Use JDK proxy for introduction interface without target

Closes gh-33985

Author: jhoeller

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

@@ -60,11 +60,12 @@ public class DefaultAopProxyFactory implements AopProxyFactory, Serializable {
 	public AopProxy createAopProxy(AdvisedSupport config) throws AopConfigException {
 		if (config.isOptimize() || config.isProxyTargetClass() || !config.hasUserSuppliedInterfaces()) {
 			Class<?> targetClass = config.getTargetClass();
-			if (targetClass == null) {
+			if (targetClass == null && config.getProxiedInterfaces().length == 0) {
 				throw new AopConfigException("TargetSource cannot determine target class: " +
 						"Either an interface or a target is required for proxy creation.");
 			}
-			if (targetClass.isInterface() || Proxy.isProxyClass(targetClass) || ClassUtils.isLambdaClass(targetClass)) {
+			if (targetClass == null || targetClass.isInterface() ||
+					Proxy.isProxyClass(targetClass) || ClassUtils.isLambdaClass(targetClass)) {
 				return new JdkDynamicAopProxy(config);
 			}
 			return new ObjenesisCglibAopProxy(config);

spring-aop/src/test/java/org/springframework/aop/framework/ProxyFactoryTests.java

@@ -340,6 +340,18 @@ void proxyTargetClassInCaseOfIntroducedInterface() {
 		assertThat(AopProxyUtils.ultimateTargetClass(proxy)).isEqualTo(MyDate.class);
 	}
 
+	@Test
+	void proxyInterfaceInCaseOfIntroducedInterfaceOnly() {
+		ProxyFactory pf = new ProxyFactory();
+		pf.addInterface(TimeStamped.class);
+		TimestampIntroductionInterceptor ti = new TimestampIntroductionInterceptor(0L);
+		pf.addAdvisor(new DefaultIntroductionAdvisor(ti, TimeStamped.class));
+		Object proxy = pf.getProxy();
+		assertThat(AopUtils.isJdkDynamicProxy(proxy)).as("Proxy is a JDK proxy").isTrue();
+		assertThat(proxy).isInstanceOf(TimeStamped.class);
+		assertThat(AopProxyUtils.ultimateTargetClass(proxy)).isEqualTo(proxy.getClass());
+	}
+
 	@Test
 	void proxyInterfaceInCaseOfNonTargetInterface() {
 		ProxyFactory pf = new ProxyFactory();