not</emphasis> actually 'picked up' until a method is called
          on the dynamic-language-backed bean. It is only when a method is called on a
          dynamic-language-backed bean that it checks to see if its underlying script
          source has changed. Any exceptions relating to refreshing the script
          (such as encountering a compilation error, or finding that the script
          file has been deleted) will result in a <emphasis>fatal</emphasis>
          exception being propagated to the calling code.
        </para>
        <para>
          The refreshable bean behavior described above does
          <emphasis>not</emphasis> apply to dynamic language source files
          defined using the <literal>&lt;lang:inline-script/&gt;</literal> element
          notation (see <xref linkend="dynamic-language-beans-inline" />).
          Additionally, it <emphasis>only</emphasis> applies to beans where
          changes to the underlying source file can actually be detected;
          for example, by code that checks the last modified date of a
          dynamic language source file that exists on the filesystem.
        </para>
      </section>

      <section id="dynamic-language-beans-inline">
        <title>Inline dynamic language source files</title>
        <para>
          The dynamic language support can also cater for dynamic language
          source files that are embedded directly in Spring bean definitions.
          More specifically, the <literal>&lt;lang:inline-script/&gt;</literal>
          element allows you to define dynamic language source immediately
          inside a Spring configuration file. An example will perhaps make the
          inline script feature crystal clear:
        </para>
        <programlisting language="xml"><![CDATA[<lang:groovy id="messenger">
    <lang:inline-script>
package org.springframework.scripting.groovy;

import org.springframework.scripting.Messenger

class GroovyMessenger implements Messenger {

    String message
}
    </lang:inline-script>
    <lang:property name="message" value="I Can Do The Frug" />
</lang:groovy>]]></programlisting>
        <para>
          If we put to one side the issues surrounding whether it is good practice
          to define dynamic language source inside a Spring configuration file, the
          <literal>&lt;lang:inline-script/&gt;</literal> element can be useful in
          some scenarios. For instance, we might want to quickly add a Spring
          <interfacename>Validator</interfacename> implementation to a Spring MVC
          <interfacename>Controller</interfacename>. This is but a moment's work
          using inline source. (See <xref linkend="dynamic-language-scenarios-validators" />
          for such an example.)
        </para>
        <para>
          Find below an example of defining the source for a JRuby-based bean
          directly in a Spring XML configuration file using the
          <literal>inline:</literal> notation. (Notice the use of the <![CDATA[&lt;]]>
          characters to denote a <literal>'&lt;'</literal> character. In such a case
          surrounding the inline source in a <literal>&lt;![CDATA[]]&gt;</literal> region might be better.)
        </para>
        <programlisting language="xml"><![CDATA[<lang:jruby id="messenger" script-interfaces="org.springframework.scripting.Messenger">
    <lang:inline-script>
require 'java'

include_class 'org.springframework.scripting.Messenger'

class RubyMessenger &lt; Messenger

 def setMessage(message)
  @@message = message
 end

 def getMessage
  @@message
 end
 
end
    </lang:inline-script>
    <lang:property name="message" value="Hello World!" />
</lang:jruby>]]></programlisting>
      </section>

      <section id="dynamic-language-beans-ctor-injection">
        <title>Understanding Constructor Injection in the context of dynamic-language-backed beans</title>
        <para>
          There is one <emphasis>very</emphasis> important thing to be aware of
          with regard to Spring's dynamic language support. Namely, it is not (currently)
          possible to supply constructor arguments to dynamic-language-backed beans (and hence
          constructor-injection is not available for dynamic-language-backed beans).
          In the interests of making this special handling of constructors and
          properties 100% clear, the following mixture of code and configuration
          will <emphasis>not</emphasis> work.
        </para>
        <programlisting source="java"><![CDATA[// from the file 'Messenger.groovy'
package org.springframework.scripting.groovy;

import org.springframework.scripting.Messenger

class GroovyMessenger implements Messenger {

    GroovyMessenger() {}

    // this constructor is not available for Constructor Injection
    GroovyMessenger(String message) {
        this.message = message;
    }

