Java Guru: Log4j

Log4j, a popular logging package for java. The package is distributed under the Apache Software License, a fully-fledge open source license.

Drawbacks : slow down an application, If two verbose.

Advantage : It provides the precise context about a run of the application. One inserted into code, the generation of logging output requires no human intervention (involvement). Log output can be saved in persistent medium to be studied at a later time. Now it was important component of the development cycle.

It has three main components:

Logger
Appender
Layout

These three type of component work together to enable developers to log message according to message type and level and to control at runtime how these message are formatted and where they are reported.

Logger:

Logger is main class which provides functionality to log information for that purpose it provides several methods. This class extends Category class. In log4j version 1.2, Category class is replaced by Logger class as main logging class. So do not use Category.getCategory to get a logger. This is deprecated.

org.apache.log4j.Logger class provides methods to get Logger:

Logger(String name) [constructor]

getRootLogger() : Logger

getLogger(String name):Logger

getLogger(Class class) :Logger

To log(register) message Methods that used by the Logger is defined its super class Category :

debug(Object Message) : void

info(Object Message) : void

warn(Object Message) : void

error(Object Message) : void

fatal(Object Message) : void

trace(Object Message):void [this method in Logger class]

log(Priority level, Object message) : void

log() is very useful method that provide to log any type of message by defining message level.

Level is sub class of Priority which defines message level as a final static constant:

Level.DEBUG

Level.INFO

Level.WARN

Level.ERROR

Level.FATAL

Level.TRACE

Level.ALL

Level.OFF

Level Description

all

All levels including custom levels

trace (since log4j 1.2.12)

developing only, can be used to follow the program execution.

debug

developing only, for debugging purpose

info

Production optionally, Course grained (rarely written informations), I use it to print that a configuration is initialized, a long running import job is starting and ending.

warn

Production, simple application error or unexpected behaviour. Application can

continue. I warn for example in case of bad login attemps, unexpected data during import jobs

error

Production, application error/exception but application can continue. Part of the application

is probably not working.

Fatal

Production, fatal application error/exception, application cannot continue, for example

database is down.

Do not log at all.

Appender:

Log4j allow logging results print to multiple destinations, an output destination is called an appender. An appender specifies where your log messages are written to. There is a wide choice of appenders available. All appenders are direct or indirect subclasses of the AppenderSkeleton.

ConsoleAppender Logs to console

FileAppender Logs to a file

SMTPAppender Logs by email

RollingFileAppender Logs to a file, starts a new file once the max size is reached. (An alternative is the DailyRollingFileAppender which creates on file per day)

But there are as well:

AsyncAppender, JDBCAppender, JMSAppender, LF5Appender, NTEventLogAppender,

NullAppender, NullAppender, SMTPAppender, SocketAppender, SocketHubAppender,

SyslogAppender, TelnetAppender, DailyRollingFileAppender, RollingFileAppender.

Custom appenders can be created as well. The log4j download comes with a whole bunch of

samples in the examples directory.

Layout:

The layout specifies how a log message looks like.

DateLayout

HTMLLayout

PatternLayout

SimpleLayout

XMLLayout

SimpleLayout has no properties to be set. It is simple. We used PatternLayout in our example and we set a property named ConversionPattern. This property allows us to define the log output.

Steps:

First you define the layout.

log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

The pattern layout requires another parameter, i.e. the pattern.

log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

%d{ABSOLUTE} Date in Format Absolute

%5p %5 defines a right justified print with 5 characters, p prints the priority of the log message

%c{1}:%L - %m%n And the other settings. Very simple. They are all explained in the API.

Steps to use log4j:

Create a Java project.
Add the log4j.jar to the build path of the project.(add in project library)
Create Configuration file (log4j.xml or log4j.properties) in src folder
Create a class to getLogger and add log message

Using properties file to configure log4j

### direct log messages to stdout using simple layout###

log4j.appender.stdout=org.apache.log4j.ConsoleAppender

log4j.appender.stdout.Target=System.out

log4j.appender.stdout.layout=org.apache.log4j.SimpleLayout

log4j.rootLogger=debug, stdout

### direct log messages to stdout using pattern layout###

log4j.appender.stdout=org.apache.log4j.ConsoleAppender

log4j.appender.stdout.Target=System.out

log4j.appender.stdout.layout=org.apache.log4j.PatternLayout

log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

log4j.rootLogger=debug, stdout

### file appender

log4j.appender.file=org.apache.log4j.RollingFileAppender

log4j.appender.file.maxFileSize=100KB

log4j.appender.file.maxBackupIndex=5

log4j.appender.file.File=test.log

log4j.appender.file.threshold=info

log4j.appender.file.layout=org.apache.log4j.PatternLayout

log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

log4j.rootLogger=warn, file

Log messages with Level info to fatal to a file and send messages from error to fatal by email. The

file should be rolled every 100 KB.

#email appender

log4j.appender.mail=org.apache.log4j.net.SMTPAppender

#defines how othen emails are send

log4j.appender.mail.BufferSize=1

log4j.appender.mail.SMTPHost="smtp.myservername.xx"

log4j.appender.mail.From=fromemail@myservername.xx

log4j.appender.mail.To=toemail@myservername.xx

log4j.appender.mail.Subject=Log ...

log4j.appender.mail.threshold=error

log4j.appender.mail.layout=org.apache.log4j.PatternLayout

log4j.appender.mail.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n

log4j.rootLogger=warn, mail

You need mail.jar and activation.jar libraries from J2EE to send emails. Further properties of the

SmtpAppender are described above.

Using xml is best option to configure Log4j

1 Add log4j.jar file

2 Create log4j.xml in src folder and Copy the log4j.dtd into the source folder as well.

3 Create a class to getLogger and add log message

< log4j:configuration > is root element which is used to configure log4j

<appender> sub element of < log4j:configuration> is used to configure appender

<layout> sub element of <appender> is used to configure layout

<root>sub element of < log4j:configuration> is used to configure root logger

Examples:-

Using console appender with simple layout and pattern layout:-

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" >

<log4j:configuration>

</appender>

<root>

<appender-ref ref="stdout"/>

</root>

</log4j:configuration>

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" >

<log4j:configuration>

</layout>

</appender>

<root>

<appender-ref ref="stdout"/>

</root>

</log4j:configuration>

Using Rolling file appender:

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" >

<log4j:configuration>

<appender name="file"

class="org.apache.log4j.RollingFileAppender">

</layout>

</appender>

<root>

<appender-ref ref="file"/>

</root>

</log4j:configuration>

Using Rolling file appender and mail appender together

<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd" >

<log4j:configuration>

<appender name="file"

class="org.apache.log4j.RollingFileAppender">

<param name="ConversionPattern"

value="%d{ABSOLUTE} %5p %c{1}:%L - %m%n" />

</layout>

</appender>

<param name="ConversionPattern"

value="%d{ABSOLUTE} %5p %c{1}:%L - %m%n" />

</layout>

</appender>

<root>

<appender-ref ref="file" />

<appender-ref ref="mail"/>

</root>

</log4j:configuration>

<root>

<appender-ref ref="file" />

<appender-ref ref="mail"/>

</root>

</log4j:configuration>

Java Guru

Log4j

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