没有参数的 java.lang.RuntimeException : Failed to invoke public com. example.syncapp.MessageBase()

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);
}

---

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

public class SyncSmsMessage extends MessageBase {
  public String SenderNum;  
}

代码一直有效，直到 MessageBase msggg=gson.fromJson(data, MessageBase.class);。我得到这个异常(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)

我需要做什么？ 我将 .jar 放在 lib 文件夹中，我认为 tomcat 可以很好地加载 .jar。

Answer 1

来自 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

你的问题是 GSON 的 Instance Creator 在你想要反序列化 JSON 响应的类中需要一个无参数构造函数，即 MessageBase。

否则，你需要自己写Instance Creator，比如this .