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POlocking on a cache key
 

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  1. POlocking on a cache key
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    <p>I've read several questions similar to this, but none of the answers provide ideas of how to clean up memory while still maintaining lock integrity. I'm estimating the number of key-value pairs at a given time to be in the tens of thousands, but the number of key-value pairs over the lifespan of the data structure is virtually infinite (realistically it probably wouldn't be more than a billion, but I'm coding to the worst case).</p> <p>I have an interface:</p> <pre><code>public interface KeyLock&lt;K extends Comparable&lt;? super K&gt;&gt; { public void lock(K key); public void unock(K key); } </code></pre> <p>with a default implementation:</p> <pre><code>public class DefaultKeyLock&lt;K extends Comparable&lt;? super K&gt;&gt; implements KeyLock&lt;K&gt; { private final ConcurrentMap&lt;K, Mutex&gt; lockMap; public DefaultKeyLock() { lockMap = new ConcurrentSkipListMap&lt;K, Mutex&gt;(); } @Override public void lock(K key) { Mutex mutex = new Mutex(); Mutex existingMutex = lockMap.putIfAbsent(key, mutex); if (existingMutex != null) { mutex = existingMutex; } mutex.lock(); } @Override public void unock(K key) { Mutex mutex = lockMap.get(key); mutex.unlock(); } } </code></pre> <p>This works nicely, but the map never gets cleaned up. What I have so far for a clean implementation is:</p> <pre><code>public class CleanKeyLock&lt;K extends Comparable&lt;? super K&gt;&gt; implements KeyLock&lt;K&gt; { private final ConcurrentMap&lt;K, LockWrapper&gt; lockMap; public CleanKeyLock() { lockMap = new ConcurrentSkipListMap&lt;K, LockWrapper&gt;(); } @Override public void lock(K key) { LockWrapper wrapper = new LockWrapper(key); wrapper.addReference(); LockWrapper existingWrapper = lockMap.putIfAbsent(key, wrapper); if (existingWrapper != null) { wrapper = existingWrapper; wrapper.addReference(); } wrapper.addReference(); wrapper.lock(); } @Override public void unock(K key) { LockWrapper wrapper = lockMap.get(key); if (wrapper != null) { wrapper.unlock(); wrapper.removeReference(); } } private class LockWrapper { private final K key; private final ReentrantLock lock; private int referenceCount; public LockWrapper(K key) { this.key = key; lock = new ReentrantLock(); referenceCount = 0; } public synchronized void addReference() { lockMap.put(key, this); referenceCount++; } public synchronized void removeReference() { referenceCount--; if (referenceCount == 0) { lockMap.remove(key); } } public void lock() { lock.lock(); } public void unlock() { lock.unlock(); } } } </code></pre> <p>This works for two threads accessing a single key lock, but once a third thread is introduced the lock integrity is no longer guaranteed. Any ideas?</p>
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    1. COIgnore the superfluous addReference() calls in the lock method. I was trying out some ideas and forgot to take them out when posting the code here.
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