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Load data from html form, update database and display data on the same page in table using java servlet

I need some help.
I have a small application which consists of Java Servlet and html form.
User puts some data in a html form and after clicking on submit button the java servlet gets the data in it's post method (loads it in a database). Till now all works fine. I am using java servlet and tomcat.
What I want to do now is to display the data in a table on the same page on html. I have found, that this would be possible through ajax get method. But somehow I can't get it right.
Here simplified code of my application:
Java Servlet:
#WebServlet(name = "MyServlet", urlPatterns = { "/MyServlet" })
public class MyServlet extends HttpServlet {
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType("text/html");
// getting the values submitted by the user from the html form
String name = request.getParameter("name");
String surname = request.getParameter("surname");
// here goes some logic to load data in the database
// data from database is recieved as an array and is then converted to
// json
// here I can only print the whole json retrieved from DB
// and it is displayed on another page with myurl/MyServlet
response.setContentType("application/json");
PrintWriter out = response.getWriter();
response.setCharacterEncoding("UTF-8");
out.println(json);
}
public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException{
// nothing happens here
}
In my html form I have the following (the most important things here only):
<form action="MyServlet" method="post">
<input class="form-control" type="text" name="name" id="name">
<input class="form-control" type="text" name="surname" id="surname">
<input type="submit" class="btn btn-info" id="MyS"value="MyServlet">
<table class="table" id="table">
<tr>
<th>name</th>
<th>surname</th>
</tr>
The user input is stored in a database and everything is working fine.
The problem is, that I get no response and I would like to display all of the data given from user in a table, which schould be updated after each submit and user schould stay on the same page (don't actually know, how to do this).
From the database I get an Array which I can convert to JSON and all this information schould be displayed in a table.
I tried to write some ajax.get like this (I am a real newby in javascript):
<script>
<script type="text/javascript">
$("#MyS").click(function() {
}
$.ajax({
url: "/MyServlet",
type: "POST",
dataType: '{json: json}',
data: myvalue,
success: function(data) {
$('#table').append(data);
},
});
</script>
But no data is displayed in the table or somewhere.
I would be glad to get some hints i what I am doing wrong and how should it be done properly?
Thanks in advance.

Execute it with these lines and tell me what response you get in your browser's console:
$.ajax({
url: "/MyServlet",
type: "POST",
dataType: '{json: json}',
data: myvalue,
success: function(data)
{
console.log( data ); // logs this on success
$('#table').append(data);
},
**error: function (textStatus, errorThrown)
{
console.log( textStatus ); // log this on error
console.log( errorThrown );
}**
});

How can I send a data and receive back with a button click with jscript ajax from servlet [duplicate]

Whenever I print something inside the servlet and call it by the webbrowser, it returns a new page containing that text. Is there a way to print the text in the current page using Ajax?
I'm very new to web applications and servlets.

