Loggers in Log4j

The first and foremost advantage of any logging API over plain System.out.println resides in its ability to disable certain log statements while allowing others to print unhindered. This capability assumes that the logging space, that is, the space of all possible logging statements, is categorized according to some developer-chosen criteria. This observation had previously led to choose category as the central concept of the package. However, since log4j version 1.2, Logger class has replaced the Category class. For those familiar with earlier versions of log4j, the Logger class can be considered as a mere alias to the Category class.

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. A logger is said to be a parent of a child logger if there are no ancestors between itself and the descendant logger.

For example, the logger named "com.parent" is a parent of the logger named "com.parent.child". Similarly, "ancestor" is a parent of "ancestor.parent" and an ancestor of " ancestor.parent.child".
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.

Invoking the class static Logger.getRootLogger method retrieves it. All other loggers are instantiated and retrieved with the class static Logger.getLogger method. This method takes the name of the desired logger as a parameter. Some of the basic methods in the Logger class are listed below.

The logger is the core component of the logging process. In log4j, there are 5 normal levels Levels of logger available (not including custom Levels), the following is borrowed from the log4j API (http://jakarta.apache.org/log4j/docs/api/index.html):

static Level DEBUG - The DEBUG Level designates fine-grained informational events that are most useful to debug an application.
static Level INFO - The INFO level designates informational messages that highlight the progress of the application at coarse-grained level.
static Level WARN - The WARN level designates potentially harmful situations.
static Level ERROR - The ERROR level designates error events that might still allow the application to continue running.
static Level FATAL - The FATAL level designates very severe error events that will presumably lead the application to abort.

In addition, there are two special levels of logging available: (descriptions borrowed from the log4j API http://jakarta.apache.org/log4j/docs/api/index.html):
static Level ALL -The ALL Level has the lowest possible rank and is intended to turn on all logging.
static Level OFF - The OFF Level has the highest possible rank and is intended to turn off logging.

There are a number of ways to create a logger, one can retrieve the root logger:
Logger logger = Logger.getRootLogger();

One can create a new logger:
Logger logger = Logger.getLogger("MyLogger");

More usually, one instantiates a static logger globally, based on the name of the class:
static Logger logger = Logger.getLogger(test.class);

All these create a logger called "logger", one can set the level with:
logger.setLevel((Level)Level.DEBUG);
You can use any of 7 levels; Level.DEBUG, Level.INFO, Level.WARN, Level.ERROR, Level.FATAL, Level.ALL and Level.OFF.