杰克逊根据类型反序列化

Question

假设我有以下格式的JSON:

{
    "type" : "Foo"
    "data" : {
        "object" : {
            "id" : "1"
            "fizz" : "bizz"
            ...
        },
        "metadata" : {
            ...
        },
        "owner" : {
            "name" : "John"
            ...
        }
    }
}

我试图避免自定义反序列化器并尝试将上述JSON(称为Wrapper.java)反序列化为Java POJO."类型"字段指示"对象"反序列化即.type = foo表示使用Foo.java反序列化"object"字段.(如果type = Bar,请使用Bar.java反序列化对象字段).元数据/所有者将始终使用简单的Jackson注释Java类以相同的方式反序列化.有没有办法使用注释来完成这个？如果没有,如何使用自定义解串器完成？

Answer 1

仅注释方法

或者在自定义解串器的方式,你可以有一个注释,唯一的解决办法如下(类似于中描述的Spunc的答案,但使用type作为外部属性):

public abstract class AbstractData {

    private Owner owner;

    private Metadata metadata;

    // Getters and setters
}

public static final class FooData extends AbstractData {

    private Foo object;

    // Getters and setters
}

public static final class BarData extends AbstractData {

    private Bar object;

    // Getters and setters
}

public class Wrapper {

    private String type;

    @JsonTypeInfo(use = Id.NAME, property = "type", include = As.EXTERNAL_PROPERTY)
    @JsonSubTypes(value = { 
            @JsonSubTypes.Type(value = FooData.class, name = "Foo"),
            @JsonSubTypes.Type(value = BarData.class, name = "Bar") 
    })
    private AbstractData data;

    // Getters and setters
}

在此方法中,@JsonTypeInfo设置为type用作外部属性来确定映射data属性的正确类.

JSON文档可以反序列化如下:

ObjectMapper mapper = new ObjectMapper();
Wrapper wrapper = mapper.readValue(json, Wrapper.class);  

Answer 2

自定义解串器方法

您可以使用自定义反序列化程序检查type属性以将object属性解析为最合适的类.

首先定义一个将由Foo和Bar类实现的接口:

public interface Model {

}

public class Foo implements Model {

    // Fields, getters and setters
}

public class Bar implements Model {

    // Fields, getters and setters
}

然后定义你的Wrapper和Data类:

public class Wrapper {

    private String type;

    private Data data;

    // Getters and setters
}

public class Data {

    @JsonDeserialize(using = ModelDeserializer.class)
    private Model object;

    private Metadata metadata;

    private Owner owner;

    // Getters and setters
}

该object字段用注释@JsonDeserialize,表示将用于该object属性的反序列化器.

解串器定义如下:

public class ModelDeserializer extends JsonDeserializer<Model> {

    @Override
    public Model deserialize(JsonParser jp, DeserializationContext ctxt)
        throws IOException, JsonMappingException {

        // Get reference to ObjectCodec
        ObjectCodec codec = jp.getCodec();

        // Parse "object" node into Jackson's tree model
        JsonNode node = codec.readTree(jp);

        // Get value of the "type" property
        String type = ((Wrapper) jp.getParsingContext().getParent()
            .getCurrentValue()).getType();

        // Check the "type" property and map "object" to the suitable class
        switch (type) {

            case "Foo":
                return codec.treeToValue(node, Foo.class);

            case "Bar":
                return codec.treeToValue(node, Bar.class);

            default:
                throw new JsonMappingException(jp, 
                    "Invalid value for the \"type\" property");
        }
    }
}

JSON文档可以反序列化如下:

ObjectMapper mapper = new ObjectMapper();
Wrapper wrapper = mapper.readValue(json, Wrapper.class);  

作为此自定义反序列化器的替代方法,请考虑仅注释方法.

Answer 3

所有这些都可以通过注释来完成.

