Question

I recently saw a piece of code which used a ThreadLocal object and kept a ConcurrentHashMap within it.

Is there any logic/benefit in this, or is it redundant?

Answer 1

If the only reference to the concurrent hashmap resides in the ThreadLocal, the hashmap is obviously only referenced from a single thread. In such case I would say it is completely redundant.

However, it’s not hard to imagine someone “sharing” the thread-locally stored hashmap with other threads:

ThreadLocal<ConcurrentHashMap<String, String>> tl = ...

// ...

final ConcurrentHashMap<String, String> props = tl.get();

EventQueue.invokeLater(new Runnable() {
    public void run() {
        props.add(key.getText(), val.getText());
    }
});