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    <p>Synchronizing on an intern'd String might not be a good idea at all - by interning it, the String turns into a global object, and if you synchronize on the same interned strings in different parts of your application, you might get really weird and basically undebuggable synchronization issues such as deadlocks. It might seem unlikely, but when it happens you are really screwed. As a general rule, only ever synchronize on a local object where you're absolutely sure that no code outside of your module might lock it.</p> <p>In your case, you can use a synchronized hashtable to store locking objects for your keys.</p> <p>E.g.:</p> <pre><code>Object data = StaticCache.get(key, ...); if (data == null) { Object lock = lockTable.get(key); if (lock == null) { // we're the only one looking for this lock = new Object(); synchronized(lock) { lockTable.put(key, lock); // get stuff lockTable.remove(key); } } else { synchronized(lock) { // just to wait for the updater } data = StaticCache.get(key); } } else { // use from cache } </code></pre> <p>This code has a race condition, where two threads might put an object into the lock table after each other. This should however not be a problem, because then you only have one more thread calling the webservice and updating the cache, which shouldn't be a problem.</p> <p>If you're invalidating the cache after some time, you should check whether data is null again after retrieving it from the cache, in the lock != null case.</p> <p>Alternatively, and much easier, you can make the whole cache lookup method ("getSomeDataByEmail") synchronized. This will mean that all threads have to synchronize when they access the cache, which might be a performance problem. But as always, try this simple solution first and see if it's really a problem! In many cases it should not be, as you probably spend much more time processing the result than synchronizing.</p>
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    1. POSynchronizing on String objects in Java
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    1. USMartin Probst
    1. USMartin Probst
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    1. COQuestion - do you mean if I synchronize on the SAME interned string elsewhere in the application?
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      1. USmatt b
    2. CO+1 for a map of strings to lock objects
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    3. COAgreed. Props for pointing out the problem and providing a pretty good solution. Maybe edit this response to include actual code to seal the deal?
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      1. USOutlaw Programmer
 

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