<servlet-mapping>
        <servlet-name>BookServlet2</servlet-name>
        <url-pattern>/BookServlet2</url-pattern>
    </servlet-mapping>

        try 
        {
                RequestBuilder requestBuilder = new RequestBuilder( RequestBuilder.GET, "/BookServlet2" );

                requestBuilder.setCallback(new RequestCallback() 
                {
                        @Override
                        public void onResponseReceived(Request request, Response response) 
                        {
                            List <IBook> books = parseJsonData (response.getText());
                            
                            BooksWidget <IBook> booksWidget = new BooksWidget <IBook>();

                            if (books != null) 
                            {
                                    booksWidget.setData(books);
                                    booksWidget.show();
                            }
                        }

                        @Override
                        public void onError(Request request, Throwable exception) 
                        {
                            Window.alert(exception.getMessage());
                        }
                });
                requestBuilder.send();
        } catch (RequestException ex) {
                Window.alert(ex.getMessage());
        }

public class MainEntryPoint implements EntryPoint {

    /**
     * Creates a new instance of MainEntryPoint
     */
    public MainEntryPoint() {
    }

    /**
     * The entry point method, called automatically by loading a module that
     * declares an implementing class as an entry-point
     */
    public void onModuleLoad() {
        final Label label = new Label("Hello, GWT!!!");
        final Button button = new Button("Click me!");
        
        button.addClickHandler(new ClickHandler() {
            public void onClick(ClickEvent event) {
                //label.setVisible(!label.isVisible());
                //getDataFromServer();
                try 
                {
                   RequestBuilder requestBuilder = new RequestBuilder( RequestBuilder.GET, "/BooksServlet" );
                   requestBuilder.send();
                } catch (RequestException ex) {
                    Window.alert(ex.getMessage());
                }
            }
        });
        
        RootPanel.get().add(button);
        RootPanel.get().add(label);
    }
        
}

    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>org.yournamehere.server.HelloServlet</servlet-class>
    </servlet>

   <servlet-mapping>
        <servlet-name>BookServlet2</servlet-name> <---- ?
        <url-pattern>/BookServlet2</url-pattern>
    </servlet-mapping>
   <servlet>
        <servlet-name>HelloServlet</servlet-name>  <---- ?
        <servlet-class>org.yournamehere.server.HelloServlet</servlet-class>
    </servlet>

<servlet-mapping>
 <servlet-name>BookServlet2</servlet-name>
 <url-pattern>/BookServlet2</url-pattern>
</servlet-mapping>

<servlet>
 <servlet-name>HelloServlet</servlet-name>
 <servlet-class>org.yournamehere.server.HelloServlet</servlet-class>
</servlet>

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="3.0" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
    <servlet>
        <servlet-name>HelloServlet</servlet-name>
        <servlet-class>org.yournamehere.client.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>HelloServlet</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <session-config>
        <session-timeout>
            30
        </session-timeout>
    </session-config>
    <welcome-file-list>
        <welcome-file>welcomeGWT.html</welcome-file>
    </welcome-file-list>
</web-app>

package org.yournamehere.client;

import com.google.gwt.core.client.EntryPoint;
import com.google.gwt.user.client.ui.Button;
import com.google.gwt.user.client.ui.Label;
import com.google.gwt.user.client.ui.RootPanel;
import com.google.gwt.event.dom.client.ClickEvent;
import com.google.gwt.event.dom.client.ClickHandler;
import com.google.gwt.user.client.Window;
import com.google.gwt.user.client.ui.TextBox;

public class MainEntryPoint implements EntryPoint {

    public MainEntryPoint() {
    }

    public void onModuleLoad() {
        final Label label = new Label("Hello, GWT!!!");
        final Button button = new Button("Click me!");
        final TextBox nameField = new TextBox();
        
        button.addClickHandler(new ClickHandler() {
            public void onClick(ClickEvent event) {
                label.setVisible(!label.isVisible());
                Window.Location.replace("/hello?name="+nameField.getText());  
            }
        });
        
        RootPanel.get().add(button);
        RootPanel.get().add(label);
        RootPanel.get().add(nameField);
    }
}

package org.yournamehere.client;

import java.io.IOException;
import java.io.PrintWriter;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class HelloServlet extends HttpServlet 
{

    /**
     * Processes requests for both HTTP <code>GET</code> and <code>POST</code>
     * methods.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    protected void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        response.setContentType("text/html;charset=UTF-8");
        PrintWriter out = response.getWriter();
        try {
            /* TODO output your page here. You may use following sample code. */
            out.println("<!DOCTYPE html>");
            out.println("<html>");
            out.println("<head>");
            out.println("<title>Servlet HelloServlet</title>");            
            out.println("</head>");
            out.println("<body>");
            out.println("<h1>Servlet HelloServlet at " + request.getContextPath() + "</h1>");
            out.println("</body>");
            out.println("</html>");
        } finally {
            out.close();
        }
    }

    // <editor-fold defaultstate="collapsed" desc="HttpServlet methods. Click on the + sign on the left to edit the code.">
    /**
     * Handles the HTTP <code>GET</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        //processRequest(request, response);
        String name = request.getParameter("name");  
        response.getWriter().write("Hello "+name+", Welcome in My Servlet");  
    }

    /**
     * Handles the HTTP <code>POST</code> method.
     *
     * @param request servlet request
     * @param response servlet response
     * @throws ServletException if a servlet-specific error occurs
     * @throws IOException if an I/O error occurs
     */
    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        processRequest(request, response);
    }

    /**
     * Returns a short description of the servlet.
     *
     * @return a String containing servlet description
     */
    @Override
    public String getServletInfo() {
        return "Short description";
    }// </editor-fold>

}