log4j logging is pretty easy. Here's a good tutorial: http://www.developer.com/open/article.php/10930_3097221_1, but you'll find better illustration on this page once you've spent the time on the concepts there. The bonafide Apache tutorial (log4j is an Apache thing) is at: http://logging.apache.org/log4j/1.2/manual.html and it's a good resource once you've read the first one and mine here unless you prefer more formal language.

Source code sample

Here is the sample code for the output we'll be studying. What's in italics and bold will vary for our examples.

	package com.etretatlogiciels.samples.log4j;

	import org.apache.log4j.Logger;

	public class Log4J
	{
	  private static Logger   logger = Logger.getLogger( Log4J.class.getName() );
	  private SimpleLayout    myLayout;
	  private Appender        myAppender;

	  public Log4J()
	  {
	    logger.setLevel( Level.INFO );
	    myLayout   = new SimpleLayout();
	    myAppender = new ConsoleAppender( myLayout );
	    logger.addAppender( myAppender );
	  }

	  public static void do_something( int a, float b )
	  {
	    double   X = b - a - 3.1415;

	    logger.info ( "Parameters passed to method do_something are: " + a + ", " + b );
	    logger.debug( "Operation performed successfully" );
	    logger.error( "Value of X is " + X );
	  }

	  public static void main( String[] args )
	  {
	    PropertyConfigurator.configure( "src/resources/log4j.properties" );
	    do_something( 99, 542 );
	  }
	}

	log4j.rootLogger=debug, myAppender
	log4j.appender.myAppender=org.apache.log4j.ConsoleAppender
	log4j.appender.myAppender.layout=org.apache.log4j.SimpleLayout

Note: the placement of the properties file and the argument passed to the PropertyConfigurator.configure() method is annoyingly difficult when using Eclipse. Please see a better working example below.

myAppender in this properties file is, of course, the very variable in the code. It must be identical. Some examples of console output while varying SimpleLayout in the properties file are:

SimpleLayout

Yeah, this is a sort of default. Here's what comes out in the Eclipse Console tab when you run the code:

INFO - Parameters passed to method do_something are: 99, 542.0
DEBUG - Operation performed successfully
ERROR - Value of X is 439.8585

PatternLayout

With this scheme, you lose the labels for whatever reason there is behind that.

Parameters passed to method do_something are: 99, 542.0
Operation performed successfully
Value of X is 439.8585

HTMLLayout

This gives you HTML output that can be nicely browsed:

Log session start time Tue Oct 14 16:27:31 MDT 2008

DateLayout

Deprecated? I don't know why this isn't working in my set up, except that I looked at the class documentation and much of it was deprecated. I'll be figuring this out because most logging is done using a timestamp for obvious reasons.

	java.lang.ClassNotFoundException:org.apache.log4j.DateLayout

XMLLayout

Didn't work. The doc doesn't say it's deprecated (unlike DateLayout) and it's also not full, legal XML, but only XML that you can tuck inside the surrounding tag of a full, legal file.

	java.lang.ClassNotFoundException:org.apache.log4j.XMLLayout

	if( log.isDebugEnabled() )
	    log.debug( "Entry number: " + i + " is " + String.valueOf( entry[ i ] ) );

The doc for this feature is found at: http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/PatternLayout.html

	package com.mycompany.pkg;

	import org.apache.log4j.Logger;

	public class MyClass
	{
	   private static final Logger log = Logger.getLogger( MyClass.class );
	   .
	   .
	   .
	   public void someMethod()
	   {
	      log.debug( "This is a message at DEBUG level in the log file" );
	      .
	      .
	      .
	}

	import org.apache.log4j.PropertyConfigurator;
	.
	.
	.
	PropertyConfigurator.configure( "properties/log4j.properties" );
		

If log4j cannot find the properties file, a nasty gram will issue such as:

	log4j:WARN No appenders could be found for logger (com.mycompany.pkg.MyClass).
	log4j:WARN Please initialize the log4j system properly.

...and the logged message will (probably) come out anyway in the Eclipse console view, but you should set this up to work correctly (the point of this blurb).

Despite our message, we'll send this to the Eclipse console during our run. In consequence, set up log4j.properties thus. This is a good place to get started.

	# The root logger is assigned priority level DEBUG and an appender named
	# myAppender.
	log4j.rootLogger=DEBUG,myAppender

	# The appender's type specified as ConsoleAppender, i.e. log output to
	# the Eclipse console. (Could have been FileAppender.)
	log4j.appender.myAppender=org.apache.log4j.ConsoleAppender

	# The appender is assigned a layout SimpleLayout. SimpleLayout will
	# include only priority level of the log statement and the log statement
	# itself in the log output.
	log4j.appender.myAppender.layout=org.apache.log4j.SimpleLayout

A service wrapper wraps around another program to allow it to live in a more useful context. For example, the Java service wrapper from Tanuki Software manages, among an array of useful solutions to problems, the logging of an application written in Java.

The Java service wrapper places an initial log line prefix prior to anything formally logged to log4j as specified in wrapper.conf. This can be confusing when attempting, without success, to tailor the lines logged to your log file. You quickly realize that there is a leftmost "column" over whose format you seemingly have no power. This is because it's defined in a different configuration file than log4j.properties, usually wrapper.conf.

See, primarily, http://wrapper.tanukisoftware.org/doc/english/introduction.html

wrapper.logfile.format

• L for log level (INFO, ERROR, WARN, DEBUG, TRACE, etc.)
• P for prefix
• D for threadname
• T for for timestamp in YYY/MM/DD HH:mm:ss.SSS
• Z for for timestamp in milliseconds
• M for the message (what you're probably using log4.j.properties to configure)

For example,

	wrapper.logfile.format=LPTM

...yields

	INFO | wrapper  | 2012/03/23 13:45:33.560 | INFO my message stuff...

You can turn lines logged into blank lines using

	wrapper.logfile.format=

...or constrain the logfile to just that content you are trying to log via the usual log4j mechanisms:

	wrapper.logfile.format=M