What is a good way to test that a Java method is synchronized?

Question

I have several classes that implement some interface. The interface has a contract, that some methods should be synchronized, and some should not, and I want to verify that contract through unit tests for all the implementations. The methods should use the synchronized keyword or be locked on this - very similar to the synchronizedCollection() wrapper. That means I should be able to observe it externally.

To continue the example of Collections.synchronizedCollection() if I have one thread calling iterator(), I should still be able to get into methods like add() with another thread because iterator() should not do any locking. On the other hand, I should be able to synchronize on the collection externally and see that another thread blocks on add().

Is there a good way to test that a method is synchronized in a JUnit test? I want to avoid long sleep statements.

Answer 1

If you just want to check if a method has the synchronized modifier, aside from the obvious (looking at the source code/Javadoc), you can also use reflection.

Modifier.isSynchronized(method.getModifiers())

The more general question of testing if a method guarantees proper synchronization in all concurrency scenarios is likely to be an undecidable problem.