What are the ways to configure log4j?

log4j can be configured in one of the following ways:

1. Programmatically: Configuration is set directly in Java code.

   // Example of programmatic configuration
   org.apache.log4j.BasicConfigurator.configure();
   org.apache.log4j.Logger rootLogger = org.apache.log4j.Logger.getRootLogger();
   rootLogger.setLevel(org.apache.log4j.Level.DEBUG);
   org.apache.log4j.ConsoleAppender consoleAppender = new org.apache.log4j.ConsoleAppender(
           new org.apache.log4j.PatternLayout("%d [%t] %-5p %c - %m%n")
   );
   rootLogger.addAppender(consoleAppender);
   
   org.apache.log4j.Logger logger = org.apache.log4j.Logger.getLogger(MyClass.class);
   logger.debug("Debug message");
   

2. Using a properties file: Configuration is specified in a .properties file. Log4j automatically searches for a file named log4j.properties in the classpath.

   # Example of log4j.properties file
   log4j.rootLogger=DEBUG, console, file
   
   # Console appender configuration
   log4j.appender.console=org.apache.log4j.ConsoleAppender
   log4j.appender.console.layout=org.apache.log4j.PatternLayout
   log4j.appender.console.layout.ConversionPattern=%d [%t] %-5p %c - %m%n
   
   # File appender configuration
   log4j.appender.file=org.apache.log4j.FileAppender
   log4j.appender.file.File=application.log
   log4j.appender.file.Append=true
   log4j.appender.file.layout=org.apache.log4j.PatternLayout
   log4j.appender.file.layout.ConversionPattern=%d [%t] %-5p %c - %m%n
   

3. Using an XML file: Configuration is specified in a .xml file. Log4j can automatically find a log4j.xml file in the classpath.

   <!-- Example of log4j.xml file -->
   <!DOCTYPE log4j:configuration PUBLIC "-//APACHE//DTD LOG4J 1.2//EN" "log4j.dtd">
   <log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
   
       <appender name="console" class="org.apache.log4j.ConsoleAppender">
           <layout class="org.apache.log4j.PatternLayout">
               <param name="ConversionPattern" value="%d [%t] %-5p %c - %m%n"/>
           </layout>
       </appender>
   
       <appender name="file" class="org.apache.log4j.FileAppender">
           <param name="File" value="application.log"/>
           <param name="Append" value="true"/>
           <layout class="org.apache.log4j.PatternLayout">
               <param name="ConversionPattern" value="%d [%t] %-5p %c - %m%n"/>
           </layout>
       </appender>
   
       <root>
           <level value="DEBUG"/>
           <appender-ref ref="console"/>
           <appender-ref ref="file"/>
       </root>
   
   </log4j:configuration>
   

Loading order of configuration (by default):

Log4j searches for configuration files in the following order:

1. System property log4j.configuration (specifies the path to the configuration file).
2. log4j.xml file in the classpath.
3. log4j.properties file in the classpath.
4. If no file is found, a default configuration is used (ConsoleAppender with ERROR level).

Configuration priority:

• Programmatic configuration has the highest priority.
• Then comes configuration specified via the system property.
• XML file has higher priority than the properties file if both are present and no system property is specified.