java - Json 和抽象类 'Can not construct instance'

Question

我在 stackoverflow 上阅读了很多有关此问题的信息，但任何解决方案都适合我。任何人都可以告诉我为什么它不起作用，因为我已经正确地描述了所有 child 对象 - 或者可能不正确？

这是我对jackson的依赖

<context:annotation-config/>    
<dependency>
    <groupId>org.codehaus.jackson</groupId>
    <artifactId>jackson-mapper-asl</artifactId>
    <version>1.9.10</version>
</dependency>
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-annotations</artifactId>
    <version>2.3.2</version>
</dependency>

以我的抽象对象作为属性的主类 (MLMObject)

public class MLMResponse {

    private int errorCode;
    private String errorMessage;
    private MLMObject object;


    public MLMResponse() {

    }


    public int getErrorCode() {
        return errorCode;
    }
    public void setErrorCode(int errorCode) {
        this.errorCode = errorCode;
    }
    public String getErrorMessage() {
        return errorMessage;
    }
    public void setErrorMessage(String errorMessage) {
        this.errorMessage = errorMessage;
    }
    public MLMObject getObject() {
        return object;
    }
    public void setObject(MLMObject object) {
        this.object = object;
    }



}

抽象类实现

import com.fasterxml.jackson.annotation.*;



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

    public MLMObject() {
        super();        
    }


}

子类实现

import com.fasterxml.jackson.annotation.JsonTypeName;


@JsonTypeName("person")
public class Person extends MLMObject{

    private String name;
    private String secondName;


    public Person() {
        super();
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public String getSecondName() {
        return secondName;
    }
    public void setSecondName(String secondName) {
        this.secondName = secondName;
    }




}

和我要读取的 json 文本 - 这只是对真实消息的测试。

String msg = "{\"errorCode\": 0,\"errorMessage\":\"\",\"object\":{\"type\":\"person\",\"name\":\"Jan\",\"secondName\":\"Jan\"}}";

我正在尝试用下面的代码阅读它

ObjectMapper objectMapper = new ObjectMapper();
 MLMResponse emp = objectMapper.readValue(msg, MLMResponse.class);

但是当我运行它时出现以下错误。

org.codehaus.jackson.map.JsonMappingException: Can not construct instance of com.pl.model.MLMObject, problem: abstract types can only be instantiated with additional type information
 at [Source: java.io.StringReader@a06816; line: 1, column: 34] (through reference chain: com.pl.model.MLMResponse["object"])
    at org.codehaus.jackson.map.JsonMappingException.from(JsonMappingException.java:163)
    at org.codehaus.jackson.map.deser.StdDeserializationContext.instantiationException(StdDeserializationContext.java:233)
    at org.codehaus.jackson.map.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:60)
    at org.codehaus.jackson.map.deser.SettableBeanProperty.deserialize(SettableBeanProperty.java:299)
    at org.codehaus.jackson.map.deser.SettableBeanProperty$MethodProperty.deserializeAndSet(SettableBeanProperty.java:414)
    at org.codehaus.jackson.map.deser.BeanDeserializer.deserializeFromObject(BeanDeserializer.java:697)
    at org.codehaus.jackson.map.deser.BeanDeserializer.deserialize(BeanDeserializer.java:580)
    at org.codehaus.jackson.map.ObjectMapper._readMapAndClose(ObjectMapper.java:2732)
    at org.codehaus.jackson.map.ObjectMapper.readValue(ObjectMapper.java:1863)
    at com.pl.service.JacksonMappingService.mapLoginMessage(JacksonMappingService.java:22)
    at com.pl.controller.HelloWorldController.hello(HelloWorldController.java:38)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
    at java.lang.reflect.Method.invoke(Method.java:597)

有人可以帮我吗？

问候简

Answer 1

同步您的 Jackson 版本。 Jackson 1 查找 Jackson 1 注释，Jackson 2 查找 Jackson 2 注释。您正在尝试让 Jackson 1 查找 Jackson 2 注释。这根本行不通。