Test log lines

Some times the only thing we are interested in testing is weather a log line correctly gets logged. This is not the most optimal test but sometimes its required... in cases where log lines are getting monitored by a monitoring agent and we want to make sure correct format gets logged etc.

The trick is to mock the Log Appender and then capture the logs on that appender in a safe way. Once these log lines are captured you can verify them to your hearts content....

A fully working example of this is given at: https://github.com/njaiswal/logLineTester

Following snippet shows how to achieve this:

	// 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")));
	}

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