    String message

    String anotherMessage
}]]></programlisting>
        <programlisting language="xml"><![CDATA[<lang:groovy id="badMessenger"
    script-source="classpath:Messenger.groovy">
]]>
    <lineannotation>&lt;!-- this next constructor argument will *not* be injected into the <classname>GroovyMessenger</classname> --&gt;</lineannotation>
    <lineannotation>&lt;!--     in fact, this isn't even allowed according to the schema --&gt;</lineannotation><![CDATA[
    <constructor-arg value="]]><lineannotation><emphasis role="bold">This will *not* work</emphasis></lineannotation><![CDATA[" />]]>
    
    <lineannotation>&lt;!-- <emphasis role="bold">only</emphasis> property values are injected into the dynamic-language-backed object --&gt;</lineannotation><![CDATA[
    <lang:property name="anotherMessage" value="Passed straight through to the dynamic-language-backed object" />

</lang>]]></programlisting>
        <para>
          In practice this limitation is not as significant as it first appears since
          setter injection is the injection style favored by the overwhelming majority
          of developers anyway (let's leave the discussion as to whether that is a good
          thing to another day).
        </para>
      </section>
    </section>

    <section id="dynamic-language-beans-jruby">
      <title>JRuby beans</title>

      <sidebar>
        <title>The JRuby library dependencies</title>
        <para>
          The JRuby scripting support in Spring requires the following
          libraries to be on the classpath of your application.
          (The  versions listed just happen to be the versions that the
          Spring team used in the development of the JRuby scripting support;
          you may well be able to use another version of a specific library.)
        </para>
        <itemizedlist>
          <listitem>
            <para><filename>jruby.jar</filename></para>
          </listitem>
          <listitem>
            <para><filename>cglib-nodep-2.1_3.jar</filename></para>
          </listitem>
        </itemizedlist>
      </sidebar>
      <para>From the JRuby homepage...</para>
      <quote>
        <emphasis>JRuby is an 100% pure-Java implementation of the Ruby programming language.</emphasis>
      </quote>
      <para>
        In keeping with the Spring philosophy of offering choice, Spring's
        dynamic language support also supports beans defined in the JRuby
        language. The JRuby language is based on the quite intuitive
        Ruby language, and has support for inline regular expressions, blocks
        (closures), and a whole host of other features that do make solutions
        for some domain problems a whole lot easier to develop.
      </para>
      <para>
        The implementation of the JRuby dynamic language support in Spring is
        interesting in that what happens is this: Spring creates a JDK dynamic
        proxy implementing all of the interfaces that are specified in the
        <literal>'script-interfaces'</literal> attribute value of the
        <literal>&lt;lang:ruby&gt;</literal> element (this is why
        you <emphasis>must</emphasis> supply at least one interface in the value
        of the attribute, and (accordingly) program to interfaces when using
        JRuby-backed beans).
      </para>
      <para>
        Let us look at a fully working example of using a JRuby-based bean. Here is
        the JRuby implementation of the <interfacename>Messenger</interfacename>
        interface that was defined earlier in this chapter (for your convenience it
        is repeated below).
      </para>
      <programlisting source="ruby"><![CDATA[package org.springframework.scripting;

public interface Messenger {

    String getMessage();
}]]></programlisting>
      <programlisting source="ruby"><![CDATA[require 'java'

class RubyMessenger
    include org.springframework.scripting.Messenger

    def setMessage(message)
        @@message = message
    end

    def getMessage
        @@message
    end
end

# this last line is not essential (but see below)
RubyMessenger.new]]></programlisting>
      <para>
        And here is the Spring XML that defines an instance of the
        <classname>RubyMessenger</classname> JRuby bean.
      </para>
      <programlisting language="xml"><![CDATA[<lang:jruby id="messageService"
            script-interfaces="org.springframework.scripting.Messenger"
            script-source="classpath:RubyMessenger.rb">
    
