Java Cached thread pool and thread local

问题

I have a question about java and concurrency.

Let say I have a ThreadLocal variable called 'a'. And I use a CachedThreadPool to obtain new threads. When a thread is reused, what happens to the ThreadLocal variable 'a'? It maintains the same value (cause it is the same thread) or it starts empty (as if the thread was new)?

回答1:

By default ThreadLocals are reused along with the thread. If you need them to be be reinitialized you can do so by overriding the methods noted below:

from javadoc for java.util.concurrent.ThreadPoolExecutor

Hook methods This class provides protected overridable beforeExecute(java.lang.Thread, java.lang.Runnable) and afterExecute(java.lang.Runnable, java.lang.Throwable) methods that are called before and after execution of each task. These can be used to manipulate the execution environment; for example, reinitializing ThreadLocals, gathering statistics, or adding log entries. Additionally, method terminated() can be overridden to perform any special processing that needs to be done once the Executor has fully terminated. If hook or callback methods throw exceptions, internal worker threads may in turn fail and abruptly terminate.

回答2:

You can clear the thread local pool for a thread using reflection. You can do

public static void clearAllThreadLocals() {
    try {
        Field threadLocals = Thread.class.getDeclaredField("threadLocals");
        threadLocals.setAccessible(true);
        threadLocals.set(Thread.currentThread(), null);
    } catch (Exception e) {
        throw new AssertionError(e);
    }
}

回答3:

If the thread is returned to the pool, the ThreadLocal variable will still be attached to it. When using ThreadLocals with pools, you have to be careful to be able to set when the thread is pulled from the pool and unset before it is returned.

来源：https://stackoverflow.com/questions/7403809/java-cached-thread-pool-and-thread-local