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copied!<p>See a simple solution here <a href="https://stackoverflow.com/questions/5639870/simple-java-name-based-locks/5640671#5640671">Simple Java name based locks?</a></p> <p>EDIT: This solution has a clear happens-before relation from unlock to lock. However, the next solution, now withdrawn, doesn't. The <code>ConcurrentMap</code> javadoc is too light to guaranteed that.</p> <hr> <p>(<em>Withdrawn</em>) If you want to reuse your map as a lock pool, </p> <pre><code>private final V LOCK = ...; // a fake value // if a key is mapped to LOCK, that means the key is locked ConcurrentMap<K,V> map = ...; V lock(key) V value; while( (value=map.putIfAbsent(key, LOCK))==LOCK ) // another thread locked it before me wait(); // now putIfAbsent() returns a real value, or null // and I just sucessfully put LOCK in it // I am now the lock owner of this key return value; // for caller to work on // only the lock owner of the key should call this method unlock(key, value) // I put a LOCK on the key to stall others // now I just need to swap it back with the real value if(value!=null) map.put(key, value); else // map doesn't accept null value map.remove(key) notifyAll(); test() V value = lock(key); // work on value // unlock. // we have a chance to specify a new value here for the next worker newValue = ...; // null if we want to remove the key from map unlock(key, newValue); // in finally{} </code></pre> <p>This is quite messy because we reuse the map for two difference purposes. It's better to have lock pool as a separate data structure, leave map simply as the k-v storage.</p>

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