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POSimple Java name based locks?
 

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    <p>MySQL has a handy function:</p> <pre><code>SELECT GET_LOCK("SomeName") </code></pre> <p>This can be used to create simple, but very specific, name based locks for an application. However, it requires a database connection.</p> <p>I have many situations like:</p> <pre><code>someMethod() { // do stuff to user A for their data for feature X } </code></pre> <p>It doesn't make sense to simply synchronize this method, because, for example, if this method is called for user B in the meantime, user B does not need to wait for user A to finish before it starts, only operations for the user A and feature X combination need to wait.</p> <p>With the MySql lock I could do something like:</p> <pre><code>someMethod() { executeQuery("SELECT GET_LOCK('userA-featureX')") // only locked for user A for their data for feature X executeQuery("SELECT RELEASE_LOCK('userA-featureX')") } </code></pre> <p>Since Java locking is based on objects, it seems like I would need to create a new object to represent the situation for this lock and then put it in a static cache somewhere so all the threads can see it. Subsequent requests to lock for that situation would then locate the lock object in the cache and acquire its lock. I tried to create something like this, but then the lock cache itself needs synchronization. Also, it is difficult to detect when a lock object is no longer being used so that it can be removed from the cache.</p> <p>I have looked at the Java concurrent packages, but nothing stands out as being able to handle something like this. Is there an easy way to implement this, or am I looking at this from the wrong perspective?</p> <p><strong>Edit:</strong></p> <p>To clarify, I am not looking to create a predefined pool of locks ahead of time, I would like to create them on demand. Some pseudo code for what I am thinking is:</p> <pre><code>LockManager.acquireLock(String name) { Lock lock; synchronized (map) { lock = map.get(name); // doesn't exist yet - create and store if(lock == null) { lock = new Lock(); map.put(name, lock); } } lock.lock(); } LockManager.releaseLock(String name) { // unlock // if this was the last hold on the lock, remove it from the cache } </code></pre>
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