Testing Multi-Threaded and Asynchronous Code

If you’ve been writing code long enough, or maybe even if you haven’t, chances are you’ve hit on a scenario where you want to test some multi-threaded code. The conventional wisdom is that threads and tests should not mix. Usually this works out fine because the thing that you really want to test just happens to run inside of a multi-threaded system and can be tested individually without the use of threads. But what if you can’t separate things out, or moreover, what if threading is the point of the code you’re testing?

I’m here to tell you that while threads in tests might not be the norm, they are ok. The software police will not arrest you for firing up a thread in a unit test, though how to actually go about testing multi-threaded code is another matter. Some excellent asynchronous technologies like Akka and Vert.x provide test kits to ease the burden. But outside of these, testing multi-threaded code usually requires a different approach than a typical synchronous unit test.

Go Parallel

The first step is to kick off whatever threaded action you wish to test the outcome of. For example, let’s use a hypothetical API to register a message handler against a message bus, and publish a message on the bus which will be delivered asynchronously, in a separate thread, to our handler:

messageBus . registerHandler ( message -> { System . out . println ( "Received " + message ); }; messageBus . publish ( "test" );

Looks good. When the test runs the bus should deliver our message to the handler on another thread, but this isn’t terribly useful since we’re not asserting anything. Let’s update our test to assert that the message bus delivers our message as expected:

String msg = "test" ; messageBus . registerHandler ( message -> { System . out . println ( "Received " + message ); assertEquals ( message , msg ); }; messageBus . publish ( msg );

This seems better. We run our test and it’s green. Awesome! But the Received message never printed - something isn’t quite right.

Wait a Second

In our test above, when a message is published to the message bus, it’s delivered by the bus to the handler on another thread. But when our unit testing tool, such as JUnit, executes our test, it doesn’t know anything about the message bus’ threads. JUnit only knows about the main thread that it executes our test in. So while the message bus is busy trying to deliver our message, the test finishes execution in the main test thread and JUnit reports success. The solution? We need the main test thread to wait for the message bus to deliver our message. So let’s add a sleep statement:

String msg = "test" ; messageBus . registerHandler ( message -> { System . out . println ( "Received " + message ); assertEquals ( message , msg ); }; messageBus . publish ( msg ); Thread . sleep ( 1000 );

Our test is green and the Received statement prints out as expected. Awesome! But having a 1 second sleep means our test takes at least one second to run - no good. We could lower the sleep time, but then we risk the test ending before the message is received. What we need is a way to coordinate between the main test thread and the message handler’s thread. Looking at the java.util.concurrent package, we’re sure to find something we can use. How about a CountDownLatch ?

String msg = "test" ; CountDownLatch latch = new CountDownLatch ( 1 ); messageBus . registerHandler ( message -> { System . out . println ( "Received " + message ); assertEquals ( message , msg ); latch . countDown (); }; messageBus . publish ( msg ); latch . await ();

In this approach, we’re sharing a CountDownLatch between the main test thread and our message handler thread. The main thread is made to wait on the latch and the test thread releases the waiting main thread by calling countDown() on the latch after the message has been received. We no longer need to sleep for 1 second, our test only takes as long as it needs to.

Ship It!?

With our new CountDownLatch awesomeness, we start writing multi-threaded tests like it’s going out of style. But pretty quickly we notice that one of our test cases blocks forever and isn’t finishing. What’s going on? Consider the message bus scenario: the latch is made to wait, but it only releases after a message is received. If the bus is broken and the message is never delivered then our test never completes. So let’s add a timeout to the latch:

latch . await ( 1 , TimeUnit . SECONDS );

Now our test that was blocking fails after 1 second with a TimeoutException . Eventually we find the problem and fix the test, but we decide to leave the timeouts in place. In case this ever happens again, we’d rather our test suite block for a second and fail than block forever and not complete at all.

Another problem we notice when writing our tests is that they all seem to pass even when they maybe shouldn’t. How can this be? Consider our message handling test again:

messageBus . registerHandler ( message -> { assertEquals ( message , msg ); latch . countDown (); };

We had to use a CountDownLatch to coordinate the completion of our test with the main test thread, but what about the assertion? If the assertion fails, will JUnit know? It turns out that because we’re not performing the assertion in the main test thread, any failure of the assertion goes completely unnoticed by JUnit. Let’s try a little scenario to verify this:

CountDownLatch latch = new CountDownLatch ( 1 ); new Thread (() -> { assertTrue ( false ); latch . countDown (); }). start (); latch . await ();

Ugh, the test is green! So now what do we do? We need a way to relay any test failures from the message handling thread back to the main test thread. If a failure occurs in the message handling thread, we need it to be re-thrown in the main thread so that the test will fail as expected. Let’s take a stab at this:

CountDownLatch latch = new CountDownLatch ( 1 ); AtomicReference < AssertionError > failure = new AtomicReference <>(); new Thread (() -> { try { assertTrue ( false ); } catch ( AssertionError e ) { failure . set ( e ); } latch . countDown (); }). start (); latch . await (); if ( failure . get () != null ) throw failure . get ();

A quick run and yes, the test fails just as it should! Now we can go back and add CountDownLatches and try/catch blocks and AtomicReferences to all of our test cases. Awesome! Actually, not awesome, that sounds like a lot of boilerplate.

Cut the Cruft

Ideally what we want is an API that allows us to coordinate waiting, asserting, and resuming execution across threads, so that unit tests can be made to pass or fail as expected regardless of where the assertion failure occurs. Luckily, ConcurrentUnit provides a lightweight construct that does just this: the Waiter. Let’s adapt our message handling test from above one last time and see what ConcurrentUnit’s Waiter can do for us:

String msg = "test" ; Waiter waiter = new Waiter (); messageBus . registerHandler ( message -> { waiter . assertEquals ( message , msg ); waiter . resume (); }; messageBus . publish ( msg ); waiter . await ( 1 , TimeUnit . SECONDS );

In this test, we can see that Waiter has taken the place of our CountDownLatch and AtomicReference . Through the Waiter we block the main test thread, perform our assertion, then resume the main test thread so that the test can complete. If the assertion were to fail, the waiter.await call automatically unblocks and throws the failure, causing our test to pass or fail as it should, even when asserting from another thread.

More Parallel

Now that we’re certified multi-threaded testers, we may want to assert that multiple asynchronous actions occur. ConcurrentUnit’s waiter makes this straightforward:

Waiter waiter = new Waiter (); messageBus . registerHandler ( message -> { waiter . resume (); }; messageBus . publish ( "one" ); messageBus . publish ( "two" ); waiter . await ( 1 , TimeUnit . SECONDS , 2 );

Here we publish two messages to the bus and verify that both messages are delivered by making the Waiter wait until resume() is called 2 times. If the messages fail to be delivered and resume is not called twice within 1 second then the test will fail with a TimeoutException .

One general tip with this approach is to make sure that your timeouts are reasonably long enough for any concurrent actions to complete. Under normal conditions when the system under test operates as expected, the timeout should not matter, and only comes into play when the system fails for some reason.

Recap

In this article we learned that multi-threaded unit testing is not evil and is fairly easy to do. We learned about a general approach where we block the main test thread, perform assertions from some other threads, then resume the main thread. And we learned about ConcurrentUnit which can help make this easier to do.

Happy testing!

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