java - 如何对 Map 使用 Jackson 自定义值序列化器？

Question

我有一个来自数据库的结果集，格式为 Map<String, Object>我应该返回 json来自 REST 服务。 map 中的值可以是各种类型，包括 PGObject , String , Integer和Date .

我为 org.postgresql.util.PGObject 编写了一个自定义序列化器类型为“jsonb”的类，在 List<?> 中工作正常，但不在 Map<String, Object> 中.

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 看起来像这样:

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 连载此PGObject时Map<String, Object> 中的值然后我得到 json值如:

  "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和定制MapType至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);
        }
    }
}

但我对每个非 PGObject 都遇到序列化错误 map 中的对象:

[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)

如何启用我的PGObjectSerializer在 Jackson 中，当 PGObject 在 Map<String, Object> 序列化期间被发现为值时？

Answer 1

一种解决方法可能是为 Object 添加序列化程序。

然后在序列化器本身中，您可以检查Object是否是instanceOf PGobject。

并且在 myMapType 中您可以指定 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);
    }
}