How can I verify in a unit test that a method will be executed asynchronously i.e. in a separate thread?

Question

The following method calls the method serveThis() of a service synchronously and the method serveThat() in a separate thread i.e. asynchronously:

public void doSomething() {
    service.serveThis();
    new Thread(() -> service.serveThat()).start();
}

I want to verify in a unit test that service.serveThat() will be executed asynchronously as according to the specification it must not be executed synchronously. So, I want to prevent that later on someone just removes starting a new thread like this:

public void doSomething() {
    service.serveThis();
    // This synchronous execution must cause the test to fail
    service.serveThat();
}

Answer 1

In order to achieve that I use Mockito:

Thread threadServeThatRunsOn;

@Test
public void serveThatWillBeExecutedAsynchronously() throws Exception {
    doAnswer(invocation -> {
        threadServeThatRunsOn = Thread.currentThread();
        return null;
    }).when(mockService).serveThat();

    testObject.doSomething();

    verify(mockService, timeout(200)).serveThat();
    assertNotEquals(threadServeThatRunsOn, Thread.currentThread());
}

Now, if someone would modify doSomething()so that service.serveThat() would run synchronously, then this test would fail.