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Toolkit for testing multi-threaded and asynchronous applications
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A simple, zero-dependency toolkit for testing multi-threaded code. Supports Java 1.6+.


ConcurrentUnit was created to help developers test multi-threaded or asynchronous code. It allows you to perform assertions and wait for operations in any thread, with failures being properly reported back to the main test thread. If an assertion fails, your test fails, regardless of which thread the assertion came from.


  1. Create a Waiter
  2. Use Waiter.await to block the main test thread.
  3. Use the Waiter.assert calls from any thread to perform assertions.
  4. Once expected assertions are completed, use Waiter.resume call to unblock the awaiting thread.

When your test runs, assertion failures will result in the main thread being interrupted and the failure thrown. If an await call times out before all expected resume calls occur, the test will fail with a TimeoutException.


Consider a test for a message bus that delivers messages asynchronously:

public void shouldDeliverMessage() throws Throwable {
  final Waiter waiter = new Waiter();

  messageBus.registerHandler(message -> {
    // Called on separate thread
    waiter.assertEquals(message, "foo");
  // Wait for resume() to be called

We can also handle wait for multiple resume calls:

public void shouldDeliverMessages() throws Throwable {
  final Waiter waiter = new Waiter();

  messageBus.registerHandler(message -> {
    waiter.assertEquals(message, "foo");
  // Wait for resume() to be called 3 times
  waiter.await(1000, 3);

If an assertion fails in any thread, the test will fail as expected:

@Test(expected = AssertionError.class)
public void shouldFail() throws Throwable {
  final Waiter waiter = new Waiter();

  new Thread(() -> {

TimeoutException is thrown if resume is not called before the await time is exceeded:

@Test(expected = TimeoutException.class)
public void shouldTimeout() throws Throwable {
  new Waiter().await(1);


As a more concise alternative to using the Waiter class, you can extend the ConcurrentTestCase:

class SomeTest extends ConcurrentTestCase {
  public void shouldSucceed() throws Throwable {
    new Thread(() -> {



ConcurrentUnit's Waiter supports the standard assertions along with Hamcrest Matcher assertions:

waiter.assertEquals(expected, result);
waiter.assertThat(result, is(equalTo(expected)));

Since Hamcrest is an optional dependency, users need to explicitly add it to their classpath (via Maven/Gradle/etc).

Other Examples

More example usages can be found in the WaiterTest or in the following projects:

Additional Notes

On await / resume Timing

Since it is not always possible to ensure that resume is called after await in multi-threaded tests, ConcurrentUnit allows them to be called in either order. If resume is called before await, the resume calls are recorded and await will return immediately if the expected number of resumes have already occurred. This ability comes with a caveat though: it is not possible to detect when additional unexpected resume calls are made since ConcurrentUnit allows an await call to follow.

Additional Resources


Copyright 2011-2016 Jonathan Halterman - Released under the Apache 2.0 license.

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