Whenever I print something inside the servlet and call it by the web browser, it returns a new page containing that text. Is there a way to print the text on the current page using Ajax?
I'm very new to web applications and servlets.
How should I use servlets and Ajax?
Kickoff example returning
Returning
Returning
Returning
Returning
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:
<!DOCTYPE html>
<html lang="en">
<head>
<title>SO question 4112686</title>
<script src="https://code.jquery.com/jquery-3.7.1.min.js"></script>
<script>
const yourServletURL = "${pageContext.request.contextPath}/yourServlet";
$(document).on("click", "#somebutton", function() { // When HTML DOM "click" event is invoked on element with ID "somebutton", execute the following function...
$.get(yourServletURL, function(responseText) { // Execute Ajax GET request on your servlet URL 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:
@WebServlet("/yourServlet")
public class YourServlet extends HttpServlet {
@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.
}
}
Obviously, the URL pattern of /yourServlet is free to your choice, but if you change it then you'll need to alter the /yourServlet string in the yourServletURL JS variable accordingly.
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 gets 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.
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(yourServletURL, function(responseJson) { // Execute Ajax GET request on your servlet URL and execute the following function with Ajax response JSON...
const $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(yourServletURL, function(responseJson) { // Execute Ajax GET request on your servlet URL and execute the following function with Ajax response JSON...
const $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(yourServletURL, function(responseJson) { // Execute Ajax GET request on your servlet URL and execute the following function with Ajax response JSON...
const $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($("<td>").text(product.price)); // Create HTML <td> element, set its text content with price of currently iterated product and append it to the <tr>.
});
});
});
Returning List<Entity> as XML
Here's an example which does effectively the same as previous example, but then with XML instead of JSON. When using JSP as XML output generator you'll see that it's less tedious to code the table and all. JSTL is this way much more helpful as you can actually use it to iterate over the results and perform server side data formatting. The servlet:
@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
List<Product> products = someProductService.list();
request.setAttribute("products", products);
request.getRequestDispatcher("/WEB-INF/xml/products.jsp").forward(request, response);
}
The JSP code (note: if you put the <table> in a <jsp:include>, it may be reusable elsewhere in a non-Ajax response):
<?xml version="1.0" encoding="UTF-8"?>
<%@page contentType="application/xml" pageEncoding="UTF-8"%>
<%@taglib prefix="c" uri="jakarta.tags.core" %>
<%@taglib prefix="fmt" uri="jakarta.tags.fmt" %>
<data>
<table>
<c:forEach items="${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(yourServletURL, function(responseXml) { // Execute Ajax GET request on your servlet URL 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 $.serialize() to easily ajaxify existing POST forms without fiddling around with collecting and passing the individual form input parameters. Assuming an existing form which works perfectly fine without JavaScript/jQuery (and thus degrades gracefully when the end user has JavaScript disabled):
<form id="someform" action="${pageContext.request.contextPath}/yourServletURL" 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) {
const $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 as per application/x-www-form-urlencoded content type, exactly as used by normal HTML forms, so that you can continue using request.getParameter(name) to extract the data.
const params = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.post(yourServletURL, $.param(params), function(response) {
// ...
});
The $.post() is essentially a shorthand for the following $.ajax() call.
$.ajax({
type: "POST",
url: yourServletURL,
data: $.param(params),
success: function(response) {
// ...
}
});
The same doPost() method as shown in previous section can be reused. Do note that above $.post() 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.
const data = {
foo: "fooValue",
bar: "barValue",
baz: "bazValue"
};
$.ajax({
type: "POST",
url: yourServletURL,
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:
need to parse the json on the last example. –
Monopteros
@kuhaku: nope. If you read post from top to bottom, you'll learn why. –
Anisotropic
This answer has been my lifeline for the last month or so lol. Learning a bunch from it. I LOVE the XML example. Thanks for putting this together! One noob question though if you have time. Is there a reason for putting the xml folder in WEB-INF ? –
Hollenbeck
@JonathanLaliberte: So users cannot download them. –
Anisotropic
@BalusC, your XML example is great, thank you. But I am getting "Unable to get property 'replace' of undefined or null reference" for
$("#somediv").html($(responseXml).find("data").html()) this line. It also says "Wrong number of arguments or invalid property assignment". I can also see that my XML is populated with data when I debug it. Any ideas ? –
Floury @629: perhaps you don't have a
<div id="somediv">? –
Anisotropic @BalusC, It worked when I open the page on Edge. Any ideas on how to make it work for IE11 as well? –
Floury
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);
}
}
}
I am unable to get any content in the ajax success function –
Dictate
$.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);
}
});
An explanation would be in order. E.g., what is the idea/gist? Please respond by editing (changing) your answer, not here in comments (without "Edit:", "Update:", or similar - the answer should appear as if it was written today). –
Fateful
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")
To post JSON data with AJAX, you can use the "POST" method of the XMLHttpRequest object and set the "Content-Type" header to "application/json". Here is the JavaScript code for sending AJAX request:
Define the data to be posted
const data = {
key1: 'value1',
key2: 'value2'
};
// Create a new XMLHttpRequest object
const xhr = new XMLHttpRequest();
// Set the URL and method
xhr.open('POST', 'http://example.com/endpoint');
// Set the content type to JSON
xhr.setRequestHeader('Content-Type', 'application/json');
// Send the request
xhr.send(JSON.stringify(data));
In this code JavaScript object called "data" that contains the JSON data to be posted. It then creates a new XMLHttpRequest object and uses the "open" and "setRequestHeader" methods to configure the request. The "Content-Type" header is set to "application/json" to indicate that the request body contains JSON data. The request is then sent using the "send" method, with the JSON data stringified using the "JSON.stringify" function.
The follonging Servlet code can read data kay1 and key2 and do necessary work.
You have to use Servlet Version 3.0
package nam;
import java.io.IOException;
import java.io.PrintWriter;
import java.io.BufferedReader;
import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.sql.*;
import java.util.*;
import com.google.gson.Gson;
import com.google.gson.*;
import databaseconnection.DBConnect;
import java.sql.*;
@WebServlet("/yearlynooflicencesservlet")
public class YearlyNoOfLicencesServlet extends HttpServlet
{
private static final long serialVersionUID = 1L;
protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
{
StringBuilder sb=new StringBuilder();
BufferedReader reader = request.getReader();
String line="";
try
{
while ((line = reader.readLine()) != null) {
sb.append(line).append('\n');
}
} finally
{
reader.close();
}
//catch(Exception ex){}
String json_str=sb.toString();
System.out.println("Client Side Data in YearlyNoOfLicencesServlet ="+sb.toString());
String state_code="";
//String scheme_category_code="";
try
{
JsonParser parser=new JsonParser();
JsonElement jsonElement=parser.parse(json_str);
JsonObject rootObject=jsonElement.getAsJsonObject();
String key1=rootObject.get("key1").getAsString();
String key2=rootObject.get("key2").getAsString();
// You may get other type of data also.
}
catch(Exception ex){System.out.println(ex);}
try
{
response.setContentType("application/json");
PrintWriter out=response.getWriter();
String data="{data: 'data has been properly receved.'}";
Gson gson=new Gson();
String gson_str=gson.toJson(data);
out.println(gson_str);
}
catch(Exception ex){System.out.println(ex);}
finally {
try{
}
catch(Exception ex){
System.out.println(ex.getMessage());
}
}
}
}
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