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    <p><strong>PermGen exhaustions</strong> in combination with <code>ThreadLocal</code> are often caused by <strong>classloader leaks</strong>.<br><br> <em>An example:</em> <br> Imagine an application server which has a pool of <em>worker threads</em>.<br> They will be kept alive until application server termination.<br> A deployed web application uses a <strong>static</strong> <code>ThreadLocal</code> in one of its classes in order to store some thread-local data, an instance of another class (lets call it <code>SomeClass</code>) of the web application. This is done within the worker thread (e.g. this action originates from a <em>HTTP request</em>).<br><br> <strong>Important:</strong><br> <a href="http://docs.oracle.com/javase/6/docs/api/java/lang/ThreadLocal.html">By definition</a>, a reference to a <code>ThreadLocal</code> <em>value</em> is kept until the "owning" thread dies or if the ThreadLocal itself is no longer reachable.<br><br> If the web application <strong>fails to clear the reference</strong> to the <code>ThreadLocal</code> <strong>on shutdown</strong>, bad things will happen:<br> Because the worker thread will usually never die and the reference to the <code>ThreadLocal</code> is static, the <code>ThreadLocal</code> value <strong>still references</strong> the instance of <code>SomeClass</code>, a web application's class - <strong>even if the web application has been stopped!</strong><br><br> As a consequence, the web application's <strong>classloader cannot be garbage collected</strong>, which means that <strong>all classes</strong> (and all static data) of the web application <strong>remain loaded</strong> (this affects the PermGen memory pool as well as the heap).<br> Every redeployment iteration of the web application will increase permgen (and heap) usage. <br><br><strong>=> This is the permgen leak</strong> <br><br>One popular example of this kind of leak is <a href="https://issues.apache.org/bugzilla/show_bug.cgi?id=50486">this bug</a> in log4j (fixed in the meanwhile).</p>
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    1. POThreadLocal & Memory Leak
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    1. USMRalwasser
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      1. VTAcceptedByOriginator
    1. COThe (may be dumb) question is - Why Web Server do not try to Kill the Worked Threads for that application when the application is stopped? I would guess/assume that Web Server would create the worker thread specific for the application. If webserver do not kill those threads, even in case of no ThreadLocal, the threads would remain there.
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      1. USSandeep Jindal
    2. COThreads should never be killed, they should only be notified/interrupted in order to terminate gently by itself. Also, threads are expensive to create and often shared across multiple applications within the same container - but this is implementation-specific. However, some application servers drop all threads of a stopped web application (depending on the product/configuration), or renew its threads periodically in order to prevent such leaks. This is implementation specific, too. Have a look at the details for tomcat: http://tomcat.apache.org/tomcat-7.0-doc/config/executor.html
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      1. USMRalwasser
    3. CO@MRalwasser: Thnaks for nice explanation. It is clear why the treadlocal vairable will not be garbage collected, but why the classsloader and other classes also not be garbage collected? other classes dont have to do naything with the class that has the threadlocal variable
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