一，Log4j Example

log4j.properties

\# For standalone Java app, make sure the log4j.properties file is under the project/classes directory\# For Java web applications, make sure the log4j.properties file is under the WEB-INF/classes directory\# Root logger optionlog4j.rootLogger=DEBUG, stdout, file\# Redirect log messages to consolelog4j.appender.stdout=org.apache.log4j.ConsoleAppenderlog4j.appender.stdout.Target=System.outlog4j.appender.stdout.layout=org.apache.log4j.PatternLayoutlog4j.appender.stdout.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n\# Redirect log messages to a log file, support file rolling.log4j.appender.file=org.apache.log4j.RollingFileAppenderlog4j.appender.file.File=D:\\log4j-application.loglog4j.appender.file.MaxFileSize=5MBlog4j.appender.file.MaxBackupIndex=10log4j.appender.file.layout=org.apache.log4j.PatternLayoutlog4j.appender.file.layout.ConversionPattern=%d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n

LOG4J有五个记录级别，分别是info，debug，error，fatal，warn但是我们通常是使用debug或者error的级别，其他的我们就很少使用。

详细解释

1. DEBUG, stdout, file：定义了日志的记录级别，输出的方式是控制台和文件的形式
2. %d{yyyy-MM-dd HH:mm:ss}：格式化时间日期
3. %-5p：记录的优先级.-5格式化输出的样式，左对齐
4. %c{1}：通过getLogger()方法，得到我们设置的日志名称
5. D:\log4j-application.log：输出文件的位置，文件内容最大5MB
6. %L：在日志请求记录在代码中行号
7. %m%n：换行和信息记录

HelloExample.java

package com.log4j.helloworld;import org.apache.log4j.*;public class HelloExample {    final static Logger logger = Logger.getLogger(HelloExample.class);    public static void main(String[] args) {        HelloExample obj = new HelloExample();        obj.runMe("mkyong");    }    private void runMe(String parameter){        if(logger.isDebugEnabled()){            logger.debug("This is debug:"+parameter);        }        if(logger.isInfoEnabled()){            logger.info("This is info:"+parameter);        }        logger.warn("This is warn:"+parameter);        logger.error("This is error:"+parameter);        logger.fatal("This is fatal:"+parameter);    }}

控制台输出

2016-12-16 16:55:19 DEBUG HelloExample:14 - This is debug:mkyong2016-12-16 16:55:19 INFO  HelloExample:17 - This is info:mkyong2016-12-16 16:55:19 WARN  HelloExample:19 - This is warn:mkyong2016-12-16 16:55:19 ERROR HelloExample:20 - This is error:mkyong2016-12-16 16:55:19 FATAL HelloExample:21 - This is fatal:mkyong

log4j-application.log