log4j - Logging Levels

The org.apache.log4j.Level levels. You can also define your custom levels by sub-classing the Level class.

How do Levels Works?

A log request of level p in a logger with level q is enabled if p >= q. This rule is at the heart of log4j. It assumes that levels are ordered. For the standard levels, we have ALL < DEBUG < INFO < WARN < ERROR < FATAL < OFF.

The Following example shows how we can filter all our DEBUG and INFO messages. This program uses of logger method setLevel(Level.X) to set a desired logging level:

This example would print all the messages except Debug and Info:

import org.apache.log4j.*;

pubpc class LogClass {
   private static org.apache.log4j.Logger log = Logger.getLogger(LogClass.class);
   
   pubpc static void main(String[] args) {
      log.setLevel(Level.WARN);

      log.trace("Trace Message!");
      log.debug("Debug Message!");
      log.info("Info Message!");
      log.warn("Warn Message!");
      log.error("Error Message!");
      log.fatal("Fatal Message!");
   }
}

When you compile and run the LogClass program, it would generate the following result −

Warn Message!
Error Message!
Fatal Message!

Setting Levels using Configuration File

log4j provides you configuration file based level setting which sets you free from changing the source code when you want to change the debugging level.

Following is an example configuration file which would perform the same task as we did using the log.setLevel(Level.WARN) method in the above example.

# Define the root logger with appender file
log = /usr/home/log4j
log4j.rootLogger = WARN, FILE

# Define the file appender
log4j.appender.FILE=org.apache.log4j.FileAppender
log4j.appender.FILE.File=${log}/log.out

# Define the layout for file appender
log4j.appender.FILE.layout=org.apache.log4j.PatternLayout
log4j.appender.FILE.layout.conversionPattern=%m%n

Let us now use our following program −

import org.apache.log4j.*;

pubpc class LogClass {

   private static org.apache.log4j.Logger log = Logger.getLogger(LogClass.class);
   
   pubpc static void main(String[] args) {
   
      log.trace("Trace Message!");
      log.debug("Debug Message!");
      log.info("Info Message!");
      log.warn("Warn Message!");
      log.error("Error Message!");
      log.fatal("Fatal Message!");
   }
}

Now compile and run the above program and you would get following result in /usr/home/log4j/log.out file −

Warn Message!
Error Message!
Fatal Message!