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

Firstly, create a mock Appender to assert logging content with JUnit. If you are dealing with Log4j2, the TestAppender functionality can be handy to capture logs, thus enhancing your assertions. Here is how you can accomplish it:

import org.apache.logging.log4j.*;
import org.apache.logging.log4j.core.*;
import org.junit.*;
import static org.junit.Assert.*;

public class LoggerTest {
    private final Logger logger = LogManager.getLogger();
    private final TestAppender testAppender = new TestAppender();

    @Before
    public void setUp() {
        // No one else is allowed in our party!
        ((LoggerContext) LogManager.getContext(false)).getConfiguration().getRootLogger().addAppender(testAppender);
    }

    @After
    public void tearDown() {
        // Party's over, folks!
        ((LoggerContext) LogManager.getContext(false)).getConfiguration().getRootLogger().removeAppender(testAppender);
    }

    @Test
    public void logsExpectedMessage() {
        logger.debug("Test Message");
        // Do we spy with our little eye the message we're looking for?
        assertTrue(testAppender.contains("Test Message"));
    }

    private static class TestAppender extends AbstractAppender {
        private final StringBuilder logContent = new StringBuilder();

        TestAppender() {
            super("TestAppender", null, null);
            // Let's get this party started!
            start();
        }

        @Override
        public void append(LogEvent event) {
            // Another one bites the dust (gets logged)!
            logContent.append(event.getMessage().getFormattedMessage());
        }

        public boolean contains(String message) {
            // Sherlock Holmes mode: ON.
            return logContent.toString().contains(message);
        }
    }
}


Attach the TestAppender before tests kick-off and remember to detach it after the tests. Assert the logged messages by using assertTrue, inserting your specific logged content.