logLineTester.java

// Fully working test at: https://github.com/njaiswal/logLineTester/blob/master/src/test/java/com/nj/Utils/UtilsTest.java

    @Test
    public void testUtilsLog() throws InterruptedException {

        Logger utilsLogger = (Logger) LoggerFactory.getLogger("com.nj.utils");

        final Appender mockAppender = mock(Appender.class);
        when(mockAppender.getName()).thenReturn("MOCK");
        utilsLogger.addAppender(mockAppender);

        final List<String> capturedLogs = Collections.synchronizedList(new ArrayList<>());
        final CountDownLatch latch = new CountDownLatch(3);

        //Capture logs
        doAnswer((invocation) -> {
            LoggingEvent loggingEvent = invocation.getArgumentAt(0, LoggingEvent.class);
            capturedLogs.add(loggingEvent.getFormattedMessage());
            latch.countDown();
            return null;
        }).when(mockAppender).doAppend(any());

        //Call method which will do logging to be tested
        Application.main(null);

        //Wait 5 seconds for latch to be true. That means 3 log lines were logged
        assertThat(latch.await(5L, TimeUnit.SECONDS), is(true));

        //Now assert the captured logs
        assertThat(capturedLogs, hasItem(containsString("One")));
        assertThat(capturedLogs, hasItem(containsString("Two")));
        assertThat(capturedLogs, hasItem(containsString("Three")));
    }