Verification with timeout on synchronized method fails

[Peter-bytes]

When using Mockito verify with a timeout to wait for an asynchronous method call, it sometimes fails if the method is synchronized.

I have observed the effect with multiple versions of Eclipse, including 2020-06 (4.16.0), and both Mockito 3.1.0 and 3.4.6. The example below illustrates the problem (both the test and the subject under test are in the same class for simplicity).

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
class A {
  @Mock
  private A a;

  public synchronized void f() {
    // Do stuff
  }

  @Test
  void test() {
    // Call a.f() asynchronously and with a small delay 
    // (to make the test thread yield).
    new Thread(() -> {
      try {
        Thread.sleep(100);
      } catch (final InterruptedException e) {
        Thread.currentThread().interrupt();
      }
      a.f();
    }).start();

    // This verification fails, but it should pass.
    Mockito.verify(a, Mockito.timeout(2000)).f();
  }
}

Noteworthy points:

• The effect is not perfectly reproducible. In fact, my real-life examples never fail on some computers while they always fail on others, suggesting a race condition.
• Removing the synchronized keyword solves the issue, even when replacing it with a synchronized(this) block in the method body.
• If the class containing the synchronized method implements an interface, mocking the interface instead of the concrete class solves the issue.
• Removing the sleep in the example above, hence increasing the chance that the call to f is finished before verify is called, solves the issue.
• The behavior is suspiciously similar to the issue described (and presumably solved) here: Can't verify on synchronized methods of objects #253.

[spmason]

Just to note that I'm seeing this bug by default in Mockito 5.0.0 - in 4.11.0 the provided test case passes but it fails in 5.0.0. I'm not able to get it to pass even without the sleep / with a longer timeout, on multiple machines. Only moving the synchronization to the body of the method works around the issue

[spmason]

Just noticed this:

Mockito 5 switches the default mockmaker to mockito-inline, and now requires Java 11.

Which I guess confirms the link to #2500