Apache log4j java logging framework - Tutorials,articles and free resources and log4j scripts

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1. Introduction

2. Installation

3. log4j Basic Concepts

4. Logger

5. Appender

6. Layout

7. Using External Configuration Files

Appender

The Appender controls how the logging is output. The Appenders available are (descriptions borrowed from the log4j API http://jakarta.apache.org/log4j/docs/api/index.html):

ConsoleAppender: appends log events to System.out or System.err using a layout specified by the user. The default target is System.out.
DailyRollingFileAppender extends FileAppender so that the underlying file is rolled over at a user chosen frequency.
FileAppender appends log events to a file.
RollingFileAppender extends FileAppender to backup the log files when they reach a certain size.
WriterAppender appends log events to a Writer or an OutputStream depending on the user's choice.
SMTPAppender sends an e-mail when a specific logging event occurs, typically on errors or fatal errors.
SocketAppender sends LoggingEvent objects to a remote a log server, usually a SocketNode.
SocketHubAppender sends LoggingEvent objects to a set of remote log servers, usually a SocketNodes
SyslogAppendersends messages to a remote syslog daemon.
TelnetAppender is a log4j appender that specializes in writing to a read-only socket.

One may also implement the Appender interface to create ones own ways of outputting log statements.

3.2.1.?Using A ConsoleAppender

A ConsoleAppender can be created like this:

ConsoleAppender appender = new ConsoleAppender(new PatternLayout());

Which creates a console appender, with a default PatternLayout. The default output of System.out is used.

3.2.2.?Using A FileAppender

A FileAppender can be created like this:

          FileAppender appender = null;
          try {
             appender = new FileAppender(new PatternLayout(),"filename");
          } catch(Exception e) {}

The constructor in use above is:

FileAppender(Layout layout, String filename) 
          Instantiate a FileAppender and open the file designated by filename.

Another useful constructor is:

FileAppender(Layout layout, String filename, boolean append) 
          Instantiate a FileAppender and open the file designated by filename.

So that one may choose whether or not to append the file specified or not. If this is not specified, the default is to append.

3.2.3.?Using A WriterAppender

A WriterAppender can be created like this:

          WriterAppender appender = null;
          try {
            appender = new WriterAppender(new PatternLayout(),new FileOutputStream("filename"));
          } catch(Exception e) {}

This WriterAppender uses the constructor that takes a PatternLayout and an OutputStream as arguments, in this case a FileOutputStream is used to output to a file, there are other constructors available.