Explicitly replace target ApplicationListener with singleton proxy, if any (avoiding double registration/invocation)

Issue: SPR-15452

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -43,6 +43,25 @@ import org.springframework.util.ObjectUtils;
  */
 public abstract class AopProxyUtils {
 
+	/**
+	 * Obtain the singleton target object behind the given proxy, if any.
+	 * @param candidate the (potential) proxy to check
+	 * @return the singleton target object managed in a {@link SingletonTargetSource},
+	 * or {@code null} in any other case (not a proxy, not an existing singleton target)
+	 * @since 4.3.8
+	 * @see Advised#getTargetSource()
+	 * @see SingletonTargetSource#getTarget()
+	 */
+	public static Object getSingletonTarget(Object candidate) {
+		if (candidate instanceof Advised) {
+			TargetSource targetSource = ((Advised) candidate).getTargetSource();
+			if (targetSource instanceof SingletonTargetSource) {
+				return ((SingletonTargetSource) targetSource).getTarget();
+			}
+		}
+		return null;
+	}
+
 	/**
 	 * Determine the ultimate target class of the given bean instance, traversing
 	 * not only a top-level proxy but any number of nested proxies as well —
@@ -59,14 +78,7 @@ public abstract class AopProxyUtils {
 		Class<?> result = null;
 		while (current instanceof TargetClassAware) {
 			result = ((TargetClassAware) current).getTargetClass();
-			Object nested = null;
-			if (current instanceof Advised) {
-				TargetSource targetSource = ((Advised) current).getTargetSource();
-				if (targetSource instanceof SingletonTargetSource) {
-					nested = ((SingletonTargetSource) targetSource).getTarget();
-				}
-			}
-			current = nested;
+			current = getSingletonTarget(current);
 		}
 		if (result == null) {
 			result = (AopUtils.isCglibProxy(candidate) ? candidate.getClass().getSuperclass() : candidate.getClass());

spring-context/src/main/java/org/springframework/context/event/AbstractApplicationEventMulticaster.java

@@ -23,6 +23,7 @@ import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
+import org.springframework.aop.framework.AopProxyUtils;
 import org.springframework.beans.factory.BeanClassLoaderAware;
 import org.springframework.beans.factory.BeanFactory;
 import org.springframework.beans.factory.BeanFactoryAware;
@@ -98,6 +99,12 @@ public abstract class AbstractApplicationEventMulticaster
 	@Override
 	public void addApplicationListener(ApplicationListener<?> listener) {
 		synchronized (this.retrievalMutex) {
+			// Explicitly remove target for a proxy, if registered already,
+			// in order to avoid double invocations of the same listener.
+			Object singletonTarget = AopProxyUtils.getSingletonTarget(listener);
+			if (singletonTarget instanceof ApplicationListener) {
+				this.defaultRetriever.applicationListeners.remove(singletonTarget);
+			}
 			this.defaultRetriever.applicationListeners.add(listener);
 			this.retrieverCache.clear();
 		}

spring-context/src/test/java/org/springframework/context/event/ApplicationContextEventTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2016 the original author or authors.
+ * Copyright 2002-2017 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.
@@ -17,6 +17,8 @@
 package org.springframework.context.event;
 
 import java.util.HashSet;
+import java.util.LinkedList;
+import java.util.List;
 import java.util.Set;
 import java.util.concurrent.Executor;
 
@@ -184,6 +186,7 @@ public class ApplicationContextEventTests extends AbstractApplicationEventListen
 
 		smc.multicastEvent(new MyEvent(this));
 		smc.multicastEvent(new MyOtherEvent(this));
+		assertEquals(2, listener1.seenEvents.size());
 	}
 
 	@Test
@@ -197,6 +200,7 @@ public class ApplicationContextEventTests extends AbstractApplicationEventListen
 
 		smc.multicastEvent(new MyEvent(this));
 		smc.multicastEvent(new MyOtherEvent(this));
+		assertEquals(2, listener1.seenEvents.size());
 	}
 
 	@Test
@@ -213,6 +217,26 @@ public class ApplicationContextEventTests extends AbstractApplicationEventListen
 
 		smc.multicastEvent(new MyEvent(this));
 		smc.multicastEvent(new MyOtherEvent(this));
+		assertEquals(2, listener1.seenEvents.size());
+	}
+
+	@Test
+	@SuppressWarnings("unchecked")
+	public void proxiedListenersMixedWithTargetListeners() {
+		MyOrderedListener1 listener1 = new MyOrderedListener1();
+		MyOrderedListener2 listener2 = new MyOrderedListener2(listener1);
+		ApplicationListener<ApplicationEvent> proxy1 = (ApplicationListener<ApplicationEvent>) new ProxyFactory(listener1).getProxy();
+		ApplicationListener<ApplicationEvent> proxy2 = (ApplicationListener<ApplicationEvent>) new ProxyFactory(listener2).getProxy();
+
+		SimpleApplicationEventMulticaster smc = new SimpleApplicationEventMulticaster();
+		smc.addApplicationListener(listener1);
+		smc.addApplicationListener(listener2);
+		smc.addApplicationListener(proxy1);
+		smc.addApplicationListener(proxy2);
+
+		smc.multicastEvent(new MyEvent(this));
+		smc.multicastEvent(new MyOtherEvent(this));
+		assertEquals(2, listener1.seenEvents.size());
 	}
 
 	@Test
@@ -459,7 +483,7 @@ public class ApplicationContextEventTests extends AbstractApplicationEventListen
 
 	public static class MyOrderedListener1 implements ApplicationListener<ApplicationEvent>, Ordered {
 
-		public final Set<ApplicationEvent> seenEvents = new HashSet<>();
+		public final List<ApplicationEvent> seenEvents = new LinkedList<>();
 
 		@Override
 		public void onApplicationEvent(ApplicationEvent event) {