Nayish Nayish - 1 month ago 25
jQuery Question

How to upload files to server using JSP/Servlet and Ajax?

I'm creating a JSP/Servlet webapplication and I'd like to upload a file to a Servlet via Ajax. How would I go on doing this? I'm using jQuery.

I've done so far:

<form class="upload-box">
<input type="file" id="file" name="file1" />
<span id="upload-error" class="error" />
<input type="submit" id="upload-button" value="upload" />
</form>


With this jQuery:

$(document).on("#upload-button", "click", function() {
$.ajax({
type: "POST",
url: "/Upload",
async: true,
data: $(".upload-box").serialize(),
contentType: "multipart/form-data",
processData: false,
success: function(msg) {
alert("File has been uploaded successfully");
},
error:function(msg) {
$("#upload-error").html("Couldn't upload file");
}
});
});


However, it doesn't appear to send the file contents.

Answer

To the point, as of the current XMLHttpRequest version 1 as used by jQuery, it is not possible to upload files using JavaScript through XMLHttpRequest. The common workaround is to let JavaScript create a hidden <iframe> and submit the form to it instead so that the impression is created that it happens asynchronously. That's also exactly what the majority of the jQuery file upload plugins are doing such as jQuery Form plugin (example here).

Assuming that your JSP with the HTML form is rewritten in such way so that it's not broken when the client has JS disabled (as you have now...), like below:

<form id="upload-form" class="upload-box" action="/Upload" method="post" enctype="multipart/form-data">
    <input type="file" id="file" name="file1" />
    <span id="upload-error" class="error">${uploadError}</span>
    <input type="submit" id="upload-button" value="upload" />
</form>

Then it's with help of jQuery Form plugin just a matter of

<script src="jquery.js"></script>
<script src="jquery.form.js"></script>
<script>
    $(function() {
        $('#upload-form').ajaxForm({
            success: function(msg) {
                alert("File has been uploaded successfully");
            },
            error: function(msg) {
                $("#upload-error").text("Couldn't upload file");
            }
        });
    });
</script>

As to the servlet side, no special stuff needs to be done here. Just implement it exactly the same way as you would do when not using Ajax: How to upload files to server using JSP/Servlet?

You'll only need an additional check in the servlet if the X-Requested-With header equals to XMLHttpRequest or not, so that you know how what kind of response to return for the case that the client has JS disabled (as of now, it are mostly the older mobile browsers which have JS disabled).

if ("XMLHttpRequest".equals(request.getHeader("X-Requested-With"))) {
    // Return ajax response (e.g. write JSON or XML).
} else {
    // Return regular response (e.g. forward to JSP).
}

Note that the relatively new XMLHttpRequest version 2 is capable of sending a selected file using the new File and FormData APIs. See also HTML5 File Upload to Java Servlet and sending a file as multipart through xmlHttpRequest.