Question

Say If have a synchronized method and within that method, I update a hashmap like this:

public synchronized void method1()
{
    myHashMap.clear();
    //populate the hashmap, takes about 5 seconds.
}

now while the method1 is running and the hashmap is being re-populated, if there are other threads tring to get the value of the hashmap, I assume they will get blocked?

Now instead of using sync method, if I change hashmap to ConcurrentHashMap like below, what’s the behaviour?

public void method1()
{
     myConcurrentHashMap.clear();
    //populate the hashmap, takes about 5 seconds.
}

what if i use Collections.synchronizedMap ? is it the same?

Answer 1

If you want to have all read and write actions to your HashMap synchronized, you need to put the synchronize on all methods accessing the HashMap; it is not enough to block just one method.

ConcurrentHashMap allows thread-safe access to your data without locking. That means you can add/remove values in one thread and at the same time get values out in another thread without running into an exception. See also the documentation of ConcurrentHashMap