java.lang.RuntimeException: Failed to invoke public com.example.syncapp.MessageBase() with no args

Question

protected void doPost(HttpServletRequest request, HttpServletResponse response)      throws ServletException, IOException {    System.out.println(request.getParameter("msg").toString());   String data = request.getParameter("msg").toString();   Gson gson = new Gson();   MessageBase msggg = gson.fromJson(data, MessageBase.class);   //System.out.println(msggg.Id + msggg.MessageText); } 

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public abstract class MessageBase implements Serializable {   public int Id;   public String MessageText;   public Date ReceiveDate; }  public class SyncSmsMessage extends MessageBase {   public String SenderNum;   } 

The code works until MessageBase msggg=gson.fromJson(data, MessageBase.class);. I get this exception:

java.lang.RuntimeException: Failed to invoke public com.example.syncapp.MessageBase() with no args   at com.google.gson.internal.ConstructorConstructor$2.construct(ConstructorConstructor.java:94)   at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:162)   at com.google.gson.Gson.fromJson(Gson.java:795)   at com.google.gson.Gson.fromJson(Gson.java:761)   at com.google.gson.Gson.fromJson(Gson.java:710)   at com.google.gson.Gson.fromJson(Gson.java:682)   at AndroidServlet.doPost(AndroidServlet.java:75)   at javax.servlet.http.HttpServlet.service(HttpServlet.java:647)   at javax.servlet.http.HttpServlet.service(HttpServlet.java:728) 

What I need to do? I put .jar in lib folder and i think the tomcat load the .jar well.

Answer 1

From GSON User Guide:

While deserializing an Object, Gson needs to create a default instance of the class [...] Well-behaved classes that are meant for serialization and deserialization should have a no-argument constructor

Your problem is that GSON's Instance Creator needs a no-argument constructor in the class in which you want to deserialize the JSON response, namely MessageBase.

Otherwise, you need to write your own Instance Creator, like this.