Indeed, the keyword is "Ajax": Asynchronous JavaScript and XML. However, last years it's more than often Asynchronous JavaScript and JSON. Basically, you let JavaScript execute an asynchronous HTTP request and update the HTML DOM tree based on the response data.
Since it's pretty tedious work to make it to work across all browsers (especially Internet Explorer versus others), there are plenty of JavaScript libraries out which simplifies this in single functions and covers as many as possible browser-specific bugs/quirks under the hoods, such as jQuery, Prototype, Mootools. Since jQuery is most popular these days, I'll use it in the below examples.
Kickoff example returning String as plain text
Create a /some.jsp like below (note: the code snippets in this answer doesn't expect the JSP file being placed in a subfolder, if you do so, alter servlet URL accordingly from "someservlet" to "${pageContext.request.contextPath}/someservlet"; it's merely omitted from the code snippets for brevity):
<!DOCTYPE html>
<html lang="en">
<head>
<title>SO question 4112686</title>
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseText) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response text...
$("#somediv").text(responseText); // Locate HTML DOM element with ID "somediv" and set its text content with the response text.
});
});
</script>
</head>
<body>
<button id="somebutton">press here</button>
<div id="somediv"></div>
</body>
</html>
Create a servlet with a doGet() method which look like this:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String text = "some text";
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(text); // Write response body.
}
Map this servlet on an URL pattern of /someservlet or /someservlet/* as below (obviously, the URL pattern is free to your choice, but you'd need to alter the someservlet URL in JS code examples over all place accordingly):
package com.example;
#WebServlet("/someservlet/*")
public class SomeServlet extends HttpServlet {
// ...
}
Or, when you're not on a Servlet 3.0 compatible container yet (Tomcat 7, GlassFish 3, JBoss AS 6, etc. or newer), then map it in web.xml the old fashioned way (see also our Servlets wiki page):
<servlet>
<servlet-name>someservlet</servlet-name>
<servlet-class>com.example.SomeServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>someservlet</servlet-name>
<url-pattern>/someservlet/*</url-pattern>
</servlet-mapping>
Now open the http://localhost:8080/context/test.jsp in the browser and press the button. You'll see that the content of the div get updated with the servlet response.
Returning List<String> as JSON
With JSON instead of plaintext as response format you can even get some steps further. It allows for more dynamics. First, you'd like to have a tool to convert between Java objects and JSON strings. There are plenty of them as well (see the bottom of this page for an overview). My personal favourite is Google Gson. Download and put its JAR file in /WEB-INF/lib folder of your web application.
Here's an example which displays List<String> as <ul><li>. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<String> list = new ArrayList<>();
list.add("item1");
list.add("item2");
list.add("item3");
String json = new Gson().toJson(list);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
The JavaScript code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $ul = $("<ul>").appendTo($("#somediv")); // Create HTML <ul> element and append it to HTML DOM element with ID "somediv".
$.each(responseJson, function(index, item) { // Iterate over the JSON array.
$("<li>").text(item).appendTo($ul); // Create HTML <li> element, set its text content with currently iterated item and append it to the <ul>.
});
});
});
Do note that jQuery automatically parses the response as JSON and gives you directly a JSON object (responseJson) as function argument when you set the response content type to application/json. If you forget to set it or rely on a default of text/plain or text/html, then the responseJson argument wouldn't give you a JSON object, but a plain vanilla string and you'd need to manually fiddle around with JSON.parse() afterwards, which is thus totally unnecessary if you set the content type right in first place.
Returning Map<String, String> as JSON
Here's another example which displays Map<String, String> as <option>:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Map<String, String> options = new LinkedHashMap<>();
options.put("value1", "label1");
options.put("value2", "label2");
options.put("value3", "label3");
String json = new Gson().toJson(options);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
And the JSP:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $select = $("#someselect"); // Locate HTML DOM element with ID "someselect".
$select.find("option").remove(); // Find all child elements with tag name "option" and remove them (just to prevent duplicate options when button is pressed again).
$.each(responseJson, function(key, value) { // Iterate over the JSON object.
$("<option>").val(key).text(value).appendTo($select); // Create HTML <option> element, set its value with currently iterated key and its text content with currently iterated item and finally append it to the <select>.
});
});
});
with
<select id="someselect"></select>
Returning List<Entity> as JSON
Here's an example which displays List<Product> in a <table> where the Product class has the properties Long id, String name and BigDecimal price. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Product> products = someProductService.list();
String json = new Gson().toJson(products);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
The JS code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $table = $("<table>").appendTo($("#somediv")); // Create HTML <table> element and append it to HTML DOM element with ID "somediv".
$.each(responseJson, function(index, product) { // Iterate over the JSON array.
$("<tr>").appendTo($table) // Create HTML <tr> element, set its text content with currently iterated item and append it to the <table>.
.append($("<td>").text(product.id)) // Create HTML <td> element, set its text content with id of currently iterated product and append it to the <tr>.
.append($("<td>").text(product.name)) // Create HTML <td> element, set its text content with name of currently iterated product and append it to the <tr>.
.append($("<td>").text(product.price)); // Create HTML <td> element, set its text content with price of currently iterated product and append it to the <tr>.
});
});
});
Returning List<Entity> as XML
Here's an example which does effectively the same as previous example, but then with XML instead of JSON. When using JSP as XML output generator you'll see that it's less tedious to code the table and all. JSTL is this way much more helpful as you can actually use it to iterate over the results and perform server side data formatting. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Product> products = someProductService.list();
request.setAttribute("products", products);
request.getRequestDispatcher("/WEB-INF/xml/products.jsp").forward(request, response);
}
The JSP code (note: if you put the <table> in a <jsp:include>, it may be reusable elsewhere in a non-Ajax response):
<?xml version="1.0" encoding="UTF-8"?>
<%#page contentType="application/xml" pageEncoding="UTF-8"%>
<%#taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%#taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<data>
<table>
<c:forEach items="${products}" var="product">
<tr>
<td>${product.id}</td>
<td><c:out value="${product.name}" /></td>
<td><fmt:formatNumber value="${product.price}" type="currency" currencyCode="USD" /></td>
</tr>
</c:forEach>
</table>
</data>
The JavaScript code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseXml) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response XML...
$("#somediv").html($(responseXml).find("data").html()); // Parse XML, find <data> element and append its HTML to HTML DOM element with ID "somediv".
});
});
You'll by now probably realize why XML is so much more powerful than JSON for the particular purpose of updating a HTML document using Ajax. JSON is funny, but after all generally only useful for so-called "public web services". MVC frameworks like JSF use XML under the covers for their ajax magic.
Ajaxifying an existing form
You can use jQuery $.serialize() to easily ajaxify existing POST forms without fiddling around with collecting and passing the individual form input parameters. Assuming an existing form which works perfectly fine without JavaScript/jQuery (and thus degrades gracefully when the end user has JavaScript disabled):
<form id="someform" action="someservlet" method="post">
<input type="text" name="foo" />
<input type="text" name="bar" />
<input type="text" name="baz" />
<input type="submit" name="submit" value="Submit" />
</form>
You can progressively enhance it with Ajax as below:
$(document).on("submit", "#someform", function(event) {
var $form = $(this);
$.post($form.attr("action"), $form.serialize(), function(response) {
// ...
});
event.preventDefault(); // Important! Prevents submitting the form.
});
You can in the servlet distinguish between normal requests and Ajax requests as below:
#Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String foo = request.getParameter("foo");
String bar = request.getParameter("bar");
String baz = request.getParameter("baz");
boolean ajax = "XMLHttpRequest".equals(request.getHeader("X-Requested-With"));
// ...
if (ajax) {
// Handle Ajax (JSON or XML) response.
} else {
// Handle regular (JSP) response.
}
}
The jQuery Form plugin does less or more the same as above jQuery example, but it has additional transparent support for multipart/form-data forms as required by file uploads.
Manually sending request parameters to servlet
If you don't have a form at all, but just wanted to interact with the servlet "in the background" whereby you'd like to POST some data, then you can use jQuery $.param() to easily convert a JSON object to an URL-encoded query string.
var params = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.post("someservlet", $.param(params), function(response) {
// ...
});
The same doPost() method as shown here above can be reused. Do note that above syntax also works with $.get() in jQuery and doGet() in servlet.
Manually sending JSON object to servlet
If you however intend to send the JSON object as a whole instead of as individual request parameters for some reason, then you'd need to serialize it to a string using JSON.stringify() (not part of jQuery) and instruct jQuery to set request content type to application/json instead of (default) application/x-www-form-urlencoded. This can't be done via $.post() convenience function, but needs to be done via $.ajax() as below.
var data = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.ajax({
type: "POST",
url: "someservlet",
contentType: "application/json", // NOT dataType!
data: JSON.stringify(data),
success: function(response) {
// ...
}
});
Do note that a lot of starters mix contentType with dataType. The contentType represents the type of the request body. The dataType represents the (expected) type of the response body, which is usually unnecessary as jQuery already autodetects it based on response's Content-Type header.
Then, in order to process the JSON object in the servlet which isn't being sent as individual request parameters but as a whole JSON string the above way, you only need to manually parse the request body using a JSON tool instead of using getParameter() the usual way. Namely, servlets don't support application/json formatted requests, but only application/x-www-form-urlencoded or multipart/form-data formatted requests. Gson also supports parsing a JSON string into a JSON object.
JsonObject data = new Gson().fromJson(request.getReader(), JsonObject.class);
String foo = data.get("foo").getAsString();
String bar = data.get("bar").getAsString();
String baz = data.get("baz").getAsString();
// ...
Do note that this all is more clumsy than just using $.param(). Normally, you want to use JSON.stringify() only if the target service is e.g. a JAX-RS (RESTful) service which is for some reason only capable of consuming JSON strings and not regular request parameters.
Sending a redirect from servlet
Important to realize and understand is that any sendRedirect() and forward() call by the servlet on an ajax request would only forward or redirect the Ajax request itself and not the main document/window where the Ajax request originated. JavaScript/jQuery would in such case only retrieve the redirected/forwarded response as responseText variable in the callback function. If it represents a whole HTML page and not an Ajax-specific XML or JSON response, then all you could do is to replace the current document with it.
document.open();
document.write(responseText);
document.close();
Note that this doesn't change the URL as end user sees in browser's address bar. So there are issues with bookmarkability. Therefore, it's much better to just return an "instruction" for JavaScript/jQuery to perform a redirect instead of returning the whole content of the redirected page. E.g., by returning a boolean, or a URL.
String redirectURL = "http://example.com";
Map<String, String> data = new HashMap<>();
data.put("redirect", redirectURL);
String json = new Gson().toJson(data);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
function(responseJson) {
if (responseJson.redirect) {
window.location = responseJson.redirect;
return;
}
// ...
}
See also:
Call Servlet and invoke Java code from JavaScript along with parameters
Access Java / Servlet / JSP / JSTL / EL variables in JavaScript
How can I switch easily between an Ajax-based website and a basic HTML website?
How can I upload files to a server using JSP/Servlet and Ajax?

