How to capture log events in tests with Log4j 2

Sometimes you want to verify that your program actually logs what it is supposed to be logging.

When using Apache Log4j 2 as logging framework, it can be done like this:

1. Include this dependency (for Maven, adjust appropriately for other build systems):

   <dependency>
       <groupId>org.apache.logging.log4j</groupId>
       <artifactId>log4j-core</artifactId>
       <type>test-jar</type>
       <scope>test</scope>
   </dependency>
   

2. Import some stuff:

   import org.apache.logging.log4j.core.Logger;
   import org.apache.logging.log4j.core.LoggerContext;
   import org.apache.logging.log4j.test.appender.ListAppender;
   

3. Add a ListAppender to the logger you are interested in:

   var loggerContext = LoggerContext.getContext(false);
   var logger = (Logger) loggerContext.getLogger(MyClass.class);
   var appender = new ListAppender("List");
   appender.start();
   loggerContext.getConfiguration().addLoggerAppender(logger, appender);
   

4. Exercise your code which should be logging.
5. Extract what was logged:

   List<String> loggedStrings = 
           appender.getEvents().stream().map(event -> event.getMessage().toString()).collect(Collectors.toList());
   

Update:

Starting with Log4j 2.20.0, this has changed to

<dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core-test</artifactId>
    <scope>test</scope>
</dependency>

and

import org.apache.logging.log4j.core.test.appender.ListAppender;