Question

I know that the first one is a class and the second is an interface but the point is,
why should client Services extends the RemoteService and for the ServiceImpl class extends RemoteServiceServlet

So What is Actually behind the Scene ?!

Answer 1

You’re trying to compare apples and oranges. Please read docs and make any simple demo project to clearly understand these concepts.

RemoteService is the interface that RPC interfaces on client side should extend. And RemoteServiceServlet is the servlet base class for your RPC service implementations on the server. It deserializes requests from the client and serializes outgoing responses.

To use the RPC you need create and define in the client package own interface should extend the RemoteService interface and specifies methods the server should implement.

package com.yourproject.client;

import com.google.gwt.user.client.rpc.RemoteService;

public interface CustomInfoService extends RemoteService {
    Info getInfo(String infoId);
}

Then you need provide implementation of this interface by server side. This should be a servlet in server package which extends RemoteServiceServlet and implements the CustomInfoService.

package com.yourproject.server;

import com.yourproject.client.CustomInfoService;
import com.google.gwt.user.server.rpc.RemoteServiceServlet;

public class InfoServiceImpl extends RemoteServiceServlet implements CustomInfoService {
   public Info getInfo(String infoId) {
       //implementation (for example getting info from database)
   }
}

Sure to work properly you also need set the servlet mapping, create an asynchronous interface, make its call and process a result in a generated callback, implement the interface Serializable or IsSerializable for classes of instances which are pass over the RPC.