Is it a good idea to use WeakHashMap in ThreadLocal

Question

For my usecase, I have to pass quite a few context information from different layers/components of the application. Since few of the components are discrete, I am thinking to use ThreadLocal to store such context information. I have an interceptor/filter in place to clean it before the the response is written back to the user. Now, my question is, is it a good idea to use WeakHashMap inside ThreadLocal (see the code snippet below)?

    private static final ThreadLocal<Map<String, Object>> context = new ThreadLocal<WeakHashMap<String, Object>>();

The doubt in my mind (with my limited knowledge of Weak references in Java) is, the weak references can return NULL (because GC collects them as per its own will).

Please help me in understanding this. Should I use a strong reference like HashMap or ConcurrentHashMap or my implementation is good to go?

Answer 1

The Javadoc for WeakHashMap states:

This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator. Once such a key is discarded it can never be recreated, so it is impossible to do a lookup of that key in a WeakHashMap at some later time and be surprised that its entry has been removed. This class will work perfectly well with key objects whose equals methods are not based upon object identity, such as String instances. With such recreatable key objects, however, the automatic removal of WeakHashMap entries whose keys have been discarded may prove to be confusing.

So if you can't tolerate entries randomly disappearing, then you really shouldn't be using WeakHashMap with String keys.