Sunday, November 11, 2007

What's New in Tally-Ho?

And just where is that 1.0 release, anyway?

Well, I made good on my threat to rip out Toplink Essentials and replace it with OpenJPA. OpenJPA is a bit more pedantic about some things. For example, this code would run fine in Toplink but would throw an IllegalStateException in OpenJPA:


While I was working on dropping in OpenJPA, I decided that I really wanted my tests to pass from within Maven, so I could be sure that run-time enhanced (woven) classes were all going to work nicely. I also wanted to make sure that none of the tests depended on any data to be in the database that wasn't put there by the SQL scripts to initialize the DB. So I modified my base test class to perform one-time database wiping/initialization prior to running any tests. This exposed a great many flaws in tests that I wrote in a fairly lazy fashion to assume that certain objects were already present.

After fixing all of that, I decided to let Eclipse clean up a lot of other code for me. Eclipse's Source-->Cleanup feature is very powerful, allowing you to "final" gobs of things and implement the default serial ID for Serializable classes in one giant swoop.

Then I got to work on what the next major project / feature is for Tally-Ho: arbitrary HTML pages. For quite a while I agonized over how to manage associations between pages. In most of the world, if a page changes its name or location, links to it break. I also needed the capability to attach images, PDFs or other documents to arbitrary HTML. It turns out that the solution to both of these problems is the same. In a massive refactoring of BinaryResource, any BinaryResourceReference can now be attached to any other BinaryResourceReference. BinaryResource is gone, and instead the relationship is now 1 BinaryResourceReference has many BinaryResourceReferenceLocales, each of which has one BinaryResourceContent. A BinaryResourceReference may also have many Attachments, which have an sequence number and a reference to the attached BinaryResourceReference. An HtmlPage is just a subclass of BinaryResourceReference with some bits added for the title, keywords, whether to include a message board, etc.

Attachments are numbered in sequence (1, 2, 3). Inside the HtmlPageService, references to attachments are converted by an AttachmentUrlProvider (an interface) and Velocity to their URLs. So if you want to refer to the URL for attachment #1, you use ${1} in the HTML. (Roughly... this bit isn't done yet.) It is up to the AttachmentUrlProvider to decide how to make the URL, given the scope, path and extension.

This refactoring is probably 75% complete. I'm too burnt out on code and too tired to work on it any more this weekend.

So to summarize what's changed:

1. OpenJPA replaced Toplink Essentials
2. Everything builds and tests in Maven (including compile time bytecode instrumentation)
3. Refactoring of binary resources
4. Initial HtmlPage work
5. Binary resource attachment support.

A couple other things I learned today about JPA:

1. If you JOIN multiple things, at least with OpenJPA you need to alias each thing you join. Ie, JOIN foo JOIN bar. The parser will complain if you leave off that last "bar" in that example.
2. Your ability to lazy load ends once the EntityManager you used to load your object is closed. I knew this before and subsequently forgot, and then learned it again the hard way. Merging the entity with a new EntityManager doesn't work either. You need to keep the original one open until you're done navigating your object graph.

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Hi Nick,
The specification says it is undefined if users access lazy loaded data once an entity is detached. In TopLink Essentials you are still able to retrieve the related entity as long as you have not serialized the owning entity.
Also, once you call the merge API you should have a managed instance returned to you from the merge(Object) call. This instance should be fully navigable without the need to keep the old EM around. That's what the spec requires and that's how TopLink Essentials works.
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