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Log4j delivers control over logging

The open source log4j API for Java offers fast, efficient log services

By Ceki Gülcü, JavaWorld.com, 11/22/00

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Log4j makes it easy to name categories by software component. That can be accomplished by statically instantiating a category in each class, with the category name equal to the fully qualified name of the class -- a useful and straightforward method of defining categories. As the log output bears the name of the generating category, such a naming strategy facilitiates identifying a log message's origin. However, that is only one possible, albeit common, strategy for naming categories. Log4j does not restrict the possible set of categories. Indeed, the developer is free to name the categories as desired.

Appenders and layouts

The ability to selectively enable or disable logging requests based on their category is only part of the picture. Log4j also allows logging requests to print to multiple output destinations called appenders in log4j speak. Currently, appenders exist for the console, files, GUI components, remote socket servers, NT Event Loggers, and remote UNIX Syslog daemons.

A category may refer to multiple appenders. Each enabled logging request for a given category will be forwarded to all the appenders in that category as well as the appenders higher in the hierarchy. In other words, appenders are inherited additively from the category hierarchy. For example, if you add a console appender to the root category, all enabled logging requests will at least print on the console. If, in addition, a file appender is added to a category, say C, then enabled logging requests for C and C's children will print on a file and on the console. Be aware that you can override that default behavior so that appender accumulation is no longer additive.

More often than not, users want to customize not only the output destination but also the output format, a feat accomplished by associating a layout with an appender. The layout formats the logging request according to the user's wishes, whereas an appender takes care of sending the formatted output to its destination. The PatternLayout, part of the standard log4j distribution, lets the user specify the output format according to conversion patterns similar to the C language printf function.

For example, the PatternLayout with the conversion pattern %r [%t]%-5p %c - %m%n will output something akin to: 

176 [main] INFO  org.foo.Bar - Located nearest gas station.


In the output above:

  • The first field equals the number of milliseconds elapsed since the start of the program
  • The second field indicates the thread making the log request
  • The third field represents the priority of the log statement
  • The fourth field equals the name of the category associated with the log request


The text after the - indicates the statement's message.

Configuration

Inserting log requests into the application code requires a fair amount of planning and effort. Observation shows that code dedicated to logging represents approximately four percent of the application's total. Consequently, even moderately sized applications will have thousands of logging statements embedded within their code. Given their number, it becomes imperative to manage those log statements without the need to modify them manually.

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