Ajax request parameter cannot be retrieved within servlet issue

As part of learning Ajax, I have created a little servlet + some HTML page + Javascript code. The page contains a couple of buttons. The idea is to pass a parameter value in a request according to the pressed button and return the value in the response. Nothing crazy.

Here is the function making the request (button are linked to this function on click):

function loadXMLDoc2() {
    var xmlhttp;
    if (window.XMLHttpRequest) {
        // code for IE7+, Firefox, Chrome, Opera, Safari
        xmlhttp=new XMLHttpRequest();
    } else {
        // code for IE6, IE5
        xmlhttp=new ActiveXObject("Microsoft.XMLHTTP");
    }
    xmlhttp.onreadystatechange=function() {
        if (xmlhttp.readyState==4 && xmlhttp.status==200) {
            document.getElementById("myDiv").innerHTML=xmlhttp.responseText;
        }
    }
    xmlhttp.open("GET","/WebFront/Ajaxlet",true);
    var retr = this.id;
    var param = "cmd=" + retr;
    document.getElementById("L1_LEFT").innerHTML = param;
    xmlhttp.send(param);
}

The L1_LEFT element allows me to check the constructed parameter (for example: cmd=bd_info).

The following method is called on the servlet side to proceed the request and response:

private void setAnswer2(HttpServletRequest req, HttpServletResponse res) throws IOException {

    Enumeration<String> iter = req.getParameterNames();

    System.out.println("Parameters=");
    while (iter.hasMoreElements()) {
        String next = iter.nextElement();
        System.out.println(next);
    }

    ...

}

However, each time I click on buttons, I get an extra parameter line printed:

Parameters=
Parameters=
Parameters=
Parameters=
...

Why? What am I doing wrong?

ED开发者_JS百科IT

Following BalusC's solution, I modified my code as following:

var retr = this.id;
var param = "cmd=" + encodeURIComponent(retr);
document.getElementById("L1_LEFT").innerHTML = param;
xmlhttp.open("POST","/WebFront/Ajaxlet",true);
xmlhttp.send(param);

and

    Map<String, String[]> en = req.getParameterMap();

    System.out.println("Parameters=");
    for (String next : en.keySet()) {
        System.out.println(next);
    }

The GET solution works, but the POST solution does not work (same issue). I read that POST is preferable to GET. Anyone knows why POST would not work?

EDIT II

Here is the servlet code:

@WebServlet(name="Ajaxlet", urlPatterns={"/Ajaxlet"}, asyncSupported=false)
public class Ajaxlet extends HttpServlet {

    private void setAnswer2(HttpServletRequest req, HttpServletResponse res) throws IOException {

        Map<String, String[]> en = req.getParameterMap();

        System.out.println("Parameters=");
        for (String next : en.keySet()) {
            System.out.println(next);
        }

        res.setContentType("text/html");
        PrintWriter out = res.getWriter();
        out.println(System.currentTimeMillis());
        out.close();        

    }

    @Override
    public void doPost(HttpServletRequest req, HttpServletResponse res)
            throws ServletException, IOException {

        System.out.println("doPost");
        setAnswer2(req, res);

    }

}

EDIT III

Here is my web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app version="2.5" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd">
    <servlet>
        <display-name>Ajaxlet</display-name>
        <servlet-name>Ajaxlet</servlet-name>
        <servlet-class>net.test.Ajaxlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>Ajaxlet</servlet-name>
        <url-pattern>/Ajaxlet/*</url-pattern>
    </servlet-mapping>
    <welcome-file-list>
            <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>
</web-app>

and my context.xml:

<?xml version="1.0" encoding="UTF-8"?>
<Context antiJARLocking="true" path="/WebFront"/>

EDIT IV

Updated my web.xml to:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
      version="3.0"> 

    <welcome-file-list>
            <welcome-file>index.jsp</welcome-file>
    </welcome-file-list>

</web-app>

Still having the same issue with POST.

You're creating a GET request and then sending the parameter in the request body. This isn't going to work. In order to send data in the request body, you have to create a POST request (and do the server side request processing job in servlet's doPost() method instead of doGet()):

xmlhttp.open("POST", "/WebFront/Ajaxlet", true);
xmlhttp.send(param);

On GET requests, you normally append the query string with the parameters to the request URL, not the request body:

xmlhttp.open("GET", "/WebFront/Ajaxlet?" + param, true);
xmlhttp.send(null);

Regardless of the request method, you should ensure that parameter names and values are properly URL encoded! You can use JS' builtin encodeURIComponent() function for this:

var param = "cmd=" + encodeURIComponent(retr);

Unrelated to the concrete problem, I know that you're learning Ajax, but I suggest to after all take a look at an existing JavaScript library which takes all the nasty and cross browser sensitive details of sending Ajax requests and traversing/manipulating the HTML DOM from your hands, such as jQuery. In this related question you can find some nice examples how to get started with jQuery + Servlets: How to use Servlets and Ajax?

Another unrelated note, you should prefer getParameterMap() to iterate over parameter names and values. It returns a Map instead of an old fashioned Enumeration; a Map allows you to use the enhanced for loop.

Had exactly this same problem, for documentations sake the solution was in my Javascript code,

When sending "form data" in your AJAX Post, you need to set a header like so..

xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded") xmlhttp.send("paramName=paramValue")

source: http://www.w3schools.com/ajax/ajax_xmlhttprequest_send.asp