How can I test with junit that a warning was logged with log4j? [duplicate]

Answer 1

I'm adapting Haim's answer to something that's more RAII:

public static class TestAppender extends AppenderSkeleton {
    @Override
    protected void append(LoggingEvent event) {
        messages.add(event.getRenderedMessage());
    }

    @Override
    public void close() { }

    @Override
    public boolean requiresLayout() { return false; }

    protected final List messages = new ArrayList<>();
}

static class LogGuard implements AutoCloseable {
    protected final TestAppender appender;

    LogGuard(Level level) {
        appender = new TestAppender();
        appender.setThreshold(level);
        Logger.getRootLogger().addAppender(appender);
    }

    @Override
    public void close() throws Exception {
        Logger.getRootLogger().removeAppender(appender);
    }
}

And then the usage is simply:

try (LogGuard log = new LogGuard(Level.WARN)) { // if you want WARN or higher
    // do what causes the logging
   Assert.assertTrue(log.appender.messages.stream().anyMatch(m -> m.equals("expected"));
}