How I Added File-Download Support To My Model Framework

Apr 12, 2014, 1:33 PM

Tags: xpages java

Unlike the last few posts, this one isn't as much a tip or tutorial as it is an attempt to share my suffering by detailing the steps I took recently to add file-download and -upload support to my current model framework.

The goal of my framework overall is to retain the benefits of normal Domino document data sources (arbitrary field access, ease of use in simple cases, and compatibility with standard XPage idoms) while adding on useful new abilities (getters/setters, relationships with other objects, abstraction between the XPage and the actual location of the data). Most of this work is easy - by implementing DataObject, my objects worked well with EL bindings and most of the work was focused on hooking up Domino Document objects in a similar way to IBM's DominoDocument class.

However, some of the more esoteric controls are not so easy, and the file controls were chief among them. As I tinkered, I found that you can't directly point a xp:fileDownload control to a normal data source and get it to work well. You can get it to list a single file, but I wasn't able to get it to recognize more than one through traditional means.

In my tinkering, I discovered that there are two main paths for the control: the traditional one and a more-complicated one, which makes a lot of assumptions about the nature and availability of the referenced data source. Importantly, unlike normal bindings (which take your expression, say #{doc.body}, have the EL processor resolve it, and deal with the final resolved entity), the control actually treats the EL expression as a String, splits it in two on the "." character, and looks up the variable on the left side plus ".DATASOURCE". This "doc.DATASOURCE" idiom matches the behavior of data-source controls on an XPage, which make themselves or an associated object available by that name. If the control finds an object by that name and if it implements DataSource (a common interface used by these controls), it assumes that the source contains a DataContainer object that also implements DocumentDataContainer specifically.

Phew, okay, so far so good. It makes some inconvenient sense that the control would expect its referenced value to conform to the norms of the built-in data sources, so I wrote controls to do just that (previously, I got by using normal EL references from managed beans). However, I still ended up getting ClassCastExceptions at runtime: it turned out that the control also uses a nebulous factory object to attempt to convert the received value into a data type it expects, specifically DominoDocument. Unfortunately, unlike the interfaces I implemented already, subclassing DominoDocument was not something I was willing to do. So I was back to digging.

What I found was that this attempted conversion was happening due to an object implementing, which has the job of taking an intermediate value used in the control (a, which contains a single-entry HashMap containing the object and requested field name) and returning the attachment data as an instance of DataModel. In addition to writing the code to do this, there's another wrinkle: the way the factory is found. The way I found to access this is thoroughly awkward: if I use the deprecated method getFactoryLookup() on the application instance, I can then retrieve the default factory by a key (""), wrap it in a new object that knows about my model objects, and replace it in the lookup.

Once I did all that, I was off to the races! Sort of! There remained another problem: the "delete attachment" icon on the control. I discovered that the way this icon works is that it composes an action that tracks down an adapter that knows how to communicate with the associated model object and sends it a deleteAttachments(...) call. As before, the way it "tracks down" the adapter is to look up another factory ("") and ask it to provide an appropriate adapter. So again I wrote my own variant of this to intercept calls dealing with model objects and carefully placed it, Indiana-Jones-style, in place of the default factory.

The net result is that it works! I can point a file-download control at a model object (as long as that object was specifically instantiated in a data source control and is referenced directly in "object.field" format) and have it list all of the attachments in the field, along with a functioning delete action if desired. The file-upload control was, fortunately, much easier: when I realized I could swap out most of the guts of my model object with a back-end DominoDocument (now that the OpenNTF API takes care of data conversions for me), I was able to just proxy the upload calls to it and let it do the heavy lifting.

Now, there's only one nagging problem remaining: inline image uploads in rich text controls. But still, even if I never get that part working, the file manipulation will be enough. And, importantly, I've paved the way for me to be able to do use the same controls with model objects that aren't associated with Domino at all, should I desire to do so down the line.

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