Logback中spring.application.name变量无法解析显示var_IS_UNDEFINED的解决方法

我正在修复logback-spring.xml文件的一个Bug，该配置用于将日志发送至Logstash服务器，尝试传递applicationName等额外参数，但Logback无法解析spring.application.name变量，仅显示varName_IS_UNDEFINED。以下是我的logback-spring.xml代码：

<property name="CONSOLE_LOG_PATTERN"
          value="%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){faint} %clr(${LOG_LEVEL_PATTERN:-%5p}) %clr(${PID:- }){magenta} %clr(---){faint} %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} %clr(:){faint} %m | %mdc %n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}"/>
<property resource="application.yml"/>
    <include resource="org/springframework/boot/logging/logback/base.xml"/>
    <property resource="application.yml"/>

    <springProfile name="test,prod">
        <springProperty scope="system" name="swarmTaskName" source="swarm.task.name" defaultValue="N.A."/>
        <springProperty scope="system" name="logstashEndpoint" source="logstash.endpoint"
                        defaultValue="localhost:1234"/>
        <appender name="LOGSTASH" class="net.logstash.logback.appender.LogstashTcpSocketAppender">
            <destination>${logstashEndpoint}</destination>
            <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
                <providers>
                    <mdc/> <!-- MDC variables on the Thread will be written as JSON fields -->
                    <context/> <!--Outputs entries from logback's context -->
                    <version/> <!-- Logstash json format version, the @version field in the output -->
                    <logLevel/>
                    <loggerName/>
                    <pattern>
                        <pattern> <!-- we can add some custom fields to be sent with all the log entries. -->
                            {      <!--make filtering easier in Logstash -->
                            "appName": "${spring.application.name}",<!--or searching with Kibana -->
                            }
                        </pattern>
                    </pattern>
                    <threadName/>
                    <message/>
                    <logstashMarkers/> <!-- Useful so we can add extra information for specific log lines as Markers -->
                    <arguments/> <!--or through StructuredArguments -->
                    <stackTrace/>
                </providers>
            </encoder>
        </appender>
    </springProfile>

<!-- Root logger writes to file, console and sends the data to Logstash -->
<root level="INFO">
    <appender-ref ref="FILE"/>
    <appender-ref ref="CONSOLE"/>
    <springProfile name="test,prod">
        <appender-ref ref="LOGSTASH"/>
    </springProfile>
</root>

<logger name="org.springframework.cache" level="DEBUG"/>

解决方法 #

核心原因 #

直接在Logstash的pattern中使用${spring.application.name}无效，因为Logback原生属性解析无法直接读取Spring环境变量，必须通过Spring Boot提供的<springProperty>标签将Spring变量导入到Logback上下文。

具体修复步骤 #

1. 导入spring.application.name到Logback上下文
   在<springProfile name="test,prod">代码块内，新增一个<springProperty>标签，将Spring的spring.application.name映射为Logback可识别的属性：

<springProperty scope="context" name="appName" source="spring.application.name" defaultValue="unknown-app"/>

• scope="context"：让该属性在Logback全局上下文可用
• defaultValue：避免变量未定义时出现异常

2. 修改Logstash编码器中的属性引用
   把原代码里的"appName": "${spring.application.name}"替换为上面定义的Logback属性：

"appName": "${appName}",

3. 清理重复配置
   配置中重复加载了application.yml，保留一个即可，避免潜在加载冲突。

修复后的完整配置片段 #

<property name="CONSOLE_LOG_PATTERN"
          value="%clr(%d{yyyy-MM-dd HH:mm:ss.SSS}){faint} %clr(${LOG_LEVEL_PATTERN:-%5p}) %clr(${PID:- }){magenta} %clr(---){faint} %clr([%15.15t]){faint} %clr(%-40.40logger{39}){cyan} %clr(:){faint} %m | %mdc %n${LOG_EXCEPTION_CONVERSION_WORD:-%wEx}"/>
<property resource="application.yml"/>
<include resource="org/springframework/boot/logging/logback/base.xml"/>

<springProfile name="test,prod">
    <springProperty scope="system" name="swarmTaskName" source="swarm.task.name" defaultValue="N.A."/>
    <springProperty scope="system" name="logstashEndpoint" source="logstash.endpoint"
                    defaultValue="localhost:1234"/>
    <!-- 新增导入spring.application.name -->
    <springProperty scope="context" name="appName" source="spring.application.name" defaultValue="unknown-app"/>
    <appender name="LOGSTASH" class="net.logstash.logback.appender.LogstashTcpSocketAppender">
        <destination>${logstashEndpoint}</destination>
        <encoder class="net.logstash.logback.encoder.LoggingEventCompositeJsonEncoder">
            <providers>
                <mdc/> <!-- MDC variables on the Thread will be written as JSON fields -->
                <context/> <!--Outputs entries from logback's context -->
                <version/> <!-- Logstash json format version, the @version field in the output -->
                <logLevel/>
                <loggerName/>
                <pattern>
                    <pattern> <!-- we can add some custom fields to be sent with all the log entries. -->
                        {      <!--make filtering easier in Logstash -->
                        "appName": "${appName}",<!--or searching with Kibana -->
                        }
                    </pattern>
                </pattern>
                <threadName/>
                <message/>
                <logstashMarkers/> <!-- Useful so we can add extra information for specific log lines as Markers -->
                <arguments/> <!--or through StructuredArguments -->
                <stackTrace/>
            </providers>
        </encoder>
    </appender>
</springProfile>

<!-- Root logger writes to file, console and sends the data to Logstash -->
<root level="INFO">
    <appender-ref ref="FILE"/>
    <appender-ref ref="CONSOLE"/>
    <springProfile name="test,prod">
        <appender-ref ref="LOGSTASH"/>
    </springProfile>
</root>

<logger name="org.springframework.cache" level="DEBUG"/>

额外注意事项 #

• <springProperty>是Spring Boot扩展标签，仅在logback-spring.xml中生效，不能用于普通的logback.xml
• 确保Spring Boot版本在1.3及以上（该标签从1.3版本开始支持）

内容的提问来源于stack exchange，提问作者notforward