使用"元数据"和"所有者"等公共字段及其getter/setter创建一个抽象超类.该类需要使用@JsonTypeInfo进行注释.它应该看起来像:

@JsonTypeInfo(use = Id.CLASS, include = As.PROPERTY, property = "type")

使用参数property = "type"指定类标识符将在JSON文档中的字段类型下序列化.

可以使用指定类标识符的值use.Id.CLASS使用完全限定的Java类名.您还可以使用Id.MINIMAL_CLASS缩写的Java类名称.要拥有自己的标识符,请使用Id.NAME.在这种情况下,您需要声明子类型:

@JsonTypeInfo(use = Id.NAME, include = As.PROPERTY, property = "type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = Foo.class, name = "Foo"),
    @JsonSubTypes.Type(value = Bar.class, name = "Bar")
})

通过从抽象超类扩展来实现您的类Foo和Bar.

Jackson的ObjectMapper将使用JSON文档的附加字段"type"进行序列化和反序列化.E. g.当您将JSON字符串反序列化为超类引用时,它将是适当的子类:

ObjectMapper om = new ObjectMapper();
AbstractBase x = om.readValue(json, AbstractBase.class);
// x will be instanceof Foo or Bar

完整的代码示例(我使用公共字段作为快捷方式,不需要编写getter/setter):

package test;

import com.fasterxml.jackson.annotation.JsonTypeInfo;
import com.fasterxml.jackson.annotation.JsonTypeInfo.Id;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.annotation.JsonTypeInfo.As;

import java.io.IOException;

import com.fasterxml.jackson.annotation.JsonSubTypes;

@JsonTypeInfo(use = Id.NAME, include = As.PROPERTY, property = "type")
@JsonSubTypes({
    @JsonSubTypes.Type(value = Foo.class, name = "Foo"),
    @JsonSubTypes.Type(value = Bar.class, name = "Bar")
})
public abstract class AbstractBase {

    public MetaData metaData;
    public Owner owner;
    @Override
    public String toString() {
        return "metaData=" + metaData + "; owner=" + owner;
    }

    public static void main(String[] args) throws IOException {

        // Common fields
        Owner owner = new Owner();
        owner.name = "Richard";
        MetaData metaData = new MetaData();
        metaData.data = "Some data";

        // Foo
        Foo foo = new Foo();
        foo.owner = owner;
        foo.metaData = metaData;
        CustomObject customObject = new CustomObject();
        customObject.id = 20l;
        customObject.fizz = "Example";
        Data data = new Data();
        data.object = customObject;
        foo.data = data;
        System.out.println("Foo: " + foo);

        // Bar
        Bar bar = new Bar();
        bar.owner = owner;
        bar.metaData = metaData;
        bar.data = "A String in Bar";

        ObjectMapper om = new ObjectMapper();

        // Test Foo:
        String foojson = om.writeValueAsString(foo);
        System.out.println(foojson);
        AbstractBase fooDeserialised = om.readValue(foojson, AbstractBase.class);
        System.out.println(fooDeserialised);

        // Test Bar:
        String barjson = om.writeValueAsString(bar);
        System.out.println(barjson);
        AbstractBase barDeserialised = om.readValue(barjson, AbstractBase.class);
        System.out.println(barDeserialised);

    }

}

class Foo extends AbstractBase {
    public Data data;
    @Override
    public String toString() {
        return "Foo[" + super.toString() + "; data=" + data + ']';
    }
}

class Bar extends AbstractBase {
    public String data;
    public String toString() {
        return "Bar[" + super.toString() + "; data=" + data + ']';
    }
}


class Data {
    public CustomObject object;
    @Override
    public String toString() {
        return "Data[object=" + object + ']';
    }
}

class CustomObject {
    public long id;
    public String fizz;
    @Override
    public String toString() {
        return "CustomObject[id=" + id + "; fizz=" + fizz + ']';
    }
}

class MetaData {
    public String data;
    @Override
    public String toString() {
        return "MetaData[data=" + data + ']';
    }
}

class Owner {
    public String name;
    @Override
    public String toString() {
        return "Owner[name=" + name + ']';
    }
}