Question 1

How to test asynchronous callbacks with ConcurrentUnit?

Accepted Answer

Use the Waiter to block the main thread with await(), and in the callback, perform assertions using waiter.assertEquals() or similar, then call resume(). This coordinates the test and ensures failures are reported, as shown in the message bus example in the README.

Question 2

ConcurrentUnit vs Awaitility: which is better for async testing?

Accepted Answer

ConcurrentUnit is simpler and zero-dependency, focusing on thread coordination and cross-thread assertions. Awaitility offers more fluent APIs for waiting on conditions but may have more dependencies. Choose based on your need for lightweight control vs declarative waiting.

Question 3

Can I use ConcurrentUnit with JUnit 5?

Accepted Answer

Yes, ConcurrentUnit is framework-agnostic and works with JUnit 5. You can use the Waiter class in JUnit 5 tests, though you might need to handle exceptions like TimeoutException within JUnit's assertion mechanisms.

Question 4

How to handle multiple threads in a ConcurrentUnit test?

Accepted Answer

Create a Waiter instance, spawn threads that perform assertions and call resume(), then use await() with the expected number of resumes. The README example shows waiting for three resume calls to synchronize multiple message deliveries.

Question 5

What are common mistakes when using ConcurrentUnit?

Accepted Answer

Common pitfalls include not accounting for the flexible order of await and resume calls, forgetting to set timeouts leading to hangs, and missing assertions in worker threads. The README warns about undetected extra resumes as a key limitation.

Question 6

How to integrate Hamcrest matchers with ConcurrentUnit?

Accepted Answer

Add Hamcrest as an optional dependency to your project via Maven or Gradle, then use waiter.assertThat() with matchers like is(equalTo(expected)). The README notes that Hamcrest support is optional and requires explicit inclusion.

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