How we can handle both Synchronous and Asynchronous request in Servlet ?

The send, receive, and reply operations may be synchronous or asynchronous. A synchronous operation blocks a process till the operation completes. An asynchronous operation is non-blocking and only initiates the operation.

While load of the page by synchronous request will refresh the total page Where as Asynchronous response will refresh only a specific fragment.

Based on our Requirement, we need to choose wheather to go with Asynchronous request or Synchronous Request. 

Here is an example containing the use of both Synchronous Request/Response and Asynchronous Request/Responsee.

Here I have used jQuery+Ajax+JSP as Client Side Resource and Servlet as Server Side Resource.

Folder Structure Should be like,

Client Side Code :

<script src="./jquery-1.11.0.min.js"></script>
<script>
$(document).ready(function(){
 $('#name').keyup(function(){
     var username=$('#name').val();
     if(username==""){
      username=" ";
     }
           $.post('SynchronousValidation?username='+username, function(responseText) { 
                $('#resultAsyn').text(responseText);  
            });
 });
});
</script>
<form action="./SynchronousValidation">
Enter Your Name : <input type="text" name="username" id="name">
<br>
<!-- For Synchronous Call Output -->
<font color=red>Synchronous Response :  </font><span id="resultSyn">
<%if(null != request.getAttribute("result")){
 out.print(request.getAttribute("result"));
}%>
</span>
<br>
<!-- Asynchronous Output -->
<font color=red>Asynchronous Response : </font><span id="resultAsyn">
</span>
<br>
<input type="submit" value="Validate User Name">
</form>

Server side Code : 

package com.skc.username.validate;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

import javax.servlet.RequestDispatcher;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class SynchronousValidation
 */
public class SynchronousValidation extends HttpServlet {
 private static final long serialVersionUID = 1L;

    /**
     * Default constructor. 
     */
    public SynchronousValidation() {
        // TODO Auto-generated constructor stub
    }

 /**
  * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
  */
 protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  response.setContentType("text/html");
  //  Getting the Request Parameter from Form Page
  String name=request.getParameter("username");
  List<String> myNameList = new ArrayList<String>();
  myNameList.add("Sitakant");
  myNameList.add("SKC");
  myNameList.add("Chaudhury");
  myNameList.add("BLGR");
  RequestDispatcher rd = request.getRequestDispatcher("/index.jsp");
  if(myNameList.contains(name)){
   request.setAttribute("result", "Username already Exist");
  }else{
   request.setAttribute("result", "No such username is registered.");
  }
  rd.forward(request, response);
  
 }

 /**
  * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
  */
 protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
  String result = request.getParameter("username");
  List<String> myNameList = new ArrayList<String>();
  myNameList.add("Sitakant");
  myNameList.add("SKC");
  myNameList.add("Chaudhury");
  myNameList.add("BLGR");
  if(myNameList.contains(result)){
   result=  "Username already Exist";
  }else{
   result = "No such username is registered.";
  }
  response.setContentType("text/plain");  // Set content type of the response so that jQuery knows what it can expect.
     response.setCharacterEncoding("UTF-8"); // You want world domination, huh?
     response.getWriter().write(result);       // Write response body.
 }

}

Output :