The right way to update the page currently displayed in the user's browser (without reloading it) is to have some code executing in the browser update the page's DOM.
That code is typically JavaScript that is embedded in or linked from the HTML page, hence the Ajax suggestion. (In fact, if we assume that the updated text comes from the server via an HTTP request, this is classic Ajax.)
It is also possible to implement this kind of thing using some browser plugin or add-on, though it may be tricky for a plugin to reach into the browser's data structures to update the DOM. (Native code plugins normally write to some graphics frame that is embedded in the page.)

I will show you a whole example of a servlet and how do an Ajax call.
Here, we are going to create the simple example to create the login form using a servlet.
File index.html
<form>
Name:<input type="text" name="username"/><br/><br/>
Password:<input type="password" name="userpass"/><br/><br/>
<input type="button" value="login"/>
</form>
An Ajax sample
$.ajax
({
type: "POST",
data: 'LoginServlet=' + name + '&name=' + type + '&pass=' + password,
url: url,
success:function(content)
{
$('#center').html(content);
}
});
LoginServlet servlet code:
package abc.servlet;
import java.io.File;
public class AuthenticationServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
doPost(request, response);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
try{
HttpSession session = request.getSession();
String username = request.getParameter("name");
String password = request.getParameter("pass");
/// Your Code
out.println("sucess / failer")
}
catch (Exception ex) {
// System.err.println("Initial SessionFactory creation failed.");
ex.printStackTrace();
System.exit(0);
}
}
}

$.ajax({
type: "POST",
url: "URL to hit on servelet",
data: JSON.stringify(json),
dataType: "json",
success: function(response){
// We have the response
if(response.status == "SUCCESS"){
$('#info').html("Info has been added to the list successfully.<br>" +
"The details are as follws: <br> Name: ");
}
else{
$('#info').html("Sorry, there is some thing wrong with the data provided.");
}
},
error: function(e){
alert('Error: ' + e);
}
});

Ajax (also AJAX), an acronym for Asynchronous JavaScript and XML, is a group of interrelated web development techniques used on the client-side to create asynchronous web applications. With Ajax, web applications can send data to, and retrieve data from, a server asynchronously.
Below is the example code:
A JSP page JavaScript function to submit data to a servlet with two variables, firstName and lastName:
function onChangeSubmitCallWebServiceAJAX()
{
createXmlHttpRequest();
var firstName = document.getElementById("firstName").value;
var lastName = document.getElementById("lastName").value;
xmlHttp.open("GET", "/AJAXServletCallSample/AjaxServlet?firstName="
+ firstName + "&lastName=" + lastName, true)
xmlHttp.onreadystatechange = handleStateChange;
xmlHttp.send(null);
}
Servlet to read data send back to JSP in XML format (you could use text as well. You just need to change the response content to text and render data on JavaScript function.)
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
response.getWriter().write("<details>");
response.getWriter().write("<firstName>" + firstName + "</firstName>");
response.getWriter().write("<lastName>" + lastName + "</lastName>");
response.getWriter().write("</details>");
}

Normally you can’t update a page from a servlet. The client (browser) has to request an update. Either the client loads a whole new page or it requests an update to a part of an existing page. This technique is called Ajax.

Using Bootstrap multi select:
Ajax
function() { $.ajax({
type: "get",
url: "OperatorController",
data: "input=" + $('#province').val(),
success: function(msg) {
var arrayOfObjects = eval(msg);
$("#operators").multiselect('dataprovider',
arrayOfObjects);
// $('#output').append(obj);
},
dataType: 'text'
});}
}
In Servlet
request.getParameter("input")

How to retrive the bean value gettting as ajax result from the servlet controller [duplicate]

Whenever I print something inside the servlet and call it by the webbrowser, it returns a new page containing that text. Is there a way to print the text in the current page using Ajax?
I'm very new to web applications and servlets.

