Setting up Log4j

Setting up Log4j for Java Web Applications 

Content of log4j.properties file would be as follows:

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

Using log4j in Java Program:

The following Java class is a very simple example that initializes, and then uses, the Log4J logging library for Java applications.

import org.apache.log4j.Logger;

import java.io.*;
import java.sql.SQLException;
import java.util.*;

public class log4jExample{
  /* Get actual class name to be printed on */
  static Logger log = Logger.getLogger(log4jExample
.class);

  public static void main(String[] args)
                throws IOException,SQLException{
   
     log.debug("Hello this is an debug message");
     log.info("Hello this is an info message");
  }
}

  Setting up Log4j for Java Web Applications 

There are different ways to setup log4j for you web application.

log4j.logger.<appname>logger=DEBUG, C, fileappender log4j.additivity.<appname>logger=false log4j.appender.C=org.apache.log4j.ConsoleAppender log4j.appender.C.layout=org.apache.log4j.PatternLayout #basic pattern log4j.appender.C.layout.ConversionPattern=[%c] [%d{dd MMM yyyy - hh:mm:ss}] %5p - %m %n #advanced pattern (slow) #log4j.appender.C.layout.ConversionPattern=[%c] [%d{dd MMM yyyy - hh:mm:ss}] %5p - %m - in %M() at line %L of class %C %n log4j.appender.fileappender=org.apache.log4j.RollingFileAppender log4j.appender.fileappender.File=${appRootPath}WEB-INF/logs/<appname>.log log4j.appender.fileappender.MaxFileSize=500KB ## Keep one backup file log4j.appender.fileappender.MaxBackupIndex=3 log4j.appender.fileappender.layout=org.apache.log4j.PatternLayout log4j.appender.fileappender.layout.ConversionPattern=%p %t %c - %m%n #log4j.appender.C.layout.ConversionPattern=[%c] [%d{dd MMM yyyy - hh:mm:ss}] %5p - %m %n

Now use one of the methods below to add and enable Log4J in your web application

Method 1: Use Servlet to initialize log4j

Step 1: Put log4j.properties file in the right place

• Place 'log4j.properties' into the root of your classpath. See an example web application layout hereTip: when web application is deployed it should be in /WEB-INF/classes/log4j.properties.
• If using eclipse place 'log4j.properties' under your project_name/src directory

Step 2: Define the servlet mapping in web.xml

Add the following code to WEB-INF/web.xml

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<servlet>      <servlet-name>log4j-init</servlet-name>      <servlet-class>com.FunkyWebapp.servlets.Log4jInit</servlet-class>      <init-param>        <param-name>log4j-init-file</param-name>        <param-value>WEB-INF/classes/log4j.properties</param-value>      </init-param>      <load-on-startup>1</load-on-startup> </servlet>

Make sure to change the java class file path to be relevant to your project. 'com.FunkyWebapp.servlets.Log4jInit'

Step 3: Add the servlet you mapped in Step 2 to your application

Add the following servlet to your project

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import javax.servlet.http.HttpServlet; import org.apache.log4j.PropertyConfigurator;   public class Log4jInit extends HttpServlet {    public void init()  {      String prefix =  getServletContext().getRealPath("/");      String file = getInitParameter("log4j-init-file");         // if the log4j-init-file context parameter is not set, then no point in trying      if(file != null){       PropertyConfigurator.configure(prefix+file);       System.out.println("Log4J Logging started: " + prefix+file);      }      else{       System.out.println("Log4J Is not configured for your Application: " + prefix + file);      }      } }

Make sure the <servlet-class> element in Step 2 matches the fully qualified name of your servlet. And make sure it's in the right package.

Step 4: Initialize the logger and start logging from other servlets in your web application

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public class MyServlet extends HttpServlet { ... private Logger log = Logger.getLogger("<appname>logger"); . . . log.debug("Some string to print out");   }

Make sure the <appname> part matches with the line

log4j.logger.<appname>logger in the log4j.properties file

Notes:
When you initialize log4j in a servlet you will only be able to log from classes which are loaded after the servlet (eg. other servlets). You won't be able to do logging from a ServletContextListener for example.

Method 2: Initialize log4j in a ServletContextListener

Step 1: Put properties file in the right place

• Place 'log4j.properties' into the root of your classpath. See an example web application layout hereTip: when web application is deployed it should be in /WEB-INF/classes/log4j.properties.
• If using eclipse place 'log4j.properties' under your project_name/src directory

Step 2: Define a listener mapping in web.xml

Add the following servlet listener mapping in web.xml:

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<listener>   <listener-class>    com.package.listeners.ApplicationServletContextListener   </listener-class> </listener>

Step 3: Add the SerlvetContextListener class to the application

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public class ApplicationServletContextListener implements ServletContextListener {  public void contextInitialized(ServletContextEvent event)  {      ServletContext ctx = e.getServletContext();        String prefix =  ctx.getRealPath("/");       String file = "WEB-INF"+System.getProperty("file.separator")+"classes"+System.getProperty("file.separator")+"log4j.properties";                  if(file != null) {        PropertyConfigurator.configure(prefix+file);        System.out.println("Log4J Logging started for application: " + prefix+file);      }      else      {       System.out.println("Log4J Is not configured for application Application: " + prefix+file);      }                  }    public void contextDestroyed(ServletContextEvent event)  {      }   }

Step 4: Define the logger and start logging from other servlets in your applications

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public class MyServlet extends HttpServlet { ... private Logger log = Logger.getLogger("<appname>logger"); . . . log.debug("Some string to print out");   }

Notes:

• Easier to implement that Method 1
• All web application now has access to log4j (including listeners)
• Placing log4j initialization in a ServletContextListener allows initialization of log4j when your web application is loaded for the first time.