Logging Basics with Log4j

Why use a smart logging tool...

• We can put log output to different presentations like console, log file.
• We can categorize log entries to several level of importance and purpose (Debug/ Info/ Error). And then restrict log output to a certain level when we want.
• Logger provides time, class, other information in log output.
• You can personalize the format of log output.
  
• And many other…

How to put log4j logging in my code…

• Configuration: Before using Logger class for logging, you need to load configuraion in your program. Otherwise your log will not be printed or put anywhere.
  
  
• write a config file "myApp.log.config". Following is a simple configuration to show log in console and store in a file "test.log"
  

  log4j.rootLogger=DEBUG, A1, F1
  

  log4j.appender.A1=org.apache.log4j.ConsoleAppender
  

  log4j.appender.A1.layout=org.apache.log4j.PatternLayout

  log4j.appender.A1.layout.ConversionPattern=%-4r [%t] %-5p %c %x -%m%n

  log4j.appender.F1=org.apache.log4j.FileAppender

  log4j.appender.F1.File=${user.home}/test.log

  log4j.appender.F1.layout=org.apache.log4j.PatternLayout

  log4j.appender.F1.layout.ConversionPattern=%p %t %c - %m%n
  details of configuration...
  

• Load the configuration from code
  

  PropertyConfigurator.configure("myApp.log.cofig");
  
  

• Log Now! You are ready to log.
  
  • Get the Logger class
    
  Logger logger = Logger.getLogger( this.getClass() );
  Logger logger = Logger.getLogger( MyClass.class );
  Logger logger = Logger.getLogger( "ClassName" );
  
  
  • Log your message in appropriate level
    
  logger.debug ( "No of content loaded : " + lenContentList );
  logger.error( "Error occurred while parsing", execption );
  logger.fatal( "Failed to connect the server" );
  

Appropriate use of logging levels

Here is a guideline for deciding the logging level when you put a message in the log.

• FATAL: at which point your application or part of application is no longer functional to serve.
• Crash of the application.
• Crash the relevant sub-component.

• ERROR
• Usually when a Java exception occurs.
  
• Processing of a request is postponed due to some error.
• Processing a request has not cover all features.

• WARN: indicates minor problems
• Factors external to the application. Such as missing or inconsistent input parameters supplied by user.
• Improper initialization, configuration etc.

• INFO:
  
  • Significant non-error events in the normal life cycle of the application.
  • When a request is successfully processed in your application.

• DEBUG
  
• Printing value of a variable or object property in log .
  
• Non-error events in the normal life cycle of the application

• TRACE
• Usually used for timing information.