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Most votes on ajax questions 5

Most votes on ajax questions 5. #41 How many concurrent AJAX (XmlHttpRequest) requests are allowed in popular browsers? #42 Getting "net::ERR_BLOCKED_BY_CLIENT" error on some AJAX calls #43 CORS: Cannot use wildcard in Access-Control-Allow-Origin when credentials flag is true #44 Why is jQuery's .ajax() method not sending my session cookie? #45 Origin is not allowed by Access-Control-Allow-Origin #46 How to use Servlets and Ajax? #47 How can I add a custom HTTP header to ajax request with js or jQuery? #48 jQuery $.ajax(), $.post sending "OPTIONS" as REQUEST_METHOD in Firefox #49 Jquery - How to make $.post() use contentType=application/json? #50 jQuery.ajax handling continue responses: "success:" vs ".done"?

Read all the top votes questions and answers in a single page.

Created: 2009-02-18 Last updated: 2013-09-04

Tags: ajax, browser, xmlhttprequest

In Firefox 3, the answer is 6 per domain: as soon as a 7th XmlHttpRequest (on any tab) to the same domain is fired, it is queued until one of the other 6 finish.

What are the numbers for the other major browsers?

Also, are there ways around these limits without having my users modify their browser settings? For example, are there limits to the number of jsonp requests (which use script tag injection rather than an XmlHttpRequest object)?

Background: My users can make XmlHttpRequests from a web page to the server, asking the server to run ssh commands on remote hosts. If the remote hosts are down, the ssh command takes a few minutes to fail, eventually preventing my users from performing any further commands.

Created: 2009-02-18 Last updated: 2016-10-07

One trick you can use to increase the number of concurrent connections is to host your images from a different sub domain. These will be treated as separate requests, each domain is what will be limited to the concurrent maximum.

IE6, IE7 - have a limit of two. IE8 is 6 if you have a broadband - 2 (if it’s a dial up).

Created: 2010-08-30

The network results at Browserscope will give you both Connections per Hostname and Max Connections for popular browsers. The data is gathered by running tests on users “in the wild,” so it will stay up to date.

See also original question in stackoverflow

#42: Getting "net::ERR_BLOCKED_BY_CLIENT" error on some AJAX calls (Score: 361)

Created: 2014-04-28 Last updated: 2016-03-04

Tags: ajax, google-chrome-extension, firefox-addon

Recently I’ve realised that, some adblocker extensions (such as adBlocker plus) block some Ajax calls. I get that error on the console:

GET http://localhost/prj/conn.php?q=users/list/ net::ERR_BLOCKED_BY_CLIENT 

Why does it block some Ajax call but not the others and what causes that? Is there any workaround other than telling user to turn off adblocker?

#42 Best answer 1 of Getting "net::ERR_BLOCKED_BY_CLIENT" error on some AJAX calls (Score: 621)

Created: 2014-04-28

AdBlockers usually have some rules, i.e. they match the URIs against some type of expression (sometimes they also match the DOM against expressions, not that this matters in this case).

Having rules and expressions that just operate on a tiny bit of text (the URI) is prone to create some false-positives…

Besides instructing your users to disable their extensions (at least on your site) you can also get the extension and test which of the rules/expressions blocked your stuff, provided the extension provides enough details about that. Once you identified the culprit, you can either try to avoid triggering the rule by using different URIs, report the rule as incorrect or overly-broad to the team that created it, or both. Check the docs for a particular add-on on how to do that.

For example, AdBlock Plus has a Blockable items view that shows all blocked items on a page and the rules that triggered the block. And those items also including XHR requests.

Blockable items

#42 Best answer 2 of Getting "net::ERR_BLOCKED_BY_CLIENT" error on some AJAX calls(Score: 88)

Created: 2015-10-02 Last updated: 2017-01-19

If your URL contains words such as “advert”, “ad”, “doubleclick”, “click”, or something similar…

For example:

  • GET googleads.g.doubleclick.net/pagead/id
  • static.doubleclick.net/instream/ad_status.js

…Then ad-blocker will block it.

