How to Convert Async Calls to Sync Using Mockito in Java?

How can I convert asynchronous calls to synchronous ones using Mockito in Java?

Mockito.when(asyncService.call()).thenReturn(CompletableFuture.completedFuture(expectedResult));

Mockito is a powerful framework used in Java for unit testing, particularly for mocking objects. When dealing with asynchronous methods, testing in synchronous contexts can be challenging. This guide outlines methods to convert async behavior to sync, enabling easier testing of asynchronous code with Mockito.

CompletableFuture<String> future = mock(CompletableFuture.class);
Mockito.when(future.get()).thenReturn("result");

String result = future.get(); // This will now return 'result' synchronously.

Causes

• Asynchronous code execution can lead to test timing issues.
• Mocking asynchronous methods requires synchronization to ensure tests complete as expected.

Solutions

• Use `CompletableFuture` to provide synchronous behavior when mocking.
• Leverage `CountDownLatch` or similar synchronization tools to wait for asynchronous calls to complete before continuing with assertions.

Mistake: Not waiting for the CompletableFuture to complete in the tests.

Solution: Ensure you call the `.get()` method on the CompletableFuture to block and wait for the result.

Mistake: Assuming async methods will behave synchronously without proper mocking.

Solution: Use Mockito's capabilities to define synchronous behavior for mocks, as shown in the examples.