what is a apachi log4j Explain Log4j / interview question

Explain Log4j / interview question



Logging within the context of program development constitutes inserting statements into the program that provide some kind of output information that is useful to the developer.

Log4j provide  mathod

1. error()

2. fatal()

3. info()

4. debug()

5. warn()


Requirtments

log4j.jar  -->lib/log4j-1.2.6.jar

log4j.xml

Finally configured the program

when will occur the erro during runtime



Eample of the Program




import org.apache.log4j.Level;
import org.apache.log4j.Logger;
import org.apache.log4j.SimpleLayout;
import org.apache.log4j.FileAppender;
public class simpandfile {
   static Logger logger = Logger.getLogger(simpandfile.class);
   public static void main(String args[]) {
      SimpleLayout layout = new SimpleLayout();

      FileAppender appender = null;
      try {
         appender = new FileAppender(layout,"output1.txt",false);
      } catch(Exception e) {}

      logger.addAppender(appender);
      logger.setLevel((Level) Level.DEBUG);

      logger.debug("Here is some DEBUG");
      logger.info("Here is some INFO");
      logger.warn("Here is some WARN");
      logger.error("Here is some ERROR");
      logger.fatal("Here is some FATAL");
   }
}
   

Configuring Log4J
As you can see above we added the log4j library. This library does like a configuration file in the source directory or it welcomes you with the following error.
log4j:WARN No appenders could be found for logger (TestClient).
log4j:WARN Please initialize the log4j system properly.
Create a file named log4j.properties in the root directory and insert the following:
### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L – %m%n
### set log levels – for more verbose logging change ‘info’ to ‘debug’ ###
log4j.rootLogger=debug, stdout
log4j.logger.org.hibernate=info
#log4j.logger.org.hibernate=debug
### log HQL query parser activity
#log4j.logger.org.hibernate.hql.ast.AST=debug
### log just the SQL
log4j.logger.org.hibernate.SQL=debug
### log JDBC bind parameters ###
log4j.logger.org.hibernate.type=info
### log schema export/update ###
log4j.logger.org.hibernate.tool.hbm2ddl=info
### log HQL parse trees
#log4j.logger.org.hibernate.hql=debug
### log cache activity ###
log4j.logger.org.hibernate.cache=info
### log transaction activity
#log4j.logger.org.hibernate.transaction=debug
### log JDBC resource acquisition
#log4j.logger.org.hibernate.jdbc=debug
### enable the following line if you want to track down connection ###
### leakages when using DriverManagerConnectionProvider ###
#log4j.logger.org.hibernate.connection.DriverManagerConnectionProvider=trace

log4j.properties

# Set root logger level to DEBUG and its only appender to Appender1.
log4j.rootLogger=INFO, Appender1,Appender2

# Appender1 is set to be a ConsoleAppender.
log4j.appender.Appender1=org.apache.log4j.ConsoleAppender
log4j.appender.Appender2=org.apache.log4j.RollingFileAppender
log4j.appender.Appender2.File=sample.log


# Appender2 uses PatternLayout.
log4j.appender.Appender1.layout=org.apache.log4j.PatternLayout
log4j.appender.Appender1.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

log4j.appender.Appender2.layout=org.apache.log4j.PatternLayout
log4j.appender.Appender2.layout.ConversionPattern=%-4r [%t] %-5p %c %x - %m%n

log4j.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">

<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/" debug="false">

   <appender name="consoleAppender" class="org.apache.log4j.ConsoleAppender">
      <param name="Threshold" value="INFO" />   
      <layout class="org.apache.log4j.PatternLayout">
         <param name="ConversionPattern" value="%d %-5p  [%c{1}] %m %n" />
      </layout>
   </appender>

   <appender name="fileAppender" class="org.apache.log4j.RollingFileAppender">
      <param name="Threshold" value="INFO" />
      <param name="File" value="sample.log"/>
      <layout class="org.apache.log4j.PatternLayout">
         <param name="ConversionPattern" value="%d %-5p  [%c{1}] %m %n" />
      </layout>
   </appender>

   <logger name="javabeat.net.log4j" additivity="false" >   
      <level value="INFO" />
      <appender-ref ref="consoleAppender"/>      
      <appender-ref ref="fileAppender"/>      
   </logger>

</log4j:configuration>

Log4jPropertyTest.java

package javabeat.net.log4j;

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

/**
 * source : www.javabeat.net
 */
public class Log4jPropertyTest {
    private static Logger logger = Logger.getLogger(Log4jPropertyTest.class);
    public static void main (String args[]){        
        PropertyConfigurator.configure("log4j.properties");
        logger.info("Test Log");
    }
}

Log4jXmlTest.java

package javabeat.net.log4j;

import org.apache.log4j.Logger;
import org.apache.log4j.xml.DOMConfigurator;

/**
 * source : www.javabeat.net
 */
public class Log4jXmlTest {
    private static Logger logger = Logger.getLogger(Log4jXmlTest.class);
    public static void main (String args[]){        
        DOMConfigurator.configure("log4j.xml");
        logger.info("Test Log");
    }

}

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