log4j path for logs file

Question

I am using log4j for my logs. My application has a simple XML configuration file and I need to have in my config file the path for the log file. At the moment, it's another XML configuration file for log4j that contains this:

<log4j:configuration>
    <appender name="file" class="org.apache.log4j.FileAppender">
    <errorHandler class="org.apache.log4j.helpers.OnlyOnceErrorHandler" />
        <param name="file" value="log.out" /> 
        (..........)

(log.out is the default log which is in the project's directory.) How can I move this configuration into my application's configuration file?

Solution

There are basically three options:

1. During the build, create a config for log4j which contains an absolute path for the file parameter. Least flexible.

2. Use a System property. While this gives you some flexibility, this causes problems when you run in a container (J2EE server) and you have several applications which all use log4j.

The second option comes in two flavors:

1. You can specify the path for the log file using ${logFile} in the XML and use -DlogFile= on the command line to specify the path.

2. You can keep the XML config in a different place and tell log4j to load it when it starts using -Dlog4j.configuration=/absolute/path/to/log4j.xml

Related articles:

• http://www.jroller.com/rmcmahon/entry/system_properties_in_log4j_properties
• Log4J change File path dynamically
• Log4j: How to configure simplest possible file logging?
• http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/FileAppender.html