See also original question in stackoverflow

#43: CORS: Cannot use wildcard in Access-Control-Allow-Origin when credentials flag is true (Score: 356)

Created: 2013-11-02 Last updated: 2015-10-01

Tags: ajax, django, node.js, cors, django-cors-headers

I have a setup involving

Frontend server (Node.js, domain: localhost:3000) <—> Backend (Django, Ajax, domain: localhost:8000)

Browser <– webapp <– Node.js (Serve the app)

Browser (webapp) –> Ajax –> Django(Serve ajax POST requests)

Now, my problem here is with CORS setup which the webapp uses to make Ajax calls to the backend server. In chrome, I keep getting

Cannot use wildcard in Access-Control-Allow-Origin when credentials flag is true.

doesn’t work on firefox either.

My Node.js setup is:

var allowCrossDomain = function(req, res, next) {
    res.header('Access-Control-Allow-Origin', 'http://localhost:8000/');
    res.header('Access-Control-Allow-Credentials', true);
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE');
    res.header("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type, Accept");
    next();
};

And in Django I’m using this middleware along with this

The webapp makes requests as such:

$.ajax({
    type: "POST",
    url: 'http://localhost:8000/blah',
    data: {},
    xhrFields: {
        withCredentials: true
    },
    crossDomain: true,
    dataType: 'json',
    success: successHandler
});

So, the request headers that the webapp sends looks like:

Access-Control-Allow-Credentials: true
Access-Control-Allow-Headers: "Origin, X-Requested-With, Content-Type, Accept"
Access-Control-Allow-Methods: 'GET,PUT,POST,DELETE'
Content-Type: application/json 
Accept: */*
Accept-Encoding: gzip,deflate,sdch
Accept-Language: en-US,en;q=0.8
Cookie: csrftoken=***; sessionid="***"

And here’s the response header:

Access-Control-Allow-Headers: Content-Type,*
Access-Control-Allow-Credentials: true
Access-Control-Allow-Origin: *
Access-Control-Allow-Methods: POST,GET,OPTIONS,PUT,DELETE
Content-Type: application/json

Where am I going wrong?!

Edit 1: I’ve been using chrome --disable-web-security, but now want things to actually work.

Edit 2: Answer:

So, solution for me django-cors-headers config:

CORS_ORIGIN_ALLOW_ALL = False
CORS_ALLOW_CREDENTIALS = True
CORS_ORIGIN_WHITELIST = (
    'http://localhost:3000' # Here was the problem indeed and it has to be http://localhost:3000, not http://localhost:3000/
)

#43 Best answer 1 of CORS: Cannot use wildcard in Access-Control-Allow-Origin when credentials flag is true (Score: 286)

Created: 2013-11-02 Last updated: 2017-05-23

This is a part of security, you cannot do that. If you want to allow credentials then your Access-Control-Allow-Origin must not use *. You will have to specify the exact protocol + domain + port. For reference see these questions :

  1. https://stackoverflow.com/questions/14003332/access-control-allow-origin-wildcard-subdomains-ports-and-protocols
  2. https://stackoverflow.com/questions/8074665/cross-origin-resource-sharing-with-credentials

Besides * is too permissive and would defeat use of credentials. So set http://localhost:3000 or http://localhost:8000 as the allow origin header.

#43 Best answer 2 of CORS: Cannot use wildcard in Access-Control-Allow-Origin when credentials flag is true(Score: 52)

Created: 2017-08-26 Last updated: 2017-09-18

If you are using CORS middleware and you want to send withCredential boolean true, you can configure CORS like this:

var cors = require('cors');    
app.use(cors({credentials: true, origin: 'http://localhost:3000'}));

See also original question in stackoverflow

Created: 2010-05-20 Last updated: 2021-04-02

Tags: jquery, ajax, session, cookies

After logging in via $.ajax() to a site, I am trying to send a second $.ajax() request to that site - but when I check the headers sent using FireBug, there is no session cookie being included in the request.

What am I doing wrong?

Created: 2011-08-25 Last updated: 2016-03-05

I am operating in cross-domain scenario. During login remote server is returning Set-Cookie header along with Access-Control-Allow-Credentials set to true.

The next ajax call to remote server should use this cookie.

CORS’s Access-Control-Allow-Credentials is there to allow cross-domain logging. Check https://developer.mozilla.org/En/HTTP_access_control for examples.

