ThreadLocalUsage

ThreadLocals should be stored in static variables to avoid memory leaks. If a ThreadLocal is stored in an instance (non-static) variable, there will be <code>M * N</code> instances of the ThreadLocal value where M is the number of threads, and N is the number of instances of the containing class. Each instance may remain live as long the thread that stored it stays live.

Example:

class C {
   private final ThreadLocal<D> local = new ThreadLocal<D>();

   public f() {
       D d = local.get();
       if (d == null) {
          d = new D(this);
          local.set(d);
       }
       d.doSomething();
   }
}

The fix is often to make the field static:

private static final ThreadLocal<D> local = new ThreadLocal<D>();