使用ConcurrentHashMap模拟锁，impl可以安全吗？

Question

I read some interesting code from an open source project, and I don't understand it really.

The concurrentMapExample below is a java.util.concurrent.ConcurrentMap. Can the code below prevent multiple threads return isLocked=true at the same time?

public boolean tryLock()
{
    isLocked = concurrentMapExample.putIfAbsent(key, "") == null;
    return isLocked;
}

Answer 1

Can the code below prevent multiple threads return isLocked=true at the same time?

Yes. This code is thread-safe and only one thread will return null. Other threads calling putIfAbsent(...) afterwards with the same key (!!) will get the "" value and isLocked will be false.

To be pedantic, I would suggest that you wrap the equality check in a parens or maybe do something like the following to improve readability:

if (concurrentMapExample.putIfAbsent(key, "") == null)
   return true;
else
   return false;

This seems like an academic question but for posterity, there obviously are better wants to do this. For example, the same logic using an AtomicBoolean would be:

private final AtomicBoolean isLocked = new AtomicBoolean(false);
...
return isLocked.compareAndSet(false, true);