For me it seems like a bug in JQuery (or at least feature-to-be in next version).

UPDATE:

  1. Cookies are not set automatically from AJAX response (citation: http://aleembawany.com/2006/11/14/anatomy-of-a-well-designed-ajax-login-experience/)

Why?

  1. You cannot get value of the cookie from response to set it manually (http://www.w3.org/TR/XMLHttpRequest/#dom-xmlhttprequest-getresponseheader)

I’m confused..

There should exist a way to ask jquery.ajax() to set XMLHttpRequest.withCredentials = "true" parameter.

ANSWER: You should use xhrFields param of http://api.jquery.com/jQuery.ajax/

The example in the documentation is:

$.ajax({
   url: a_cross_domain_url,
   xhrFields: {
      withCredentials: true
   }
});

It’s important as well that server answers correctly to this request. Copying here great comments from @Frédéric and @Pebbl:

Important note: when responding to a credentialed request, server must specify a domain, and cannot use wild carding. The above example would fail if the header was wildcarded as: Access-Control-Allow-Origin: *

So when the request is:

Origin: http://foo.example
Cookie: pageAccess=2

Server should respond with:

Access-Control-Allow-Origin: http://foo.example
Access-Control-Allow-Credentials: true

[payload]

Otherwise payload won’t be returned to script. See: https://developer.mozilla.org/en-US/docs/Web/HTTP/Access_control_CORS#Requests_with_credentials

#44 Best answer 2 of Why is jQuery's .ajax() method not sending my session cookie?(Score: 234)

Created: 2011-03-24 Last updated: 2015-09-14

AJAX calls only send Cookies if the url you’re calling is on the same domain as your calling script.

This may be a Cross Domain Problem.

Maybe you tried to call a url from www.domain-a.com while your calling script was on www.domain-b.com (In other words: You made a Cross Domain Call in which case the browser won’t sent any cookies to protect your privacy).

In this case your options are:

  • Write a small proxy which resides on domain-b and forwards your requests to domain-a. Your browser will allow you to call the proxy because it’s on the same server as the calling script.
    This proxy then can be configured by you to accept a cookie name and value parameter which it can send to domain-a. But for this to work you need to know the cookie’s name and value your server on domain-a wants for authentication.
  • If you’re fetching JSON objects try to use a JSONP request instead. jQuery supports these. But you need to alter your service on domain-a so that it returns valid JSONP responds.

Glad if that helped even a little bit.

See also original question in stackoverflow

#45: Origin is not allowed by Access-Control-Allow-Origin (Score: 347)

Created: 2012-04-13 Last updated: 2019-02-02

Tags: javascript, ajax, cors, xmlhttprequest, cross-domain

I’m making an Ajax.request to a remote PHP server in a Sencha Touch 2 application (wrapped in PhoneGap).

The response from the server is the following:

XMLHttpRequest cannot load http://nqatalog.negroesquisso.pt/login.php. Origin http://localhost:8888 is not allowed by Access-Control-Allow-Origin.

How can I fix this problem?

#45 Best answer 1 of Origin is not allowed by Access-Control-Allow-Origin (Score: 380)

Created: 2012-04-13 Last updated: 2019-03-01

I wrote an article on this issue a while back, Cross Domain AJAX.

The easiest way to handle this if you have control of the responding server is to add a response header for:

Access-Control-Allow-Origin: *

This will allow cross-domain Ajax. In PHP, you’ll want to modify the response like so:

<?php header('Access-Control-Allow-Origin: *'); ?>

You can just put the Header set Access-Control-Allow-Origin * setting in the Apache configuration or htaccess file.

It should be noted that this effectively disables CORS protection, which very likely exposes your users to attack. If you don’t know that you specifically need to use a wildcard, you should not use it, and instead you should whitelist your specific domain:

<?php header('Access-Control-Allow-Origin: http://example.com') ?>

#45 Best answer 2 of Origin is not allowed by Access-Control-Allow-Origin(Score: 64)

Created: 2012-04-13 Last updated: 2017-05-23

If you don’t have control of the server, you can simply add this argument to your Chrome launcher: --disable-web-security.

