Log4j Example : Using various levels of logging using Apache Log4j

Suppose there is a log file to be analyzed for certain issues while
tracing activity is performed. This activity can be real difficult
if one has to find/look for a pin from a box full of sand, type of
situation. This may happen if number of tracing / logging lines of
statements ranges from few hundred to thousand lines for a real
big application with many classes being executed with an operation.

So what do you think can be possible way to simplify this particular
problem?? In my thinking if there could be some way to filter only
those log statements, those are related to one particular class
or component in hand.

Yes, You are right!!! Apache Log4j has a provision for defining
which class to be considered for logging and leaving rest of the
classes not eligible for logging purpose.

In this example I shall try to demonstrate this objective of 
configuring log4j.properties file in such a way so as to provide
a filter kind of functionality and this should be done without
any code change as far as the Java class files are concerned.
So this is pretty configuration time setting up activity.

In order to show a method execution flow, I have defined three dummy
class files, such as ExampleController, ExampleService, ExampleDAO
with some dummy kind of methods too.


demo.example.ExampleController.java


package demo.example;

import org.apache.log4j.Logger;

public class ExampleController {
  private static Logger logger = Logger.getLogger(ExampleController.class);
  public void delegateCall() {
	logger.info("Entering delegateMethod");
	new ExampleService().doBusiness();  
	logger.info("Leaving delegateMethod");
  }
}
demo.example.ExampleService.java


package demo.example;

import org.apache.log4j.Logger;

public class ExampleService {
  private static Logger logger = Logger.getLogger(ExampleService.class);	
  public void doBusiness() {
    logger.info("Entering doBusiness");
    new ExampleDAO().doStore();  
    logger.info("Leaving doBusiness");

  }
}
demo.example.ExampleDAO.java


package demo.example;

import org.apache.log4j.Logger;

public class ExampleDAO {
   private static Logger logger = Logger.getLogger(ExampleDAO.class);	
   public void doStore() {
    logger.info("Entering doStore");
    logger.debug("TODO database call");  
    logger.info("Leaving doStore");
   }
}



As these files shows that ExampleController.delegateCall method
is calling ExampleService.doBusiness and this method is again 
calling ExampleDAO.doStore, so there is a chain of method
execution with some logging statements being coded.

Now a very common (I think so) log4j properties file for this example
as follows:



log4j.rootLogger=INFO, A
log4j.appender.A=org.apache.log4j.ConsoleAppender
log4j.appender.A.layout=org.apache.log4j.PatternLayout
log4j.appender.A.layout.ConversionPattern=%d [%t] %-5p %C - %m%n


This log4j.properties file prints out all the logging statements
that Log4j encounters while these method execution, as shown below:



[main] INFO  demo.example.ExampleController - Entering delegateMethod
[main] INFO  demo.example.ExampleService - Entering doBusiness
[main] INFO  demo.example.ExampleDAO - Entering doStore
[main] INFO  demo.example.ExampleDAO - Leaving doStore
[main] INFO  demo.example.ExampleService - Leaving doBusiness
[main] INFO  demo.example.ExampleController - Leaving delegateMethod



Now I have modified log4j.properties file and added the line marked
as bold RED in color



log4j.rootLogger=FATAL
log4j.logger.demo.example=INFO, A
log4j.appender.A=org.apache.log4j.ConsoleAppender
log4j.appender.A.layout=org.apache.log4j.PatternLayout
log4j.appender.A.layout.ConversionPattern=%d [%t] %-5p %C - %m%n


This makes no difference so far as the number of logging statements
on console, for this example is concerned :

But the real difference appeared when I added the class file name
within the second line of log4j properties file as follows:




log4j.rootLogger=FATAL
log4j.logger.demo.example.ExampleController=INFO, A
log4j.appender.A=org.apache.log4j.ConsoleAppender
log4j.appender.A.layout=org.apache.log4j.PatternLayout
log4j.appender.A.layout.ConversionPattern=%d [%t] %-5p %C - %m%n


This makes the log statement being printed on console, restricted
only to the demo.example.ExampleController class file, as show below:



[main] INFO  demo.example.ExampleController - Entering delegateMethod
[main] INFO  demo.example.ExampleController - Leaving delegateMethod

By this way one can apply some filter/restrictions (Just of understanding)
and log only those statements those are real important to track.


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