In JBoss log4j is used for logging. If you are not familiar with the log4j package and would like to use it in your applications, you can read more about it at the Jakarta web site (http://jakarta.apache.org/log4j/).

Logging is controlled from a central conf/jboss-log4j.xml file. This file defines a set of appenders specifying the log files, what categories of messages should go there, the message format and the level of filtering. By default, JBoss produces output to both the console and a log file (log/server.log).

There are 5 basic log levels used: DEBUG, INFO, WARN, ERROR and FATAL. The logging threshold on the console is INFO, which means that you will see informational messages, warning messages and error messages on the console but not general debug messages. In contrast, there is no threshold set for the server.log file, so all generated logging messages will be logged there.

If things are going wrong and there doesn’t seem to be any useful information in the console, always check the server.log file to see if there are any debug messages which might help you to track down the problem. However, be aware that just because the logging threshold allows debug messages to be displayed, that doesn't mean that all of JBoss will produce detailed debug information for the log file. You will also have to boost the logging limits set for individual categories. Take the following category for example.

<!-- Limit JBoss categories to INFO --> 
<category name="org.jboss"> 
    <priority value="INFO"/> 
</category>    

This limits the level of logging to INFO for all JBoss classes, apart from those which have more specific overrides provided. If you were to change this to DEBUG, it would produce much more detailed logging output.

As another example, let’s say you wanted to set the output from the container-managed persistence engine to DEBUG level and to redirect it to a separate file, cmp.log, in order to analyze the generated SQL commands. You would add the following code to the conf/jboss-log4j.xml file:

<appender name="CMP" class="org.jboss.logging.appender.RollingFileAppender"> 
    <errorHandler class="org.jboss.logging.util.OnlyOnceErrorHandler"/> 
    <param name="File" value="${jboss.server.home.dir}/log/cmp.log"/> 
    <param name="Append" value="false"/> 
    <param name="MaxFileSize" value="500KB"/> 
    <param name="MaxBackupIndex" value="1"/> 
 
    <layout class="org.apache.log4j.PatternLayout"> 
        <param name="ConversionPattern" value="%d %-5p [%c] %m%n"/> 
    </layout> 
</appender> 
 
<category name="org.jboss.ejb.plugins.cmp"> 
    <priority value="DEBUG" /> 
    <appender-ref ref="CMP"/> 
</category> 

This creates a new file appender and specifies that it should be used by the logger (or category) for the package org.jboss.ejb.plugins.cmp.

The file appender is set up to produce a new log file every day rather than producing a new one every time you restart the server or writing to a single file indefinitely. The current log file is cmp.log. Older files have the date they were written added to the name. You will notice that the log directory also contains HTTP request logs which are produced by the web container.