Log4j 2 简介及示例

1、获得Logger实例

import org.apache.logging.log4j.Logger;

Logger logger=LogManager.getLogger( “logger的名字” );

Logger logger=LogManager.getLogger(HelloWorld.class);

      （1）The LogManager will locate the appropriate LoggerContext and then obtain the Logger from it.

     （2）The logger simply has a name and is associated with a LoggerConfig.

 Logger x = Logger.getLogger("wombat");
 Logger y = Logger.getLogger("wombat");

  　（3）通过相同的名字获得对同一个logger对象的引用。

2、LoggerConfig的层级

     The root LoggerConfig resides at the top of the LoggerConfig hierarchy. it always exists and it is part of every hierarchy. A Logger that is directly linked to the root LoggerConfig. 

     The LoggerConfig named "com.foo" is a parent of the LoggerConfig named"com.foo.Bar".

     层级关系： root  > com.foo （包名）> com.foo.Bar （类名）

3、Log levels

每个Logger引用一个合适的LoggerConfig，LoggerConfig也引用了它的父类对象。

Level of LogEvent的等级（从底到高）：TRACE, DEBUG, INFO, WARN, ERROR ,FATAL

例如：

Logger Name	Assigned LoggerConfig	Level
root	root	Debug
x	x	Error
x.y	x	Error
x.y.z	x	Error

1、the loggers root and X each have a Configured LoggerConfig with the same name. 

2、The loggers X.Y and X.Y.Z do not have configured LoggerConfigs and so get their Level from the LoggerConfig assigned to them,X, since it is the LoggerConfig whose name has the longest match to the start of the Logger's name.

4、Appender

(1) An output destination is called an Appender. 

(2) Currently, appenders exist for the console, files, remote socket servers, Apache Flume, JMS, and remote UNIX Syslog daemons. More than one Appender can be attached to a Logger.

(3) Each enabled logging request for a given logger will be forwarded to all the appenders in that Logger's LoggerConfig as well as the Appenders of the LoggerConfig's parents.

The table below shows an example:

Logger Name	Added Appenders	Additivity Flag	Output Targets
root	A1	not applicable	A1
x	A-x1, A-x2	true	A1, A-x1, A-x2
x.y	none	true	A1, A-x1, A-x2
x.y.z	A-xyz1	true	A1, A-x1, A-x2, A-xyz1
security	A-sec	false	A-sec
security.access	none	true	A-sec

5、示例程序

（1）jar包：log4j-core-2.0-beta4.jar  log4j-api-2.0-beta4.jar

（2）配置文件：log4j2.xml

<?xml version="1.0" encoding="UTF-8"?> 
<configuration status="OFF"> 
    <properties> 
        <property name="filename">target/test.log</property> 
    </properties>

    <appenders> 
        <Console name="Console" target="SYSTEM_OUT"> 
            <PatternLayout 
                pattern="%d{yyyy-MM-dd HH:mm:ss,SSS} %-5level - [%logger{1} - %M] %msg%n" /> 
        </Console> 
        <File name="File" fileName="${filename}"> 
            <PatternLayout 
                pattern="%d{yyyy-MM-dd HH:mm:ss,SSS} %-5level - [%logger{1} - %M] %msg%n" /> 
        </File> 
    </appenders>

    <loggers> 
        <root level="trace"> 
            <appender-ref ref="Console" /> 
        </root> 
        <logger name="edu.shao.log4j.sample" level="debug" additivity="true"> 
            <appender-ref ref="File" /> 
        </logger> 
    </loggers>

</configuration>

 (3)程序

import org.apache.logging.log4j.LogManager; 
import org.apache.logging.log4j.Logger;

public class HelloWorld { 
    private static Logger logger=LogManager.getLogger(HelloWorld.class); 
    
    public static boolean judge(){ 
        logger.entry(); 
        logger.debug("In doIt()."); 
        logger.info("In doIt().."); 
        logger.warn("In doIt()..."); 
        return logger.exit(true); 
    }

    public static void main(String[] args) { 
        logger.trace("begin..."); 
        if (judge()) { 
            logger.error("Do it again!"); 
            logger.fatal("Do it again!!"); 
        } 
        logger.trace("end!"); 
    } 
} 

（4）文件和控制台的输出

2013-03-28 16:48:33,047 DEBUG - [HelloWorld - judge] In doIt().
2013-03-28 16:48:33,049 INFO  - [HelloWorld - judge] In doIt()..
2013-03-28 16:48:33,049 WARN  - [HelloWorld - judge] In doIt()...
2013-03-28 16:48:33,049 ERROR - [HelloWorld - main] Do it again!
2013-03-28 16:48:33,050 FATAL - [HelloWorld - main] Do it again!!