Indeed, the keyword is "Ajax": Asynchronous JavaScript and XML. However, last years it's more than often Asynchronous JavaScript and JSON. Basically, you let JavaScript execute an asynchronous HTTP request and update the HTML DOM tree based on the response data.
Since it's pretty tedious work to make it to work across all browsers (especially Internet Explorer versus others), there are plenty of JavaScript libraries out which simplifies this in single functions and covers as many as possible browser-specific bugs/quirks under the hoods, such as jQuery, Prototype, Mootools. Since jQuery is most popular these days, I'll use it in the below examples.
Kickoff example returning String as plain text
Create a /some.jsp like below (note: the code snippets in this answer doesn't expect the JSP file being placed in a subfolder, if you do so, alter servlet URL accordingly from "someservlet" to "${pageContext.request.contextPath}/someservlet"; it's merely omitted from the code snippets for brevity):
<!DOCTYPE html>
<html lang="en">
<head>
<title>SO question 4112686</title>
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseText) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response text...
$("#somediv").text(responseText); // Locate HTML DOM element with ID "somediv" and set its text content with the response text.
});
});
</script>
</head>
<body>
<button id="somebutton">press here</button>
<div id="somediv"></div>
</body>
</html>
Create a servlet with a doGet() method which look like this:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String text = "some text";
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(text); // Write response body.
}
Map this servlet on an URL pattern of /someservlet or /someservlet/* as below (obviously, the URL pattern is free to your choice, but you'd need to alter the someservlet URL in JS code examples over all place accordingly):
package com.example;
#WebServlet("/someservlet/*")
public class SomeServlet extends HttpServlet {
// ...
}
Or, when you're not on a Servlet 3.0 compatible container yet (Tomcat 7, GlassFish 3, JBoss AS 6, etc. or newer), then map it in web.xml the old fashioned way (see also our Servlets wiki page):
<servlet>
<servlet-name>someservlet</servlet-name>
<servlet-class>com.example.SomeServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>someservlet</servlet-name>
<url-pattern>/someservlet/*</url-pattern>
</servlet-mapping>
Now open the http://localhost:8080/context/test.jsp in the browser and press the button. You'll see that the content of the div get updated with the servlet response.
Returning List<String> as JSON
With JSON instead of plaintext as response format you can even get some steps further. It allows for more dynamics. First, you'd like to have a tool to convert between Java objects and JSON strings. There are plenty of them as well (see the bottom of this page for an overview). My personal favourite is Google Gson. Download and put its JAR file in /WEB-INF/lib folder of your web application.
Here's an example which displays List<String> as <ul><li>. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<String> list = new ArrayList<>();
list.add("item1");
list.add("item2");
list.add("item3");
String json = new Gson().toJson(list);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
The JavaScript code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $ul = $("<ul>").appendTo($("#somediv")); // Create HTML <ul> element and append it to HTML DOM element with ID "somediv".
$.each(responseJson, function(index, item) { // Iterate over the JSON array.
$("<li>").text(item).appendTo($ul); // Create HTML <li> element, set its text content with currently iterated item and append it to the <ul>.
});
});
});
Do note that jQuery automatically parses the response as JSON and gives you directly a JSON object (responseJson) as function argument when you set the response content type to application/json. If you forget to set it or rely on a default of text/plain or text/html, then the responseJson argument wouldn't give you a JSON object, but a plain vanilla string and you'd need to manually fiddle around with JSON.parse() afterwards, which is thus totally unnecessary if you set the content type right in first place.
Returning Map<String, String> as JSON
Here's another example which displays Map<String, String> as <option>:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Map<String, String> options = new LinkedHashMap<>();
options.put("value1", "label1");
options.put("value2", "label2");
options.put("value3", "label3");
String json = new Gson().toJson(options);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
And the JSP:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $select = $("#someselect"); // Locate HTML DOM element with ID "someselect".
$select.find("option").remove(); // Find all child elements with tag name "option" and remove them (just to prevent duplicate options when button is pressed again).
$.each(responseJson, function(key, value) { // Iterate over the JSON object.
$("<option>").val(key).text(value).appendTo($select); // Create HTML <option> element, set its value with currently iterated key and its text content with currently iterated item and finally append it to the <select>.
});
});
});
with
<select id="someselect"></select>
Returning List<Entity> as JSON
Here's an example which displays List<Product> in a <table> where the Product class has the properties Long id, String name and BigDecimal price. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Product> products = someProductService.list();
String json = new Gson().toJson(products);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
The JS code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $table = $("<table>").appendTo($("#somediv")); // Create HTML <table> element and append it to HTML DOM element with ID "somediv".
$.each(responseJson, function(index, product) { // Iterate over the JSON array.
$("<tr>").appendTo($table) // Create HTML <tr> element, set its text content with currently iterated item and append it to the <table>.
.append($("<td>").text(product.id)) // Create HTML <td> element, set its text content with id of currently iterated product and append it to the <tr>.
.append($("<td>").text(product.name)) // Create HTML <td> element, set its text content with name of currently iterated product and append it to the <tr>.
.append($("<td>").text(product.price)); // Create HTML <td> element, set its text content with price of currently iterated product and append it to the <tr>.
});
});
});
Returning List<Entity> as XML
Here's an example which does effectively the same as previous example, but then with XML instead of JSON. When using JSP as XML output generator you'll see that it's less tedious to code the table and all. JSTL is this way much more helpful as you can actually use it to iterate over the results and perform server side data formatting. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Product> products = someProductService.list();
request.setAttribute("products", products);
request.getRequestDispatcher("/WEB-INF/xml/products.jsp").forward(request, response);
}
The JSP code (note: if you put the <table> in a <jsp:include>, it may be reusable elsewhere in a non-Ajax response):
<?xml version="1.0" encoding="UTF-8"?>
<%#page contentType="application/xml" pageEncoding="UTF-8"%>
<%#taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%#taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<data>
<table>
<c:forEach items="${products}" var="product">
<tr>
<td>${product.id}</td>
<td><c:out value="${product.name}" /></td>
<td><fmt:formatNumber value="${product.price}" type="currency" currencyCode="USD" /></td>
</tr>
</c:forEach>
</table>
</data>
The JavaScript code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseXml) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response XML...
$("#somediv").html($(responseXml).find("data").html()); // Parse XML, find <data> element and append its HTML to HTML DOM element with ID "somediv".
});
});
You'll by now probably realize why XML is so much more powerful than JSON for the particular purpose of updating a HTML document using Ajax. JSON is funny, but after all generally only useful for so-called "public web services". MVC frameworks like JSF use XML under the covers for their ajax magic.
Ajaxifying an existing form
You can use jQuery $.serialize() to easily ajaxify existing POST forms without fiddling around with collecting and passing the individual form input parameters. Assuming an existing form which works perfectly fine without JavaScript/jQuery (and thus degrades gracefully when the end user has JavaScript disabled):
<form id="someform" action="someservlet" method="post">
<input type="text" name="foo" />
<input type="text" name="bar" />
<input type="text" name="baz" />
<input type="submit" name="submit" value="Submit" />
</form>
You can progressively enhance it with Ajax as below:
$(document).on("submit", "#someform", function(event) {
var $form = $(this);
$.post($form.attr("action"), $form.serialize(), function(response) {
// ...
});
event.preventDefault(); // Important! Prevents submitting the form.
});
You can in the servlet distinguish between normal requests and Ajax requests as below:
#Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String foo = request.getParameter("foo");
String bar = request.getParameter("bar");
String baz = request.getParameter("baz");
boolean ajax = "XMLHttpRequest".equals(request.getHeader("X-Requested-With"));
// ...
if (ajax) {
// Handle Ajax (JSON or XML) response.
} else {
// Handle regular (JSP) response.
}
}
The jQuery Form plugin does less or more the same as above jQuery example, but it has additional transparent support for multipart/form-data forms as required by file uploads.
Manually sending request parameters to servlet
If you don't have a form at all, but just wanted to interact with the servlet "in the background" whereby you'd like to POST some data, then you can use jQuery $.param() to easily convert a JSON object to an URL-encoded query string.
var params = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.post("someservlet", $.param(params), function(response) {
// ...
});
The same doPost() method as shown here above can be reused. Do note that above syntax also works with $.get() in jQuery and doGet() in servlet.
Manually sending JSON object to servlet
If you however intend to send the JSON object as a whole instead of as individual request parameters for some reason, then you'd need to serialize it to a string using JSON.stringify() (not part of jQuery) and instruct jQuery to set request content type to application/json instead of (default) application/x-www-form-urlencoded. This can't be done via $.post() convenience function, but needs to be done via $.ajax() as below.
var data = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.ajax({
type: "POST",
url: "someservlet",
contentType: "application/json", // NOT dataType!
data: JSON.stringify(data),
success: function(response) {
// ...
}
});
Do note that a lot of starters mix contentType with dataType. The contentType represents the type of the request body. The dataType represents the (expected) type of the response body, which is usually unnecessary as jQuery already autodetects it based on response's Content-Type header.
Then, in order to process the JSON object in the servlet which isn't being sent as individual request parameters but as a whole JSON string the above way, you only need to manually parse the request body using a JSON tool instead of using getParameter() the usual way. Namely, servlets don't support application/json formatted requests, but only application/x-www-form-urlencoded or multipart/form-data formatted requests. Gson also supports parsing a JSON string into a JSON object.
JsonObject data = new Gson().fromJson(request.getReader(), JsonObject.class);
String foo = data.get("foo").getAsString();
String bar = data.get("bar").getAsString();
String baz = data.get("baz").getAsString();
// ...
Do note that this all is more clumsy than just using $.param(). Normally, you want to use JSON.stringify() only if the target service is e.g. a JAX-RS (RESTful) service which is for some reason only capable of consuming JSON strings and not regular request parameters.
Sending a redirect from servlet
Important to realize and understand is that any sendRedirect() and forward() call by the servlet on an ajax request would only forward or redirect the Ajax request itself and not the main document/window where the Ajax request originated. JavaScript/jQuery would in such case only retrieve the redirected/forwarded response as responseText variable in the callback function. If it represents a whole HTML page and not an Ajax-specific XML or JSON response, then all you could do is to replace the current document with it.
document.open();
document.write(responseText);
document.close();
Note that this doesn't change the URL as end user sees in browser's address bar. So there are issues with bookmarkability. Therefore, it's much better to just return an "instruction" for JavaScript/jQuery to perform a redirect instead of returning the whole content of the redirected page. E.g., by returning a boolean, or a URL.
String redirectURL = "http://example.com";
Map<String, String> data = new HashMap<>();
data.put("redirect", redirectURL);
String json = new Gson().toJson(data);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
function(responseJson) {
if (responseJson.redirect) {
window.location = responseJson.redirect;
return;
}
// ...
}
See also:
Call Servlet and invoke Java code from JavaScript along with parameters
Access Java / Servlet / JSP / JSTL / EL variables in JavaScript
How can I switch easily between an Ajax-based website and a basic HTML website?
How can I upload files to a server using JSP/Servlet and Ajax?

