如何为 Map<String, Object> 使用 Jackson 自定义值序列化程序？

Question

我有一个来自 DB 的结果集，格式为 Map<String, Object>，我应该从 REST 服务返回为 json。映射中的值可以是各种类型，包括PGObject、String、Integer 和Date。

我为org.postgresql.util.PGObject 类编写了一个自定义序列化程序，类型为“jsonb”，在List<?> 中可以正常工作，但在Map<String, Object> 中却不行。

public class PgObjectSerializer extends JsonSerializer<PGobject>{

    @Override
    public void serialize(PGobject pgObject, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        switch (pgObject.getType()) {
            case "json":
            case "jsonb":
                gen.writeRawValue(pgObject.getValue());
                break;
            default:
                gen.writeString(pgObject.getValue());
        }
    }
}

目标 PGObject 如下所示：

PGObject pgo = new PGObject();
pgo.setType("jsonb");
pgo.setValue("[{"id": 6, "name": "Foo"}, {"id": 7, "name": "Bar"}, {"id": 8, "name": "Baz"}]"); map.put("reason", pgo);

当 Jackson 在 Map<String, Object> 中序列化这个 PGObject 值时，我会得到 json 值，例如：

  "reason": {
    "type": "jsonb",
    "value": "[{\"id\": 6, \"name\": \"Foo\"}, {\"id\": 7, \"name\": \"Bar\"}, {\"id\": 8, \"name\": \"Baz\"}]"
  }

我需要什么：

  "reason": [
    {
      "id": 6,
      "name": "Foo"
    },
    {
      "id": 7,
      "name": "Bar"
    },
    {
      "id": 8,
      "name": "Baz"
    },
  ],

我尝试将模块添加到ObjectMapper 并将自定义MapType 添加到ObjectWriter，如Serializing Map<Date, String> with Jackson 的答案所示：

@Service
@Transactional
public class MyClass {
    private final ObjectWriter writer;
    private final MyRepo repository;

    @Autowired
    public MyClass(MyRepo repository) {
        this.repository = repository;

        SimpleModule module = new SimpleModule();
        module.addKeySerializer(PGobject.class, new PgObjectSerializer());

        ObjectMapper mapper = new ObjectMapper();
        JavaType myMapType = mapper.getTypeFactory().
                constructMapType(HashMap.class, String.class, PGobject.class);
        writer = mapper.registerModule(module).writerFor(myMapType);
    }

    ...

    private String toJsonString(Map<String, Object> map) {
        try {
            return writer.writeValueAsString(map);
        } catch (JsonProcessingException e) {
            throw new RuntimeException(e);
        }
    }
}

但是对于地图中的每个非PGObject 对象，我都会收到序列化错误：

[Test worker] ERROR 
com.fasterxml.jackson.databind.JsonMappingException: object is not an instance of declaring class (through reference chain: java.util.HashMap["end_date"]->java.lang.String["type"])
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:388)
    at com.fasterxml.jackson.databind.JsonMappingException.wrapWithPath(JsonMappingException.java:348)
    at com.fasterxml.jackson.databind.ser.std.StdSerializer.wrapAndThrow(StdSerializer.java:343)
    at com.fasterxml.jackson.databind.ser.std.BeanSerializerBase.serializeFields(BeanSerializerBase.java:698)
    at com.fasterxml.jackson.databind.ser.BeanSerializer.serialize(BeanSerializer.java:155)
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFieldsUsing(MapSerializer.java:736)
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:534)
    at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:30)
    at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:416)
    at com.fasterxml.jackson.databind.ObjectWriter$Prefetch.serialize(ObjectWriter.java:1425)
    at com.fasterxml.jackson.databind.ObjectWriter._configAndWriteValue(ObjectWriter.java:1158)
    at com.fasterxml.jackson.databind.ObjectWriter.writeValueAsString(ObjectWriter.java:1031)

当在Map<String, Object> 序列化过程中发现 PGObject 作为值时，如何在 Jackson 中启用我的 PGObjectSerializer？

Answer 1

一种解决方法是为Object 添加序列化程序。

然后在序列化程序本身中，您可以检查 Object 是否为 instanceOf PGobject。

在 myMapType 中你可以指定Object.class:

JavaType myMapType = mapper.getTypeFactory().
                constructMapType(HashMap.class, String.class, Object.class);

序列化器将是：

class PgObjectSerializer extends JsonSerializer<Object> {
@Override
public void serialize(Object object, JsonGenerator gen, SerializerProvider serializers) throws IOException {
    if (object instanceof PGobject) {
        PGobject pgObject = (PGobject) object;
        switch (pgObject.getType()) {
        case "json":
        case "jsonb":
                gen.writeRawValue(pgObject.getType());
                break;
        default:
                gen.writeString(pgObject.getType());
        }
        }else{
                ObjectMapper mapper = new ObjectMapper();
                mapper.writeValue(gen, object);
        }
    }
}