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I often see environments where web applications use log4j for logging into files using various appenders. That is all well and good until I see that the logs are getting written into the application server’s logs. In JBOSS for example this is server.log. So why is this a bad idea ?

Why not to write into server.log:

1. An application server’s log is supposed to be used by the app server and not by your application.
2. This log is supposed to contain app server level information like loading war files / exceptions that were handed over to the container etc.
3. Weeding through the logs of about 10 applications to find a particular debug / error line is going to be crazy.
4. A PROD logging configuration should never have a console appender. The reason your application logs are getting logged into the server’s logs could be that your log4j configuration defines a console appender. This could mean you are logging the same thing in 2 places (assuming there is another file appender defined for your app).

Ideally each application should manage its own logging needs. Many developers seem to be unaware of a class named DomConfigurator in the log4j API that can load a log4j XML and configure it for your application. Here are the steps I usually take to prepare an application for logging

1. Write a custom class that implements ServletContextListener like the one shown below. The class looks for a context param named log4jFileName that is defined in web.xml. This locates the log4j XML configuration file. This will configure log4j before other classes use it.

Custom listener:

public class StartupListener implements ServletContextListener
{
    @Override
    public void contextDestroyed(ServletContextEvent arg0)
    {
        // Cleanup code goes here
    }
 
    @Override
    public void contextInitialized(ServletContextEvent sce)
    {
        Logger logger = null;
        ServletContext servletContext = sce.getServletContext();
        String log4jFile = servletContext.getInitParameter("log4jFileName");
        DOMConfigurator.configure(log4jFile);
        logger = LogManager.getLogger(StartupListener.class.getName());
        logger.debug("Loaded: " + log4jFile);
    }
}

2. In your web.xml define the listener and a context param like so

Web.xml entries:

<context-param>
    <param-name>log4jFileName</param-name>
    <param-value>
         /usr/local/app/log4j/config/log4jConfig.xml
    </param-value>
</context-param>
 
<listener>
    <listener-class>
        com.yourpackage.YourListener
    </listener-class>
</listener>

3. Now configure your log4jConfig.xml appropriately to ensure that your application logs are logged under an appropriate path. Use a file appender like RollingFileAppender to handle the retention policy for your applications.

Here is a sample log4j XML to help you

Sample log4j XML: (Customize as necessary)

<?xml version="1.0" encoding="UTF-8"?>
<log4j:configuration xmlns:log4j = "http://jakarta.apache.org/log4j/" debug = "false">
    <appender name = "FILE" class = "org.apache.log4j.RollingFileAppender">
        <param name = "File" value = "/usr/local/path/to/application.log"/>
        <param name = "MaxFileSize" value = "5MB"/>
        <param name = "MaxBackupIndex" value = "50"/>
        <layout class = "org.apache.log4j.PatternLayout">
            <param name = "ConversionPattern" value = "[%d{dd/MM/yy hh:mm:ss:sss z}] %5p %c{2}: %m%n"/>
        </layout>
    </appender>
    <category name = "com.your.application.package">
        <priority value = "debug"/>
    </category>
    <category name = "org.apache.catalina">
        <priority value = "error"/>
    </category>
    <category name = "org.hibernate">
        <priority value = "error"/>
    </category>
    <root>
        <priority value = "info"/>
        <appender-ref ref = "FILE"/>
    </root>
</log4j:configuration>

That is pretty much it. This will ensure that your application’s logs are logged elsewhere from the server’s own logs. Makes managing log files a little easier.