Question

I want to test if a ThreadLocal has been initialized without actually initializing it. Of course, the code needs to be thread-safe. Ideally I want something like this:

class TestableThreadLocal<T> extends ThreadLocal<T> {
    public boolean isInitialized() {
        ...
    }
}

But how would I implement this method?

Edit: Motivation: I have subclassed a ThreadLocal to override initialValue(). However, I do not always need the initialization, in particular because it could cause a memory leak in multi-classloader environments. A simple test would help me write code to avoid the accidental initialization.

Answer 1

I tend to use simple rules to avoid classloader leaks, admittedly they are cumbersome but with some wrapping it’s not that bad. Yes, leaks are evil.

• do no override initialValue, ever – you are just asking for trouble, just forget it exists.
• do not store non-system/non-bootstap classes in the ThreadLocal unless you do threadLocal.set(xxx); try{...process...}finally{threadLocal.set(null);}
• if you still override initialValue, use threadLocal.remove() not threadLocal.set(null)
• or use WeakReferene(i.e. ThreadLocal<WeakReference<Foo>>) for the value from a pool that keeps the hard references. It might look counter intuitive but once the pool is cleared the values disappear and the classes are free to be GC’d

I realize the post is not a direct reply to your question, however there is no simple way to achieve what you wish and keep the leaks at bay.