Deserializing JSON with Jackson - Why JsonMappingException "No suitable constructor"?

Question

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

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-string:

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

I try to deserialize like this:

ProtocolContainer ret = ProtocolContainer.Create(jsonString); 

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

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 - a container class that encapsulates my "SubPackets":

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 - a superclass for all my datapackets

public class DataPacket  {  } 

LoginRequestReply.java - a DataPacket

package MyPackage.DataPackets;  import MyPackage.DataPacket;  public class LoginRequestReply extends DataPacket {     public boolean LoginOK;     public int UserId; } 

Answer 1

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.)

Answer 2

In this case, you could add @JsonCreator annotation to constructor. There are two ways it could be done:

• If you only add that annotation, then the whole matching JSON is first bound to type of the only argument (`DataPacket'). I assume you do not want to do that.
• If you also add @JsonProperty annotation before the argument, then JSON property matching that name is passed to constructor (annotation is mandatory because Java byte code does NOT contain name of method or constructor arguments) -- I suspect you want @JsonProperty("SubPacket")

This works if necessary information for constructor comes from JSON. If not, you do need to add alternate no-arg constructor.

By the way, the error message does sound wrong in this case. It should only be given if JSON data matching expected value if a JSON String.