Note that I wouldn’t use this for normal “web surfing”. For reference, see this post: https://stackoverflow.com/questions/3102819/chrome-disable-same-origin-policy.

One you use Phonegap to actually build the application and load it onto the device, this won’t be an issue.

See also original question in stackoverflow

#46: How to use Servlets and Ajax? (Score: 345)

Created: 2010-11-06 Last updated: 2017-03-22

Tags: java, ajax, jsp, servlets

I’m very new to web apps and Servlets and I have the following question:

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?

#46 Best answer 1 of How to use Servlets and Ajax? (Score: 574)

Created: 2010-11-06 Last updated: 2021-01-06

Indeed, the keyword is “ajax”: Asynchronous JavaScript and XML. However, last years it’s more than often Asynchronous JavaScript and JSON. Basically, you let JS execute an asynchronous HTTP request and update the HTML DOM tree based on the response data.

Since it’s pretty a 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 webapplication.

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 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 $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 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(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 enduser 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 enduser 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 an 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:

#46 Best answer 2 of How to use Servlets and Ajax?(Score: 14)

Created: 2010-11-06 Last updated: 2014-02-16

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.)

See also original question in stackoverflow

#47: How can I add a custom HTTP header to ajax request with js or jQuery? (Score: 340)

Created: 2011-10-07 Last updated: 2019-04-09

Tags: javascript, jquery, ajax, http-headers, httprequest

Does anyone know how to add or create a custom HTTP header using JavaScript or jQuery?

#47 Best answer 1 of How can I add a custom HTTP header to ajax request with js or jQuery? (Score: 621)

Created: 2013-02-01 Last updated: 2013-07-15

There are several solutions depending on what you need…

If you want to add a custom header (or set of headers) to an individual request then just add the headers property:

// Request with custom header
$.ajax({
    url: 'foo/bar',
    headers: { 'x-my-custom-header': 'some value' }
});

If you want to add a default header (or set of headers) to every request then use $.ajaxSetup():

$.ajaxSetup({
    headers: { 'x-my-custom-header': 'some value' }
});

// Sends your custom header
$.ajax({ url: 'foo/bar' });

// Overwrites the default header with a new header
$.ajax({ url: 'foo/bar', headers: { 'x-some-other-header': 'some value' } });

If you want to add a header (or set of headers) to every request then use the beforeSend hook with $.ajaxSetup():

$.ajaxSetup({
    beforeSend: function(xhr) {
        xhr.setRequestHeader('x-my-custom-header', 'some value');
    }
});

// Sends your custom header
$.ajax({ url: 'foo/bar' });

// Sends both custom headers
$.ajax({ url: 'foo/bar', headers: { 'x-some-other-header': 'some value' } });

Edit (more info): One thing to be aware of is that with ajaxSetup you can only define one set of default headers and you can only define one beforeSend. If you call ajaxSetup multiple times, only the last set of headers will be sent and only the last before-send callback will execute.

#47 Best answer 2 of How can I add a custom HTTP header to ajax request with js or jQuery?(Score: 55)

Created: 2012-07-16

Or, if you want to send the custom header for every future request, then you could use the following:

$.ajaxSetup({
    headers: { "CustomHeader": "myValue" }
});

This way every future ajax request will contain the custom header, unless explicitly overridden by the options of the request. You can find more info on ajaxSetup here

See also original question in stackoverflow

#48: jQuery $.ajax(), $.post sending "OPTIONS" as REQUEST_METHOD in Firefox (Score: 334)

Created: 2009-07-08 Last updated: 2013-01-22

Tags: ajax, firefox, jquery-plugins, jquery

Having trouble with what I thought was a relatively simple jQuery plugin…

The plugin should fetch data from a php script via ajax to add options to a <select>. The ajax request is pretty generic:

$.ajax({
  url: o.url,
  type: 'post',
  contentType: "application/x-www-form-urlencoded",
  data: '{"method":"getStates", "program":"EXPLORE"}',
  success: function (data, status) {
    console.log("Success!!");
    console.log(data);
    console.log(status);
  },
  error: function (xhr, desc, err) {
    console.log(xhr);
    console.log("Desc: " + desc + "\nErr:" + err);
  }
});

This seems to work fine in Safari. In Firefox 3.5, the REQUEST_TYPE on the server is always ‘OPTIONS’, and the $_POST data does not appear. Apache logs the request as type ‘OPTIONS’:

::1 - - [08/Jul/2009:11:43:27 -0500] "OPTIONS sitecodes.php HTTP/1.1" 200 46

Why would this ajax call work in Safari, but not Firefox, and how do I fix it for Firefox?

