Mockito.verify not behaving properly when using a capture

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Summary

The issue at hand involves Mockito.verify not behaving as expected when using a capture in conjunction with times(2). This problem arose after upgrading to JDK 21. The test fails when both times(2) and myCaptor.capture() are used, but passes if either is removed.

Root Cause

The root cause of this issue can be attributed to several factors:

  • Incompatible versions: The upgrade to JDK 21 might have introduced compatibility issues with the version of Mockito being used.
  • Capture limitations: The use of capture with verify and times might be causing Mockito to malfunction.
  • Verification logic: The logic behind Mockito.verify might not be correctly handling the combination of times and capture.

Why This Happens in Real Systems

This issue occurs in real systems due to:

  • Version mismatches: Using outdated or incompatible versions of libraries like Mockito with newer JDK versions.
  • Complex test scenarios: Tests involving multiple verifications with capture and times can lead to unexpected behavior.
  • Library limitations: Mockito or other mocking libraries might have inherent limitations or bugs that surface under specific conditions.

Real-World Impact

The real-world impact of this issue includes:

  • Test failures: Tests that were previously passing might start failing after upgrading to JDK 21 or changing the test setup.
  • Debugging challenges: Identifying the root cause of the issue can be time-consuming and require significant debugging effort.
  • Development delays: Resolving this issue might delay the development and deployment of software applications.

Example or Code

import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.ArgumentCaptor;
import org.mockito.Captor;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.junit.MockitoJUnitRunner;

import static org.mockito.Mockito.times;
import static org.mockito.Mockito.verify;

@RunWith(MockitoJUnitRunner.class)
public class MyServiceTest {

    @Mock
    private MyService myService;

    @Captor
    private ArgumentCaptor myCaptor;

    @Test
    public void testMyMethod() {
        // Given
        MyObject obj = new MyObject();

        // When
        myService.myMethod(obj);
        myService.myMethod(obj);

        // Then
        verify(myService, times(2)).myMethod(myCaptor.capture());
    }
}

How Senior Engineers Fix It

Senior engineers fix this issue by:

  • Checking library versions: Ensuring that all libraries, including Mockito, are up-to-date and compatible with the current JDK version.
  • Simplifying test scenarios: Breaking down complex test scenarios into simpler, more manageable parts to isolate the issue.
  • Using alternative verification methods: Exploring alternative verification methods, such as using atLeast or atMost instead of times, to achieve the desired test behavior.

Why Juniors Miss It

Junior engineers might miss this issue due to:

  • Lack of experience: Limited experience with Mockito and JDK upgrades can make it difficult to identify the root cause of the problem.
  • Insufficient testing: Inadequate testing and debugging can lead to overlooking the issue or failing to identify the correct solution.
  • Inadequate knowledge of library limitations: Junior engineers might not be aware of the limitations and potential bugs in Mockito and other libraries, making it harder to troubleshoot the issue.