// This creates the "VERBOSE" level if it does not exist yet.
final Level VERBOSE = Level.forName("VERBOSE", 550);

final Logger logger = LogManager.getLogger();
logger.log(VERBOSE, "a verbose message"); // use the custom VERBOSE level

// Create and use a new custom level "DIAG".
logger.log(Level.forName("DIAG", 350), "a diagnostic message");

// Use (don't create) the "DIAG" custom level.
// Only do this *after* the custom level is created!
logger.log(Level.getLevel("DIAG"), "another diagnostic message");

// Using an undefined level results in an error: Level.getLevel() returns null,
// and logger.log(null, "message") throws an exception.
logger.log(Level.getLevel("FORGOT_TO_DEFINE"), "some message"); // throws exception!

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
  <!-- Define custom levels before using them for filtering below. -->
  <CustomLevels>
    <CustomLevel name="DIAG" intLevel="350" />
    <CustomLevel name="NOTICE" intLevel="450" />
    <CustomLevel name="VERBOSE" intLevel="550" />
  </CustomLevels>

  <Appenders>
    <Console name="Console" target="SYSTEM_OUT">
      <PatternLayout pattern="%d %-7level %logger{36} - %msg%n"/>
    </Console>
    <File name="MyFile" fileName="logs/app.log">
      <PatternLayout pattern="%d %-7level %logger{36} - %msg%n"/>
    </File>
  </Appenders>
  <Loggers>
    <Root level="trace">
      <!-- Only events at DIAG level or more specific are sent to the console. -->
      <AppenderRef ref="Console" level="diag" />
      <AppenderRef ref="MyFile" level="trace" />
    </Root>
  </Loggers>
</Configuration>

// convenience methods for the built-in DEBUG level
debug(Marker, Message)
debug(Marker, Message, Throwable)
debug(Marker, Object)
debug(Marker, Object, Throwable)
debug(Marker, String)
debug(Marker, String, Object...)
debug(Marker, String, Throwable)
debug(Message)
debug(Message, Throwable)
debug(Object)
debug(Object, Throwable)
debug(String)
debug(String, Object...)
debug(String, Throwable)
// lambda support methods added in 2.4
debug(Marker, MessageSupplier)
debug(Marker, MessageSupplier, Throwable)
debug(Marker, String, Supplier<?>...)
debug(Marker, Supplier<?>)
debug(Marker, Supplier<?>, Throwable)
debug(MessageSupplier)
debug(MessageSupplier, Throwable)
debug(String, Supplier<?>...)
debug(Supplier<?>)
debug(Supplier<?>, Throwable)

// need to pass the custom level as a parameter
logger.log(VERBOSE, "a verbose message");
logger.log(Level.forName("DIAG", 350), "another message");

// nice to have: descriptive methods and no need to pass the level as a parameter
logger.verbose("a verbose message");
logger.diag("another message");
logger.diag("java 8 lambda expression: {}", () -> someMethod());

// ExtLogger is a generated logger wrapper
import com.mycompany.myproject.ExtLogger;

public class MyService {
    // instead of Logger logger = LogManager.getLogger(MyService.class):
    private static final ExtLogger logger = ExtLogger.create(MyService.class);

    public void demoExtendedLogger() {
        // ...
        logger.trace("the built-in TRACE level");
        logger.verbose("a custom level: a VERBOSE message");
        logger.debug("the built-in DEBUG level");
        logger.notice("a custom level: a NOTICE message");
        logger.info("the built-in INFO level");
        logger.diag("a custom level: a DIAG message");
        logger.warn("the built-in WARN level");
        logger.error("the built-in ERROR level");
        logger.fatal("the built-in FATAL level");
        logger.notice("java 8 lambda expression only executed if NOTICE is enabled: {}", () -> someMethod());
        // ...
    }
    ...
}

java -cp log4j-core-2.23.1.jar org.apache.logging.log4j.core.tools.ExtendedLoggerGenerator \
        com.mycomp.ExtLogger DIAG=350 NOTICE=450 VERBOSE=550 > com/mycomp/ExtLogger.java

java -cp log4j-core-2.23.1.jar org.apache.logging.log4j.core.tools.CustomLoggerGenerator \
        com.mycomp.MyLogger DEFCON1=350 DEFCON2=450 DEFCON3=550 > com/mycomp/MyLogger.java