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apache log4j - apache logging services
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"With log4j it is possible to enable logging at runtime 
without modifying the application binary. The log4j package 
is designed so that these statements can remain in shipped 
code without incurring a heavy performance cost. Logging 
behavior can be controlled by editing a configuration file, 
without touching the application binary."

For more information:
http://logging.apache.org/log4j/1.2/