Spy on Thread raise unexpected exception while stack walking

[BegBang]

Hello,

when I switched from Mockito 3.9.0 to 5.1.1, one test scenario started to raise an expcetion (scenario that on 3.1.1 worked fine).

Exception in thread "test-pq" org.mockito.exceptions.base.MockitoException: Caught an unexpected exception while stack walking.
This is unexpected and is likely due to a change in either Java's StackWalker or Reflection APIs.
It's worth trying to upgrade to a newer version of Mockito, or otherwise to file a bug report.
	at cz.aimtec.dci.label.PrinterQueue.run(PrinterQueue.java:375)

Simplified testing code bellow.
mockito: 5.1.1
jdk: java-11-openjdk-11.0.18.10-1.windows.redhat.x86_64
os: Win 10
mvn: 3.6.3

public class ThreadSpyTest {
    @Test
    void cleanup() {
        MyThread t = spy(new MyThread());
        t.start();

        await().atMost(10, TimeUnit.SECONDS)
                .until(() -> t.methodCalled);

        verify(t, times(1)).callMethod();
    }

    private class MyThread extends Thread {
        boolean methodCalled = false;

        @Override
        public void run() {
            if (isTrue()) {
                callMethod();
            }
        }

        boolean isTrue() {
            return true;
        }

        void callMethod() {
            System.out.println("Method called!");
            methodCalled = true;
        }
    }
}

I will be glad about any information about this issue.

[TimvdLippe]

Unfortunately, Thread is one of the classes that Mockito relies on internally for its behavior. Stubbing Thread will therefore lead to undefined behavior. Additionally, it is advised not to mock classes you don't own: https://github.com/mockito/mockito/wiki/How-to-write-good-tests#dont-mock-a-type-you-dont-own We are working on improving the user experience by working on a DoNotMock feature to avoid mocking classes/methods that are known to crash Mockito internals (#1833). Therefore, I am closing this as "Infeasible". Apologies for the uninformative exception that is thrown.

[nodece]

I have the same issue, the MyThread extends Thread, why cannot spy this object?

I am not an expert in this field, so I am very curious.

A workaround to fix this issue by Wrapper:

    public class MyThread {
        private Thread thread;

        public MyThread() {
            thread = new Thread(this::run);
        }

        public void start() {
            thread.start();
        }

        private void run() {
            // TODO
        }
    }