I have a Struts2 action in the server side for file downloading.

<action name="download" class="com.xxx.DownAction">
    <result name="success" type="stream">
        <param name="contentType">text/plain</param>
        <param name="inputName">imageStream</param>
        <param name="contentDisposition">attachment;filename={fileName}</param>
        <param name="bufferSize">1024</param>
    </result>
</action>

However when I call the action using the jQuery:

$.post(
  "/download.action",{
    para1:value1,
    para2:value2
    ....
  },function(data){
      console.info(data);
   }
);

in Firebug I see the data is retrieved with the Binary stream. I wonder how to open the file downloading window with which the user can save the file locally?

Answer

2019 modern browsers update

This is the approach I’d now recommend with a few caveats:

• A relatively modern browser is required
• If the file is expected to be very large you should likely do something similar to the original approach (iframe and cookie) because some of the below operations could likely consume system memory at least as large as the file being downloaded and/or other interesting CPU side effects.

fetch('https://jsonplaceholder.typicode.com/todos/1')
  .then(resp => resp.blob())
  .then(blob => {
    const url = window.URL.createObjectURL(blob);
    const a = document.createElement('a');
    a.style.display = 'none';
    a.href = url;
    // the filename you want
    a.download = 'todo-1.json';
    document.body.appendChild(a);
    a.click();
    window.URL.revokeObjectURL(url);
    alert('your file has downloaded!'); // or you know, something with better UX...
  })
  .catch(() => alert('oh no!'));

2012 Original jQuery/iframe/Cookie based approach

Bluish is completely right about this, you can’t do it through Ajax because JavaScript cannot save files directly to a user’s computer (out of security concerns). Unfortunately pointing the main window’s URL at your file download means you have little control over what the user experience is when a file download occurs.

I created jQuery File Download which allows for an “Ajax like” experience with file downloads complete with OnSuccess and OnFailure callbacks to provide for a better user experience. Take a look at my blog post on the common problem that the plugin solves and some ways to use it and also a demo of jQuery File Download in action. Here is the source

Here is a simple use case demo using the plugin source with promises. The demo page includes many other, ‘better UX’ examples as well.

$.fileDownload('some/file.pdf')
    .done(function () { alert('File download a success!'); })
    .fail(function () { alert('File download failed!'); });

Depending on what browsers you need to support you may be able to use https://github.com/eligrey/FileSaver.js/ which allows more explicit control than the IFRAME method jQuery File Download uses.