MapStruct使用映射对象作为入参生成代码抛UnsupportedOperationException问题

• 你用到的PredictionRequest和Destination都是Protobuf生成的类，MapStruct内置了Protobuf类型的专属映射逻辑，默认会自动同步两个Protobuf对象的unknownFields以及allFields集合
• Protobuf生成的getAllFields()方法返回的是不可变Map，不支持putAll操作，所以执行时直接抛出UnsupportedOperationException
• 当传入非映射生成的对象时，MapStruct识别不到对应Protobuf字段的匹配逻辑，就不会生成这段同步代码，因此不会报错

方案1：针对当前Mapper方法单独忽略系统字段 #

直接在PredictionMapper的map方法上追加两个忽略字段的注解即可，改完重新编译后生成的代码就只会保留你需要的setDestination逻辑：

@Mapping(source = "request.idOrder", target = "referenceId")
@Mapping(source = "request.orderDateUTC", target = "expeditionDate")
@Mapping(source = "protoDestination", target = "destination")
@Mapping(source = "notDeliverableLocalDates", target = "notDeliverableLocalDatesList")
// 追加以下两行，忽略Protobuf自动同步的系统字段
@Mapping(target = "unknownFields", ignore = true)
@Mapping(target = "allFields", ignore = true)
PredictionRequest map(Request request, List<String> notDeliverableLocalDates, Destination protoDestination);

方案2：全局关闭Protobuf字段自动同步逻辑 #

如果项目里多处都有同类问题，可以全局关闭MapStruct的Protobuf自动字段同步配置，无需每个Mapper单独配置，在maven-compiler-plugin编译参数中添加以下配置即可：

<plugin>
  <groupId>org.apache.maven.plugins</groupId>
  <artifactId>maven-compiler-plugin</artifactId>
  <version>3.8.1</version>
  <configuration>
    <source>11</source>
    <target>11</target>
    <annotationProcessorPaths>
      <path>
        <groupId>org.mapstruct</groupId>
        <artifactId>mapstruct-processor</artifactId>
        <version>你的MapStruct版本号</version>
      </path>
    </annotationProcessorPaths>
    <compilerArgs>
      <!-- 全局关闭Protobuf自动同步字段逻辑 -->
      <arg>-Amapstruct.protobuf.auto-configure-fields=false</arg>
    </compilerArgs>
  </configuration>
</plugin>

方案3：严格控制只映射指定字段 #

如果需要完全避免MapStruct自动生成未明确声明的映射逻辑，可以给方法加上@BeanMapping(ignoreByDefault = true)，只有你标注的@Mapping规则才会生效：

@BeanMapping(ignoreByDefault = true)
@Mapping(source = "request.idOrder", target = "referenceId")
@Mapping(source = "request.orderDateUTC", target = "expeditionDate")
@Mapping(source = "protoDestination", target = "destination")
@Mapping(source = "notDeliverableLocalDates", target = "notDeliverableLocalDatesList")
PredictionRequest map(Request request, List<String> notDeliverableLocalDates, Destination protoDestination);

内容的提问来源于stack exchange，提问作者Roberto Rodriguez