如何编写创建新线程的单元测试

Question

I have following method for test:

public class classToTest{
    @Autowired
    private Alternator alternator;

     public void methodToTest(){
         Thread t = new Thread(new Runnable() {
             public void run() {
                 while(true) {
                     if(alternator.get()) {
                         System.out.print("Hello");
                         alternator.set(false);
                     }
                 }
             }
          };
          t.start()
    }
}

I need to check that was invoked method

alternator.set(false);

How can I do it?

Answer 1

Instead of starting a thread directly, can you pass in an "Executor" instance?

For example...

public class ClassToTest{
    @Autowired
    private Alternator alternator;

    @Autowired @Qualifier("myExecutor")
    private java.util.concurrent.Executor executor;

    public void methodToTest() {
        Runnable runnable = new Runnable() {
           public void run() {
               while(true) {
                 if(alternator.get()) {
                     System.out.print("Hello");
                     alternator.set(false);
                 }
             }
        };

        executor.execute(runnable);
    }
}

Now you can test this easier...

public class ClassToTestTest {

    ...

    @Before
    public void setup() {
        alternator = mock(Alternator.class);
        executor = mock(Executor.class);

        obj = new ClassToTest();
        ReflectionTestUtils.setField(obj, "alternator", alternator);
        ReflectionTestUtils.setField(obj, "executor", executor);
    }

    @Test
    public void shouldStartRunnable() {
        obj.methodToTest();

        ArgumentCaptor<Runnable> runnableCaptor = ArgumentCaptor.forClass(Runnable.class);

        verify(executor).execute(runnableCaptor.capture());

        Runnable runnable = runnableCaptor.getValue();

        // Now test your actual "runnable"...
        when(alternator.get()).thenReturn(true);
        runnable.run();

        verify(alternator).set(false);
    }
}

(Have not tried to compile this, so I apologise if there are any mistakes!!)

Answer 2

Though Bret's post of passing in an executor is very much recommended, you can use the timeout() mock verification setting to test for asynchronous conditions.

verify(alternator, timeout(500)).set(false);

Note that this will necessarily increase the flakiness of your test (ie the likelihood that the test fails when the code passes). With a sensible timeout value, that flakiness should be negligible, but if you're making this a part of your core test infrastructure you may consider refactoring to allow for synchronous execution in the test.