How to use Log4j 2 with Spring Boot2.X

Adding Log4j2
All the Spring Boot starters depend on spring-boot-starter-logging, which uses Logback by default. For using Log4j2, you need to exclude spring-boot-starter-logging and add spring-boot-starter-log4j2 dependency.

如果引用的log4j2没有起作用，因为pom文件中没有正确的排除掉Logback，请检查pom文件。

Open pom.xml file and add the following snippet to the “dependencies” section

---

<!-- Exclude Spring Boot's Default Logging -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter</artifactId>
    <exclusions>
        <exclusion>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-logging</artifactId>
        </exclusion>
    </exclusions>
</dependency>

<!-- Add Log4j2 Dependency -->
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-log4j2</artifactId>
</dependency>

Configuring Log4j2

Spring Boot automatically configures Log4j if it finds a file named log4j2.xml or log4j2.json or log4j2.yaml in the classpath.

In this article, we’ll configure log4j 2 using XML. Create a new file log4j2.xml inside src/main/resources directory, and add the following configuration to it -

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN" monitorInterval="30">
    <Properties>
        <Property name="LOG_PATTERN">
            %d{yyyy-MM-dd HH:mm:ss.SSS} %5p ${hostName} --- [%15.15t] %-40.40c{1.} : %m%n%ex
        </Property>
    </Properties>
    <Appenders>
        <Console name="ConsoleAppender" target="SYSTEM_OUT" follow="true">
            <PatternLayout pattern="${LOG_PATTERN}"/>
        </Console>
    </Appenders>
    <Loggers>
        <Logger name="com.example.log4j2demo" level="debug" additivity="false">
            <AppenderRef ref="ConsoleAppender" />
        </Logger>

        <Root level="info">
            <AppenderRef ref="ConsoleAppender" />
        </Root>
    </Loggers>
</Configuration>

The above configuration defines a simple ConsoleAppender and declares two loggers - an application specific logger and the the root logger.

Using Log4j 2 in the app

Let’s add some logging code to check our logger configuration. Open Log4j2DemoApplication.java and replace it with the following code -

package com.example.log4j2demo;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.springframework.boot.ApplicationArguments;
import org.springframework.boot.ApplicationRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class Log4j2DemoApplication implements ApplicationRunner {
    private static final Logger logger = LogManager.getLogger(Log4j2DemoApplication.class);

    public static void main(String[] args) {
        SpringApplication.run(Log4j2DemoApplication.class, args);
    }

    @Override
    public void run(ApplicationArguments applicationArguments) throws Exception {
        logger.debug("Debugging log");
        logger.info("Info log");
        logger.warn("Hey, This is a warning!");
        logger.error("Oops! We have an Error. OK");
        logger.fatal("Damn! Fatal error. Please fix me.");
    }
}

We have added 5 simple logs of different log levels. When you run the app, it logs everything on the console.

Adding a Rolling File Appender

If you want your logs to be written to a file, then you can use a RollingFile appender. RollingFile appender creates a new file whenever the log file reaches a certain threshold specified by the triggering policy.

It stores the old log files with names matching the pattern specified by the filePattern parameter -

<!-- Rolling File Appender -->
<RollingFile name="FileAppender" fileName="logs/log4j2-demo.log" 
             filePattern="logs/log4j2-demo-%d{yyyy-MM-dd}-%i.log">
    <PatternLayout>
        <Pattern>${LOG_PATTERN}</Pattern>
    </PatternLayout>
    <Policies>
        <SizeBasedTriggeringPolicy size="10MB" />
    </Policies>
    <DefaultRolloverStrategy max="10"/>
</RollingFile>

log4j2.xml文件输入mybatis的sql日志

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="INFO">
    <Appenders>
        <Console name="SYSOUT" target="SYSTEM_OUT">
            <PatternLayout patter="%d [%t] %-5p [%c] - %m%n  "/>
        </Console>

        <!-- name可以自定义，作用是在Loggers中AppenderRef中使用 -->
        <!-- fileName定义输出文件名称（当前文件） -->
        <!-- filePattern定义输出文件名称（文件满足条件后自动截断，生成历史文件） -->
        <RollingFile name="DEBUG_ROLLING_FILE"
                     fileName="~/logs/xxx/logs.logs"
                     filePattern="~/logs/xxx/logs/%d{yyyy-MM-dd}-debugs.log">
            <PatternLayout>
                <Pattern>%d [%t] %-5p [%c] - %m%n  </Pattern>
            </PatternLayout>

            <!-- 文件截断的条件，具体参考文档 -->
            <Policies>
                <TimeBasedTriggeringPolicy interval="24"/>
                <SizeBasedTriggeringPolicy size="250 MB"/>
            </Policies>
        </RollingFile>

        <!-- 同一来源的Appender可以定义多个 -->
        <RollingFile name="ERROR_ROLLING_FILE"
                     fileName="~/logs/xxx/logs/error-logs.logs"
                     filePattern="~/logs/xxx/logs/%d{yyyy-MM-dd}-error.log">
            <!-- 可以通过该参数来设置获取日志的权限 -->
            <ThresholdFilter level="error" onMatch="ACCEPT" onMismatch="DENY"/>
            <PatternLayout>
                <Pattern>%d [%t] %-5p [%c] - %m%n  </Pattern>
            </PatternLayout>
            <Policies>
                <TimeBasedTriggeringPolicy interval="24"/>
                <SizeBasedTriggeringPolicy size="250 MB"/>
            </Policies>
        </RollingFile>
    </Appenders>

    <Loggers>
        <Root level="debug">
            <AppenderRef ref="SYSOUT"/>
            <AppenderRef ref="DEBUG_ROLLING_FILE"/>
            <AppenderRef ref="ERROR_ROLLING_FILE"/>
        </Root>
        <logger name="org.apache.http" level="ERROR" />
        <!-- Spring -->
        <logger name="org.springframework" level="ERROR" />
        <!-- mybatis loggers -->
        <logger name="com.ibatis" level="INFO" />
        <logger name="com.ibatis.common.jdbc.SimpleDataSource" level="DEBUG" />
        <logger name="com.ibatis.common.jdbc.ScriptRunner" level="DEBUG" />
        <logger name="com.ibatis.sqlmap.engine.impl.SqlMapClientDelegate" level="DEBUG" />
        <!-- sql loggers -->
        <logger name="java.sql.Connection" level="DEBUG"  additivity="true"/>
        <logger name="java.sql.Statement" level="DEBUG" additivity="true" />
        <logger name="java.sql.PreparedStatement" level="=debug,stdout"  additivity="true"/>
        <logger name="java.sql.ResultSet" level="DEBUG"  additivity="true"/>
    </Loggers>
</Configuration>

更多详细操作见下面链接
https://www.callicoder.com/spring-boot-log4j-2-example/