The right way to update the page currently displayed in the user's browser (without reloading it) is to have some code executing in the browser update the page's DOM.
That code is typically JavaScript that is embedded in or linked from the HTML page, hence the Ajax suggestion. (In fact, if we assume that the updated text comes from the server via an HTTP request, this is classic Ajax.)
It is also possible to implement this kind of thing using some browser plugin or add-on, though it may be tricky for a plugin to reach into the browser's data structures to update the DOM. (Native code plugins normally write to some graphics frame that is embedded in the page.)

I will show you a whole example of a servlet and how do an Ajax call.
Here, we are going to create the simple example to create the login form using a servlet.
File index.html
<form>
Name:<input type="text" name="username"/><br/><br/>
Password:<input type="password" name="userpass"/><br/><br/>
<input type="button" value="login"/>
</form>
An Ajax sample
$.ajax
({
type: "POST",
data: 'LoginServlet=' + name + '&name=' + type + '&pass=' + password,
url: url,
success:function(content)
{
$('#center').html(content);
}
});
LoginServlet servlet code:
package abc.servlet;
import java.io.File;
public class AuthenticationServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
doPost(request, response);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
try{
HttpSession session = request.getSession();
String username = request.getParameter("name");
String password = request.getParameter("pass");
/// Your Code
out.println("sucess / failer")
}
catch (Exception ex) {
// System.err.println("Initial SessionFactory creation failed.");
ex.printStackTrace();
System.exit(0);
}
}
}

$.ajax({
type: "POST",
url: "URL to hit on servelet",
data: JSON.stringify(json),
dataType: "json",
success: function(response){
// We have the response
if(response.status == "SUCCESS"){
$('#info').html("Info has been added to the list successfully.<br>" +
"The details are as follws: <br> Name: ");
}
else{
$('#info').html("Sorry, there is some thing wrong with the data provided.");
}
},
error: function(e){
alert('Error: ' + e);
}
});

Ajax (also AJAX), an acronym for Asynchronous JavaScript and XML, is a group of interrelated web development techniques used on the client-side to create asynchronous web applications. With Ajax, web applications can send data to, and retrieve data from, a server asynchronously.
Below is the example code:
A JSP page JavaScript function to submit data to a servlet with two variables, firstName and lastName:
function onChangeSubmitCallWebServiceAJAX()
{
createXmlHttpRequest();
var firstName = document.getElementById("firstName").value;
var lastName = document.getElementById("lastName").value;
xmlHttp.open("GET", "/AJAXServletCallSample/AjaxServlet?firstName="
+ firstName + "&lastName=" + lastName, true)
xmlHttp.onreadystatechange = handleStateChange;
xmlHttp.send(null);
}
Servlet to read data send back to JSP in XML format (you could use text as well. You just need to change the response content to text and render data on JavaScript function.)
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
response.getWriter().write("<details>");
response.getWriter().write("<firstName>" + firstName + "</firstName>");
response.getWriter().write("<lastName>" + lastName + "</lastName>");
response.getWriter().write("</details>");
}

Normally you can’t update a page from a servlet. The client (browser) has to request an update. Either the client loads a whole new page or it requests an update to a part of an existing page. This technique is called Ajax.

