Support for generics-based events

Update the event publishing infrastructure to support generics-based
events, that is support ApplicationListener implementations that define
a generic event, something like:

public class MyListener
        implements ApplicationListener<GenericEvent<String>> { ... }

This listener should only receive events that are matching the generic
signature, for instance:

public class StringEvent extends GenericEvent<String> { ... }

Note that because of type erasure, publishing an event that defines the
generic type at the instance level will not work. In other words,
publishing "new GenericEvent<String>" will not work as expected as type
erasure will define it as GenericEvent<?>.

To support this feature, use the new GenericApplicationListener that
supersedes SmartApplicationListener to handle generics-based even types via
`supportsEventType` that takes a ResolvableType instance instead of the
simple Class of the event. ApplicationEventMulticaster has an additional
method to multicast an event based on the event and its ResolvableType.

Issue: SPR-8201

Author: snicoll

spring-context/src/main/java/org/springframework/context/ApplicationEventPublisher.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2005 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -30,9 +30,9 @@
 public interface ApplicationEventPublisher {
 
 	/**
-	 * Notify all listeners registered with this application of an application
-	 * event. Events may be framework events (such as RequestHandledEvent)
-	 * or application-specific events.
+	 * Notify all <strong>matching</strong> listeners registered with this
+	 * application of an application event. Events may be framework events
+	 * (such as RequestHandledEvent) or application-specific events.
 	 * @param event the event to publish
 	 * @see org.springframework.web.context.support.RequestHandledEvent
 	 */

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -31,6 +31,7 @@
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationListener;
 import org.springframework.core.OrderComparator;
+import org.springframework.core.ResolvableType;
 import org.springframework.util.ClassUtils;
 import org.springframework.util.ObjectUtils;
 
@@ -49,8 +50,9 @@
  * Alternative implementations could be more sophisticated in those respects.
  *
  * @author Juergen Hoeller
+ * @author Stephane Nicoll
  * @since 1.2.3
- * @see #getApplicationListeners(ApplicationEvent)
+ * @see #getApplicationListeners(ApplicationEvent, ResolvableType)
  * @see SimpleApplicationEventMulticaster
  */
 public abstract class AbstractApplicationEventMulticaster
@@ -145,13 +147,16 @@ protected Collection<ApplicationListener<?>> getApplicationListeners() {
 	 * event type. Non-matching listeners get excluded early.
 	 * @param event the event to be propagated. Allows for excluding
 	 * non-matching listeners early, based on cached matching information.
+	 * @param eventType the event type
 	 * @return a Collection of ApplicationListeners
 	 * @see org.springframework.context.ApplicationListener
 	 */
-	protected Collection<ApplicationListener<?>> getApplicationListeners(ApplicationEvent event) {
+	protected Collection<ApplicationListener<?>> getApplicationListeners(
+			ApplicationEvent event, ResolvableType eventType) {
+
 		Object source = event.getSource();
 		Class<?> sourceType = (source != null ? source.getClass() : null);
-		ListenerCacheKey cacheKey = new ListenerCacheKey(event.getClass(), sourceType);
+		ListenerCacheKey cacheKey = new ListenerCacheKey(eventType, sourceType);
 
 		// Quick check for existing entry on ConcurrentHashMap...
 		ListenerRetriever retriever = this.retrieverCache.get(cacheKey);
@@ -169,26 +174,28 @@ protected Collection<ApplicationListener<?>> getApplicationListeners(Application
 					return retriever.getApplicationListeners();
 				}
 				retriever = new ListenerRetriever(true);
-				Collection<ApplicationListener<?>> listeners = retrieveApplicationListeners(event, sourceType, retriever);
+				Collection<ApplicationListener<?>> listeners =
+						retrieveApplicationListeners(event, eventType, sourceType, retriever);
 				this.retrieverCache.put(cacheKey, retriever);
 				return listeners;
 			}
 		}
 		else {
 			// No ListenerRetriever caching -> no synchronization necessary
-			return retrieveApplicationListeners(event, sourceType, null);
+			return retrieveApplicationListeners(event, eventType, sourceType, null);
 		}
 	}
 