    <lang:property name="message" value="Hello World!" />

</lang:jruby>]]></programlisting>
      <para>
        Take note of the last line of that JRuby source (<literal>'RubyMessenger.new'</literal>).
        When using JRuby in the context of Spring's dynamic language support, you are encouraged
        to instantiate and return a new instance of the JRuby class that you want to use as a
        dynamic-language-backed bean as the result of the execution of your JRuby source. You
        can achieve this by simply instantiating a new instance of your JRuby class on the last
        line of the source file like so:
      </para>
      <programlisting source="ruby"><![CDATA[require 'java'

include_class 'org.springframework.scripting.Messenger'

# class definition same as above...

# instantiate and return a new instance of the RubyMessenger class
RubyMessenger.new]]></programlisting>
      <para>
        If you forget to do this, it is not the end of the world; this will however result in
        Spring having to trawl (reflectively) through the type representation of your JRuby class
        looking for a class to instantiate. In the grand scheme of things this will be so fast
        that you'll never notice it, but it is something that can be avoided by simply
        having a line such as the one above as the last line of your JRuby script. If you don't
        supply such a line, or if Spring cannot find a JRuby class in your script to instantiate
        then an opaque <classname>ScriptCompilationException</classname>
        will be thrown immediately after the source is executed by the JRuby
        interpreter. The key text that identifies this as the root cause of an
        exception can be found immediately below (so if your Spring container
        throws the following exception when creating your dynamic-language-backed bean
        and the following text is there in the corresponding stacktrace, this will hopefully
        allow you to identify and then easily rectify the issue):
      </para>
      <computeroutput><![CDATA[org.springframework.scripting.ScriptCompilationException: Compilation of JRuby script returned '']]></computeroutput>
      <para>
        To rectify this, simply instantiate a new instance of whichever class
        you want to expose as a JRuby-dynamic-language-backed bean (as shown above). Please
        also note that you can actually define as many classes and objects
        as you want in your JRuby script; what is important is that the
        source file as a whole must return an object (for Spring to configure).
      </para>
      <para>
        See <xref linkend="dynamic-language-scenarios" /> for some
        scenarios where you might want to use JRuby-based beans.
      </para>
    </section>

    <section id="dynamic-language-beans-groovy">
      <title>Groovy beans</title>
      <sidebar>
        <title>The Groovy library dependencies</title>
        <para>
          The Groovy scripting support in Spring requires the following
          libraries to be on the classpath of your application.
        </para>
        <itemizedlist>
          <listitem>
            <para><filename>groovy-1.5.5.jar</filename></para>
          </listitem>
          <listitem>
            <para><filename>asm-2.2.2.jar</filename></para>
          </listitem>
          <listitem>
            <para><filename>antlr-2.7.6.jar</filename></para>
          </listitem>
        </itemizedlist>
      </sidebar>
      <para>From the Groovy homepage...</para>
      <quote>
        <emphasis>Groovy is an agile dynamic language for the Java 2 Platform that has
        many of the  features that people like so much in languages like Python, Ruby
        and Smalltalk, making them available to Java developers using a Java-like syntax.
        </emphasis>
      </quote>
      <para>
        If you have read this chapter straight from the top, you will already have
        <link linkend="dynamic-language-a-first-example">seen an example</link> of a
        Groovy-dynamic-language-backed bean. Let's look at another example (again
        using an example from the Spring test suite).
      </para>
      <programlisting source="java"><![CDATA[package org.springframework.scripting;

public interface Calculator {

    int add(int x, int y);
}]]></programlisting>
      <para>
        Here is an implementation of the <interfacename>Calculator</interfacename>
        interface in Groovy.
      </para>
      <programlisting source="java"><lineannotation>// from the file 'calculator.groovy'</lineannotation><![CDATA[
package org.springframework.scripting.groovy