Response Headers
Date: Wed, 08 Jul 2009 21:22:17 GMT
Server:Apache/2.0.59 (Unix) PHP/5.2.6 DAV/2
X-Powered-By: PHP/5.2.6
Content-Length	46
Keep-Alive	timeout=15, max=100
Connection	Keep-Alive
Content-Type	text/html

Request Headers
Host	orderform:8888
User-Agent	Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US; rv:1.9.1) Gecko/20090624 Firefox/3.5
Accept	text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Accept-Language	en-us,en;q=0.5
Accept-Encoding	gzip,deflate
Accept-Charset	ISO-8859-1,utf-8;q=0.7,*;q=0.7
Keep-Alive	300
Connection	keep-alive
Origin	http://ux.inetu.act.org
Access-Control-Request-Method	POST
Access-Control-Request-Headers	x-requested-with

Here is a picture of the Firebug output:

#48 Best answer 1 of jQuery $.ajax(), $.post sending "OPTIONS" as REQUEST_METHOD in Firefox (Score: 170)

Created: 2009-07-10 Last updated: 2012-12-27

The reason for the error is the same origin policy. It only allows you to do XMLHTTPRequests to your own domain. See if you can use a JSONP callback instead:

$.getJSON( 'http://<url>/api.php?callback=?', function ( data ) { alert ( data ); } );

#48 Best answer 2 of jQuery $.ajax(), $.post sending "OPTIONS" as REQUEST_METHOD in Firefox(Score: 57)

Created: 2010-08-19 Last updated: 2012-10-10

I used the following code on Django side to interpret the OPTIONS request and to set the required Access-Control headers. After this my cross domain requests from Firefox started working. As said before, the browser first sends the OPTIONS request and then immediately after that the POST/GET

def send_data(request):
    if request.method == "OPTIONS": 
        response = HttpResponse()
        response['Access-Control-Allow-Origin'] = '*'
        response['Access-Control-Allow-Methods'] = 'POST, GET, OPTIONS'
        response['Access-Control-Max-Age'] = 1000
        # note that '*' is not valid for Access-Control-Allow-Headers
        response['Access-Control-Allow-Headers'] = 'origin, x-csrftoken, content-type, accept'
        return response
    if request.method == "POST":
        # ... 

Edit: it seems to be that at least in some cases you also need to add the same Access-Control headers to the actual response. This can be a little bit confusing, since the request seems to succeed, but Firefox does not pass the contents of the response to the Javascript.

See also original question in stackoverflow

#49: Jquery - How to make $.post() use contentType=application/json? (Score: 329)

Created: 2010-05-16 Last updated: 2017-08-17

Tags: jquery, ajax, content-type

I’ve noticed that when using $.post() in jquery that the default contentType is application/x-www-form-urlencoded - when my asp.net mvc code needs to have contentType=application/json

(See this question for why I must use application/json: https://stackoverflow.com/questions/2792603/aspnet-mvc-why-is-modelstate-isvalid-false-the-x-field-is-required-when-that)

How can I make $.post() send contentType=application/json? I already have a large number of $.post() functions, so I don’t want to change to $.ajax() because it would take too much time

If I try

$.post(url, data, function(), "json") 

It still has contentType=application/x-www-form-urlencoded. So what exactly does the “json” param do if it does not change the contenttype to json?

If I try

$.ajaxSetup({
  contentType: "application/json; charset=utf-8"
});

That works but affects every single $.get and $.post that I have and causes some to break.

So is there some way that I can change the behavior of $.post() to send contentType=application/json?

