How to use log4j to record logs in Java

1. Preface

log4j is a reliable, fast and flexible logging framework (API) written in Java, which is released under the Apache Software License. Log4j has been ported to languages ​​such as C, C++, C#, Perl, Python and Ruby.

Log4j is highly configurable and can be configured through external files at runtime. It prioritizes logging and provides mechanisms to direct logging information to many destinations, such as databases, files, consoles, UNIX system logs, etc.

There are three main components in Log4j:

Loggers: Responsible for capturing logging information.

Appenders: Responsible for publishing log information to different preferred destinations.

Layouts: Responsible for formatting log information in different styles.

Note: This article is based on log4j 2.X and above.

2. Installation

log4j-core-xx.jar

log4j-api-xx.jar

log4j-web -xx.jar (required reference for web projects)

3. Configuration

Prepare some log classes and add the following references:

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

static Logger logger = LogManager.getLogger(Test.class.getName());

The configuration file location is in: src root directory , even if there is no configuration file, no error will be reported, and the output will be in the form of console by default.

The log4j2 configuration file is very different from log4 (the 1. ):

<?xml version="1.0" encoding="UTF-8"?>
 
<configuration status="error">
 <!--先定义所有的appender-->
 <appenders>
  <!--这个输出控制台的配置-->
  <Console name="Console" target="SYSTEM_OUT">
   <!--这个是输出日志的格式-->
   <PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %class{36} %L %M - %msg%xEx%n"/>
  </Console>
  <!--文件会打印出所有信息，这个log每次运行程序会自动清空，由append属性决定，适合临时测试用-->
  <File name="Error" fileName="${web:rootDir}/logs/error.log" append="false">
   <!--文件只记录level及以上级别的信息（onMatch），其他的直接拒绝（onMismatch）-->
   <ThresholdFilter level="error" onMatch="ACCEPT" onMismatch="DENY"/>
   <PatternLayout charset="UTF-8" pattern="%d{HH:mm:ss.SSS} %-5level %class{36} %L %M - %msg%xEx%n"/>
  </File>
 
  <!--这个会打印出所有的信息，每次大小超过size，则这size大小的日志会自动存入按年份-月份建立的文件夹下面并进行压缩，作为存档-->
  <RollingFile name="RollingFile" fileName="${web:rootDir}/logs/history.log"
      filePattern="log/$${date:yyyy-MM}/history-%d{MM-dd-yyyy}-%i.log.gz">
   <PatternLayout charset="UTF-8" pattern="%d{yyyy-MM-dd &#39;at&#39; HH:mm:ss z} %-5level %class{36} %L %M - %msg%xEx%n"/>
   <SizeBasedTriggeringPolicy size="50MB"/>
  </RollingFile>
 </appenders>
 <!--然后定义logger，只有定义了logger并引入的appender，appender才会生效-->
 <loggers>
  <!--建立一个默认的root的logger-->
  <root level="trace">
   <appender-ref ref="Error"/>
   <appender-ref ref="RollingFile"/>
   <appender-ref ref="Console"/>
  </root>
 </loggers>
</configuration>

4. Ordinary projects and web projects

For ordinary projects, it can be used normally after the above configuration is completed. For web projects, no log files will be generated. of. You need to add the following configuration under the root node of in web. The content can be of certain help to everyone's study or work. If you have any questions, you can leave a message to communicate. Thank you for your support to the PHP Chinese website.

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