Fix scoped-proxy memory leak w/ @Resource injection

Prior to this change, request-scoped components having
@Resource-injected dependencies caused a memory leak in
DefaultListableBeanFactory#dependenciesForBeanMap.

Consider the following example:

    @Component
    @Scope(value="request", proxyMode=ScopedProxyMode.TARGET_CLASS)
    public class MyComponent {

        @Resource
        private HttpServletRequest request;

        // ...
    }

The bean name for "MyComponent" will end up being
'scopedTarget.myComponent', which will become a key in
the #dependenciesForBeanMap structure.

On the first request, the injected HttpServletRequest bean will be a
proxy and will internally have a bean name of the form
"$Proxy10@1a3a2a52". This name will be added to the Set value associated
with the 'scopedTarget.myComponent' entry in #dependenciesForBeanMap.

On the second request, the process will repeat, but the injected
HttpServletRequest will be a different proxy instance, thus having a
different identity hex string, e.g. "$Proxy10@5eba06ff". This name will
also be added to the Set value associated with the
'scopedTarget.myComponent' entry in #dependenciesForBeanMap, and this
is the source of the leak: a new entry is added to the set on each
request but should be added only once.

This commit fixes the leak by introducing caching to
CommonAnnotationBeanPostProcessor#ResourceElement similar to that already
present in AutowiredAnnotationBeanPostProcessor#AutowiredFieldElement
and #AutowiredMethodElement. Essentially, each ResourceElement instance
now tracks whether it has been created, caches the ultimate value to be
injected and returns it eagerly if necessary. Besides solving the memory
leak, this has the side effect of avoiding unnecessary proxy creation.

This fix also explains clearly why injection into request-scoped
components using @Autowired never suffered this memory leak: because the
correct caching was already in place. Because @Resource is considerably
less-frequently used than @Autowired, and given that this particular
injection arrangement is relatively infrequent, it becomes
understandable how this bug has been present without being reported
since the introduction of @Resource support in Spring 2.5: developers
were unlikely to encounter it in the first place; and if they did, the
leak was minor enough (adding strings to a Set), that it could
potentially go unnoticed indefinitely depending on request volumes and
available memory.

Issue: SPR-9176

spring-context/src/main/java/org/springframework/context/annotation/CommonAnnotationBeanPostProcessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2011 the original author or authors.
+ * Copyright 2002-2012 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.
@@ -35,6 +35,7 @@ import java.util.LinkedList;
 import java.util.Map;
 import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
+
 import javax.annotation.PostConstruct;
 import javax.annotation.PreDestroy;
 import javax.annotation.Resource;
@@ -509,9 +510,12 @@ public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBean
 	 */
 	private class ResourceElement extends LookupElement {
 
-		@SuppressWarnings("unused")
 		protected boolean shareable = true;
 
+		private volatile boolean cached = false;
+
+		private volatile Object cachedFieldValue;
+
 		public ResourceElement(Member member, PropertyDescriptor pd) {
 			super(member, pd);
 		}
@@ -546,7 +550,20 @@ public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBean
 
 		@Override
 		protected Object getResourceToInject(Object target, String requestingBeanName) {
-			return getResource(this, requestingBeanName);
+			Object value = null;
+			if (this.cached && this.shareable) {
+				value = this.cachedFieldValue;
+			}
+			synchronized (this) {
+				if (!this.cached) {
+					value = getResource(this, requestingBeanName);
+					if (value != null && this.shareable) {
+						this.cachedFieldValue = value;
+						this.cached = true;
+					}
+				}
+			}
+			return value;
 		}
 	}
 
@@ -724,4 +741,4 @@ public class CommonAnnotationBeanPostProcessor extends InitDestroyAnnotationBean
 		}
 	}
 
-}
+}