How should I unit test threaded code?

Question

I have thus far avoided the nightmare that is testing multi-threaded code since it just seems like too much of a minefield. I\'d like to ask how people have gone about test

Answer 1

If you are testing simple new Thread(runnable).run() You can mock Thread to run the runnable sequentially

For instance, if the code of the tested object invokes a new thread like this

Class TestedClass {
    public void doAsychOp() {
       new Thread(new myRunnable()).start();
    }
}

Then mocking new Threads and run the runnable argument sequentially can help

@Mock
private Thread threadMock;

@Test
public void myTest() throws Exception {
    PowerMockito.mockStatic(Thread.class);
    //when new thread is created execute runnable immediately 
    PowerMockito.whenNew(Thread.class).withAnyArguments().then(new Answer() {
        @Override
        public Thread answer(InvocationOnMock invocation) throws Throwable {
            // immediately run the runnable
            Runnable runnable = invocation.getArgumentAt(0, Runnable.class);
            if(runnable != null) {
                runnable.run();
            }
            return threadMock;//return a mock so Thread.start() will do nothing         
        }
    }); 
    TestedClass testcls = new TestedClass()
    testcls.doAsychOp(); //will invoke myRunnable.run in current thread
    //.... check expected 
}