log4j

log4j: Logging for Java
-open source logging tool from apache.

Log Levels:

There are 8 levels

• ALL
• TRACE
• DEBUG
• INFO
• WARN
• ERROR
• FATAL
• OFF

The specified level and all higher level messages will be printed.
For example, if the level is set to INFO, then only INFO,WARN, ERROR and FATAL messages are printed and TRACE, DEBUG messages are skipped.

Steps to configure Logger in your class programmatically:
1. Get logger instance on your class
Logger logger = Logger.getLogger(Test.class);

2. Create a Layout(determines the format of the print message)
Layout layout = new PatternLayout();

3. Create an Appender passing the layout
Appender appender = new FileAppender(layout, "file.log");

4. Add the Appender reference to the class Logger
logger.addAppender(appender);

5. Set the log level of the Logger
logger.setLevel(Level.INFO);

Thats it...Now you can write log messages, with different levels.
logger.info("This is an info message");
logger.warn("This is a warning message");

Main Components:
1. org.apache.log4j.Logger(class)
It is the main class to handle log messages
2. org.apache.log4j.Layout(abstract class)
It is used to display log messages in our own styles and formats
3. org.apache.log4j.Appender(interface)
It is used to display logger messages in different repositories.

If no appenders added to Logger, then we get a warning message saying "No appenders could be found for logger (<class>)."
Layouts:
1. SimpleLayout (%p - %m%n)
2. HtmlLayout
3. XmlLayout
4. PatternLayout (we can use any customized pattern here)

Patterns:
Few useful pattern strings:
%d  : Date
%m : Message
%n  : new line
%p  : Priority/Level
%t  : Thread
%c  : Class name 
%L  : Line number

Please have a look at the below link, for all available patterns:
http://logging.apache.org/log4j/2.x/manual/layouts.html#Patterns

Appenders:
1. ConsoleAppender
2. FileAppender
3. RollingFileAppender (Controls number of log files and size of each file)
4. JDBCAppender (Writes messages to database)
5. JMSAppender (sends messages over queues)
6. SMTPAppender (Sends mail for each message)

Declarative Approach to configure Log4j:
Two ways
1. log4j.xml
2. log4j.properties

If both xml and properties files are present, then log4j.xml takes higher priority.

1. log4j.xml:
eg:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE log4j:configuration SYSTEM "log4j.dtd">
<log4j:configuration xmlns:log4j="http://jakarta.apache.org/log4j/">
    <appender name="ConsoleApp" class="org.apache.log4j.ConsoleAppender">
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="%L [%m][%p][%d]%n" />
        </layout>
    </appender>
    <appender name="FileApp" class="org.apache.log4j.FileAppender">
        <param name="file" value="profile.log" />
        <layout class="org.apache.log4j.PatternLayout">
            <param name="ConversionPattern" value="[%m][%p][%d][%t]%n" />
        </layout>
    </appender>
    <root>
        <level value="trace" />
        <appender-ref ref="FileApp" />
        <appender-ref ref="ConsoleApp" />
    </root>
    <logger name="com.Log4jTest">
        <level value="warn" />
        <appender-ref ref="ConsoleApp" />
    </logger>
    <logger name="com.test">
        <level value="trace" />
        <appender-ref ref="ConsoleApp" />
    </logger>
</log4j:configuration>

2. log4j.properties:
eg:

# Define the root logger with appender file
log4j.rootLogger = DEBUG, FileApp

# Define the file appender with file name
log4j.appender.FileApp=org.apache.log4j.FileAppender
log4j.appender.FileApp.File=log.out

# Define the layout for file appender
log4j.appender.FileApp.layout=org.apache.log4j.PatternLayout
log4j.appender.FileApp.layout.conversionPattern=%L [%d] [%t] [%p] %m%n


Adding multiple appenders:

<root>
   <level value="trace" />
   <appender-ref ref="FileApp" />
   <appender-ref ref="ConsoleApp" />
</root>

Different log level for different packages:

<logger name="com.Log4jTest">
   <level value="warn" />
   <appender-ref ref="ConsoleApp" />
</logger>
<logger name="com.test">
   <level value="trace" />
   <appender-ref ref="ConsoleApp" />
</logger>

Configurators:

1. BasicConfigurator:
This does not require any log4j file.

Logger logger = Logger.getLogger(Log4jTest.class);
BasicConfigurator.configure();

This used ConsoleAppender and PatternLayout with pattern "%r [%t] %p %c %x - %m%n"

2. DomConfigurator:
This is used to specify the log4j xml configuration file.
File name can be anything


3. PropertyConfigurator:
This is used to specify the log4j properties configuration file.
File name can be anything.