java - 使用 StdDeserializer Jackson 2.5 处理多态

Question

我有三个类，它们继承自父类(super class)(SensorData)

@JsonDeserialize(using = SensorDataDeserializer.class)
public abstract class SensorData {

}

public class HumiditySensorData extends SensorData {
}

public class LuminositySensorData extends SensorData {
}

public class TemperatureSensorData extends SensorData {
}

我想根据参数将 json 输入转换为此类之一。我正在尝试使用 Jackson StdDeserializer 并创建一个自定义反序列化器

@Component
public class SensorDataDeserializer extends StdDeserializer<SensorData> {

    private static final long serialVersionUID = 3625068688939160875L;

    @Autowired
    private SensorManager sensorManager;

    private static final String discriminator = "name";

    public SensorDataDeserializer() {
        super(SensorData.class);
        SpringBeanProvider.getInstance().autowireBean(this);
    }

    @Override
    public SensorData deserialize(JsonParser parser,
            DeserializationContext context) throws IOException,
            JsonProcessingException {
        ObjectMapper mapper = (ObjectMapper) parser.getCodec();
        ObjectNode root = (ObjectNode) mapper.readTree(parser);
        ObjectNode sensor = (ObjectNode) root.get("data");
        String type = root.get(discriminator).asText();
        Class<? extends SensorData> clazz = this.sensorManager
                .getCachedSensorsMap().get(type).sensorDataClass();
        if (clazz == null) {
            // TODO should throw exception
            return null;
        }
        return mapper.readValue(sensor.traverse(), clazz);
    }
}

我的问题是，当我确定映射具体类的正确类型时，映射器再次调用自定义 StdDeserializer。所以我需要一个方法当我有正确的类型时打破循环。堆栈跟踪是下一个

java.lang.NullPointerException
at com.hp.psiot.mapping.SensorDataDeserializer.deserialize(SensorDataDeserializer.java:38)
at com.hp.psiot.mapping.SensorDataDeserializer.deserialize(SensorDataDeserializer.java:1)
at com.fasterxml.jackson.databind.ObjectMapper._readValue(ObjectMapper.java:3532)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:1868)
at com.hp.psiot.mapping.SensorDataDeserializer.deserialize(SensorDataDeserializer.java:47)
at com.hp.psiot.mapping.SensorDataDeserializer.deserialize(SensorDataDeserializer.java:1)
at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:3560)
at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:2660)
at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.readJavaType(AbstractJackson2HttpMessageConverter.java:205)
at org.springframework.http.converter.json.AbstractJackson2HttpMessageConverter.read(AbstractJackson2HttpMessageConverter.java:200)
at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver.readWithMessageConverters    (AbstractMessageConverterMethodArgumentResolver.java:138)
at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.readWithMessageConverters(RequestResponseBodyMethodProcessor.java:184)
at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.resolveArgument(RequestResponseBodyMethodProcessor.java:105)

输入示例

{
    "name":"temperature",
    "data": {
        "value":20
    }
}

我只包含堆栈跟踪来显示映射器正在再次调用解串器。 nullPointerException 的原因是当第二次调用 ObjectMapper 时，输入是

"value":20

因此，抛出异常是因为我们没有确定类型的信息，并且它不会检查输入是否正确

如果可能的话，我想避免使用 JsonSubTypes 和 JsonTypeInfo。

提前致谢!

<小时/>

部分解决方案

在我的例子中，SensorData 被包装在其他类 (ServiceData) 中

class ServiceData {
    @JsonDeserialize(using = SensorDataDeserializer.class)
    List<SensorData> sensors;

}

因此，我摆脱了 SensorData 类中的 JsonDeserializer 并将其放入字段中以避免循环。该解决方案不是最好的，但就我而言它对我有帮助。但如果该类没有包装在另一个类中，我们仍然会遇到同样的问题。

请注意，如果您有一个 Collection 并且使用 JsonDeserialize 该字段进行注释，则必须处理所有集合。这是修改就我而言

 @Component
 public class SensorDataDeserializer extends StdDeserializer<List<SensorData>> {

      private static final long serialVersionUID = 3625068688939160875L;

      @Autowired
      private SensorManager sensorManager;

      private static final String discriminator = "name";

      public SensorDataDeserializer() {
           super(SensorData.class);
            SpringBeanProvider.getInstance().autowireBean(this);
      }

      @Override
      public List<SensorData> deserialize(JsonParser parser,
                DeserializationContext context) throws IOException,
                JsonProcessingException {
           try {
                ObjectMapper mapper = (ObjectMapper) parser.getCodec();
                ArrayNode root = (ArrayNode) mapper.readTree(parser);
                int size = root.size();
                List<SensorData> sensors = new ArrayList<SensorData>();
                for (int i = 0; i < size; ++i) {
                     ObjectNode sensorHead = (ObjectNode) root.get(i);
                     ObjectNode sensorData = (ObjectNode) sensorHead.get("data");
                     String tag = sensorHead.get(discriminator).asText();
                     Class<? extends SensorData> clazz = this.sensorManager
                               .getCachedSensorsMap().get(tag).sensorDataClass();
                     if (clazz == null) {
                          throw new InvalidJson("unbound sensor");
                     }
                     SensorData parsed = mapper.readValue(sensorData.traverse(),
                               clazz);
                     if (parsed == null) {
                          throw new InvalidJson("unbound sensor");
                     }
                     sensors.add(parsed);
                }
                return sensors;
           } catch (Throwable e) {
                throw new InvalidJson("invalid data");
           }

      }
 }

希望它能帮助别人:)

Answer 1

为什么不直接使用@JsonTypeInfo？多态处理是它的具体用例。

在这种情况下，您可能需要使用以下内容:

@JsonTypeInfo(use=Id.NAME, include=As.PROPERTY, property="name")
@JsonSubTypes({ HumiditySensorData.class, ... }) // or register via mapper
public abstract class SensorData { ... }

@JsonTypeName("temperature")
public class TemperaratureSensorData extends SensorData {
   public TemperaratureSensorData(@JsonProperty("data") JsonNode data) {
     // extract pieces out
  }
}

它将处理从“名称”到子类型的解析，将“数据”的内容绑定(bind)为JsonNode(或者，如果您愿意，可以使用Map或对象或任何匹配的类型)。