Using Bootstrap multi select:
Ajax
function() { $.ajax({
type: "get",
url: "OperatorController",
data: "input=" + $('#province').val(),
success: function(msg) {
var arrayOfObjects = eval(msg);
$("#operators").multiselect('dataprovider',
arrayOfObjects);
// $('#output').append(obj);
},
dataType: 'text'
});}
}
In Servlet
request.getParameter("input")

How to use ajax in jsp [duplicate]

Whenever I print something inside the servlet and call it by the webbrowser, it returns a new page containing that text. Is there a way to print the text in the current page using Ajax?
I'm very new to web applications and servlets.

Indeed, the keyword is "Ajax": Asynchronous JavaScript and XML. However, last years it's more than often Asynchronous JavaScript and JSON. Basically, you let JavaScript execute an asynchronous HTTP request and update the HTML DOM tree based on the response data.
Since it's pretty tedious work to make it to work across all browsers (especially Internet Explorer versus others), there are plenty of JavaScript libraries out which simplifies this in single functions and covers as many as possible browser-specific bugs/quirks under the hoods, such as jQuery, Prototype, Mootools. Since jQuery is most popular these days, I'll use it in the below examples.
Kickoff example returning String as plain text
Create a /some.jsp like below (note: the code snippets in this answer doesn't expect the JSP file being placed in a subfolder, if you do so, alter servlet URL accordingly from "someservlet" to "${pageContext.request.contextPath}/someservlet"; it's merely omitted from the code snippets for brevity):
<!DOCTYPE html>
<html lang="en">
<head>
<title>SO question 4112686</title>
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseText) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response text...
$("#somediv").text(responseText); // Locate HTML DOM element with ID "somediv" and set its text content with the response text.
});
});
</script>
</head>
<body>
<button id="somebutton">press here</button>
<div id="somediv"></div>
</body>
</html>
Create a servlet with a doGet() method which look like this:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String text = "some text";
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(text); // Write response body.
}
Map this servlet on an URL pattern of /someservlet or /someservlet/* as below (obviously, the URL pattern is free to your choice, but you'd need to alter the someservlet URL in JS code examples over all place accordingly):
package com.example;
#WebServlet("/someservlet/*")
public class SomeServlet extends HttpServlet {
// ...
}
Or, when you're not on a Servlet 3.0 compatible container yet (Tomcat 7, GlassFish 3, JBoss AS 6, etc. or newer), then map it in web.xml the old fashioned way (see also our Servlets wiki page):
<servlet>
<servlet-name>someservlet</servlet-name>
<servlet-class>com.example.SomeServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>someservlet</servlet-name>
<url-pattern>/someservlet/*</url-pattern>
</servlet-mapping>
Now open the http://localhost:8080/context/test.jsp in the browser and press the button. You'll see that the content of the div get updated with the servlet response.
Returning List<String> as JSON
With JSON instead of plaintext as response format you can even get some steps further. It allows for more dynamics. First, you'd like to have a tool to convert between Java objects and JSON strings. There are plenty of them as well (see the bottom of this page for an overview). My personal favourite is Google Gson. Download and put its JAR file in /WEB-INF/lib folder of your web application.
Here's an example which displays List<String> as <ul><li>. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<String> list = new ArrayList<>();
list.add("item1");
list.add("item2");
list.add("item3");
String json = new Gson().toJson(list);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
The JavaScript code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $ul = $("<ul>").appendTo($("#somediv")); // Create HTML <ul> element and append it to HTML DOM element with ID "somediv".
$.each(responseJson, function(index, item) { // Iterate over the JSON array.
$("<li>").text(item).appendTo($ul); // Create HTML <li> element, set its text content with currently iterated item and append it to the <ul>.
});
});
});
Do note that jQuery automatically parses the response as JSON and gives you directly a JSON object (responseJson) as function argument when you set the response content type to application/json. If you forget to set it or rely on a default of text/plain or text/html, then the responseJson argument wouldn't give you a JSON object, but a plain vanilla string and you'd need to manually fiddle around with JSON.parse() afterwards, which is thus totally unnecessary if you set the content type right in first place.
Returning Map<String, String> as JSON
Here's another example which displays Map<String, String> as <option>:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Map<String, String> options = new LinkedHashMap<>();
options.put("value1", "label1");
options.put("value2", "label2");
options.put("value3", "label3");
String json = new Gson().toJson(options);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
And the JSP:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $select = $("#someselect"); // Locate HTML DOM element with ID "someselect".
$select.find("option").remove(); // Find all child elements with tag name "option" and remove them (just to prevent duplicate options when button is pressed again).
$.each(responseJson, function(key, value) { // Iterate over the JSON object.
$("<option>").val(key).text(value).appendTo($select); // Create HTML <option> element, set its value with currently iterated key and its text content with currently iterated item and finally append it to the <select>.
});
});
});
with
<select id="someselect"></select>
Returning List<Entity> as JSON
Here's an example which displays List<Product> in a <table> where the Product class has the properties Long id, String name and BigDecimal price. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Product> products = someProductService.list();
String json = new Gson().toJson(products);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
}
The JS code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseJson) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response JSON...
var $table = $("<table>").appendTo($("#somediv")); // Create HTML <table> element and append it to HTML DOM element with ID "somediv".
$.each(responseJson, function(index, product) { // Iterate over the JSON array.
$("<tr>").appendTo($table) // Create HTML <tr> element, set its text content with currently iterated item and append it to the <table>.
.append($("<td>").text(product.id)) // Create HTML <td> element, set its text content with id of currently iterated product and append it to the <tr>.
.append($("<td>").text(product.name)) // Create HTML <td> element, set its text content with name of currently iterated product and append it to the <tr>.
.append($("<td>").text(product.price)); // Create HTML <td> element, set its text content with price of currently iterated product and append it to the <tr>.
});
});
});
Returning List<Entity> as XML
Here's an example which does effectively the same as previous example, but then with XML instead of JSON. When using JSP as XML output generator you'll see that it's less tedious to code the table and all. JSTL is this way much more helpful as you can actually use it to iterate over the results and perform server side data formatting. The servlet:
#Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Product> products = someProductService.list();
