log4j 介绍（4）-- Layout篇

Layout of the log file
More often than not, users wish to customize not only the output destination but also the output format. This is accomplished by associating a layout with an appender. The layout is responsible for formatting the logging request according to the user's wishes, whereas an appender takes care of sending the formatted output to its destination.

你经常会在log4j.properties 里看到这样两行：
log4j.appender.EVENTS.layout=org.apache.log4j.PatternLayout
log4j.appender.EVENTS.layout.ConversionPattern=%d{dd-MM-yyyy}, %d{HH:mm:ss}, %-5.5p, %m %n

The best up-to-date documentation about available layouts can be found in the API documentation:
http://logging.apache.org/log4j/docs/api/org/apache/log4j/Layout.html
There you can see that we have DateLayout, HTMLLayout, PatternLayout, SimpleLayout, XMLLayout as options.


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.

The first field is the number of milliseconds elapsed since the start of the program. The second field is the thread making the log request. The third field is the level of the log statement. The fourth field is the name of the logger associated with the log request. The text after the '-' is the message of the statement.

Just as importantly, log4j will render the content of the log message according to user specified criteria. For example, if you frequently need to log Oranges, an object type used in your current project, then you can register an OrangeRenderer that will be invoked whenever an orange needs to be logged.

Object rendering follows the class hierarchy. For example, assuming oranges are fruits, if you register an FruitRenderer, all fruits including oranges will be rendered by the FruitRenderer, unless of course you registered an orange specific OrangeRenderer.

Object renderers have to implement the ObjectRenderer interface.


Layout最好理解了，看看就好了。