Fall back to JDK proxy if CGLIB proxy creation fails

Prior to this change, if the default CGLIB proxy creation failed for a
non-subclassable target class (e.g., due to a private constructor), the
ApplicationContext would fail to start.

Signed-off-by: yongjunhong <yongjunh@apache.org>

spring-aop/src/main/java/org/springframework/aop/framework/CglibAopProxy.java

@@ -79,6 +79,7 @@ import org.springframework.util.ObjectUtils;
  * @author Chris Beams
  * @author Dave Syer
  * @author Sebastien Deleuze
+ * @author Yongjun Hong
  * @see org.springframework.cglib.proxy.Enhancer
  * @see AdvisedSupport#setProxyTargetClass
  * @see DefaultAopProxyFactory
@@ -233,9 +234,11 @@ class CglibAopProxy implements AopProxy, Serializable {
 			}
 		}
 		catch (CodeGenerationException | IllegalArgumentException ex) {
-			throw new AopConfigException("Could not generate CGLIB subclass of " + this.advised.getTargetClass() +
-					": Common causes of this problem include using a final class or a non-visible class",
-					ex);
+			if (logger.isWarnEnabled()) {
+				logger.warn("CGLIB subclass failed: " + ex.getMessage() +
+						" - falling back to JDK dynamic proxy for " + this.advised.getTargetSource(), ex);
+			}
+			return handleCglibFailure(classLoader, ex);
 		}
 		catch (Throwable ex) {
 			// TargetSource.getTarget() failed
@@ -243,6 +246,23 @@ class CglibAopProxy implements AopProxy, Serializable {
 		}
 	}
 
+	private Object handleCglibFailure(@Nullable ClassLoader classLoader, Exception originalException) {
+		Class<?>[] interfaces = this.advised.getProxiedInterfaces();
+
+		if (interfaces.length == 0) {
+			throw new AopConfigException("Could not generate CGLIB subclass of " +
+					this.advised.getTargetClass() + " and JDK proxy fallback not possible", originalException);
+		}
+		try {
+			JdkDynamicAopProxy jdkProxy = new JdkDynamicAopProxy(this.advised);
+			return jdkProxy.getProxy(classLoader);
+		}
+		catch (Throwable ex) {
+			throw new AopConfigException("Could not generate CGLIB subclass of " +
+					this.advised.getTargetClass() + " and JDK proxy fallback failed for interfaces " + ex);
+		}
+	}
+
 	protected Class<?> createProxyClass(Enhancer enhancer) {
 		enhancer.setInterceptDuringConstruction(false);
 		return enhancer.createClass();

spring-context/src/test/java/org/springframework/aop/framework/CglibProxyTests.java

@@ -50,6 +50,7 @@ import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException
  * @author Rob Harrop
  * @author Ramnivas Laddad
  * @author Chris Beams
+ * @author Yongjun Hong
  */
 class CglibProxyTests extends AbstractAopProxyTests {
 
@@ -429,6 +430,48 @@ class CglibProxyTests extends AbstractAopProxyTests {
 		assertThat(proxy.doWithVarargs(MyEnum.A, MyOtherEnum.C)).isTrue();
 	}
 
+	@Test
+	void testProxyCreationAndPrivateAccess() {
+		ProxyFactory proxyFactory = new ProxyFactory(new TestServiceImpl());
+		proxyFactory.setProxyTargetClass(true);
+
+		TestService proxy = (TestService) proxyFactory.getProxy();
+
+		String result = proxy.publicMethod();
+		assertThat(result).isEqualTo("Private Access Success");
+	}
+
+	@Test
+	void testProxyCreationWithoutInterfaceShouldThrowException() {
+		ProxyFactory proxyFactory = new ProxyFactory(new TestServiceImplWithOutInterface());
+		proxyFactory.setProxyTargetClass(true);
+
+		assertThatExceptionOfType(AopConfigException.class)
+				.isThrownBy(proxyFactory::getProxy)
+				.withMessageContaining("Could not generate CGLIB subclass of");
+	}
+
+	interface TestService {
+		String publicMethod();
+	}
+
+	static class TestServiceImpl implements TestService {
+		private TestServiceImpl() { }
+
+		@Override
+		public String publicMethod() {
+			return "Private Access Success";
+		}
+	}
+
+	static class TestServiceImplWithOutInterface {
+
+		private TestServiceImplWithOutInterface() { }
+
+		public String publicMethod() {
+			return "Private Access Success";
+		}
+	}
 
 	public static class MyBean {