Log4j

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Apache log4j is a Java-based logging utility. It was originally written by Ceki Gülcü and is now a project of the Apache Software Foundation. log4j is one of several Java Logging Frameworks.

Gülcü has since started the SLF4J and Logback^[1] projects, with the intention of offering a compatible successor to log4j.

Log level

The following table defines the log levels and messages in Log4j, in decreasing order of severity. The left column lists the log level designation in Log4j and the right column provides a brief description of each log level.

Configuration

There are two ways to configure log4j. One is with a properties file and the other is with an XML file. Within either you can define 3 main components: Loggers, Appenders and Layouts. Configuring logging via a file has the advantage of turning logging on or off without modifying the application that uses log4j. The application can be allowed to run with logging off until there's a problem, for example, and then logging can be turned back on simply by modifying the configuration file.

Loggers are logical log file names. They are the names that are known to the Java application. Each logger is independently configurable as to what level of logging (FATAL, ERROR, etc) it currently logs. In early versions of log4j, these were called category and priority, but now they're called logger and level, respectively.

The actual outputs are done by Appenders. There are numerous Appenders available, with descriptive names, such as FileAppender, ConsoleAppender, SocketAppender, SyslogAppender, NTEventLogAppender and even SMTPAppender. Multiple Appenders can be attached to any Logger, so it's possible to log the same information to multiple outputs; for example to a file locally and to a socket listener on another computer.

Appenders use Layouts to format log entries. A popular way to format one-line-at-a-time log files is PatternLayout, which uses a pattern string, much like the C / C++ function printf. There are also HTMLLayout and XMLLayout formatters for use when HTML or XML formats are more convenient, respectively.

To debug a misbehaving configuration use the Java VM Property -Dlog4j.debug which will output to standard out. To find out where a log4j.properties was loaded from inspect getClass().getResource("/log4j.properties").

Example

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration PUBLIC
"http://logging.apache.org/log4j/docs/api/org/apache/log4j/xml/log4j.dtd">
<log4j:configuration>
    <!-- 
         an appender is an output destination, such as the console or a file;
         names of appenders are arbitrarily chosen
    -->
    <appender name="stdout" class="org.apache.log4j.ConsoleAppender">
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern"
                value="%d{ABSOLUTE} %5p %c{1}:%L - %m%n" />
        </layout>
    </appender>
 
    <!-- 
         loggers of category 'org.springframework' will only log messages of level "info" or higher;
         if you retrieve Loggers by using the class name (e.g. Logger.getLogger(AClass.class))
         and if AClass is part of the springframework.org package, t will belong to this category
    -->
    <logger name="org.springframework">
        <level value="info"/>
    </logger>
 
    <!-- 
         everything of spring was set to "info" but for class 
         PropertyEditorRegistrySupport we want "debug" logging 
    -->
    <logger name="org.springframework.beans.PropertyEditorRegistrySupport">
        <level value="debug"/>
    </logger>
 
    <logger name="org.acegisecurity">
        <level value="info"/>
    </logger>
 
    <!-- the root category -->
    <root>
        <!-- 
            all log messages of level "debug" or higher will be logged, unless defined otherwise 
            all log messages will be logged to the appender "stdout", unless defined otherwise 
        -->
        <level value="debug" />
        <appender-ref ref="stdout" />
    </root>
</log4j:configuration>

Log4j in application servers

Apache Tomcat

log4j can be activated in many application servers, including Tomcat, however an extra jar library must be added. Other application servers are available and may have log4j built in.

There are separate instructions on how to use log4j with Tomcat

Log viewer

Apache has another project named Chainsaw which was intended to use log4j generated log files. Chainsaw is a java-based GUI viewer with a lot of functionality. Chainsaw also uses similar configuration file as log4j. Other log viewing tools can be used with log4j too, for example log2web, which is open source and web-based, but has fewer features than Chainsaw. The web-based service StackHub can consume Log4J events and alert on errors and user-defined thresholds. Another .NET based viewer with lots of functionality is Log4View, which is a commercial product but also offers a free Community Edition. Log4J log file contain semi structured information about application problems, in some cases you would run log analysis on the logs in order to detect bugs, errors and generate statistics.

References

Bibliography

See also

Free software portal

• SLF4J
• Chainsaw

External links

• Short introduction to log4j
• The complete log4j manual
• Official log4j Homepage
• Log4View
• Log Viewer: a commercial, log4j log viewer and log monitor
• log4j Javadocs
• Setting up a log4j Socket Server to Email You Errors From Your Java Programs
• Simple real-time log4j demonstration
• log4eclipse
• StackHub

Ports

• log5j - A 'modern' log4j that supports printf and has additional performance features.
• Log4cxx Homepage - Apache port for C++
• Log4cplus Homepage - Another port for C++
• dlib C++ library Homepage - A port for C++
• Log4cpp Homepage - Yet another port for C++ (outdated)
• Log4plsql HomePage - A port to the Oracle PL/SQL
• Log4c - A port for C
• Log4perl - A port for Perl
• log4js - A port for JavaScript
• log4javascript - Another port for JavaScript
• logging - The official logging module for Python inspired by log4j.
• Log4r - Ruby logging module inspired by log4j
• Log4net - A port to the Microsoft .NET Framework
• log4php a port for PHP
• logback a purported successor to log4j, designed by log4j's founder
• log4sh a port for various Unix shells including sh, bash, dash and ksh
• Log4Cocoa - A Port for Cocoa
• Log4D - A Port for Delphi
• Microlog - A logging framework for Java ME and Android inspired by Log4j.
• Log5F - A port for ActionScript 3.0