Log4j

Concepts

Log4j has teh following fundamental objects:

• logger
• categories,
• priorities (levels),
• appenders and
• layouts.

Logger

Loggers are named entities. Logger names are case-sensitive and they follow the hierarchical naming rule. A logger is said to be an ancestor of another logger if its name followed by a dot is a prefix of the descendant logger name. The root logger resides at the top of the logger hierarchy. It is exceptional in two ways:

1. it always exists,
2. it cannot be retrieved by name, but by Logger.getRootLogger().

Priority

Loggers may be assigned levels. The set of possible levels, that is:

• TRACE
• DEBUG
• INFO
• WARN
• ERROR
• FATAL

Priority objects have both a string name and an integer value. The name is simply a mnemonic label for the priority. The integer value defines a relative order amongst priorities. The Level class extends the Priority class to be conform with java.util.logging. For the standard levels, we have DEBUG < INFO < WARN < ERROR < FATAL.

Category

A category is a like java named entity. There exists a special root category that simply is, but has no name.

Configuration

Properties can be defined by a properties file or by an XML file. The xml configuration class offers more flexibility and the benefits, and drawbacks, of an XML based configuration.

Logging Level are:

• ALL
• DEBUG
• ERROR
• FATAL
• INFO
• OFF
• TRACE
• WARN