核心包:

org.apache.log4j

三大组件

Loggers

日志操作

Appenders 

日志的展现形式

Layouts

日志的展现格式

日志等级

TRACE
DEBUG
INFO
WARN
ERROR
FATAL

TRACE < DEBUG < INFO < WARN < ERROR < FATAL

日志请求等级

logger.debug(Object message) //日志请求等级为:DEBUG
info(Object message) //日志请求等级为:INFO
logger.error(Object message) //日志请求等级为:ERROR

日志请求等级必须>=日志等级，否则请求日志操作无效

demo:

### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss,SSS} %5p %c{1}:%L - %m%n

### direct messages to file myfile.log ###
log4j.appender.myfile=org.apache.log4j.DailyRollingFileAppender
log4j.appender.myfile.File=logs/myfile.log
log4j.appender.myfile.DatePattern='.'yyyy-MM-dd
log4j.appender.myfile.layout=org.apache.log4j.PatternLayout
log4j.appender.myfile.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss,SSS} %5p %c{1}:%L - %m%n

log4j.rootLogger=warn, stdout,myfile

说明：

log4j.rootLogger=warn, stdout,myfile
定义主日志工具:rootLogger，其它日志工具可以通过日志名称从它继承
日志级别：warn
定义2两个Appender
stdout:
	org.apache.log4j.ConsoleAppender
	对应log4j.appender.stdout及log4j.appender.stdout.layout的配置

myfile:
	org.apache.log4j.DailyRollingFileAppender
	对应log4j.appender.myfile及log4j.appender.myfile.layout的配置

代码：

public class LogTest {
	Logger logger=Logger.getLogger("test");
	@Test
	public void testLog(){
		logger.debug("debug...");
		logger.info("info...");
		logger.warn("warn...");
		logger.error("error...");
		logger.fatal("fatal...");
	}
}

运行结果:

2014-06-29 05:46:50,805  WARN test:15 - warn...
2014-06-29 05:46:50,807 ERROR test:16 - error...
2014-06-29 05:46:50,807 FATAL test:17 - fatal...

日志继承:

### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss,SSS} %5p %c:%L - %m%n


### direct messages to file myfile.log ###
log4j.appender.myfile=org.apache.log4j.DailyRollingFileAppender
log4j.appender.myfile.File=logs/myfile.log
log4j.appender.myfile.DatePattern='.'yyyy-MM-dd
log4j.appender.myfile.layout=org.apache.log4j.PatternLayout
log4j.appender.myfile.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss,SSS} %5p %c:%L - %m%n


log4j.rootLogger=warn, stdout,myfile


### name:com.demo,level:info,Appender and layout inherit rootLogger ###
log4j.logger.com.demo=info

说明:

log4j.logger.com.demo=info
日志名称:com.demo
日志等级为:info
Appender and layout 继承 rootLogger

代码:

package org.log.test;

import static org.junit.Assert.*;

import org.apache.log4j.Logger;
import org.junit.Test;


public class LogTest {
	Logger logger=Logger.getLogger("com.demo.test");
	@Test
	public void testLog(){
		logger.debug("debug...");
		logger.info("info...");
		logger.warn("warn...");
		logger.error("error...");
		logger.fatal("fatal...");
	}
}

com.demo.test 继承com.demo,通过包名形式的名称来实现

结果:

2014-06-29 06:12:13,488  INFO com.demo.test:14 - info...
2014-06-29 06:12:13,491  WARN com.demo.test:15 - warn...
2014-06-29 06:12:13,492 ERROR com.demo.test:16 - error...
2014-06-29 06:12:13,492 FATAL com.demo.test:17 - fatal...

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log4j小结

原文：http://blog.csdn.net/wobendiankun/article/details/35633321