• logback-spring.xml如下：

由于当前1.2.3版本的logback中有这个bug，据说在1.3版本修复(https://stackoverrun.com/cn/q/13074956)，所以尽量使用gz压缩，将totalSizeCap设置小于磁盘可用空间的一半

<?xml version="1.0" encoding="UTF-8"?>
<!-- 每隔30s自动检查配置文件是否改变，改变的话重新加载配置 -->
<configuration scan="true" scanPeriod="5 seconds">
    <!-- 设置日志输出根目录为用户主目录下的logs目录，没有会新建 -->
    <property name="log.dir" value="${user.home}/logs"/>
    <!-- 使用springboot默认配置，其中有CONSOLE_LOG_PATTERN和FILE_LOG_PATTERN的配置 -->
    <include resource="org/springframework/boot/logging/logback/defaults.xml"/>
    <!-- 增加一个jvm关闭时的hook，释放logback的线程和资源，jvm关闭时延迟3s关闭logback context -->
    <shutdownHook class="ch.qos.logback.core.hook.DelayingShutdownHook">
        <delay>3000</delay>
    </shutdownHook>

    <appender name="consoleAppender" class="ch.qos.logback.core.ConsoleAppender">
        <encoder>
            <charset>UTF-8</charset>
            <pattern>${CONSOLE_LOG_PATTERN}</pattern>
        </encoder>
    </appender>

    <appender name="infoAppender" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <!-- info日志中记录日记级别大于等于info级别的 -->
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>INFO</level>
        </filter>
        <file>${log.dir}/info.log</file>
        <encoder>
            <charset>UTF-8</charset>
            <pattern>${FILE_LOG_PATTERN}</pattern>
        </encoder>
        <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
            <fileNamePattern>${log.dir}/info.%d{yyyy-MM-dd}.%i.log.gz</fileNamePattern>
            <maxHistory>30</maxHistory>
            <maxFileSize>5GB</maxFileSize>
            <totalSizeCap>30GB</totalSizeCap>
            <cleanHistoryOnStart>true</cleanHistoryOnStart>
        </rollingPolicy>
    </appender>

    <appender name="async_infoAppender" class="ch.qos.logback.classic.AsyncAppender">
        <appender-ref ref="infoAppender"/>
        <!-- 队列大小 -->
        <queueSize>2048</queueSize>
        <!-- 队列满了也不丢弃日志，会阻塞住。如果设置了neverBlock为true(默认false)，队列满会丢弃日志，不阻塞 -->
        <discardingThreshold>0</discardingThreshold>
        <!-- logback context关闭后，处理队列中未写入日志的最大时间，到时间还没有写入磁盘的就丢弃 -->
        <maxFlushTime>2500</maxFlushTime>
    </appender>

    <appender name="errorAppender" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>WARN</level>
        </filter>
        <file>${log.dir}/error.log</file>
        <encoder>
            <charset>UTF-8</charset>
            <pattern>${FILE_LOG_PATTERN}</pattern>
        </encoder>
        <rollingPolicy class="ch.qos.logback.core.rolling.SizeAndTimeBasedRollingPolicy">
            <fileNamePattern>${log.dir}/error.%d{yyyy-MM-dd}.%i.log.gz</fileNamePattern>
            <maxHistory>30</maxHistory>
            <maxFileSize>5GB</maxFileSize>
            <totalSizeCap>30GB</totalSizeCap>
            <cleanHistoryOnStart>true</cleanHistoryOnStart>
        </rollingPolicy>
    </appender>

    <appender name="async_errorAppender" class="ch.qos.logback.classic.AsyncAppender">
        <appender-ref ref="errorAppender"/>
        <discardingThreshold>0</discardingThreshold>
        <maxFlushTime>2500</maxFlushTime>
        <queueSize>2048</queueSize>
    </appender>

    <root level="INFO">
        <!-- 只在spring.profiles.active为local时向控制台打印 -->
        <springProfile name="local">
            <appender-ref ref="consoleAppender"/>
        </springProfile>
        <appender-ref ref="async_infoAppender"/>
        <appender-ref ref="async_errorAppender"/>
    </root>
</configuration>

• 使用异步appender，运行机制主要看AsyncAppenderBase，默认配置如下

• 可以看出为什么官方文档说：when the blocking queue has 20% capacity remaining, it will drop events of level TRACE, DEBUG and INFO, keeping only events of level WARN and ERROR；discardingThreshold 未指定时用的是队列大小/5

• neverBlock的作用，异步appender中的queue在上图可以看到是ArrayBlockingQueue，neverBlock为true时，调用的offer方法，队列满直接丢弃不阻塞

• 使用logback严格控制日志的大小可以如下配置和

   <appender name="infoAppender" class="ch.qos.logback.core.rolling.RollingFileAppender">
        <file>${LOG_PATH}/application.log</file>
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>INFO</level>
        </filter>
        <encoder>
            <charset>UTF-8</charset>
            <pattern>${FILE_LOG_PATTERN}</pattern>
        </encoder>
        <rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
            <fileNamePattern>${LOG_PATH}/application.%i.log.gz</fileNamePattern>
            <minIndex>1</minIndex>
            <maxIndex>10</maxIndex>
        </rollingPolicy>
        <triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
            <maxFileSize>1GB</maxFileSize>
        </triggeringPolicy>
    </appender>