如何抑制Log4j2 Socket Appender异常或按环境变量禁用它？

我使用的Log4j2 Socket Appender配置如下：

<Socket name="Socket" host="localhost" port="24224" reconnectionDelay="5000" immediateFlush="true">
    <PatternLayout>
        <charset>UTF-8</charset>
        <alwaysWriteExceptions>false</alwaysWriteExceptions>
        <Pattern>
            {"key": "value"}
        </Pattern>
    </PatternLayout>
</Socket>

但在未安装对应日志服务器的环境中，日志会抛出如下异常：

2023-08-04 13:56:58,708 main ERROR TcpSocketManager (TCP:localhost:24224) caught exception and will continue: java.io.IOException: Unable to create socket for localhost at port 24224 using ip addresses and ports , 0:0:0:0:0:0:0:1:24224
    at org.apache.logging.log4j.core.net.TcpSocketManager$TcpSocketManagerFactory.createSocket(TcpSocketManager.java:506)
    at org.apache.logging.log4j.core.net.TcpSocketManager$TcpSocketManagerFactory.createManager(TcpSocketManager.java:475)
    at org.apache.logging.log4j.core.net.TcpSocketManager$TcpSocketManagerFactory.createManager(TcpSocketManager.java:456)
    at org.apache.logging.log4j.core.appender.AbstractManager.getManager(AbstractManager.java:113)
    at org.apache.logging.log4j.core.appender.OutputStreamManager.getManager(OutputStreamManager.java:114)
    at org.apache.logging.log4j.core.net.TcpSocketManager.getSocketManager(TcpSocketManager.java:202)
    at org.apache.logging.log4j.core.appender.SocketAppender.createSocketManager(SocketAppender.java:443)
    at org.apache.logging.log4j.core.appender.SocketAppender$Builder.build(SocketAppender.java:221)
    at org.apache.logging.log4j.core.appender.SocketAppender$Builder.build(SocketAppender.java:195)
    at org.apache.logging.log4j.core.config.plugins.util.PluginBuilder.build(PluginBuilder.java:122)
    at org.apache.logging.log4j.core.config.AbstractConfiguration.createPluginObject(AbstractConfiguration.java:1002)
    at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:942)
    at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:934)
    at org.apache.logging.log4j.core.config.AbstractConfiguration.doConfigure(AbstractConfiguration.java:552)
    at org.apache.logging.log4j.core.config.AbstractConfiguration.initialize(AbstractConfiguration.java:241)
    at org.apache.logging.log4j.core.config.AbstractConfiguration.start(AbstractConfiguration.java:288)
    at org.apache.logging.log4j.core.LoggerContext.setConfiguration(LoggerContext.java:579)
    at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:651)
    at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:668)
    at org.apache.logging.log4j.core.LoggerContext.start(LoggerContext.java:253)
    at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:153)
    at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:45)
    at org.apache.logging.log4j.LogManager.getContext(LogManager.java:194)
    at org.apache.logging.log4j.LogManager.getLogger(LogManager.java:581)
    at p_outils.httptest_0_1.httpTest.<clinit>(httpTest.java:59)

想了解如何抑制这些错误，或者通过环境变量让Log4j2不实例化该Appender。

1. 抑制连接错误日志 #

要屏蔽TcpSocketManager输出的连接失败错误，直接在Log4j2配置的<Loggers>节点下添加一条规则，关闭该类的日志输出：

<Loggers>
    <!-- 保留原有Logger配置 -->
    <Logger name="org.apache.logging.log4j.core.net.TcpSocketManager" level="OFF"/>
</Loggers>

这条配置会完全阻止该类产生任何日志，包括你遇到的连接异常信息。如果需要保留其他级别日志，也可以将level设为FATAL，不过这类连接错误属于ERROR级别，设为OFF是最彻底的解决方式。

2. 通过环境变量控制Appender实例化 #

利用Log4j2的条件配置特性，结合环境变量来决定是否加载Socket Appender。修改你的Appender配置，添加条件判断：

<Appenders>
    <!-- 保留原有Appender配置 -->
    <Socket name="Socket" host="localhost" port="24224" reconnectionDelay="5000" immediateFlush="true">
        <PatternLayout>
            <charset>UTF-8</charset>
            <alwaysWriteExceptions>false</alwaysWriteExceptions>
            <Pattern>{"key": "value"}</Pattern>
        </PatternLayout>
        <Condition>
            <Equals target="${env:ENABLE_SOCKET_APPENDER}" value="true"/>
        </Condition>
    </Socket>
</Appenders>

• 在需要启用Socket Appender的环境中，设置环境变量：
  • Linux/macOS终端：export ENABLE_SOCKET_APPENDER=true
  • Windows命令行：set ENABLE_SOCKET_APPENDER=true
  • Windows PowerShell：$env:ENABLE_SOCKET_APPENDER = "true"
• 在未设置该变量或变量值不为true的环境中，Log4j2会跳过这个Appender的初始化，不会尝试建立Socket连接，也就不会产生错误日志。

如果需要默认启用、仅在特定环境关闭，也可以反转条件逻辑：

<Condition>
    <Not>
        <Equals target="${env:DISABLE_SOCKET_APPENDER}" value="true"/>
    </Not>
</Condition>

此时只要设置DISABLE_SOCKET_APPENDER=true，就会关闭该Appender。

内容的提问来源于stack exchange，提问作者Ibrahim Mezouar