Java logging framework

A Java logging framework is a computer data logging package for the Java platform.

In software, logging refers to the recording of activity, not cutting down trees. Logging is a common issue for development teams. Several frameworks ease and standardize the process of logging for the Java platform. This article covers general purpose logging frameworks.

Functionality overview

Logging a message is broken into three major pieces: the Logger, Formatter and the Appender (Handler). The Logger is responsible for capturing the message to be logged, along with certain meta-data like level, and passing that to the logging framework. After receiving the message, the logging framework calls the Formatter on the message. The Formatter accepts an object and formats it for proper logging. The logging framework then hands the formatted message to the appropriate Appender for disposition of the message. This might include displaying on a console, writing to disk, appending to a database, or notification via email.

Simpler logging frameworks, like Java Logging Framework by the Object Guy, combine the logger and the appender together. This makes for simple initial configuration, but less configurable, especially as the project is moved across environments.

Logger

Most frameworks support the notion of a Logger. A Logger is an object that allows the application to log data without regard to where the data is actually logged. The application logs a message in the form of an object or an object and exception. When a Logger is created, it is given a name or an identifier. When logging a message, it is logged at a certain level or priority.

Name

A logger has a name. The name is usually hierarchical, with periods (.) separating the levels. A common naming scheme is to use the name of the class or package that is doing the loggings. Both log4j and the Java API supported defining Handlers higher up the hierarchy.

For example, the logger might be named "com.sun.some.UsefulClass". The handler can be defined for any of the following:

com
com.sun
com.sun.some
com.sun.some.UsefulClass

Level

The message is logged at a certain level. The common levels are copied from Apache Commons Logging:

**Common levels**
Level	Description
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.
WARNING	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	more detailed information. Expect these to be written to logs only.

The logging framework maintains the current logging level for each logger. The logging level can be set more or less restrictive. For example, if the logging level is set to "WARNING", then all messages of that level or higher are logged, ERROR and FATAL.

Formatters or renderers

A Formatter is an object that formats a given object for logging by the Appender. Mostly this consists of taking the object and converting it to a string representation.

Appenders or handlers

The appenders are configured to listen for messages of a certain log level or above. The Appender takes the message it is passed and disposes of the messages. Some message dispositions include:

display on the console
write to a file or syslog
append to a database table
distribute via JMS
send via email
write to a listening socket
bit-bucket (/dev/null)

Comparison of features

**Features**
Framework	Supported log levels	Standard appenders	Popularity	Cost / licence
Log4J	FATAL ERROR WARN INFO DEBUG TRACE	AsyncAppender, JDBCAppender, JMSAppender, LF5Appender, NTEventLogAppender, NullAppender, SMTPAppender, SocketAppender, SocketHubAppender, SyslogAppender, TelnetAppender, WriterAppender	Widely used in many project and platforms	Apache License, Version 2.0
Java Logging API	SEVERE WARNING INFO CONFIG FINE FINER FINEST	Depends on the underlying framework; Default Sun JVM implementation has the following: ConsoleHandler, FileHandler, SocketHandler, MemoryHandler	Not widely used^{[citation needed]}	Comes with the JRE
Apache Commons Logging	FATAL ERROR WARN INFO DEBUG TRACE	Depends on the underlying framework	Used by many, in conjunction with log4j	Apache License, Version 2.0
SLF4J	ERROR WARN INFO DEBUG TRACE	Depends on the underlying framework, which is pluggable	Probably small but growing	MIT License

Summary

Of the major players, log4j is still the front runner in the Java Logging ___domain^{[citation needed]}. The log4j project has been around for a long time and has lots of support from the development community. It's simple to implement, yet has powerful tools built in to accomplish most logging tasks. It is also easily extensible to handle proprietary needs.

The Apache Commons Logging isn't really a logging framework, but a logging framework wrapper. As such, it requires a logging framework underneath it. It would be useful in an heterogeneous environment where the logging framework is likely to change. However, in most cases, once a suitable logging framework has been chosen, there is little need to change it over the life of the project.

The Java Logging API is also not a logging framework, but standard API for accessing a logging framework. Compatible frameworks can be loaded into JVM and accessed via the API. There is also a logging implementation supplied with the Sun JVM which is the default logging framework access by the API. Many developers confuse this implementation with the Java Logging API.

SLF4J and Logback, both originally written by the same original writer of log4j, are growing potential replacements in particular for log4j and Apache Commons Logging.

External links

Java 6.0 Logging API
Commons Logging
Protomatter
Open Source Logging Tools in Java
Logging in Java Applications article by Thornton Rose
The Apache 2.0 license.