确定序列化对象的类型

我需要通过套接字发送消息（从用户到引擎的请求，以及从引擎到用户的响应）。所以流程本质上是

       +--------+ serialized  request +--------+      
       | Server | <==== network ====> | Client | 
       +--------+ serialized response +--------+      
           ^                              ^
           | request/response             | mouse/keyclicks
           |    object                    |
           v                              v
       +--------+                     +--------+
       | Engine |                     |  User  |
       +--------+                     +--------+

现在，不重新发明轮子才有意义。我在两边都处理 Java，所以我计划像这样使用对象：

/**
 * A Client makes a Request.
 */
abstract class UserRequest implements Serializable {
    /**
     * The engine will call request.engineCallback(this);
     * This will tell the Engine what the request is, at which point
     * the Engine will determine its response.
     */
    abstract EngineResponse engineCallback(Engine engine);
}

/**
 * The Engine has calculated a Response.
 */
abstract class EngineResponse implements Serializable {
    /**
     * The User will call response.userCallback(this);
     * This tells the User what the Engine thought of its request,
     * and what happened as a result.
     */
     abstract void userCallback(User user);
}

我没有遵循的是，在我的服务器和客户端套接字中，我如何知道请求和响应的子类正在进入？我看到这样的情况

Object request = in.readObject();

// now what? How do I know what to cast it to?

// Can I just cast it like
UserRequest request = (UserRequest)(in.readObject());
engine.addToRequestQueue(request); // let it take care of implementation details?

我的第一个想法是通过字符串传递所有内容，但是当 Java 提供序列化时，这似乎有点愚蠢。但我如何确保我知道哪个班级通过了网络？就此而言，我是否需要知道，只要我只将 UserRequest 的后代发送到服务器并将 EngineResponse 发送到客户端？

只需使用instanceof提供的关键字：

Object o = in.readObject();

if (o instanceof SomeUserRequest)
{
  SomeUserRequest sur = (SomeUserRequest)o;
  ..
}
else if (o instanceof OtherUserRequest)
{
  ..
}