Log4j2

https://logging.apache.org/log4j/2.x/
Apache Log4j 2 is an upgrade to Log4j that provides significant improvements over its predecessor, Log4j 1.x, and provides many of the improvements available in Logback while fixing some inherent problems in Logback's architecture.
插件模式
Logger logger = LoggerFactory.getLogger(Demo2.class);

执行原理
SLF4J 门面调用 Log4j 2 门面，Log4j 2 门面调用 Log4j 2 实现
可单独使用 log4j2 Logger logger = LogManager.getLogger(Demo2.class); 但建议用门面方式
虽然 Log4j 2 也是日志门面，但它的日志实现功能也很强大，一般用作日志实现，门面用 SLF4J

log4j-api: 日志门面
log4j-core: 日志实现

<dependencies>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-api</artifactId>
    <version>2.18.0</version>
  </dependency>
  <dependency>
    <groupId>org.apache.logging.log4j</groupId>
    <artifactId>log4j-core</artifactId>
    <version>2.18.0</version>
  </dependency>
</dependencies>

2. 核心概念

LoggerContext: log的上下文环境
Configuration: 每一个 LoggerContext 都有一个有效的 Configuration
Logger
LoggerConfig 记录日志

3. log4j2.xml

Log4j 2 格式参考的是 logback, 但标签名首字母都是大写字母开头

3.1 Layouts

https://logging.apache.org/log4j/2.x/manual/layouts.html

<?xml version="1.0" encoding="UTF-8" ?>
<Configuration>
    <Properties>
        <property name="fileDir">/Users/xingxing.xiao/Downloads</property>
    </Properties>
    <Appenders>
        <Console name="consoleAppender" target="SYSTEM_ERR">
            <PatternLayout pattern="%-5level %d{yyyy-MM-dd HH:mm:ss.SSS} %m%n" />
        </Console>
        <File name="fileAppender" fileName="${fileDir}/log4j2.log">
            <PatternLayout pattern="%-5level %d{yyyy-MM-dd HH:mm:ss.SSS} %m%n" />
        </File>
        <!-- $${date:yyyy-MM-dd} 指定文件夹 -->
        <RollingFile name="rollingFileAppender" fileName="${fileDir}/log.log" filePattern="${fileDir}/$${date:yyyy-MM-dd}/log-%d{yyyy-MM-dd-HH-mm}-%i.log">
            <PatternLayout pattern="%-5level %d{yyyy-MM-dd HH:mm:ss.SSS} %m%n" />
            <Policies>
                <SizeBasedTriggeringPolicy size="10KB" />
            </Policies>
            <DefaultRolloverStrategy max="30" /> <!-- 同一目录下文件数量限制 -->
        </RollingFile>
    </Appenders>
    <Loggers>
        <Root level="debug">
            <AppenderRef ref="consoleAppender" />
            <AppenderRef ref="fileAppender" />
            <AppenderRef ref="rollingFileAppender" />
        </Root>
    </Loggers>
</Configuration>