Apache log4j is a Java-based logging utility. It was originally written by Ceki Gülcü and is now a project of the Apache Software Foundation. log4j is one of several Java logging frameworks.
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.
There are three ways to configure log4j: with a properties file, with an XML file and through Java code.
The following list defines the log levels and messages in log4j, in decreasing order of severity.
• OFF: The highest possible rank and is intended to turn off logging.
• 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.
• WARN: 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: Most detailed information. Expect these to be written to logs only.
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