#49 Best answer 1 of Jquery - How to make $.post() use contentType=application/json? (Score: 407)

Created: 2010-05-16

$.ajax({
  url:url,
  type:"POST",
  data:data,
  contentType:"application/json; charset=utf-8",
  dataType:"json",
  success: function(){
    ...
  }
})

See : jQuery.ajax()

#49 Best answer 2 of Jquery - How to make $.post() use contentType=application/json?(Score: 93)

Created: 2012-01-26

Finally I found the solution, that works for me:

jQuery.ajax ({
    url: myurl,
    type: "POST",
    data: JSON.stringify({data:"test"}),
    dataType: "json",
    contentType: "application/json; charset=utf-8",
    success: function(){
        //
    }
});

See also original question in stackoverflow

#50: jQuery.ajax handling continue responses: "success:" vs ".done"? (Score: 317)

Created: 2012-01-12 Last updated: 2020-06-20

Tags: ajax, jquery

I have been working with jQuery and AJAX for a few weeks now and I saw two different ways to ‘continue’ the script once the call has been made: success: and .done.

From the synopsis from the jQuery documentation we get:

.done(): Description: Add handlers to be called when the Deferred object is resolved.

success: (.ajax() option): A function to be called if the request succeeds.

So, both do something after the AJAX call has been completed/resolved. Can I use one or the other randomly? What is the difference and when one is used instead of the other?

#50 Best answer 1 of jQuery.ajax handling continue responses: "success:" vs ".done"? (Score: 482)

Created: 2012-01-12 Last updated: 2013-11-06

success has been the traditional name of the success callback in jQuery, defined as an option in the ajax call. However, since the implementation of $.Deferreds and more sophisticated callbacks, done is the preferred way to implement success callbacks, as it can be called on any deferred.

For example, success:

$.ajax({
  url: '/',
  success: function(data) {}
});

For example, done:

$.ajax({url: '/'}).done(function(data) {});

The nice thing about done is that the return value of $.ajax is now a deferred promise that can be bound to anywhere else in your application. So let’s say you want to make this ajax call from a few different places. Rather than passing in your success function as an option to the function that makes this ajax call, you can just have the function return $.ajax itself and bind your callbacks with done, fail, then, or whatever. Note that always is a callback that will run whether the request succeeds or fails. done will only be triggered on success.

For example:

function xhr_get(url) {

  return $.ajax({
    url: url,
    type: 'get',
    dataType: 'json',
    beforeSend: showLoadingImgFn
  })
  .always(function() {
    // remove loading image maybe
  })
  .fail(function() {
    // handle request failures
  });

}

xhr_get('/index').done(function(data) {
  // do stuff with index data
});

xhr_get('/id').done(function(data) {
  // do stuff with id data
});

An important benefit of this in terms of maintainability is that you’ve wrapped your ajax mechanism in an application-specific function. If you decide you need your $.ajax call to operate differently in the future, or you use a different ajax method, or you move away from jQuery, you only have to change the xhr_get definition (being sure to return a promise or at least a done method, in the case of the example above). All the other references throughout the app can remain the same.

There are many more (much cooler) things you can do with $.Deferred, one of which is to use pipe to trigger a failure on an error reported by the server, even when the $.ajax request itself succeeds. For example:

function xhr_get(url) {

  return $.ajax({
    url: url,
    type: 'get',
    dataType: 'json'
  })
  .pipe(function(data) {
    return data.responseCode != 200 ?
      $.Deferred().reject( data ) :
      data;
  })
  .fail(function(data) {
    if ( data.responseCode )
      console.log( data.responseCode );
  });
}

xhr_get('/index').done(function(data) {
  // will not run if json returned from ajax has responseCode other than 200
});

Read more about $.Deferred here: http://api.jquery.com/category/deferred-object/

NOTE: As of jQuery 1.8, pipe has been deprecated in favor of using then in exactly the same way.

#50 Best answer 2 of jQuery.ajax handling continue responses: "success:" vs ".done"?(Score: 5)

Created: 2017-12-17

If you need async: false in your ajax, you should use success instead of .done. Else you better to use .done. This is from jQuery official site:

As of jQuery 1.8, the use of async: false with jqXHR ($.Deferred) is deprecated; you must use the success/error/complete callback options instead of the corresponding methods of the jqXHR object such as jqXHR.done().

See also original question in stackoverflow


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