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

In the setup of the unit test:

1. Get the same logger
2. Make it non-additive

3. Add an appender which remembers the messages in a list:

   public class TestAppender extends AppenderSkeleton {
       public List<String> messages = new ArrayList<String>();
   
       public void doAppend(LoggingEvent event) {
           messages.add( event.getMessage().toString() );
       }
   }
   

4. Add the appender to the logger

Now you can call your code. After the test, you will find all log messages in the list. Add the log level if you want (messages.add( event.getLevel() + " " + event.getMessage() );).

In tearDown(), remove the appender again and enable additivity.