Question

What are the main components of log4j?

Answer 1

Hi Kiran, Log4j has four main components:

1. LogManager: This is the static class that helps us get loggers with different names and hierarchy. You can consider LogManager as a factory producing logger objects. A sample code will be:

   • package Log4jSample;
     
     import org.apache.log4j.BasicConfigurator;
     import org.apache.log4j.LogManager;
     import org.apache.log4j.Logger;
     
     public class SampleEntry {
     
      //mainLogger is a logger object that we got from LogManager. All loggers are
             //using this method only. We can consider LogManager as a factory to create
             //Logger objects
      static Logger mainLogger = LogManager.getLogger(SampleEntry.class.getName());
     
      public static void main(String[] args) {
       // TODO Auto-generated method stub
     
       BasicConfigurator.configure();
       mainLogger.info("This is just a logger"); 
     
      }
     }

2. Loggers: This class which helps you log information at different logging levels. In the above sample code you can see that we have created a logger named mainLogger using the LogManager static class. Now we can use it to write logs. As you can see we have mainLogger.info(“Comments that you want to log”) statement which logs the string.

3. Appenders: Appenders are objects which help Logger objects write logs to different outputs. Appenders can specify a file, console or a data base as the output location. In this code sample you will see that we have used a console appender to print logs like we would do using System.out or System.err

   • package Log4jSample;
     
     import org.apache.log4j.BasicConfigurator;
     import org.apache.log4j.ConsoleAppender;
     import org.apache.log4j.LogManager;
     import org.apache.log4j.Logger;
     
     public class SampleEntry {
     
      //All the loggers that can be used
      static Logger mainLogger = LogManager.getLogger(SampleEntry.class.getName());
     
      public static void main(String[] args) {
       // TODO Auto-generated method stub
     
       BasicConfigurator.configure();
     
       //Create a console appender and attach it to our mainLogger
       ConsoleAppender appender = new ConsoleAppender();
       mainLogger.addAppender(appender);
       mainLogger.info("This is just a logger"); 
      }
     }

4. Layouts: Layout class help us define how the log information should appear in the outputs. Here is a sample code which uses PatternLayout Class to change the formatting of logs:

   • package Log4jSample;
     
     import java.util.Enumeration;
     
     import org.apache.log4j.Appender;
     import org.apache.log4j.BasicConfigurator;
     import org.apache.log4j.ConsoleAppender;
     import org.apache.log4j.Layout;
     import org.apache.log4j.LogManager;
     import org.apache.log4j.Logger;
     import org.apache.log4j.PatternLayout;
     
     public class SampleEntry {
     
      //All the loggers that can be used
      static Logger mainLogger = LogManager.getLogger(SampleEntry.class.getName());
     
      public static void main(String[] args) {
       // TODO Auto-generated method stub
     
       BasicConfigurator.configure();
       ConsoleAppender appender = new ConsoleAppender();
       appender.activateOptions();
       PatternLayout layoutHelper = new PatternLayout();
       layoutHelper.setConversionPattern("%-5p [%t]: %m%n");
       layoutHelper.activateOptions();
             //mainLogger.getAppender("ConsoleAppender").setLayout(layoutHelper); 
       appender.setLayout(layoutHelper);
       mainLogger.addAppender(appender);
       //Create a console appender and attach it to our mainLogger
       mainLogger.info("Pattern 1 is displayed like this");
       layoutHelper.setConversionPattern("%C %m%n");
       mainLogger.info("Pattern 2 is displayed like this");
     
      }
     
     }