Simon Palmer’s blog

March 5, 2010

*Really* simple asynchronous file upload in Grails

Filed under: code, Grails — simonpalmer @ 4:11 pm

I spent ages trying to decide which was the best way for me to implement file upload in my grails app. What I wanted was the ability for a user to select an image from their local machine and for it to be uploaded into a tag on my page. The biggest issue I faced was that the upload button, and in fact the whole DIV that contained the image and other stuff, is generated at runtime. The other mandatory condition was that it had to happen asynchronously and without re-loading the page.

I tried out 3 or 4 solutions, including several grails plug-ins (google for “grails file upload” and you’ll find them), several ajax javascript solutions, which I have now lost, and a couple of Flash uploader implementations. The trouble with all of them was a combination of complexity and size. In the end I found a really simple alternative which seems to work just fine and builds on adding the Grails UI plugin into my application.

My solution was in two parts, first was to give myself the ability to have a modal dialog box on my page and second was to utilise the Yahoo GUI library to make an asynchronous connection back to the server.

The modal dialog came directly from the Grails UI plugin, which has a very rich set of excellent features and I recommend it. Installing that plugin gave me a dialog box, which was as simple as including a resource reference and a GSP tag in a hidden div on my page…

        <gui:resources components="['dialog']"/>
                title="Upload Image"
                buttons="[[text:'Upload', handler: 'onUploadButtonClick', isDefault: true],[text:'Cancel', handler: 'function() {this.cancel();}', isDefault: false]]">
                <form action="uploadfile" enctype="multipart/form-data" method="post" id="uploadForm">
                <input type="file" name="testFile"/>

The important thing to notice in here is that the dialog box contains a form which defines the multipart enctype, the method as POST. The action refers to the code in my Grails controller which handled persistence of the file. The gui:resources tag is exploded by Grails into a set of script includes for the Yahoo UI libraries. This is important because the YUI library is very large and there are many interdependencies, and this single tag hides all that, which is a major time saver.

The other thing that the Grails UI plugin gave me was the whole of the YUI library, so the next bit was to write some code to handle the file upload itself. Rather than lots of fancy components I realised that I could just use the YAHOO.util.Connect object to make an asynchronous call back to the server, in much that same way that I might otherwise have used an XMLHTTPRequest object in Javascript. Because the whole YUI library is included with the plugin I already had the code. To get at the Connect object I had to include the appropriate script because the gui:resources tag seemed not to add it as a dependence…

        <script type="text/javascript" src="/js/yui/2.7.0/utilities/utilities.js" ></script>

From that point it was a simple matter of writing a function to handle the click on the upload button in my dialog which made the request back to the server, and handled the response…

        function onUploadButtonClick(e)
            var uploadHandler =
                upload: function(o)
            //the second argument of setForm is crucial,
            //which tells Connection Manager this is an file upload form
            YAHOO.util.Connect.setForm('uploadForm', true);
            YAHOO.util.Connect.asyncRequest('POST', 'uploadfile', uploadHandler);

The important thinhg to notice in here is that the setForm call requires the second argument to be set to true so it recognises the form settings for the POST. Otherwise, the two lines of code invoking YAHOO connect do everything you need to send a file back to the server.

My server side code processes the multipart file upload and persists the file into Amazon S3 (see my other post about that). I then render the file as a virtual location relative to the current page back into my page so I can simply update the src property of the right IMG tag on my page – that’s what refreshActiveImage(o.responseText); does.

Eh voila! A common plug-in, a great utility library, some really basic Javascript and a form and I have the simplest file upload I could imagine.


1 Comment »

  1. […] 21, 2010 10:16 pm I recently posted about really simple file upload in my app and once I had that working I moved onto my next problem, which was in two parts, 1) […]

    Pingback by *Really* simple S3 persistence from Grails | Simon Palmer’s blog — March 21, 2010 @ 10:16 pm

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