Log4j has three main components: loggers, appenders and layouts. These three types of components work together to enable developers to log messages according to message type and level, and to control at runtime how these messages are formatted and where they are reported. Logger hierarchy 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.

Apache log4j 1.2 - Short introduction to log4j

Apache log4j 1.2 - Short introduction to log4j
OJB - ObJectRelationalBridge Summary Apache ObJectRelationalBridge (OJB) is an Object/Relational mapping tool that allows transparent persistence for Java Objects against relational databases. flexibility OJB - ObJectRelationalBridge
Commons - Commons