如何在Log4j2的JsonTemplateLayout中屏蔽默认的contextMap输出？

问题场景 #

需要通过Log4j2的JSON格式输出日志，包含两个自定义字段：

• 固定值字段myApp
• 从ThreadContext（MDC）读取的customKey1

当前使用JsonTemplateLayout时，customKey1会重复输出：一次在JSON根层级（自定义配置的），一次在默认生成的contextMap节点下，需要屏蔽contextMap的输出。

当前配置 #

<?xml version="1.0" encoding="UTF-8"?>
<Configuration>
   <Appenders>
      <Console name="console" target="SYSTEM_OUT">
          <JsonTemplateLayout eventTemplateUri="classpath:JsonLayout.json">
              <EventTemplateAdditionalField key="myApp" value="MYAPPLICATION" />
             
              <EventTemplateAdditionalField key="customKey1"
                              format="JSON"
                              value='{"$resolver": "mdc", "key": "customKey1"}'/>  
                        
          </JsonTemplateLayout>
      </Console>
   </Appenders>
    <Loggers>
        <Root level="info" additivity="false">
            <AppenderRef ref="console" />
        </Root>
    </Loggers>
</Configuration>

当前输出（含重复字段） #

{
    "instant": {
        "epochSecond": 1712749405,
        "nanoOfSecond": 600717045
    },
    "thread": "t1",
    "level": "ERROR",
    "loggerName": "com.example.myapp.Log",
    "message": "custom log msg",
    "contextMap": {
        "customKey1": "customValue"
    },
    "myApp": "MYAPPLICATION",
    "customKey1": "customValue"
}

使用版本 #

• org.springframework.boot:spring-boot-starter-log4j2:2.7.18
• org.apache.logging.log4j:log4j-layout-template-json:2.21.1

解决方法 #

JsonTemplateLayout的输出结构完全由指定的JSON模板文件控制，默认模板会包含contextMap字段，只需修改模板移除该字段即可。

步骤 #

1. 找到项目中classpath:JsonLayout.json模板文件（如果不存在，可从Log4j2的json-layout模块中复制默认模板进行修改）
2. 移除模板中定义contextMap的行（通常是类似"contextMap": {"$resolver": "mdc"}的配置）
3. 保存修改后的模板，重启应用即可生效

修改后的示例模板（JsonLayout.json） #

{
  "instant": {
    "$resolver": "timestamp",
    "pattern": {
      "$resolver": "pattern",
      "pattern": "{epochSecond} {nanoOfSecond}",
      "format": "array"
    }
  },
  "thread": {
    "$resolver": "thread",
    "field": "name"
  },
  "level": {
    "$resolver": "level",
    "field": "name"
  },
  "loggerName": {
    "$resolver": "logger",
    "field": "name"
  },
  "message": {
    "$resolver": "message",
    "stringified": true
  },
  // 移除下面这行即可屏蔽contextMap输出
  // "contextMap": {"$resolver": "mdc"},
  "myApp": {
    "$resolver": "eventTemplateAdditionalField",
    "key": "myApp"
  },
  "customKey1": {
    "$resolver": "eventTemplateAdditionalField",
    "key": "customKey1"
  }
}

修改后的输出（无contextMap） #

{
    "instant": {
        "epochSecond": 1712749405,
        "nanoOfSecond": 600717045
    },
    "thread": "t1",
    "level": "ERROR",
    "loggerName": "com.example.myapp.Log",
    "message": "custom log msg",
    "myApp": "MYAPPLICATION",
    "customKey1": "customValue"
}

内容的提问来源于stack exchange，提问作者Sumi Silky