Simple tool for testing asynchronous code. It allows you to assert operations in threads. It will wait for threads to finish and will propagate errors to the main thread.

Inspiration got from concurrentunit.

Advantages over concurentunit:

• simpler usage
• no restrictions on assertion actions
• more control over exception propagation

Wrap asynchronous part of the code using AsyncFlow.prepare and use AsyncFlow.await to block the main thread. If asynchronous part fails exceptionally, the test will also fail.

@Test
public void doesSomeWorkInSeparateThread() throws Exception {

    new Thread(
        AsyncFlow.prepare(() -> {
            work = doSomeWork();
            assertNotNull(work);
        })
    ).start();
    
    AsyncFlow.await();
}

Assertions in multiple threads.

@Test
public void doesMultipleWorkInThreads() throws Throwable {

    for (int i = 0; i < 4; i++)
    {
        new Thread(
            AsyncFlow.prepare(() -> {
                work = doSomeWork();
                assertNotNull(work);
            })
        ).start();
    }
    
    AsyncFlow.await(1000, 4);
}

AsyncFlow.await will work properly only if AsyncFlow.prepare is called in the same thread where await is triggered. This will usually be the main test thread. However, if you need flexibility, you can instantiate AsyncFlow and prepare flow lazy:

@Test
public void supportsLazyFlowPreparation() throws Exception {

    AsyncFlow.Single flow = new AsyncFlow.Single();

    new Thread(() -> {
        flow.prepare(() -> {
             work = doSomeWork();
             assertNotNull(work);
        }).run();
    }).start();
    
    flow.await();
}

By default, tool propagates every Throwable from an async flow back to the main thread. You can customize this by specifying which exceptions you want to propagate. For example:

new AsyncFlow.Single(
    IllegalStateException.class,
    IOException.class,
    CustomException.class    
);

You can contribute by forking the repo and sending a pull request. Make sure your branch builds without any warnings/issues:

mvn clean install