Common Logging

1. Specify the log implementation.

 

    commons-logging.properties

 

2. Guess which log implementation should be used.

 

There are two base abstractions used by JCL: Log (the basic logger) and LogFactory (which knows how to create Log instances). Specifying a particular Log implementation is very useful (whether that is one provided by commons-logging or a user-defined one). Specifying a LogFactory implementation other than the default is a subject for advanced users only, so will not be addressed here. The default LogFactory implementation uses the following discovery process to determine what type of Log implementation it should use (the process terminates when the first positive match - in order - is found):

1. Look for a configuration attribute of this factory named org.apache.commons.logging.Log (for backwards compatibility to pre-1.0 versions of this API, an attribute org.apache.commons.logging.log is also consulted). Configuration attributes can be set explicitly by java code, but they are more commonly set by placing a file named commons-logging.properties in the classpath. When such a file exists, every entry in the properties file becomes an "attribute" of the LogFactory. When there is more than one such file in the classpath, releases of commons-logging prior to 1.1 simply use the first one found. From release 1.1, each file may define a priority key, and the file with the highest priority is used (no priority definition implies priority of zero). When multiple files have the same priority, the first one found is used. Defining this property in a commons-logging.properties file is the recommended way of explicitly selecting a Log implementation.
2. Look for a system property named org.apache.commons.logging.Log (for backwards compatibility to pre-1.0 versions of this API, a system property org.apache.commons.logging.log is also consulted).
3. If the Log4J logging system is available in the application class path, use the corresponding wrapper class (Log4JLogger ).
4. If the application is executing on a JDK 1.4 system, use the corresponding wrapper class (Jdk14Logger ).
5. Fall back to the default simple logging wrapper (SimpleLog ).

Consult the JCL javadocs for details of the various Log implementations that ship with the component. (The discovery process is also covered in more detail there.)

 

3. Help

 

Apache Commons-logging使用实例: http://zhangjunhd.blog.51cto.com/113473/25135