class GroovyCalculator implements Calculator {

    int add(int x, int y) {
        x + y
    }
}]]></programlisting>
      <programlisting language="xml"><lineannotation>&lt;-- from the file 'beans.xml' --&gt;</lineannotation><![CDATA[
<beans>
    <lang:groovy id="calculator" script-source="classpath:calculator.groovy"/>
</beans>]]></programlisting>
      <para>
        Lastly, here is a small application to exercise the above configuration.
      </para>
      <programlisting source="java"><![CDATA[package org.springframework.scripting;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class Main {

    public static void Main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("beans.xml");
        Calculator calc = (Calculator) ctx.getBean("calculator");
        System.out.println(calc.add(2, 8));
    }
}]]></programlisting>
      <para>
        The resulting output from running the above program will be
        (unsurprisingly) <computeroutput>10</computeroutput>.
        (Exciting example, huh? Remember that the intent is to illustrate the
        concept. Please consult the dynamic language showcase project for a
        more complex example, or indeed <xref linkend="dynamic-language-scenarios" />
        later in this chapter).
      </para>
      <para>
        It is important that you <emphasis>do not</emphasis> define more than one
        class per Groovy source file. While this is perfectly legal in Groovy, it
        is (arguably) a bad practice: in the interests of a consistent approach,
        you should (in the opinion of this author) respect the standard Java
        conventions of one (public) class per source file.
      </para>

      <section id="dynamic-language-beans-groovy-customizer">
        <title>Customising Groovy objects via a callback</title>
        <para>
          The <interfacename>GroovyObjectCustomizer</interfacename>
          interface is a callback that allows you to hook additional
          creation logic into the process of creating a Groovy-backed bean.
          For example, implementations of this interface could invoke
          any required initialization method(s), or set some default property
          values, or specify a custom <classname>MetaClass</classname>.
        </para>
        <programlisting source="java"><![CDATA[public interface GroovyObjectCustomizer {

   void customize(GroovyObject goo);
}]]></programlisting>
        <para>
          The Spring Framework will instantiate an instance of your Groovy-backed
          bean, and will then pass the created <interfacename>GroovyObject</interfacename>
          to the specified <interfacename>GroovyObjectCustomizer</interfacename>
          if one has been defined. You can do whatever you like with the supplied
          <interfacename>GroovyObject</interfacename> reference: it is expected
          that the setting of a custom <classname>MetaClass</classname> is what most
          folks will want to do with this callback, and you can see an example
          of doing that below.
        </para>
        <programlisting source="java"><![CDATA[public final class SimpleMethodTracingCustomizer implements GroovyObjectCustomizer {

   public void customize(GroovyObject goo) {
      DelegatingMetaClass metaClass = new DelegatingMetaClass(goo.getMetaClass()) {

         public Object invokeMethod(Object object, String methodName, Object[] arguments) {
            System.out.println("Invoking '" + methodName + "'.");
            return super.invokeMethod(object, methodName, arguments);
         }
      };
      metaClass.initialize();
      goo.setMetaClass(metaClass);
   }
}]]></programlisting>
        <para>
          A full discussion of meta-programming in Groovy is beyond the scope of the
          Spring reference manual. Consult the relevant section of the Groovy
          reference manual, or do a search online: there are plenty of articles
          concerning this topic.
          Actually making use of a <interfacename>GroovyObjectCustomizer</interfacename>
          is easy if you are using the Spring 2.0 namespace support.
        </para>
        <programlisting language="xml"><lineannotation>&lt;!-- define the <interfacename>GroovyObjectCustomizer</interfacename> just like any other bean --&gt;</lineannotation>
<![CDATA[<bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer" />

   ]]><lineannotation>&lt;!-- ... and plug it into the desired Groovy bean via the '<literal>customizer-ref</literal>' attribute --&gt;</lineannotation><![CDATA[
   <lang:groovy id="calculator"
      script-source="classpath:org/springframework/scripting/groovy/Calculator.groovy"
      customizer-ref="tracingCustomizer" />]]></programlisting>
        <para>
          If you are not using the Spring 2.0 namespace support, you can still
          use the <interfacename>GroovyObjectCustomizer</interfacename> functionality.
        </para>
        <programlisting language="xml"><![CDATA[<bean id="calculator" class="org.springframework.scripting.groovy.GroovyScriptFactory">
      <constructor-arg value="classpath:org/springframework/scripting/groovy/Calculator.groovy"/>
      ]]><lineannotation>&lt;!-- define the <interfacename>GroovyObjectCustomizer</interfacename> (as an inner bean) --&gt;</lineannotation><![CDATA[
      <constructor-arg>
         <bean id="tracingCustomizer" class="example.SimpleMethodTracingCustomizer" />
      </constructor-arg>
</bean>

