Jon Jon - 4 months ago 15
Java Question

How to do a JUnit assert on a message in a logger

I have some code-under-test that calls on a Java logger to report its status.
In the JUnit test code, I would like to verify that the correct log entry was made in this logger. Something along the following lines:

methodUnderTest(bool x){
if(x)
logger.info("x happened")
}

@Test tester(){
// perhaps setup a logger first.
methodUnderTest(true);
assertXXXXXX(loggedLevel(),Level.INFO);
}


I suppose that this could be done with a specially adapted logger (or handler, or formatter), but I would prefer to re-use a solution that already exists. (And, to be honest, it is not clear to me how to get at the logRecord from a logger, but suppose that that's possible.)

Jon Jon
Answer

Thanks a lot for these (surprisingly) quick and helpful answers; they put me on the right way for my solution.

The codebase were I want to use this, uses java.util.logging as its logger mechanism, and I don't feel at home enough in those codes to completely change that to log4j or to logger interfaces/facades. But based on these suggestions, I 'hacked-up' a j.u.l.handler extension and that works as a treat.

A short summary follows. Extend java.util.logging.Handler:

class LogHandler extends Handler
{
    Level lastLevel = Level.FINEST;

    public Level  checkLevel() {
        return lastLevel;
    }    

    public void publish(LogRecord record) {
        lastLevel = record.getLevel();
    }

    public void close(){}
    public void flush(){}
}

Obviously, you can store as much as you like/want/need from the LogRecord, or push them all into a stack until you get an overflow.

In the preparation for the junit-test, you create a java.util.logging.Logger and add such a new LogHandler to it:

@Test tester() {
    Logger logger = Logger.getLogger("my junit-test logger");
    LogHandler handler = new LogHandler();
    handler.setLevel(Level.ALL);
    logger.setUseParentHandlers(false);
    logger.addHandler(handler);
    logger.setLevel(Level.ALL);

The call to setUseParentHandlers() is to silence the normal handlers, so that (for this junit-test run) no unnecessary logging happens. Do whatever your code-under-test needs to use this logger, run the test and assertEquality:

    libraryUnderTest.setLogger(logger);
    methodUnderTest(true);  // see original question.
    assertEquals("Log level as expected?", Level.INFO, handler.checkLevel() );
}

(Of course, you would move large part of this work into a @Before method and make assorted other improvements, but that would clutter this presentation.)