For a specific JUnit integration test I was coding, I wanted the test to fail if an errors were logged to logback. I created this appender to record if errors were logged. Once the test is finished, the JUnit test asks the appender if errors were logged and fails the test if so.

LogLevelTripwireAppender

import java.io.IOException;
 
import ch.qos.logback.classic.Level;
import ch.qos.logback.classic.encoder.PatternLayoutEncoder;
import ch.qos.logback.classic.spi.ILoggingEvent;
import ch.qos.logback.core.AppenderBase;
 
 
public class LogLevelTripwireAppender extends AppenderBase<ILoggingEvent> {
 
  private PatternLayoutEncoder encoder;
  private boolean thresholdWasReached = false;
  private Level logLevelThreshold;
 
 
  public LogLevelTripwireAppender(Level aLogLevel) {
    this.logLevelThreshold = aLogLevel;
  }
 


  @Override
  public void start() {
    if( !isEncoderValid())
      return;
    try {
      encoder.init(System.out);
    } catch (IOException e) {
      throw new RuntimeException(e);
    }
    super.start();
  }
 
  private boolean isEncoderValid() {
    boolean isValid = true;
    if (this.encoder == null) {
      addError("No encoder set for the appender named ["+ name +"].");
      isValid = false;
    }
    return isValid;
  }
 
  public void append(ILoggingEvent event) {
    Level eventLevel = event.getLevel();
    if( eventLevel.isGreaterOrEqual(this.logLevelThreshold)) {
      thresholdWasReached = true;
    }
  }
 
 
  public boolean wasThresholdReached() {
    return this.thresholdWasReached;
  }
 
  public void setEncoder(PatternLayoutEncoder anEncoder) {
    this.encoder = anEncoder;
  }

}




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Example usage


private LogLevelTripwireAppender appender 

private void setupAppender() {
    LoggerContext loggerContext = (LoggerContext) LoggerFactory.getILoggerFactory();
    appender = new LogLevelTripwireAppender(Level.WARN);
    appender.setContext(loggerContext);
    PatternLayoutEncoder encoder = createEncoder(loggerContext);
    appender.setEncoder(encoder);
    appender.start();
    Logger logbackLogger = loggerContext.getLogger(Logger.ROOT_LOGGER_NAME);
    logbackLogger.addAppender(appender);
}


private PatternLayoutEncoder createEncoder(LoggerContext loggerContext) {
    PatternLayoutEncoder encoder = new PatternLayoutEncoder();
    encoder.setContext(loggerContext);
    encoder.setPattern("%-4relative [%thread] %-5level %logger{35} - %msg%n");
    encoder.start();
    return encoder;
}