<bean class="org.springframework.scripting.support.ScriptFactoryPostProcessor"/>]]></programlisting>
      </section>
    </section>

    <section id="dynamic-language-beans-bsh">
      <title>BeanShell beans</title>
      <sidebar>
        <title>The BeanShell library dependencies</title>
        <para>
          The BeanShell scripting support in Spring requires the following
          libraries to be on the classpath of your application.
        </para>
        <itemizedlist>
          <listitem>
            <para><filename>bsh-2.0b4.jar</filename></para>
          </listitem>
          <listitem>
            <para><filename>cglib-nodep-2.1_3.jar</filename></para>
          </listitem>
        </itemizedlist>
        <para>
          All of these libraries are available in the Spring-with-dependencies
          distribution of Spring (in addition to also being freely available
          on the web).
        </para>
      </sidebar>
      <para>From the BeanShell homepage...</para>
      <quote>
        <emphasis>BeanShell is a small, free, embeddable Java source interpreter
        with dynamic language features, written in Java. BeanShell dynamically
        executes standard Java syntax and extends it with common scripting
        conveniences such as loose types, commands, and method closures like those
        in Perl and JavaScript.</emphasis>
      </quote>
      <para>
        In contrast to Groovy, BeanShell-backed bean definitions require some (small)
        additional configuration. The implementation of the BeanShell dynamic language
        support in Spring is interesting in that what happens is this: Spring creates
        a JDK dynamic proxy implementing all of the interfaces that are specified in the
        <literal>'script-interfaces'</literal> attribute value of the
        <literal>&lt;lang:bsh&gt;</literal> element (this is why
        you <emphasis>must</emphasis> supply at least one interface in the value
        of the attribute, and (accordingly) program to interfaces when using
        BeanShell-backed beans). This means that every method call on a BeanShell-backed
        object is going through the JDK dynamic proxy invocation mechanism.
      </para>
      <para>
        Let's look at a fully working example of using a BeanShell-based bean
        that implements the <interfacename>Messenger</interfacename> interface
        that was defined earlier in this chapter (repeated below for your
        convenience).
      </para>
      <programlisting source="java"><![CDATA[package org.springframework.scripting;

public interface Messenger {

    String getMessage();
}]]></programlisting>
      <para>Here is the BeanShell 'implementation' (the term is used loosely here) of the
          <interfacename>Messenger</interfacename> interface.</para>
      <programlisting source="java"><![CDATA[String message;

String getMessage() {
    return message;
}

void setMessage(String aMessage) {
    message = aMessage;
}]]></programlisting>
      <para>
        And here is the Spring XML that defines an 'instance' of the above 'class'
        (again, the term is used very loosely here).
      </para>
      <programlisting language="xml"><![CDATA[<lang:bsh id="messageService" script-source="classpath:BshMessenger.bsh"
    script-interfaces="org.springframework.scripting.Messenger">

    <lang:property name="message" value="Hello World!" />
</lang:bsh>]]></programlisting>
      <para>See <xref linkend="dynamic-language-scenarios" /> for some
      scenarios where you might want to use BeanShell-based beans.</para>
    </section>
  </section>

  <section id="dynamic-language-scenarios">
    <title>Scenarios</title>
    <para>
      The possible scenarios where defining Spring managed beans in a scripting
      language would be beneficial are, of course, many and varied. This section
      describes two possible use cases for the dynamic language support in Spring.
    </para>

