使用 Jackson 反序列化 JSON - 为什么 JsonMappingException “没有合适的构造函数"? [英] Deserializing JSON with Jackson - Why JsonMappingException "No suitable constructor"?

问题描述

我在使用 Jackson 反序列化 JSON 字符串时遇到问题(但我在将对象序列化为 JSON 时没有问题).

I have a problem deserializing a JSON string using Jackson (but I have no problem serializing an object to JSON).

下面我介绍我使用的类.当我收到一个 JSON 字符串(一个在别处序列化并通过 web 服务检索的 ProtocolContainer)并想要反序列化它时，问题就出现了:

Below I present the classes I use. The problem comes when I rececive a JSON-string (a ProtocolContainer that was serialized elsewhere and retrieved via webservice) and want to de-serialize it:

JSON 字符串:

{"DataPacketJSONString":null,"DataPacketType":"MyPackage.DataPackets.LoginRequestReply","MessageId":6604,"SenderUsername":null,"SubPacket":{"__type":"LoginRequestReply:#MyPackage.DataPackets","Reason":"错误的通行证或用户名","成功":false,"用户名":"User1"}}

{"DataPacketJSONString":null,"DataPacketType":"MyPackage.DataPackets.LoginRequestReply","MessageId":6604,"SenderUsername":null,"SubPacket":{"__type":"LoginRequestReply:#MyPackage.DataPackets","Reason":"Wrong pass or username","Success":false,"Username":"User1"}}

我尝试像这样反序列化:

I try to deserialize like this:

ProtocolContainer ret = ProtocolContainer.Create(jsonString);

和在 ProtocolContainer 中执行的代码可以在下面看到.例外:

and the code that executes in ProtocolContainer can be seen below. The exception:

org.codehaus.jackson.map.JsonMappingException:没有合适的构造函数找到类型 [简单类型，类MyPackage.ProtocolContainer]: 不能从 JSON 对象实例化(需要添加/启用类型信息?)在 [来源:java.io.StringReader@4059dcb0;行:1，列:2]

org.codehaus.jackson.map.JsonMappingException: No suitable constructor found for type [simple type, class MyPackage.ProtocolContainer]: can not instantiate from JSON object (need to add/enable type information?) at [Source: java.io.StringReader@4059dcb0; line: 1, column: 2]

ProtocolContainer.java - 封装我的SubPackets"的容器类:

import java.io.IOException;

import org.codehaus.jackson.JsonGenerationException;
import org.codehaus.jackson.JsonParseException;
import org.codehaus.jackson.map.JsonMappingException;
import org.codehaus.jackson.map.ObjectMapper;

import MyPackage.DataPackets.*;

public class ProtocolContainer 
{
    public String SenderUsername;
    public String DataPacketType;
    public long MessageId;
    public String DataPacketJSONString;
    public DataPacket SubPacket;

    public ProtocolContainer(DataPacket dp)
    {
        DataPacketType = dp.getClass().toString().substring(6);
        SubPacket = dp;
    }

    public String toJSON()
    {
        try {
            if (SubPacket != null)
                this.DataPacketJSONString = ProtocolContainer.mapper.writeValueAsString(SubPacket);

            return ProtocolContainer.mapper.writeValueAsString(this);
        } catch (JsonGenerationException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JsonMappingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        return null;
    }

    public static ObjectMapper mapper = new ObjectMapper();

    public static ProtocolContainer Create(String jsonString)
    {
        ProtocolContainer pc = null;
        try {
            pc = mapper.readValue(jsonString, ProtocolContainer.class); // error here!
        } catch (JsonParseException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (JsonMappingException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();  // Exception when deserializing
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }


        try 
        {
            if (pc != null && pc.DataPacketType == "LoginRequest")
                pc.SubPacket = mapper.readValue(jsonString, LoginRequest.class);
    }
        catch (JsonParseException e) 
        {
            e.printStackTrace();
        }
        catch (JsonMappingException e) 
        {
            e.printStackTrace();
        }
        catch (IOException e) 
        {
            e.printStackTrace();
        }
        return pc;
    }
}

DataPacket.java - 我所有数据包的超类

public class DataPacket 
{

}

LoginRequestReply.java - 一个数据包

package MyPackage.DataPackets;

import MyPackage.DataPacket;

public class LoginRequestReply extends DataPacket
{
    public boolean LoginOK;
    public int UserId;
}

推荐答案

错误消息说明了一切，您的 ProtocolContainer 没有默认构造函数，因此 Jackson 无法创建它的实例.(因为当前创建 ProtocolContainer 的唯一方法是传入 DataPacket.)

The error messages says it all, your ProtocolContainer does not have a default constructor so Jackson is unable to create an instance of it. (Since the only current way of creating a ProtocolContainer is by passing in a DataPacket.)

这篇关于使用 Jackson 反序列化 JSON - 为什么 JsonMappingException “没有合适的构造函数"?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！