Testing that a Logback Log statment was called using Junit?

Question

Below is a method that I am attempting to write Junit test for:

Method I wish to test:

   //logger declared outside of method in Class
   private static Logger LOGGER = LoggerFactory.getLogger(PersonService.class);

    public void showOutputOfIdentifications(int age) {

        if(age>25){

            LOGGER.info("Over 25");

        }else{

            LOGGER.info("25 or Under");

        }
     }

How can I test to verify that the correct Logback Log statement has been called?

In my tests I have tried to mock the logger and verify that it was called, however this has not worked?

Current Test Attempt:

 @Test
    public void testShowOutputOfIdentifications() throws ParseException{

    int age = 10;

    Logger LOGGER_mock = mock(Logger.class);

    //call method under test
    showOutputOfIdentifications(age);

    verify(LOGGER_mock).info("25 or under");


}

Current failing test output:

Wanted but not invoked:
logger.info(
    "25 or under "
);

Actually, there were zero interactions with this mock.

Answer 1

You could add your own appender and assert that the log message was written that way, see Programmatically configure LogBack appender.

Something like this:

// create the mock appender
Appender mockedAppender = Mockito.mock(Appender.class);

// inject it
((ch.qos.logback.classic.Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME)).addAppender(mockedAppender);

// run your test
LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME).error("Test msg");

// verify using ArgumentCaptor
ArgumentCaptor<Appender> argumentCaptor = ArgumentCaptor.forClass(Appender.class);
Mockito.verify(mockedAppender).doAppend(argumentCaptor.capture());

// assert against argumentCaptor.getAllValues()
Assert.assertEquals(1, argumentCaptor.getAllValues().size());
Assert.assertEquals("Test msg", ((LoggingEvent)argumentCaptor.getAllValues().get(0)).getMessage());

// remove the mock appender from static context
((Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME)).detachAppender(mockedAppender);