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    copied!<p>Based on the <a href="https://stackoverflow.com/a/5640192/337221">answer of McDowell</a> and his class <a href="http://illegalargumentexception.blogspot.ch/2008/04/java-synchronizing-on-transient-id.html" rel="nofollow noreferrer">IdMutexProvider</a>, I have written the generic class <code>LockMap</code> that uses <a href="http://docs.oracle.com/javase/6/docs/api/java/util/WeakHashMap.html" rel="nofollow noreferrer">WeakHashMap</a> to store lock objects. <code>LockMap.get()</code> can be used to retrieve a lock object for a key, which can then be used with the Java <code>synchronized (...)</code> statement to apply a lock. Unused lock objects are automatically freed during garbage collection.</p> <pre><code>import java.lang.ref.WeakReference; import java.util.WeakHashMap; // A map that creates and stores lock objects for arbitrary keys values. // Lock objects which are no longer referenced are automatically released during garbage collection. // Author: Christian d'Heureuse, www.source-code.biz // Based on IdMutexProvider by McDowell, http://illegalargumentexception.blogspot.ch/2008/04/java-synchronizing-on-transient-id.html // See also https://stackoverflow.com/questions/5639870/simple-java-name-based-locks public class LockMap&lt;KEY&gt; { private WeakHashMap&lt;KeyWrapper&lt;KEY&gt;,WeakReference&lt;KeyWrapper&lt;KEY&gt;&gt;&gt; map; public LockMap() { map = new WeakHashMap&lt;KeyWrapper&lt;KEY&gt;,WeakReference&lt;KeyWrapper&lt;KEY&gt;&gt;&gt;(); } // Returns a lock object for the specified key. public synchronized Object get (KEY key) { if (key == null) { throw new NullPointerException(); } KeyWrapper&lt;KEY&gt; newKeyWrapper = new KeyWrapper&lt;KEY&gt;(key); WeakReference&lt;KeyWrapper&lt;KEY&gt;&gt; ref = map.get(newKeyWrapper); KeyWrapper&lt;KEY&gt; oldKeyWrapper = (ref == null) ? null : ref.get(); if (oldKeyWrapper != null) { return oldKeyWrapper; } map.put(newKeyWrapper, new WeakReference&lt;KeyWrapper&lt;KEY&gt;&gt;(newKeyWrapper)); return newKeyWrapper; } // Returns the number of used entries in the map. public synchronized int size() { return map.size(); } // KeyWrapper wraps a key value and is used in three ways: // - as the key for the internal WeakHashMap // - as the value for the internal WeakHashMap, additionally wrapped in a WeakReference // - as the lock object associated to the key private static class KeyWrapper&lt;KEY&gt; { private KEY key; private int hashCode; public KeyWrapper (KEY key) { this.key = key; hashCode = key.hashCode(); } public boolean equals (Object obj) { if (obj == this) { return true; } if (obj instanceof KeyWrapper) { return ((KeyWrapper)obj).key.equals(key); } return false; } public int hashCode() { return hashCode; }} } // end class LockMap </code></pre> <p>Example of how to use the LockMap class:</p> <pre><code>private static LockMap&lt;String&gt; lockMap = new LockMap&lt;String&gt;(); synchronized (lockMap.get(name)) { ... } </code></pre> <p>A simple test program for the LockMap class:</p> <pre><code>public static Object lock1; public static Object lock2; public static void main (String[] args) throws Exception { System.out.println("TestLockMap Started"); LockMap&lt;Integer&gt; map = new LockMap&lt;Integer&gt;(); lock1 = map.get(1); lock2 = map.get(2); if (lock2 == lock1) { throw new Error(); } Object lock1b = map.get(1); if (lock1b != lock1) { throw new Error(); } if (map.size() != 2) { throw new Error(); } for (int i=0; i&lt;10000000; i++) { map.get(i); } System.out.println("Size before gc: " + map.size()); // result varies, e.g. 4425760 System.gc(); Thread.sleep(1000); if (map.size() != 2) { System.out.println("Size after gc should be 2 but is " + map.size()); } System.out.println("TestLockMap completed"); } </code></pre> <p>If anyone knows a better way to automatically test the LockMap class, please write a comment.</p>
 

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