request.setAttribute("products", products);
request.getRequestDispatcher("/WEB-INF/xml/products.jsp").forward(request, response);
}
The JSP code (note: if you put the <table> in a <jsp:include>, it may be reusable elsewhere in a non-Ajax response):
<?xml version="1.0" encoding="UTF-8"?>
<%#page contentType="application/xml" pageEncoding="UTF-8"%>
<%#taglib prefix="c" uri="http://java.sun.com/jsp/jstl/core" %>
<%#taglib prefix="fmt" uri="http://java.sun.com/jsp/jstl/fmt" %>
<data>
<table>
<c:forEach items="${products}" var="product">
<tr>
<td>${product.id}</td>
<td><c:out value="${product.name}" /></td>
<td><fmt:formatNumber value="${product.price}" type="currency" currencyCode="USD" /></td>
</tr>
</c:forEach>
</table>
</data>
The JavaScript code:
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get("someservlet", function(responseXml) { // Execute Ajax GET request on URL of "someservlet" and execute the following function with Ajax response XML...
$("#somediv").html($(responseXml).find("data").html()); // Parse XML, find <data> element and append its HTML to HTML DOM element with ID "somediv".
});
});
You'll by now probably realize why XML is so much more powerful than JSON for the particular purpose of updating a HTML document using Ajax. JSON is funny, but after all generally only useful for so-called "public web services". MVC frameworks like JSF use XML under the covers for their ajax magic.
Ajaxifying an existing form
You can use jQuery $.serialize() to easily ajaxify existing POST forms without fiddling around with collecting and passing the individual form input parameters. Assuming an existing form which works perfectly fine without JavaScript/jQuery (and thus degrades gracefully when the end user has JavaScript disabled):
<form id="someform" action="someservlet" method="post">
<input type="text" name="foo" />
<input type="text" name="bar" />
<input type="text" name="baz" />
<input type="submit" name="submit" value="Submit" />
</form>
You can progressively enhance it with Ajax as below:
$(document).on("submit", "#someform", function(event) {
var $form = $(this);
$.post($form.attr("action"), $form.serialize(), function(response) {
// ...
});
event.preventDefault(); // Important! Prevents submitting the form.
});
You can in the servlet distinguish between normal requests and Ajax requests as below:
#Override
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String foo = request.getParameter("foo");
String bar = request.getParameter("bar");
String baz = request.getParameter("baz");
boolean ajax = "XMLHttpRequest".equals(request.getHeader("X-Requested-With"));
// ...
if (ajax) {
// Handle Ajax (JSON or XML) response.
} else {
// Handle regular (JSP) response.
}
}
The jQuery Form plugin does less or more the same as above jQuery example, but it has additional transparent support for multipart/form-data forms as required by file uploads.
Manually sending request parameters to servlet
If you don't have a form at all, but just wanted to interact with the servlet "in the background" whereby you'd like to POST some data, then you can use jQuery $.param() to easily convert a JSON object to an URL-encoded query string.
var params = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.post("someservlet", $.param(params), function(response) {
// ...
});
The same doPost() method as shown here above can be reused. Do note that above syntax also works with $.get() in jQuery and doGet() in servlet.
Manually sending JSON object to servlet
If you however intend to send the JSON object as a whole instead of as individual request parameters for some reason, then you'd need to serialize it to a string using JSON.stringify() (not part of jQuery) and instruct jQuery to set request content type to application/json instead of (default) application/x-www-form-urlencoded. This can't be done via $.post() convenience function, but needs to be done via $.ajax() as below.
var data = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.ajax({
type: "POST",
url: "someservlet",
contentType: "application/json", // NOT dataType!
data: JSON.stringify(data),
success: function(response) {
// ...
}
});
Do note that a lot of starters mix contentType with dataType. The contentType represents the type of the request body. The dataType represents the (expected) type of the response body, which is usually unnecessary as jQuery already autodetects it based on response's Content-Type header.
Then, in order to process the JSON object in the servlet which isn't being sent as individual request parameters but as a whole JSON string the above way, you only need to manually parse the request body using a JSON tool instead of using getParameter() the usual way. Namely, servlets don't support application/json formatted requests, but only application/x-www-form-urlencoded or multipart/form-data formatted requests. Gson also supports parsing a JSON string into a JSON object.
JsonObject data = new Gson().fromJson(request.getReader(), JsonObject.class);
String foo = data.get("foo").getAsString();
String bar = data.get("bar").getAsString();
String baz = data.get("baz").getAsString();
// ...
Do note that this all is more clumsy than just using $.param(). Normally, you want to use JSON.stringify() only if the target service is e.g. a JAX-RS (RESTful) service which is for some reason only capable of consuming JSON strings and not regular request parameters.
Sending a redirect from servlet
Important to realize and understand is that any sendRedirect() and forward() call by the servlet on an ajax request would only forward or redirect the Ajax request itself and not the main document/window where the Ajax request originated. JavaScript/jQuery would in such case only retrieve the redirected/forwarded response as responseText variable in the callback function. If it represents a whole HTML page and not an Ajax-specific XML or JSON response, then all you could do is to replace the current document with it.
document.open();
document.write(responseText);
document.close();
Note that this doesn't change the URL as end user sees in browser's address bar. So there are issues with bookmarkability. Therefore, it's much better to just return an "instruction" for JavaScript/jQuery to perform a redirect instead of returning the whole content of the redirected page. E.g., by returning a boolean, or a URL.
String redirectURL = "http://example.com";
Map<String, String> data = new HashMap<>();
data.put("redirect", redirectURL);
String json = new Gson().toJson(data);
response.setContentType("application/json");
response.setCharacterEncoding("UTF-8");
response.getWriter().write(json);
function(responseJson) {
if (responseJson.redirect) {
window.location = responseJson.redirect;
return;
}
// ...
}
See also:
Call Servlet and invoke Java code from JavaScript along with parameters
Access Java / Servlet / JSP / JSTL / EL variables in JavaScript
How can I switch easily between an Ajax-based website and a basic HTML website?
How can I upload files to a server using JSP/Servlet and Ajax?

The right way to update the page currently displayed in the user's browser (without reloading it) is to have some code executing in the browser update the page's DOM.
That code is typically JavaScript that is embedded in or linked from the HTML page, hence the Ajax suggestion. (In fact, if we assume that the updated text comes from the server via an HTTP request, this is classic Ajax.)
It is also possible to implement this kind of thing using some browser plugin or add-on, though it may be tricky for a plugin to reach into the browser's data structures to update the DOM. (Native code plugins normally write to some graphics frame that is embedded in the page.)

