What is the best way to unit-test SLF4J log messages?

For testing slf4j without relying on a specific implementation (such as log4j), you can provide your own slf4j logging implementation as described in this SLF4J FAQ. Your implementation can record the messages that were logged and then be interrogated by your unit tests for validation.

The slf4j-test package does exactly this. It's an in-memory slf4j logging implementation that provides methods for retrieving logged messages.

Create a test rule:

    import ch.qos.logback.classic.Logger;
    import ch.qos.logback.classic.spi.ILoggingEvent;
    import ch.qos.logback.core.read.ListAppender;
    import org.junit.rules.TestRule;
    import org.junit.runner.Description;
    import org.junit.runners.model.Statement;
    import org.slf4j.LoggerFactory;
    
    import java.util.List;
    import java.util.stream.Collectors;
    
    public class LoggerRule implements TestRule {
    
      private final ListAppender<ILoggingEvent> listAppender = new ListAppender<>();
      private final Logger logger = (Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);
    
      @Override
      public Statement apply(Statement base, Description description) {
        return new Statement() {
          @Override
          public void evaluate() throws Throwable {
            setup();
            base.evaluate();
            teardown();
          }
        };
      }
    
      private void setup() {
        logger.addAppender(listAppender);
        listAppender.start();
      }
    
      private void teardown() {
        listAppender.stop();
        listAppender.list.clear();
        logger.detachAppender(listAppender);
      }
    
      public List<String> getMessages() {
        return listAppender.list.stream().map(e -> e.getMessage()).collect(Collectors.toList());
      }
    
      public List<String> getFormattedMessages() {
        return listAppender.list.stream().map(e -> e.getFormattedMessage()).collect(Collectors.toList());
      }
    
    }

Then use it:

    @Rule
    public final LoggerRule loggerRule = new LoggerRule();
    
    @Test
    public void yourTest() {
        // ...
        assertThat(loggerRule.getFormattedMessages().size()).isEqualTo(2);
    }

----- JUnit 5 with Extension Oct 2021 -----

LogCapture:

public class LogCapture {

  private ListAppender<ILoggingEvent> listAppender = new ListAppender<>();

  LogCapture() {
  }

  public String getFirstFormattedMessage() {
    return getFormattedMessageAt(0);
  }

  public String getLastFormattedMessage() {
    return getFormattedMessageAt(listAppender.list.size() - 1);
  }

  public String getFormattedMessageAt(int index) {
    return getLoggingEventAt(index).getFormattedMessage();
  }

  public LoggingEvent getLoggingEvent() {
    return getLoggingEventAt(0);
  }

  public LoggingEvent getLoggingEventAt(int index) {
    return (LoggingEvent) listAppender.list.get(index);
  }

  public List<LoggingEvent> getLoggingEvents() {
    return listAppender.list.stream().map(e -> (LoggingEvent) e).collect(Collectors.toList());
  }

  public void setLogFilter(Level logLevel) {
    listAppender.clearAllFilters();
    listAppender.addFilter(buildLevelFilter(logLevel));
  }

  public void clear() {
    listAppender.list.clear();
  }

  void start() {
    setLogFilter(Level.INFO);
    listAppender.start();
  }

  void stop() {
    if (listAppender == null) {
      return;
    }

    listAppender.stop();
    listAppender.list.clear();
    listAppender = null;
  }

  ListAppender<ILoggingEvent> getListAppender() {
    return listAppender;
  }

  private Filter<ILoggingEvent> buildLevelFilter(Level logLevel) {
    LevelFilter levelFilter = new LevelFilter();
    levelFilter.setLevel(logLevel);
    levelFilter.setOnMismatch(FilterReply.DENY);
    levelFilter.start();

    return levelFilter;
  }

}

LogCaptureExtension:

public class LogCaptureExtension implements ParameterResolver, AfterTestExecutionCallback {

  private Logger logger = (Logger) LoggerFactory.getLogger(Logger.ROOT_LOGGER_NAME);

  private LogCapture logCapture;

  @Override
  public boolean supportsParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
    return parameterContext.getParameter().getType() == LogCapture.class;
  }

  @Override
  public Object resolveParameter(ParameterContext parameterContext, ExtensionContext extensionContext) {
    logCapture = new LogCapture();

    setup();

    return logCapture;
  }

  @Override
  public void afterTestExecution(ExtensionContext context) {
    teardown();
  }

  private void setup() {
    logger.addAppender(logCapture.getListAppender());
    logCapture.start();
  }

  private void teardown() {
    if (logCapture == null || logger == null) {
      return;
    }

    logger.detachAndStopAllAppenders();
    logCapture.stop();
  }

}

then use it:

@ExtendWith(LogCaptureExtension.class)
public class SomeTest {

  @Test
  public void sometest(LogCapture logCapture)  {
    // do test here

    assertThat(logCapture.getLoggingEvents()).isEmpty();
  }

  // ...
}

I think you could solve your problem with a custom appender. Create a test appender which implements the org.apache.log4j.Appender, and set your appender in the log4j.properties and load it when you execute test cases.

If you call back to the test harness from that appender you can check the logged messages