210470: weakly wrap a classloader

diff --git a/weaver/src/org/aspectj/weaver/WeakClassLoaderReference.java b/weaver/src/org/aspectj/weaver/WeakClassLoaderReference.java
new file mode 100644 (file)
index 0000000..e50303b
--- /dev/null
+++ b/weaver/src/org/aspectj/weaver/WeakClassLoaderReference.java
@@ -0,0 +1,50 @@
+/* *******************************************************************
+ * Copyright (c) 2008 Contributors
+ * All rights reserved. 
+ * This program and the accompanying materials are made available 
+ * under the terms of the Eclipse Public License v1.0 
+ * which accompanies this distribution and is available at 
+ * http://www.eclipse.org/legal/epl-v10.html 
+ *  
+ * Contributors: 
+ *     Andy Clement     initial implementation 
+ * ******************************************************************/
+package org.aspectj.weaver;
+
+import java.lang.ref.WeakReference;
+
+/**
+ * Wraps a reference to a classloader inside a WeakReference. This should be used where we do not want the existence of
+ * a classloader reference to prevent garbage collection of that classloader (and possibly an associated weaver instance
+ * in the case of load time weaving).
+ * <p>
+ * In more detail:<br>
+ * When load time weaving, the class Aj maintains a WeakHashMap from the classloader instance to a weaver instance. The
+ * aim is that the weaver is around as long as the classloader is and should the classloader be dereferenced then the
+ * weaver can also be garbage collected. The problem is that if there are many references to the classloader from within
+ * the weaver, these are considered hard references and cause the classloader to be long lived - even if the user of the
+ * classloader has dereferenced it in their code. The solution is that the weaver should use instances of
+ * WeakClassLoaderReference objects - so that when the users hard reference to the classloader goes, nothing in the
+ * weaver will cause it to hang around. There is a big assertion here that the WeakClassLoaderReference instances will
+ * not 'lose' their ClassLoader references until the top level ClassLoader reference is null'd. This means there is no
+ * need to check for the null case on get() in this WeakReference logic below, because we shouldn't be using this weaver
+ * if its associated ClassLoader has been collected. See https://bugs.eclipse.org/bugs/show_bug.cgi?id=210470
+ * 
+ * 
+ * @author Andy Clement
+ */
+public class WeakClassLoaderReference {
+
+    private WeakReference loaderRef;
+    
+    public WeakClassLoaderReference(ClassLoader loader) {
+        loaderRef = new WeakReference(loader);
+    }
+    
+    public ClassLoader getClassLoader() {
+        ClassLoader instance = (ClassLoader) loaderRef.get();
+        // Assert instance!=null
+        return instance;
+    }
+    
+}