 	/**
 	 * Actually retrieve the application listeners for the given event and source type.
 	 * @param event the application event
+	 * @param eventType the event type
 	 * @param sourceType the event source type
 	 * @param retriever the ListenerRetriever, if supposed to populate one (for caching purposes)
 	 * @return the pre-filtered list of application listeners for the given event and source type
 	 */
 	private Collection<ApplicationListener<?>> retrieveApplicationListeners(
-			ApplicationEvent event, Class<?> sourceType, ListenerRetriever retriever) {
+			ApplicationEvent event, ResolvableType eventType, Class<?> sourceType, ListenerRetriever retriever) {
 
 		LinkedList<ApplicationListener<?>> allListeners = new LinkedList<ApplicationListener<?>>();
 		Set<ApplicationListener<?>> listeners;
@@ -198,7 +205,7 @@ private Collection<ApplicationListener<?>> retrieveApplicationListeners(
 			listenerBeans = new LinkedHashSet<String>(this.defaultRetriever.applicationListenerBeans);
 		}
 		for (ApplicationListener<?> listener : listeners) {
-			if (supportsEvent(listener, event.getClass(), sourceType)) {
+			if (supportsEvent(listener, eventType, sourceType)) {
 				if (retriever != null) {
 					retriever.applicationListeners.add(listener);
 				}
@@ -210,10 +217,10 @@ private Collection<ApplicationListener<?>> retrieveApplicationListeners(
 			for (String listenerBeanName : listenerBeans) {
 				try {
 					Class<?> listenerType = beanFactory.getType(listenerBeanName);
-					if (listenerType == null || supportsEvent(listenerType, event)) {
+					if (listenerType == null || supportsEvent(listenerType, eventType)) {
 						ApplicationListener<?> listener =
 								beanFactory.getBean(listenerBeanName, ApplicationListener.class);
-						if (!allListeners.contains(listener) && supportsEvent(listener, event.getClass(), sourceType)) {
+						if (!allListeners.contains(listener) && supportsEvent(listener, eventType, sourceType)) {
 							if (retriever != null) {
 								retriever.applicationListenerBeans.add(listenerBeanName);
 							}
@@ -236,37 +243,38 @@ private Collection<ApplicationListener<?>> retrieveApplicationListeners(
 	 * type before trying to instantiate it.
 	 * <p>If this method returns {@code true} for a given listener as a first pass,
 	 * the listener instance will get retrieved and fully evaluated through a
-	 * {@link #supportsEvent(ApplicationListener, Class, Class)} call afterwards.
+	 * {@link #supportsEvent(ApplicationListener,ResolvableType, Class)}  call afterwards.
 	 * @param listenerType the listener's type as determined by the BeanFactory
-	 * @param event the event to check
+	 * @param eventType the event type to check
 	 * @return whether the given listener should be included in the candidates
 	 * for the given event type
 	 */
-	protected boolean supportsEvent(Class<?> listenerType, ApplicationEvent event) {
-		if (SmartApplicationListener.class.isAssignableFrom(listenerType)) {
+	protected boolean supportsEvent(Class<?> listenerType, ResolvableType eventType) {
+		if (GenericApplicationListener.class.isAssignableFrom(listenerType)
+				|| SmartApplicationListener.class.isAssignableFrom(listenerType)) {
 			return true;
 		}
-		Class<?> declaredEventType = GenericApplicationListenerAdapter.resolveDeclaredEventType(listenerType);
-		return (declaredEventType == null || declaredEventType.isInstance(event));
+		ResolvableType declaredEventType = GenericApplicationListenerAdapter.resolveDeclaredEventType(listenerType);
+		return (declaredEventType == null || declaredEventType.isAssignableFrom(eventType));
 	}
 