    <section id="dynamic-language-scenarios-controllers">
      <title>Scripted Spring MVC Controllers</title>
      <para>
        One group of classes that may benefit from using dynamic-language-backed
        beans is that of Spring MVC controllers. In pure Spring MVC applications,
        the  navigational flow through a web application is to a large extent
        determined by code encapsulated within your Spring MVC controllers.
        As the navigational flow and other presentation layer logic of a web
        application needs to be updated to respond to support issues or changing
        business requirements, it may well be easier to effect any such required
        changes  by editing one or more dynamic language source files and seeing
        those changes being immediately reflected in the state of a running
        application.
      </para>
      <para>
        Remember that in the lightweight architectural model espoused by projects
        such as Spring, you are typically aiming to have a really
        <emphasis>thin</emphasis> presentation layer, with all the meaty business
        logic of an application being contained in the domain and service layer
        classes. Developing Spring MVC controllers as dynamic-language-backed beans
        allows you to change presentation layer logic by simply editing and saving
        text files; any changes to such dynamic language source files will (depending
        on the configuration) automatically be reflected in the beans that are backed
              by dynamic language source files.
      </para>
      <note>
        <para>
          In order to effect this automatic 'pickup' of any changes
          to dynamic-language-backed beans, you will have had to enable the
          'refreshable beans' functionality. See
           <xref linkend="dynamic-language-refreshable-beans"/> for a full treatment
           of this feature.
        </para>
      </note>
      <para>
        Find below an example of an
        <interfacename>org.springframework.web.servlet.mvc.Controller</interfacename>
        implemented using the Groovy dynamic language.
      </para>
      <programlisting source="java"><![CDATA[// from the file '/WEB-INF/groovy/FortuneController.groovy'
package org.springframework.showcase.fortune.web

import org.springframework.showcase.fortune.service.FortuneService
import org.springframework.showcase.fortune.domain.Fortune
import org.springframework.web.servlet.ModelAndView
import org.springframework.web.servlet.mvc.Controller

import javax.servlet.http.HttpServletRequest
import javax.servlet.http.HttpServletResponse

class FortuneController implements Controller {

    @Property FortuneService fortuneService

    ModelAndView handleRequest(
            HttpServletRequest request, HttpServletResponse httpServletResponse) {

        return new ModelAndView("tell", "fortune", this.fortuneService.tellFortune())
    }
}]]></programlisting>
      <programlisting language="xml"><![CDATA[<lang:groovy id="fortune"
             refresh-check-delay="3000"
             script-source="/WEB-INF/groovy/FortuneController.groovy">
    <lang:property name="fortuneService" ref="fortuneService"/>
</lang:groovy>
]]></programlisting>
    </section>

    <section id="dynamic-language-scenarios-validators">
      <title>Scripted Validators</title>
      <para>
        Another area of application development with Spring that may benefit
        from the flexibility afforded by dynamic-language-backed beans is that of
        validation. It <emphasis>may</emphasis> be easier to express complex validation
        logic using a loosely typed dynamic language (that may also have support
        for inline regular expressions) as opposed to regular Java.
      </para>
      <para>
        Again, developing validators as dynamic-language-backed beans allows you to change
        validation logic by simply editing and saving a simple text file; any such
        changes will (depending on the configuration) automatically be reflected
        in the execution of a running application and would not require the restart
        of an application.
      </para>
      <note>
        <para>
          Please note that in order to effect the automatic 'pickup' of any changes
          to dynamic-language-backed beans, you will have had to enable the
          'refreshable beans' feature. See 
          <xref linkend="dynamic-language-refreshable-beans"/> for a full and
          detailed treatment of this feature.
        </para>
      </note>
      <para>
        Find below an example of a Spring
        <interfacename>org.springframework.validation.Validator</interfacename>
        implemented using the Groovy dynamic language. (See <xref linkend="validator"/>
        for a discussion of the <interfacename>Validator</interfacename> interface.)
      </para>
      <programlisting source="java"><![CDATA[import org.springframework.validation.Validator
import org.springframework.validation.Errors
import org.springframework.beans.TestBean

class TestBeanValidator implements Validator {

    boolean supports(Class clazz) {
        return TestBean.class.isAssignableFrom(clazz)
    }
    
    void validate(Object bean, Errors errors) {
        if(bean.name?.trim()?.size() > 0) {
            return
        }
        errors.reject("whitespace", "Cannot be composed wholly of whitespace.")
    }
}]]></programlisting>
    </section>
  </section>

