Logging, in short, is required to track programming flow. It is very difficult to track code flow or to find faults in Big applications. Log4j makes it easier.

Log4j is usually configured using a property file or XML file externally. You can easily control its behavior through configuration file without modifying the source code.

Prerequisite

1. What is log4j and why to use it – A short review
2. Download and import log4j.jar to your project.

Main components

1. Logger – used to log messages
2. Appender – specifies the output destination like console or a file (default ConsoleAppender)
3. Layout – specify the format in which the log messages should be logged (default PatternLayout)

In short to remember

1. Logger – Who writes to log (Worker)
2. Appender – Tells where to write (Navigator)
3. Layout – format the log message (Designer)

Sample configuration file

log4j.rootLogger=WARN, CA
log4j.appender.CA=org.apache.log4j.ConsoleAppender
log4j.appender.CA.layout=org.apache.log4j.PatternLayout
log4j.appender.CA.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n<span style="font-size: 10pt; font-family: "Courier New";">
</span></pre>
<pre>

Explanation -1^st line in above configuration line is used to declare logger (who will write). 2^nd line is declaring appender (Where he will write). 3^rd & 4^th line are saying in what format logger will write.

Level – Many times we need to write error log in a separate log file while debug logs in separate log files. Log4j ease to write logs in separate log files for a same logger based on their levels.

The log4j levels follow the following order.

• DEBUG
• INFO
• WARN
• ERROR
• FATAL

In the above sample configuration file, logger level is set to WARN. It means, all log message up to WARN level (WARN, ERROR and FATAL) will be written to console (since appender type is ConsoleAppender). Rest logs would be dropped.

How to write logs

Once you have done all settings in configuration file ie log4j.properties, place it on the root of your project folder. Your application will pick it automatically. Now

1. You just need to create an object of logger class in your application.

   public static Logger as_looger = Logger.getLogger("CA");

2. And write log statement as we write System.out.println(). You can pass Exception object. It automatically prints stack trace.

   as_logger.debug(“article-stack.com has been started.”);

Sample Application

1. Create a java project
2. Create a class, say basic.java

   package com.articlestack.log4j;
   import org.apache.log4j.Logger;
   /**
    * This class fetch log settings from log4j.properties
    * @author amty
    */
   public class basic {
   	static final Logger logger = Logger.getLogger("CA");
   	public static void main(String[] args) {
   		logger.debug("Sample debug message");
   		logger.info("Sample info message");
   		logger.warn("Sample warn message");
   		logger.error("Sample error message");
   		logger.fatal("Sample fatal message");
   	}
   }
   

3. Create log4j.properties. And place it to root of your project folder (refer above screen shot)

   log4j.rootLogger=WARN, CA
   log4j.appender.CA=org.apache.log4j.ConsoleAppender
   log4j.appender.CA.layout=org.apache.log4j.PatternLayout
   #log4j.appender.CA.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n
   #log4j.appender.CA.layout.ConversionPattern=%5r %-5p [%t] %c{2} - %m%n
   log4j.appender.CA.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %c{1} [%p] %m%n
   

   *Change the level WARN to DEBUG or to ERROR or something else. Run the application. And check what you get on console.

Some Alternates

1. My own property file
   Don’t you wanna refer log4j.properties? You can create your own property file similar to log4j.properties. Now place it somewhere. And configure its path through code.

   PropertyConfigurator.configure("amtyLog.properties");

Complete class

package com.articlestack.log4j;

import org.apache.log4j.Logger;
import org.apache.log4j.PropertyConfigurator;

public class anotherFile {

	static final Logger logger = Logger.getLogger("AC");
	public static void main(String[] args) {
		PropertyConfigurator.configure("amtyLog.properties");
		logger.debug("Sample debug message"); 
		logger.info("Sample info message"); 
		logger.warn("Sample warn message"); 
		logger.error("Sample error message"); 
		logger.fatal("Sample fatal message");

	}
}

2. No property file
   Don’t you wanna refer any property file?You can create a log class. You can do all settings from java code instead property file. Refer the below class.

   package com.articlestack.cofig;
   
   import org.apache.log4j.ConsoleAppender;
   import org.apache.log4j.Level;
   import org.apache.log4j.Logger;
   import org.apache.log4j.PatternLayout;
   
   public class Log
   {
        public static Logger as_looger;
       public Log(){}
       static
       {
           as_looger = Logger.getLogger("articles logging");
           ConsoleAppender as_appender = new ConsoleAppender(new PatternLayout("%d{dd-MM-yyyy HH:mm:ss} %C %L %-5p: %m%n"));
           as_looger.setLevel(Level.DEBUG);
           as_looger.addAppender(as_appender);
       }
   }
   
   

   Now you can log from any java class very easily

   Log.as_looger.debug("DEBUG");
   

   Please note this

   You can skip reading this. This is just an Appendix.

   The org.apache.log4j.Level class provides following levels but you can also define your custom levels by sub-classing the Level class.

   Level	Description
   ALL	All levels including custom levels.
   DEBUG	Designates fine-grained informational events that are most useful to debug an application.
   ERROR	Designates error events that might still allow the application to continue running.
   FATAL	Designates very severe error events that will presumably lead the application to abort.
   INFO	Designates informational messages that highlight the progress of the application at coarse-grained level.
   OFF	The highest possible rank and is intended to turn off logging.
   TRACE	Designates finer-grained informational events than the DEBUG.
   WARN	Designates potentially harmful situations.

   *Above levels are alphabetically short not by priority.
   

   I had tried to cover all basic concepts of log4j in this article. Tell me if something is not clear to your or missing. Now the time is to use the power of log4j. That we’ll discuss in next article.

   Amit Gupta

   Hey! this is Amit Gupta (amty). By profession, I am a Software Eng. And teaching is my passion. Sometimes I am a teacher, as you can see many technical tutorials on my site, sometimes I am a poet, And sometime just a friend of friends...
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