 	/**
 	 * Determine whether the given listener supports the given event.
 	 * <p>The default implementation detects the {@link SmartApplicationListener}
-	 * interface. In case of a standard {@link ApplicationListener}, a
-	 * {@link GenericApplicationListenerAdapter} will be used to introspect
-	 * the generically declared type of the target listener.
+	 * and {@link GenericApplicationListener} interfaces. In case of a standard
+	 * {@link ApplicationListener}, a {@link GenericApplicationListenerAdapter}
+	 * will be used to introspect the generically declared type of the target listener.
 	 * @param listener the target listener to check
 	 * @param eventType the event type to check against
 	 * @param sourceType the source type to check against
 	 * @return whether the given listener should be included in the candidates
 	 * for the given event type
 	 */
 	protected boolean supportsEvent(ApplicationListener<?> listener,
-			Class<? extends ApplicationEvent> eventType, Class<?> sourceType) {
+			ResolvableType eventType, Class<?> sourceType) {
 
-		SmartApplicationListener smartListener = (listener instanceof SmartApplicationListener ?
-				(SmartApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
+		GenericApplicationListener smartListener = (listener instanceof GenericApplicationListener ?
+				(GenericApplicationListener) listener : new GenericApplicationListenerAdapter(listener));
 		return (smartListener.supportsEventType(eventType) && smartListener.supportsSourceType(sourceType));
 	}
 
@@ -276,11 +284,11 @@ protected boolean supportsEvent(ApplicationListener<?> listener,
 	 */
 	private static class ListenerCacheKey {
 
-		private final Class<?> eventType;
+		private final ResolvableType eventType;
 
 		private final Class<?> sourceType;
 
-		public ListenerCacheKey(Class<?> eventType, Class<?> sourceType) {
+		public ListenerCacheKey(ResolvableType eventType, Class<?> sourceType) {
 			this.eventType = eventType;
 			this.sourceType = sourceType;
 		}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2009 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -18,6 +18,7 @@
 
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationListener;
+import org.springframework.core.ResolvableType;
 
 /**
  * Interface to be implemented by objects that can manage a number of
@@ -29,6 +30,7 @@
  *
  * @author Rod Johnson
  * @author Juergen Hoeller
+ * @author Stephane Nicoll
  */
 public interface ApplicationEventMulticaster {
 
@@ -65,8 +67,19 @@ public interface ApplicationEventMulticaster {
 
 	/**
 	 * Multicast the given application event to appropriate listeners.
+	 * <p>Consider using {@link #multicastEvent(ApplicationEvent, ResolvableType)}
+	 * if possible as it provides a better support for generics-based events.
 	 * @param event the event to multicast
 	 */
 	void multicastEvent(ApplicationEvent event);
 
+	/**
+	 * Multicast the given application event to appropriate listeners.
+	 * <p>If the {@code eventType} is {@code null}, a default type is built
+	 * based on the {@code event} instance.
+	 * @param event the event to multicast
+	 * @param eventType the type of event (can be null)
+	 */
+	void multicastEvent(ApplicationEvent event, ResolvableType eventType);
+
 }

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

@@ -0,0 +1,46 @@
+/*
+ * Copyright 2002-2015 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.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.context.event;
+
+import org.springframework.context.ApplicationEvent;
+import org.springframework.context.ApplicationListener;
+import org.springframework.core.Ordered;
+import org.springframework.core.ResolvableType;
+
+/**
+ * Extended variant of the standard {@link ApplicationListener} interface,
+ * exposing further metadata such as the supported event type.
+ *
+ * <p>As of Spring Framework 4.2, supersedes {@link SmartApplicationListener} with
+ * proper handling of generics-based event.
+ *
+ * @author Stephane Nicoll
+ * @since 4.2
+ */
+public interface GenericApplicationListener extends ApplicationListener<ApplicationEvent>, Ordered {
+
+	/**
+	 * Determine whether this listener actually supports the given event type.
+	 */
+	boolean supportsEventType(ResolvableType eventType);
+
+	/**
+	 * Determine whether this listener actually supports the given source type.
+	 */
+	boolean supportsSourceType(Class<?> sourceType);
+
+}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2014 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -19,22 +19,24 @@
 import org.springframework.aop.support.AopUtils;
 import org.springframework.context.ApplicationEvent;
 import org.springframework.context.ApplicationListener;
-import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.Ordered;
+import org.springframework.core.ResolvableType;
 import org.springframework.util.Assert;
 
