Make proxyTargetClass=true with introduction advice work for JDK proxy targets

Closes gh-27044

(cherry picked from commit c45c46dad7)

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/AbstractAutoProxyCreator.java

@@ -17,6 +17,7 @@
 package org.springframework.aop.framework.autoproxy;
 
 import java.lang.reflect.Constructor;
+import java.lang.reflect.Proxy;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.Collections;
@@ -449,7 +450,17 @@ public abstract class AbstractAutoProxyCreator extends ProxyProcessorSupport
 		ProxyFactory proxyFactory = new ProxyFactory();
 		proxyFactory.copyFrom(this);
 
-		if (!proxyFactory.isProxyTargetClass()) {
+		if (proxyFactory.isProxyTargetClass()) {
+			// Explicit handling of JDK proxy targets (for introduction advice scenarios)
+			if (Proxy.isProxyClass(beanClass)) {
+				// Must allow for introductions; can't just set interfaces to the proxy's interfaces only.
+				for (Class<?> ifc : beanClass.getInterfaces()) {
+					proxyFactory.addInterface(ifc);
+				}
+			}
+		}
+		else {
+			// No proxyTargetClass flag enforced, let's apply our default checks...
 			if (shouldProxyTargetClass(beanClass, beanName)) {
 				proxyFactory.setProxyTargetClass(true);
 			}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,6 +28,7 @@ import org.junit.jupiter.api.Test;
 import org.springframework.aop.TargetSource;
 import org.springframework.aop.framework.ProxyFactory;
 import org.springframework.aop.support.AopUtils;
+import org.springframework.aop.support.DefaultIntroductionAdvisor;
 import org.springframework.aop.target.SingletonTargetSource;
 import org.springframework.beans.MutablePropertyValues;
 import org.springframework.beans.factory.BeanFactory;
@@ -219,7 +220,7 @@ public class AutoProxyCreatorTests {
 
 		MutablePropertyValues pvs = new MutablePropertyValues();
 		pvs.add("proxyFactoryBean", "false");
-		sac.registerSingleton("testAutoProxyCreator", TestAutoProxyCreator.class, pvs);
+		sac.registerSingleton("testAutoProxyCreator", IntroductionTestAutoProxyCreator.class, pvs);
 
 		sac.registerSingleton("noInterfaces", NoInterfaces.class);
 		sac.registerSingleton("containerCallbackInterfacesOnly", ContainerCallbackInterfacesOnly.class);
@@ -248,9 +249,9 @@ public class AutoProxyCreatorTests {
 		singletonNoInterceptor.getName();
 		assertThat(tapc.testInterceptor.nrOfInvocations).isEqualTo(0);
 		singletonToBeProxied.getAge();
-		assertThat(tapc.testInterceptor.nrOfInvocations).isEqualTo(1);
-		prototypeToBeProxied.getSpouse();
 		assertThat(tapc.testInterceptor.nrOfInvocations).isEqualTo(2);
+		prototypeToBeProxied.getSpouse();
+		assertThat(tapc.testInterceptor.nrOfInvocations).isEqualTo(4);
 	}
 
 	@Test
@@ -404,7 +405,7 @@ public class AutoProxyCreatorTests {
 			else if (name.endsWith("ToBeProxied")) {
 				boolean isFactoryBean = FactoryBean.class.isAssignableFrom(beanClass);
 				if ((this.proxyFactoryBean && isFactoryBean) || (this.proxyObject && !isFactoryBean)) {
-					return new Object[] {this.testInterceptor};
+					return getAdvicesAndAdvisors();
 				}
 				else {
 					return DO_NOT_PROXY;
@@ -414,6 +415,10 @@ public class AutoProxyCreatorTests {
 				return PROXY_WITHOUT_ADDITIONAL_INTERCEPTORS;
 			}
 		}
+
+		protected Object[] getAdvicesAndAdvisors() {
+			return new Object[] {this.testInterceptor};
+		}
 	}
 
 
@@ -426,6 +431,17 @@ public class AutoProxyCreatorTests {
 	}
 
 
+	@SuppressWarnings("serial")
+	public static class IntroductionTestAutoProxyCreator extends TestAutoProxyCreator {
+
+		protected Object[] getAdvicesAndAdvisors() {
+			DefaultIntroductionAdvisor advisor = new DefaultIntroductionAdvisor(this.testInterceptor);
+			advisor.addInterface(Serializable.class);
+			return new Object[] {this.testInterceptor, advisor};
+		}
+	}
+
+
 	/**
 	 * Interceptor that counts the number of non-finalize method calls.
 	 */