MockitoException when trying to mock java.lang.System

I have a test case that mock a static method of java.lang.System class:

@Test
fun `getLocalTime()`() {
    // Arrange
    val staticMock = Mockito.mockStatic(System::class.java)
    Mockito.`when`(System.currentTimeMillis()).thenReturn(1000L)

    // Action
    val res = deviceTimeProvider.getLocalTime()

    // Assert
    Truth.assertThat(res).isEqualTo(1000L)

    staticMock.close()
}

But when I run the test, I got this error:

> org.mockito.exceptions.base.MockitoException: It is not possible to
> mock static methods of java.lang.System to avoid interfering with
> class loading what leads to infinite loops

Why does this happen? How can I mock methods of java.lang.System class?

While Mockito since 3.4.0 version allows mocking static methods it is not allowed to mock the Thread and System static methods, see this comment on github

> Finally note that Mockito forbids mocking the static methods of System (and Thread). Those methods are to much cemented into class loading which happens in the same thread. At some point, we might add instrumentation to class loading to temporarily disable the static mocks within it to make mocking these classes, too, where we also would need to disable their intensification properties. You can however easily mock Instant.now().

If you like ugly solutions you can still mock System with PowerMockito

@PrepareForTest(System.class)
public class TestCase {
    @BeforeClass
    public void setup() {
        PowerMockito.mockStatic(System.class);
        PowerMockito.when(System.currentTimeMillis()).thenReturn(1000L);
    }
...

But I would avoid mocking System classes if possible. You can still wrap it in method and mock this method.

To mock the static methods of java.lang.System class with the help of Mockito.

1. Create an interface i.e ISystem.java

public interface ISystem {

    String getProperty(String name);

    Long getCurrentTimeInMillis();

}

2- Create the implementation class of ISystem interface i.e ISystemImpl.java

public class ISystemImpl implements ISystem {

    @Override
    public String getProperty(final String name) {
        return System.getProperty(name);
    }

    @Override
    public Long getCurrentTimeInMillis() {
        return System.currentTimeMillis();
    }

}

3- Use Isystem.java inside your DeviceTimeProvider.java class.

public class DeviceTimeProvider {

   @NonNull private final ISystem mISystem;

   public DeviceTimeProvider(ISystem iSystem){
       mIsystem = iSystem;
   }

   public Long getLocalTime(){
       return mIsystem.getCurrentTimeInMillis()
   }

}

4- Now finally mock the ISystem interface inside your test class.

public class DeviceTimeProviderTest {

   private ISystem mISystem;
   private DeviceTimeProvider sut;

   @Before
   public setup(){
       mIsystem = mockito.mock(ISystem.class)
       sut = new DeviceTimeProvider(mISystem);
   }

   @Test
   public void getDeviceLocalTime(){
       Long expectedTime = 1000L;
       mockit.when(mISystem.getCurrentTimeInMillis()).thenReturn(expectedTime);
       
       Long actualTime = sut.getLocalTime();

       Assert.assertEquals(actualTime, expectedTime);
   }

}

OUTPUT