java - 不带参数的java.lang.RuntimeException : Failed to invoke public com. example.syncapp.MessageBase()

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);
}

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

public class SyncSmsMessage extends MessageBase {
  public String SenderNum;
}

该代码在MessageBase msggg=gson.fromJson(data, MessageBase.class);之前有效。我得到这个异常:

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)

我需要做什么？
我把.jar放到lib文件夹中，我认为tomcat很好地加载了.jar。

最佳答案

从GSON User Guide:



您的问题是GSON的实例创建者需要在要反序列化JSON响应的类中使用无参数构造函数，即MessageBase。

否则，您需要编写自己的实例创建器，例如this。