Memory leak in j.l.ThreadLocal

[Victor Polischuk]

Good day everyone,

There is a quite old problem within ThreadLocal semantic which causes memory leak in case an object put in ThreadLocal has hard reference (direct or indirect) to ThreadLocal itself. Some references can be found:
 * http://stackoverflow.com/questions/17968803/threadlocal-memory-leak
 * http://plumbr.eu/blog/how-to-shoot-yourself-in-foot-with-threadlocals
 * many other sites which consider it as a problem.

In a word, since values put in a ThreadLocal are stored not in the instance but in a Thread as *hard* references there is no chance the instance will be collected unless all threads died (which is not an option in case of using a thread pool).

The most obvious solution is to introduce "ConcurrentWeakHashMap" or something like it and store the values in ThreadLocal itself (in the case we probably lose performance). 

Another solution: currently, a thread has weak reference to ThreadLocal but a strong reference to its value. Let's invert it: a weak reference to a ThreadLocalEntry (couple ThreadLocal and value) in Thread but ThreadLocal stores a strong reference to ThreadLocalEntry. In this case the only synchronized thing we need to implement - write-only storage of ThreadLocalEntry in ThreadLocal (updates will be done from Thread so no sync needed).

Since many developers assume that TreadLocal's behavior should be close to Map (where a Thread instance is the key) and all of us get used to see that GC cleans circular references - I believe that the fix should be applied to the issue with ThreadLocal.

Please let me know what do you think of it.

Sincerely Yours,
Victor Polischuk