How to use JUnit to test asynchronous processes

Question

How do you test methods that fire asynchronous processes with JUnit?

I don\'t know how to make my test wait for the process to end (it is not exactly a unit test, it

Answer 1

If you want to test the logic just don´t test it asynchronously.

For example to test this code which works on results of an asynchronous method.

public class Example {
    private Dependency dependency;

    public Example(Dependency dependency) {
        this.dependency = dependency;            
    }

    public CompletableFuture someAsyncMethod(){
        return dependency.asyncMethod()
                .handle((r,ex) -> {
                    if(ex != null) {
                        return "got exception";
                    } else {
                        return r.toString();
                    }
                });
    }
}

public class Dependency {
    public CompletableFuture asyncMethod() {
        // do some async stuff       
    }
}

In the test mock the dependency with synchronous implementation. The unit test is completely synchronous and runs in 150ms.

public class DependencyTest {
    private Example sut;
    private Dependency dependency;

    public void setup() {
        dependency = Mockito.mock(Dependency.class);;
        sut = new Example(dependency);
    }

    @Test public void success() throws InterruptedException, ExecutionException {
        when(dependency.asyncMethod()).thenReturn(CompletableFuture.completedFuture(5));

        // When
        CompletableFuture result = sut.someAsyncMethod();

        // Then
        assertThat(result.isCompletedExceptionally(), is(equalTo(false)));
        String value = result.get();
        assertThat(value, is(equalTo("5")));
    }

    @Test public void failed() throws InterruptedException, ExecutionException {
        // Given
        CompletableFuture c = new CompletableFuture();
        c.completeExceptionally(new RuntimeException("failed"));
        when(dependency.asyncMethod()).thenReturn(c);

        // When
        CompletableFuture result = sut.someAsyncMethod();

        // Then
        assertThat(result.isCompletedExceptionally(), is(equalTo(false)));
        String value = result.get();
        assertThat(value, is(equalTo("got exception")));
    }
}

You don´t test the async behaviour but you can test if the logic is correct.