Difference between synchronized and re-entrant lock? [duplicate]

Question

I have used the synchronized keyword and re-entrant locks in Java, but I don't understand how they differ, or which is appropriate for a given situation.

How do I decide when should I use synchronized and when I should use re-entrant locks?

Answer 1

A ReentrantLock is:

A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.

Extended capabilities include:

1. The ability to have more than one condition variable per monitor. Monitors that use the synchronized keyword can only have one. This means reentrant locks support more than one wait()/notify() queue.
2. The ability to make the lock fair. Synchronized blocks are unfair.

   "[fair] locks favor granting access to the longest-waiting thread. Otherwise this lock does not guarantee any particular access order."

3. The ability to check if the lock is being held.
4. The ability to get the list of threads waiting on the lock.

The disadvantages of reentrant locks are:

1. Need to add import statement.
2. Need to wrap lock acquisitions in a try/finally block. This makes it more ugly than the synchronized keyword.
3. The synchronized keyword can be put in method definitions which avoids the need for a block which reduces nesting.

Summary

The synchronized keyword is syntactically nicer, but the Reentrant lock has more features.