Synchronized Block and Method

The synchronized(obj) statement in Java is the equivalent of the lock(obj) statement in C#.

It prevents more than one thread from entering the synchronized block at any one time. Subsequent threads must wait until the first thread has finished executing the code in the block.

This is achieved by the first thread taking a mutually exclusive lock on 'obj' which it releases when it exits the synchronized block. The next thread then takes a lock on 'obj' and so on.

The synchronized method in Java (which has no equivalent in C#) locks an entire method whilst a thread is executing it. Subsequent threads are unable to enter the method until the first thread has returned.

The mutually exclusive lock in this situation is taken (in the case of an instance method) on the object on which the synchonized method is called or (in the case of a static method) using a class-wide object.