log4j - Architecture

The logger component is accessible through the Logger class of the log4j API.This class provides a static method Logger.getLogger(name).

getLogger(name) retrieves an existing logger object by the given name or , creates a new logger of given name if none exists.

The logger object is then used to set properties of logger component. It will invoke methods which generate log requests.These methods are debug(), info(), warn(), error(), fatal(), and log()

Each class in the Java application being logged can have an individual logger assigned to it or share a common logger with other classes.

Any number of loggers can be created for the application to suit specific logging needs

It is a common practice to create one logger for each class, with a name same as the fully-qualified class name.

Getting Logger Object

1) Create a new logger:

Logger logger = Logger.getLogger("MyLogger");

2) Instantiate a static logger globally, based on the name of the class :

static Logger log = Logger.getLogger(YourClassName.class.getName())

Note: While creating a Logger object we need to pass either fully qualified class name or class object as a parameter, class means current class for which we are going to use Log4j.

public class Test {

   static Logger l = Logger.getLogger(Test.class.getName());

   public static void main(String[] args) {
     // Our logic will goes here
   }
}

Now ,log4j API is accessible to user's application classes and can be used for logging.

Logger - Priority levels

Loggers can be assigned different levels of priorities.priority levels in ascending order of priority is given below

1. TRACE

This log4j level gives more detailed information than the DEBUG level and sits top of the hierarchy. This level is introduced from version 1.2.12 in log4j.

2. DEBUG

This log4j level helps developer to debug the application.It is used to print the fine-grained informational events that are most useful to debug an application.

3. INFO

The INFO level is associated with significant events in the normal life cycle of the application. It is used to print message related to progress of the application.

4. WARN

The WARN level designates potentially harmful situations of the application.The messages coming out of this level may not halt the progress of the system.

5. ERROR

This level gives information about a serious error which needs to be resolved and may result in unstable state. Error conditions do not necessarily cause the application to crash and the application may continue to service subsequent requests.

6. FATAL

The FATAL level represents very severe error situation that will lead the application to abort.

There are printing methods for each of these priority levels for example logger.info("logstatement1"); generates log request of priority level INFO for logstatement1.

To summarize priority level is given below.

trace < debug < info < warn < error < fatal

FATAL is largest whereas TRACE has lowest priority

log4j Appenders

Appender job is to write the log statement into the external file or database or repository.

In log4j there are various appenders available such as..

ConsoleAppender

FileAppender

WriterAppender

RollingFileAppender

DailyRollingFileAppender

SMTPAppender

TelnetAppender

SyslogAppender

Layout

The Layout component defines the format in which the log statements are written to the log destination by appender

Layout is an abstract class in log4j API; it can be extended to create user-defined layouts

Some ready-made layouts are also available in log4j package; they are PatternLayout, SimpleLayout, DateLayout, HTMLLayout,and XMLLayout.

♦ Type of Layouts

HTMLLayout

PatternLayout

SimpleLayout