 /**
- * {@link SmartApplicationListener} adapter that determines supported event types
+ * {@link GenericApplicationListener} adapter that determines supported event types
  * through introspecting the generically declared type of the target listener.
  *
  * @author Juergen Hoeller
+ * @author Stephane Nicoll
  * @since 3.0
  * @see org.springframework.context.ApplicationListener#onApplicationEvent
  */
-public class GenericApplicationListenerAdapter implements SmartApplicationListener {
+public class GenericApplicationListenerAdapter implements GenericApplicationListener {
 
 	private final ApplicationListener<ApplicationEvent> delegate;
 
+	private final ResolvableType declaredEventType;
 
 	/**
 	 * Create a new GenericApplicationListener for the given delegate.
@@ -44,6 +46,7 @@ public class GenericApplicationListenerAdapter implements SmartApplicationListen
 	public GenericApplicationListenerAdapter(ApplicationListener<?> delegate) {
 		Assert.notNull(delegate, "Delegate listener must not be null");
 		this.delegate = (ApplicationListener<ApplicationEvent>) delegate;
+		this.declaredEventType = resolveDeclaredEventType(this.delegate);
 	}
 
 
@@ -53,30 +56,51 @@ public void onApplicationEvent(ApplicationEvent event) {
 	}
 
 	@Override
-	public boolean supportsEventType(Class<? extends ApplicationEvent> eventType) {
-		Class<?> declaredEventType = resolveDeclaredEventType(this.delegate.getClass());
-		if (declaredEventType == null || declaredEventType.equals(ApplicationEvent.class)) {
-			Class<?> targetClass = AopUtils.getTargetClass(this.delegate);
-			if (targetClass != this.delegate.getClass()) {
-				declaredEventType = resolveDeclaredEventType(targetClass);
-			}
+	@SuppressWarnings("unchecked")
+	public boolean supportsEventType(ResolvableType eventType) {
+		if (this.delegate instanceof SmartApplicationListener) {
+			Class<? extends ApplicationEvent> eventClass = (Class<? extends ApplicationEvent>) eventType.getRawClass();
+			return ((SmartApplicationListener) this.delegate).supportsEventType(eventClass);
+		}
+		else {
+			return (this.declaredEventType == null || this.declaredEventType.isAssignableFrom(eventType));
 		}
-		return (declaredEventType == null || declaredEventType.isAssignableFrom(eventType));
 	}
 
 	@Override
 	public boolean supportsSourceType(Class<?> sourceType) {
-		return true;
+		if (this.delegate instanceof SmartApplicationListener) {
+			return ((SmartApplicationListener) this.delegate).supportsSourceType(sourceType);
+		}
+		else {
+			return true;
+		}
 	}
 
 	@Override
 	public int getOrder() {
 		return (this.delegate instanceof Ordered ? ((Ordered) this.delegate).getOrder() : Ordered.LOWEST_PRECEDENCE);
 	}
 
+	static ResolvableType resolveDeclaredEventType(Class<?> listenerType) {
+		ResolvableType resolvableType = ResolvableType.forClass(listenerType).as(ApplicationListener.class);
+		if (resolvableType == null || !resolvableType.hasGenerics()) {
+			return null;
+		}
+		return resolvableType.getGeneric();
+	}
+
+	private static ResolvableType resolveDeclaredEventType(ApplicationListener<ApplicationEvent> listener) {
+		ResolvableType declaredEventType = resolveDeclaredEventType(listener.getClass());
+		if (declaredEventType == null || declaredEventType.isAssignableFrom(
+				ResolvableType.forClass(ApplicationEvent.class))) {
 
-	static Class<?> resolveDeclaredEventType(Class<?> listenerType) {
-		return GenericTypeResolver.resolveTypeArgument(listenerType, ApplicationListener.class);
+			Class<?> targetClass = AopUtils.getTargetClass(listener);
+			if (targetClass != listener.getClass()) {
+				declaredEventType = resolveDeclaredEventType(targetClass);
+			}
+		}
+		return declaredEventType;
 	}
 
 }