What is Log4J. An absolute beginners tutorial.

The below are the logging level provided by the org.apache.log4j level class.

How Level Works?

The log request level r in a logger with level s, is enabled if r >= s. This is primary rule of log4j. For standard levels the rules are ALL < DEBUG < INFO < WARN < ERROR < FATAL < OFF.

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager; 
 
public class LogClassExample
{
    static final Logger logger_c = LogManager.getLogger(Log4jConfigurationExample.class.getName());
    public static void main(String[] args)
    {
			logger_c.setLevel(Level.WARN);
			logger_c.debug("Message Debug!!");
			logger_c.info( " Message Info!!");
			logger_c.trace("Message Trace!!");
			logger_c.warn( " Message Warn!!");
			logger_c.error("Message Error!!");
			logger_c.fatal( " Message Fatal!!");

    }
}

In the above the example since the set level is set to desired logging level anything lesser than that will not be displayed. Here Debug and INFO message will be filtered out.

Message Warn!!

Message Error!!

Message Fatal!!

Setting Levels using Configuration File:

Using configuration file level setting can be changed based on level setting. In below example we are setting it to the LEVEL.WARN

<?xml version="1.0" encoding="UTF-8"?>
<configuration status="WARN">
  <appenders>
    <File name="MyFile" fileName=" Applicationdemo.log ">
        <PatternLayout pattern="%d{yyyy-mm-dd HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
    </File>            
  </appenders>
  <loggers>    
    <root level="debug">
      <appender-ref ref="MyFile " level="warn"/>
    </root>   
  </loggers>
</configuration>

Example Program

import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.LogManager;
 
public class Log4jConfigurationExample
{
    static final Logger logger_c = LogManager.getLogger(Log4jConfigurationExample.class.getName());
    public static void main(String[] args)
    {
        logger_c.debug("Message Debug!!");
        logger_c.info( " Message Info!!");
        logger_c.trace("Message Trace!!");
        logger_c.warn( " Message Warn!!");
        logger_c.error("Message Error!!");
        logger_c.fatal( " Message Fatal!!");
    }
}

Compile and run the program to get output in Applicationdemo.log in project folder.

Message Warn!!
Message Error!!
Message Fatal!!