Log4j is an excellent logging package for Java. I've been using it since version 0.7.4 in early 2000 and we use it at work for all our logging. Presenting the information below may seem to many log4j users like teaching your grandmother to suck eggs but I think it's useful so I though I'd write it down.

Log4j's free documentation is good for describing the various configuration and logging choices you can make but doesn't present any task based or how-to information. This note describes a simple log4j setup that uses four files to record debug, informational, warning and error messages. The debug file will contain all messages, the informational file only informational, warning and error messages, the warning file only warnings, and errors and the error file only errors.

In each Java source file I define a private static final Logger instance and use that for logging. Using the class the logger is in as the value for the getLogger method means each class gets it's own Logger instance, and the logger hierarchy parallels the class hierarchy. This gives you good information about where messages are coming from and good control over which ones are logged. So in a class called Test I'd use:

private static final Logger log =
    Logger.getLogger(Test.class);

When logging a debug or info message it's good practice to preceed each with a test using the isDebugEnabled or isInfoEnabled method. This will save you cycles later when debug or info messages are disabled, especially if the message being logged includes objects that are expesive to compute. For example:

	if (log.isDebugEnabled())
	    log.debug("A Debug Message");

In the log4j configuration file the first stage is to define four appenders, one for each output file. If this file is named log4j.xml and is on the classpath then it is found automatically by log4j when it is needed. Use the threshold parameter to limit the messages that are written to the file. As threshold is a property of the AppenderSkeleton class the same technique should work for any appender. Here's the definition of the DEBUG appender.

<appender name="DEBUG" class="org.apache.log4j.FileAppender">
  <param name="File" value="logs/debug.log" />
  <param name="Threshold" value="DEBUG" />
  <layout class="org.apache.log4j.PatternLayout">
    <param name="ConversionPattern"
	value="%d{ISO8601} %-5p %c - %m%n"/>
  </layout>
</appender>

After all of the appenders the lowest level logger to be controlled is defined. It is associated with all four appenders so that all messages sent to the logger are presented to all appenders. As explained in the log4j manual appenders are additive Each enabled logging request for a given logger will be forwarded to all the appenders in that logger as well as the appenders higher in the hierarchy. So the single logger definition below will see messages logged by any loggers in the com.zanthan hierarchy.

<logger name="com.zanthan">
  <level value="debug"/>
  <appender-ref ref="DEBUG"/>
  <appender-ref ref="INFO"/>
  <appender-ref ref="WARN"/>
  <appender-ref ref="ERROR"/>
</logger>

You can turn on or off logging of various levels by class or package by adding extra logger entries. For example the entry below will meant that classes whose names start with com.zanthan.xsltxt will only output warning and error messages.

<logger name="com.zanthan.xsltxt">
  <level value="warn"/>
</logger>