log4j

Apache log4j

Developer(s)	Apache Software Foundation
Initial release	January 8, 2001 (2001-01-08)
Stable release	2.2 / February 22, 2015 (2015-02-22)
Written in	Java
Operating system	Cross-platform
Type	Logging Tool
License	Apache License 2.0
Website	http://logging.apache.org/log4j

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 projects, with the intention of offering a successor to log4j.

The log4j team has created a successor to log4j with version number 2.0. log4j 2.0 was developed with a focus on the problems of log4j 1.2, 1.3, java.util.logging and logback, and addresses issues which appeared in those frameworks. In addition, log4j 2.0 offers a plugin architecture which makes it more extensible than its predecessor. log4j 2.0 is not backwards compatible with 1.x versions, although an "adapter" is available.

Log4j 1 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.

Level	Description
OFF	The highest possible rank and is intended to turn off logging.
FATAL	Severe errors that cause premature termination. Expect these to be immediately visible on a status console.
ERROR	Other runtime errors or unexpected conditions. Expect these to be immediately visible on a status console.
WARN	Use of deprecated APIs, poor use of API, 'almost' errors, other runtime situations that are undesirable or unexpected, but not necessarily "wrong". Expect these to be immediately visible on a status console.
INFO	Interesting runtime events (startup/shutdown). Expect these to be immediately visible on a console, so be conservative and keep to a minimum.
DEBUG	Detailed information on the flow through the system. Expect these to be written to logs only.
TRACE	Most detailed information. Expect these to be written to logs only. Since version 1.2.12.

Configuration of log4j 1.2

There are three ways to configure log4j: with a properties file, with an XML file and through Java code. Within either you can define three 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") or getClass().getResource("/log4j.xml").

There is also an implicit "unconfigured" configuration of log4j, that of a log4j-instrumented Java application which lacks any log4j configuration. This prints to stdout a warning that the program is unconfigured, and the URL to the log4j web site where details on the warning and configuration may be found. As well as printing this warning, an unconfigured log4j application does not print messages at INFO, DEBUG or TRACE levels -and possibly not higher level messages.

Example for log4j 1.2

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration PUBLIC "-//LOGGER" "http://logging.apache.org/log4j/1.2/apidocs/org/apache/log4j/xml/doc-files/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 org.springframework package, it 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>
    
    
    <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>

TTCC

TTCC is a message format used by log4j. TTCC is an acronym for Time Thread Category Component. It uses the following pattern:

 %r [%t] %-5p %c %x - %m%n

Where

Mnemonic	Description
%r	Used to output the number of milliseconds elapsed from the construction of the layout until the creation of the logging event.
%t	Used to output the name of the thread that generated the logging event.
%p	Used to output the priority of the logging event.
%c	Used to output the category of the logging event.
%x	Used to output the NDC (nested diagnostic context) associated with the thread that generated the logging event.
%X{key}	Used to output the MDC (mapped diagnostic context) associated with the thread that generated the logging event for specified key.
%m	Used to output the application supplied message associated with the logging event.
%n	Used to output the platform-specific newline character or characters.

Example output
467 [main] INFO org.apache.log4j.examples.Sort - Exiting main method.

Ports

• log4c - A port for C. Log4C is a C-based logging library, released on SourceForge under the LGPL license. For various Unix operating systems the autoconf and automake files are provided. On Windows a Makefile is provided for use with MSVC. Developers may also choose to use their own make system to compile the source, depending on their build engineering requirements. An instance of the log4c library may be configured via three methods: using environment variables, programmatically, or via XML configuration file. Last version is 1.2.4, released in 2013, and the project is no longer actively developed.
• log4js - A port for JavaScript. Log4js is available under the licence of Apache Software Foundation. One special feature of Log4js is the ability to log the events of the browser remotely on the server. Using Ajax it is possible to send the logging events in several formats (XML, JSON, plain ASCII, etc.) to the server to be evaluated there. The following appenders are implemented for log4js: AjaxAppender, ConsoleAppender, FileAppender, JSConsoleAppender, MetatagAppender, and WindowsEventsAppender. The following Layout classes are provided: BasicLayout, HtmlLayout, JSONLayout, and XMLLayout. Last version is 1.1, released in 2008.
• log4javascript - Another port for JavaScript. log4javascript is a JavaScript logging framework based on the log4j. The latest version is 1.4.9, released in May 2014.
• JSNLog - A port for JavaScript. Automatically places messages from JavaScript loggers in server side logs using a .Net server side component that interfaces with Log4Net, NLog, Elmah or Common.Logging. This to provide an integrated log for client and server side events. Request ids correlate events related to a specific user. Configuration is via a server side web.config file. Supports exception logging including stack traces. In July 2014 the latest version was 2.7.1 and updates were made regularly.
• Apache Log4net - A port to the Microsoft .NET Framework. The initial work was done by Neoworks and was donated to the Apache Software Foundation in February 2004. The framework is similar to the original log4j while taking advantage of new features in the .NET runtime. Provides Nested Diagnostic Context (NDC) and Mapped Diagnostic Context (MDC). Last version is 1.2.13, released in 2013.
• log4perl - A Perl port of the widely popular log4j logging package. Last version is 1.44, released in May 2014.
• Apache log4php - "A versatile logging framework for PHP. Originally a port of Apache log4j to PHP, it has grown to include various PHP specific features."
• PL-SQL-Logging-Utility is an adaptation of log4j in PL/SQL.
• Log4db2 is a logging utility for DB2 for LUW that uses SQL instructions with SQL PL code.

Apache Log4j 2

Apache Log4j 2 is the successor of Log4j 1 which was released as GA version in July 2014. The framework was rewritten from scratch and has been inspired by existing logging solutions, including Log4j 1 and JUL. The main differences to Log4j 1 are:

• Improved configuration syntax
• Support for xml and json configuration
• Improved Filters
• Property support
• Markers
• Improved speed
• Modularity: Log4j 2 supports a plugin system
• Improved reliability. Messages are not lost while reconfiguring the framework like in Log4j 1 or Logback
• Automatic reloading of configuration

One of the most recognized features of Log4j 2 is the performance of the "Asynchronous Loggers". Log4j 2 makes use of the LMAX Disruptor. The library reduces the need for kernel logging and increases the logging performance by the factor 12. For example, in the same environment Log4j 2 can write more than 18,000,000 messages per second, whereas other frameworks like Logback and Log4j 1 just write < 2,000,000 messages per second.

Log4j 2 provides support for SLF4J, Commons Logging, Apache Flume and Log4j 1.

See also

• Chainsaw (log file viewer)
• JLV - Java logging viewer which is currently available as a plugin for Eclipse IDE