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POAvoid synchronized(this) in Java?
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  1. POAvoid synchronized(this) in Java?
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    copied!<p>Whenever a question pops up on SO about Java synchronization, some people are very eager to point out that <code>synchronized(this)</code> should be avoided. Instead, they claim, a lock on a private reference is to be preferred.</p> <p>Some of the given reasons are:</p> <ul> <li><a href="https://stackoverflow.com/questions/416183/in-java-critical-sections-what-should-i-synchronize-on#416198">some <em>evil code</em> may steal your lock</a> (very popular this one, also has an "accidentally" variant)</li> <li><a href="https://stackoverflow.com/questions/309631/what-cases-require-synchronized-method-access-in-java#309677">all synchronized methods within the same class use the exact same lock, which reduces throughput</a></li> <li><a href="https://stackoverflow.com/questions/439302/allowing-a-method-to-lock-its-parent-object-in-java#439317">you are (unnecessarily) exposing too much information</a></li> </ul> <p>Other people, including me, argue that <code>synchronized(this)</code> is an idiom that is used a lot (also in Java libraries), is safe and well understood. It should not be avoided because you have a bug and you don't have a clue of what is going on in your multithreaded program. In other words: if it is applicable, then use it.</p> <p>I am interested in seeing some real-world examples (no foobar stuff) where avoiding a lock on <code>this</code> is preferable when <code>synchronized(this)</code> would also do the job.</p> <p>Therefore: <strong>should you always avoid <code>synchronized(this)</code> and replace it with a lock on a private reference?</strong> </p> <hr> <p>Some further info (updated as answers are given):</p> <ul> <li>we are talking about instance synchronization</li> <li>both implicit (<code>synchronized</code> methods) and explicit form of <code>synchronized(this)</code> are considered</li> <li>if you quote Bloch or other authorities on the subject, don't leave out the parts you don't like (e.g. Effective Java, item on Thread Safety: <em>Typically it is the lock on the instance itself, but there are exceptions.)</em></li> <li>if you need granularity in your locking other than <code>synchronized(this)</code> provides, then <code>synchronized(this)</code> is not applicable so that's not the issue</li> </ul>
 

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