I will show you a whole example of a servlet and how do an Ajax call.
Here, we are going to create the simple example to create the login form using a servlet.
File index.html
<form>
Name:<input type="text" name="username"/><br/><br/>
Password:<input type="password" name="userpass"/><br/><br/>
<input type="button" value="login"/>
</form>
An Ajax sample
$.ajax
({
type: "POST",
data: 'LoginServlet=' + name + '&name=' + type + '&pass=' + password,
url: url,
success:function(content)
{
$('#center').html(content);
}
});
LoginServlet servlet code:
package abc.servlet;
import java.io.File;
public class AuthenticationServlet extends HttpServlet {
private static final long serialVersionUID = 1L;
protected void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException
{
doPost(request, response);
}
protected void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
try{
HttpSession session = request.getSession();
String username = request.getParameter("name");
String password = request.getParameter("pass");
/// Your Code
out.println("sucess / failer")
}
catch (Exception ex) {
// System.err.println("Initial SessionFactory creation failed.");
ex.printStackTrace();
System.exit(0);
}
}
}

$.ajax({
type: "POST",
url: "URL to hit on servelet",
data: JSON.stringify(json),
dataType: "json",
success: function(response){
// We have the response
if(response.status == "SUCCESS"){
$('#info').html("Info has been added to the list successfully.<br>" +
"The details are as follws: <br> Name: ");
}
else{
$('#info').html("Sorry, there is some thing wrong with the data provided.");
}
},
error: function(e){
alert('Error: ' + e);
}
});

Ajax (also AJAX), an acronym for Asynchronous JavaScript and XML, is a group of interrelated web development techniques used on the client-side to create asynchronous web applications. With Ajax, web applications can send data to, and retrieve data from, a server asynchronously.
Below is the example code:
A JSP page JavaScript function to submit data to a servlet with two variables, firstName and lastName:
function onChangeSubmitCallWebServiceAJAX()
{
createXmlHttpRequest();
var firstName = document.getElementById("firstName").value;
var lastName = document.getElementById("lastName").value;
xmlHttp.open("GET", "/AJAXServletCallSample/AjaxServlet?firstName="
+ firstName + "&lastName=" + lastName, true)
xmlHttp.onreadystatechange = handleStateChange;
xmlHttp.send(null);
}
Servlet to read data send back to JSP in XML format (you could use text as well. You just need to change the response content to text and render data on JavaScript function.)
/**
* #see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
*/
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String firstName = request.getParameter("firstName");
String lastName = request.getParameter("lastName");
response.setContentType("text/xml");
response.setHeader("Cache-Control", "no-cache");
response.getWriter().write("<details>");
response.getWriter().write("<firstName>" + firstName + "</firstName>");
response.getWriter().write("<lastName>" + lastName + "</lastName>");
response.getWriter().write("</details>");
}

Normally you can’t update a page from a servlet. The client (browser) has to request an update. Either the client loads a whole new page or it requests an update to a part of an existing page. This technique is called Ajax.

Using Bootstrap multi select:
Ajax
function() { $.ajax({
type: "get",
url: "OperatorController",
data: "input=" + $('#province').val(),
success: function(msg) {
var arrayOfObjects = eval(msg);
$("#operators").multiselect('dataprovider',
arrayOfObjects);
// $('#output').append(obj);
},
dataType: 'text'
});}
}
In Servlet
request.getParameter("input")

ajax + servlet country state city list

I am trying to populate city, country and statelist using ajax, and servlets.
Now I know how to get the XMLhttpRequest object. There is a standard mechanism to do that and depending on the cross browser compatibility, you do get a ActiveX or a xml object..
Then you send a request to the actionservlet, using xmlhttprequest.open() and you send the request and you have an event handler function to take care of onreadystatechange issue, Now, when it comes to receiving the response, I get an error stating that the response is not completely received, i.e the status != 4... now.
I was wondering, how does the entire mechanism work..
how to put the parameters in the request, send it to servlet, and then I know how to retreieve the param from the URL.. but then how to send a valid response...??
I am confused on the ajax part, because I am not using / not use PHP. It is harder to think. Please suggest what should be done.
Is there any easier way to populate a city, country and state list??

Just let the servlet return the desired dropdown options in the desired format based on the request parameters in the doGet() method. Here's an example which returns it in JSON format with help of Google Gson:
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
String type = request.getParameter("type"); // Returns "country" or "state".
String value = request.getParameter("value"); // Value of selected country or state.
Map<String, String> options = optionDAO.find(type, id); // Do your thing to obtain them from DB. Map key is option value and map value is option label.
String json = new Gson().toJson(options); // Convert Java object to JSON string.
response.setContentType("application/json"); // Inform client that you're returning JSON.
response.setCharacterEncoding("UTF-8"); // Important if you want world domination.
response.getWriter().write(json); // Write JSON string to response.
}
Assuming that the above servlet is mapped on an url-pattern of /options, you could use it as in the following JSP example with help of jQuery. I am recommending jQuery because you would otherwise end up with 10 times as much JavaScript code needed for this "simple" task.
<%# page pageEncoding="UTF-8" %>
<%# taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html>
<html lang="en">
<head>
<title>SO question 3983929</title>
<script src="http://code.jquery.com/jquery-latest.min.js"></script>
<script>
$(document).ready(function() {
$('#country').change(function() { fillOptions('state'); });
$('#state').change(function() { fillOptions('city'); });
});
function fillOptions(dropdownId) {
var dropdown = $('#' + dropdownId);
$.getJSON('options?type=' + dropdownId + '&value=' + $(this).val(), function(opts) {
$('>option', dropdown).remove(); // Clean old options first.
if (opts) {
$.each(opts, function(key, value) {
dropdown.append($('<option/>').val(key).text(value));
});
} else {
dropdown.append($('<option/>').text('Please select ' + dropdownId));
}
});
}
</script>
</head>
<body>
<form>
<select id="country" name="country">
<c:forEach items="${countries}" var="country">
<option value="${country.key}" ${param.country == option.key ? 'selected' : ''}>${country.value}</option>
</c:forEach>
</select>
<select id="state" name="state">
<option>Please select country</option>
</select>
<select id="city" name="city">
<option>Please select state</option>
</select>
</form>
</body>
</html>
Here I am assuming that you've already prepopulated the ${countries} as being a Map<String, String> in a servlet which has preprocessed the request to this JSP for the initial display.