ThreadLocal & Memory Leak

Question

It is mentioned at multiple posts: improper use of ThreadLocal causes Memory Leak. I am struggling to understand how Memory Leak would happen using Thread

Answer 1

Here is an alternative to ThreadLocal that doesn't have the memory leak problem:

class BetterThreadLocal {
  Map map = Collections.synchronizedMap(new WeakHashMap());

  A get() {
    ret map.get(Thread.currentThread());
  }

  void set(A a) {
    if (a == null)
      map.remove(Thread.currentThread());
    else
      map.put(Thread.currentThread(), a);
  }
}

Note: There is a new memory leak scenario, but it is highly unlikely and can be avoided by following a simple guide line. The scenario is keeping a strong reference to a Thread object in a BetterThreadLocal.

I never keep strong references to threads anyways because you always want to allow the thread to be GC'd when its work is done... so there you go: a memory leak-free ThreadLocal.

Someone should benchmark this. I expect it to be about as fast as Java's ThreadLocal (both essentially do a weak hash map lookup, just one looks up the thread, the other the ThreadLocal).

Sample program in JavaX.

And a final note: My system (JavaX) also keeps track of all WeakHashMaps and cleans them up regularly, so the last super-unlikely hole is plugged (long-lived WeakHashMaps that are never queried, but still have stale entries).