Question

I was wondering if in Java I would get any odd behaviour if I synchronise twice on the same object?

The scenario is as follows

pulbic class SillyClassName {      object moo;     ...     public void method1(){         synchronized(moo)         {             ....             method2();             ....         }     }      public void method2(){         synchronized(moo)         {             doStuff();         }     } } 

Both methods use the object and are synchronised on it. Will the second method when called by the first method stop because it’s locked?

I don’t think so because it’s the same thread but I’m unsure of any other odd results that might occur.

Answer 1

Reentrant

Synchronized blocks use reentrant locks, which means if the thread already holds the lock, it can re-aquire it without problems. Therefore your code will work as you expect.

See the bottom of the Java Tutorial page Intrinsic Locks and Synchronization.

To quote as of 2015-01…

Reentrant Synchronization

Recall that a thread cannot acquire a lock owned by another thread. But a thread can acquire a lock that it already owns. Allowing a thread to acquire the same lock more than once enables reentrant synchronization. This describes a situation where synchronized code, directly or indirectly, invokes a method that also contains synchronized code, and both sets of code use the same lock. Without reentrant synchronization, synchronized code would have to take many additional precautions to avoid having a thread cause itself to block.