About the reload of log4j2 and the output of different levels of logs to different log files

log4j2[2.1]

log4j2 The configuration file format can be JSON, perhaps XML.
Usually in classpath Find... In the path log4j2.json, perhaps log4j2.xml,
Alternatively, you can set the configuration file through system parameters , such as （ Generally, this is used for applications launched from the command line ）：

-Dlog4j.configurationFile=path/to/log4j2.xml

When the app starts , Except for Jean log4j2 stay classpath And system parameters , Other than initialization , We can also reload at any time in the code log4j2 Configuration file for , Reconfigure .

for example , Consider the following common scenarios ：WEB application , We don't want to put log4j2.xml Pack into your own jar In file （ Such changes log4j2 The configuration of is troublesome ）, I don't want to log4j2.xml Put it in WEB-INF/classes below （ I don't want users to operate casually WEB-INF The files under the ）, Then we can put log4j2.xml And configuration files used in other projects , Put it in a centralized place , such as TOMCAT/bin/config Next , At this time , Change if initialization log4j2 Well ？ We can provide one InitServlet, for example ：

...
public void init() throws ServletException {
   
    String configRoot = this.getInitParameter("configRoot");
    String log4j2Config = configRoot + File.separator + this.getInitParameter("log4j2Config");
    File file = new File(log4j2Config);
    try {
   
        LoggerContext context =(LoggerContext)LogManager.getContext(false);
        context.setConfigLocation(file.toURI());

        // Reinitialize Log4j2 Configuration context for 
        context.reconfigure();
    } catch (MalformedURLException e) {
   
        e.printStackTrace();
    }

    //todo:  I got it here logger, It has been initialized according to the new configuration file 
    logger = LogManager.getLogger(DefaultInitServlet.class);
}

Corresponding , As long as web.xml Configure this InitServlet, And provide configRoot and log4j2Config Just two paths （ You can also not configure configRoot, But through System.getProperty("user.dir") To get the running directory of the application , Yes tomcat for , This directory is tomcat/bin, Other applications such as command line , Namely bat/sh Directory of ）
web.xml
[quote]
<servlet>
<servlet-name>InitServlet</servlet-name>
<servlet-class>test.InitServlet</servlet-class>
<load-on-startup>0</load-on-startup>
<init-param>
<!-- Profile root -->
<param-name>configRoot</param-name>
<param-value>d://config</param-value>
</init-param>
<init-param>
<!-- log4j2 Relative path of configuration file -->
<param-name>log4j2Config</param-name>
<param-value>log4j2/log4j2.xml</param-value>
</init-param>
</servlet>
[/quote]

Okay , Talked about how to reinitialize log4j2, Let's talk about how to put different levels of logs , Output to different log files . This is online , Including on the official website , None of them is clear .
such as , hope trace/debug Level log output to debug.log, and info Level log output to info.log, Other such as warn/error/fatal Level logs are output to error.log, It's good to separate the output in this way . We follow the following log42j.xml Configuration of , Such output can be realized .

<?xml version="1.0" encoding="UTF-8"?>

<configuration debug="off" monitorInterval="1800">
    <Properties>
        <Property name="log-path">d://logs</Property>
    </Properties>

    <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %class{36}.%M()/%L  - %msg%xEx%n"/>
        </Console>

        <File name="app_debug" fileName="${log-path}/app/debug.log" append="false">
            <Filters>
                <ThresholdFilter level="info" onMatch="DENY" onMismatch="NEUTRAL"/>
                <ThresholdFilter level="debug" onMatch="ACCEPT" onMismatch="NEUTRAL"/>
            </Filters>
            <PatternLayout pattern="%d{yyyy.MM.dd HH:mm:ss z} %-5level %class{36}.%M()/%L - %msg%xEx%n"/>
        </File>
        <File name="app_info" fileName="${log-path}/app/info.log" append="false">
            <Filters>
                <ThresholdFilter level="warn" onMatch="DENY" onMismatch="NEUTRAL"/>
                <ThresholdFilter level="info" onMatch="ACCEPT" onMismatch="DENY"/>
            </Filters>

            <PatternLayout pattern="%d{yyyy.MM.dd HH:mm:ss z} %-5level %class{36}.%M()/%L - %msg%xEx%n"/>
        </File>
        <File name="app_error" fileName="${log-path}/app/error.log" append="false">
            <Filters>
                <ThresholdFilter level="warn" onMatch="ACCEPT" onMismatch="DENY"/>
            </Filters>
            <PatternLayout pattern="%d{yyyy.MM.dd HH:mm:ss z} %-5level %class{36}.%M()/%L - %msg%xEx%n"/>
        </File>
    </Appenders>
    <Loggers>
        <Logger name="com.test.app" level="trace" additivity="false">
            <appender-ref ref="Console"/>
            <appender-ref ref="app_debug"/>
            <appender-ref ref="app_info"/>
            <appender-ref ref="app_error"/>
        </Logger>
    </Loggers>
</configuration>

The main thing is to understand ThresholdFilter Of onMatch/onMismatch The three option values of ：ACCEPT/DENY/NEUTRAL, Actually , According to the literal meaning , It's easy to understand .
It is important to , If there are more than one ThresholdFilter, that Filters Is a must , At the same time Filters in , First, filter the non-conforming log level , Put what you don't need first DENY fall , And then again ACCEPT Required log level , This order cannot be reversed .