Thread Local State and its Interaction with Thread Pools

I recently blogged about solving problems with thread local storage. In that instance, thread local storage was a way to add a form of ‘after the fact’ synchronization to an interface that was initially designed to be called from a single thread.  Thread local storage is useful for this purpose, because it allows each thread to easily isolate the data it manipulates from other threads. Unfortunately, while this is the strength of Thread Local storage, it is also the weakness. In modern multi-threaded designs, threading issues are often abstracted behind thread pools and work queues. With these abstractions at work, the threads themselves become an implementation detail, and thread local variables are too low level to serve some useful scenarios.

One way to address this issue is to use some of the techniques from an earlier post on dynamic extent. The general gist of the idea is to provide Runnable‘s with way of reconstructing relevant thread local state at they time they are invoked on a pool thread. This maps well to the idea of ‘dynamic extent’, as presented in the earlier post. ‘Establishing the precondition’ is initializing the thread locals for the run, and ‘Establishing the post condition’ is restoring their original values. Here’s how it might look in code:

// This is the thread local storage that we want to migrate to worker threads.
ThreadLocal<Object> tls = new ThreadLocal<Object>();

Runnable bindToTls(final Runnable runnable)
{
   return new Runnable() {
      // Captured at the time/thread making the initial call to bindToTls
      final Object boundTls = tls.get();

      public void run() {
         // Captured at the time the Runnable is invoked
         Object initialTls = tls.get();

         try {
            tls.set(boundTls);

            runnable.run();
         } finally {
            tls.set(initialTls);
         }
   };
}

Calling bindToTls on a Runnable then gives a new instance of Runnable that remembers the preserved thread local state at the time of the call to bindToTls. The returned Runnable can then be provided to a thread pool, and when it is run, it will remember the thread local state. At the end of the run, it then ensures that the thread local is restored to its original value. (Which eliminates the possibility of certain classes of errors, including potential memory leaks.)

One caveat to this version of bindToTls is that is only migrates the one thread local variable that it’s been written to migrate. While this could be made considerably more general, my suggestion is that it’s usually unnecessary to do so. My current project uses one instance of bindToTls to provide a custom Spring scope. From there, Spring provides all the generality we need, and the lower level thread management code is kept contained, which is as it should be.

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