Jackson 反序列化：我可以在可反序列化对象的字段上注入带有注释的值吗？

Question

我有一个这样的对象要反序列化：

public class RelationsInput {

   Relation relation1;

   Relation relation2;

}

而 class Relation 看起来像这样：

public class Relation {

   RelationType relationtype;
   ... (more fields)

}

RelationType 是一个枚举，不是一个将被反序列化的值，而所有其他值都是。

我是否可以在 class RelationInput 的字段上“注入”带有注释的字段 relationType 的枚举值？喜欢以下


public class RelationsInput {

   @RelationType(RelationType.OWNER)
   Relation owner;

   @RelationType(RelationType.TENANT)
   Relation tenant;

}

Jackson 提供这样的东西吗？

Answer 1

恐怕没有这样的事情。如果你想使用一些注释，你可以使用自定义反序列化器。首先创建类似的东西：

@RequiredArgsConstructor
public abstract class RelationTypeDeserializer extends JsonDeserializer<Relation> {
    private final RelationType relationType;

    @Override
    public Relation deserialize(JsonParser p, DeserializationContext ctxt)
            throws IOException, JsonProcessingException {
        Relation r = p.readValueAs(Relation.class);
        r.setRelationtype(relationType);
        return r;
    }
}

然后实现实际的：

public class OwnerDeserializer extends RelationTypeDeserializer {
    public OwnerDeserializer() {
        super(RelationType.OWNER);
    }   
}

和

public class TenantDeserializer extends RelationTypeDeserializer {
    public TenantDeserializer() {
        super(RelationType.TENANT);
    }
}

然后使用像这样的：

@Getter @Setter
public class RelationsInput {
    @JsonDeserialize(using = OwnerDeserializer.class)
    private Relation owner;
    @JsonDeserialize(using = TenantDeserializer.class)
    private Relation tenant;
}

Answer 2

您可以尝试使用 com.fasterxml.jackson.databind.deser.ContextualDeserializer 接口实现自定义解串器。它允许创建带有上下文的反序列化器实例。

见下例：

import com.fasterxml.jackson.core.JsonParser;
import com.fasterxml.jackson.databind.BeanProperty;
import com.fasterxml.jackson.databind.DeserializationContext;
import com.fasterxml.jackson.databind.JsonDeserializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.annotation.JsonDeserialize;
import com.fasterxml.jackson.databind.deser.ContextualDeserializer;
import com.fasterxml.jackson.databind.deser.std.StdDeserializer;
import com.fasterxml.jackson.databind.json.JsonMapper;
import lombok.Data;

import java.io.File;
import java.io.IOException;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;

public class JsonContextualDeserializerApp {
    public static void main(String[] args) throws IOException {
        File jsonFile = new File("./resource/test.json").getAbsoluteFile();

        ObjectMapper mapper = JsonMapper.builder().build();
        RelationsInput info = mapper.readValue(jsonFile, RelationsInput.class);

        System.out.println(info.toString());
    }
}

@Data
class RelationsInput {

    @JsonDeserialize(using = RelationStdDeserializer.class)
    @RelationTypeInfo(RelationType.OWNER)
    private Relation owner;

    @JsonDeserialize(using = RelationStdDeserializer.class)
    @RelationTypeInfo(RelationType.TENANT)
    private Relation tenant;

}

@Data
class Relation {

    private int id;
    private RelationType relationtype;
}

enum RelationType {OWNER, TENANT}

@Retention(RetentionPolicy.RUNTIME)
@interface RelationTypeInfo {
    RelationType value();
}

class RelationStdDeserializer extends StdDeserializer<Relation> implements ContextualDeserializer {

    private RelationType propertyRelationType;

    public RelationStdDeserializer() {
        this(null);
    }

    public RelationStdDeserializer(RelationType relationType) {
        super(Relation.class);
        this.propertyRelationType = relationType;
    }

    @Override
    public Relation deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        JsonDeserializer<Object> deser = ctxt.findRootValueDeserializer(ctxt.getTypeFactory().constructType(Relation.class));
        Relation instance = (Relation) deser.deserialize(p, ctxt);
        if (this.propertyRelationType != null) {
            instance.setRelationtype(this.propertyRelationType);
        }
        return instance;
    }

    @Override
    public JsonDeserializer<?> createContextual(DeserializationContext ctxt, BeanProperty property) {
        RelationTypeInfo typeInfo = property.getMember().getAllAnnotations().get(RelationTypeInfo.class);

        return new RelationStdDeserializer(typeInfo.value());
    }
}

上面的有效载荷代码：

{
  "owner": {
    "id": 1
  },
  "tenant": {
    "id": 2
  }
}

打印：

RelationsInput(owner=Relation(id=1, relationtype=OWNER), tenant=Relation(id=2, relationtype=TENANT))

另请参阅：

How to inject dependency into Jackson Custom deserializer

Answer 3

如果您按要求行事，RelationType 字段的值将始终相同。无论如何，一种可能的解决方案是像这样使用个性化的序列化器-解串器：

public class RelationTypeJsonSerializer extends JsonSerializer<RelationType> {

    @Override
    public void serialize(RelationType value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        String string = value.toString();//or something like that, convert the enum to string as you like
        gen.writeString(string);
    }
}

public class RelationTypeJsonDeserializer extends JsonDeserializer<RelationType> {

    @Override
    public RelationType deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        String toString = p.getCodec().readValue(p, String.class);//read the value as string
        return RelationType.build(toString);//convert back the value to object, use switch if needed)
    }
}

ObjectMapper om = new ObjectMapper();
SimpleModule localDateModule = new SimpleModule("RelationType Module");
localDateModule.addSerializer(RelationType.class, new RelationTypeJsonSerializer());
localDateModule.addDeserializer(RelationType.class, new RelationTypeJsonDeserializer());
om.registerModule(localDateModule);

为了前后转换枚举，我推荐使用 map，超级简单而且工作完美，像这样：

Map<String, RelationType> map = new HashMap<String, RelationType>();
map.put("Some Type", RelationType.SOME_TYPE);
map.put("Some Other Type", RelationType.SOME_OTHER_TYPE);

并使用get(string)序列化并 get(value) 反序列化（找到某个值的键）这是一个通用示例，当您想要序列化-反序列化没有默认序列化器-反序列化器的东西时查看此以获取更多信息

Jackson 反序列化：我可以在可反序列化对象的字段上注入带有注释的值吗？

Jackson deserialization: Can I inject a value with an annotation on the field of the to deserializable object?

java

json

jackson

deserialization

json-deserialization