  <section id="dynamic-language-final-notes">
    <title>Bits and bobs</title>
    <para>
      This last section contains some bits and bobs related to the dynamic language
      support.
    </para>

    <section id="dynamic-language-final-notes-aop">
      <title>AOP - advising scripted beans</title>
      <para>
        It is possible to use the Spring AOP framework to advise scripted beans.
        The Spring AOP framework actually is unaware that a bean that is being
        advised might be a scripted bean, so all of the AOP use cases and functionality
        that you may be using or aim to use will work with scripted beans. There is
        just one (small) thing that you need to be aware of when advising scripted
        beans... you cannot use class-based proxies, you must use
        <link linkend="aop-proxying">interface-based proxies</link>.
      </para>
      <para>
        You are of course not just limited to advising scripted beans... you can
        also write aspects themselves in a supported dynamic language and use such
        beans to advise other Spring beans. This really would be an advanced use of
        the dynamic language support though.
      </para>
    </section>

    <section id="dynamic-language-final-notes-scopes">
      <title>Scoping</title>
      <para>
        In case it is not immediately obvious, scripted beans can of course be scoped
        just like any other bean. The <literal>scope</literal> attribute on the
        various <literal>&lt;lang:language/&gt;</literal> elements allows you to
        control the scope of the underlying scripted bean, just as it does with a
        regular bean. (The default scope is
        <link linkend="beans-factory-scopes-singleton">singleton</link>, just as it
        is with 'regular' beans.)
      </para>
      <para>
        Find below an example of using the <literal>scope</literal> attribute
        to define a Groovy bean scoped as a
        <link linkend="beans-factory-scopes-prototype">prototype</link>.
      </para>
      <programlisting language="xml"><![CDATA[<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns:lang="http://www.springframework.org/schema/lang"
       xsi:schemaLocation="
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/lang http://www.springframework.org/schema/lang/spring-lang-3.0.xsd">

    <lang:groovy id="messenger" script-source="classpath:Messenger.groovy" ]]><lineannotation><emphasis role="bold">scope="prototype"</emphasis></lineannotation><![CDATA[>
        <lang:property name="message" value="I Can Do The RoboCop" />
    </lang:groovy>

    <bean id="bookingService" class="x.y.DefaultBookingService">
        <property name="messenger" ref="messenger" />
    </bean>

</beans>]]></programlisting>
      <para>
        See <xref linkend="beans-factory-scopes"/> in <xref linkend="beans"/>
        for a fuller discussion of the scoping support in the Spring Framework.
      </para>
    </section>
  </section>

  <section id="dynamic-language-resources">
    <title>Further Resources</title>
    <para>
      Find below links to further resources about the various dynamic languages described
      in this chapter.
    </para>
    <itemizedlist>
      <listitem>
        <para>The <ulink url="http://jruby.codehaus.org/">JRuby</ulink> homepage</para>
      </listitem>
      <listitem>
        <para>The <ulink url="http://groovy.codehaus.org/">Groovy</ulink> homepage</para>
      </listitem>
      <listitem>
        <para>The <ulink url="http://www.beanshell.org/">BeanShell</ulink> homepage</para>
      </listitem>
    </itemizedlist>
    <para>
      Some of the more active members of the Spring community have also added support for
      a number of additional dynamic languages above and beyond the ones covered in this
      chapter. While it is possible that such third party contributions may be added to the
      list of languages supported by the main Spring distribution, your best bet for seeing
      if your favourite scripting language is supported is the
      <ulink url="https://springmodules.dev.java.net/">Spring Modules project</ulink>.
    </para>
  </section>

</chapter>