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How do you test tag libraries?

Posted by simongbrown on October 7, 2003 at 11:58 AM PDT

Matt is looking for a way to test tag libraries and rather than write a lengthy comment, I thought I'd follow it up here.

In the first part of his post, Matt says,

... I've looked briefly at TagUnit, but aren't you just writing JSPs (with custom tags) to test JSPs?

In answer to this question, yes, you're exactly rght in saying that you're just writing JSPs. This is the essence of TagUnit, and why I believe it's a better way to test tags than using something like Cactus. Sure, you can test the various classes with JUnit, but since you don't use the classes in this way from a JSP page, this approach can only take you so far. JSP tags are reusable components and I think that they should be tested as such. For me, this means testing those tags in the same way that they will be used on a JSP page, and that's by writing the tests and assertions as JSP pages.

Doing this has several benefits. First of all, you don't have to start mocking out parts of the servlet specification just to test the tag handlers. Instead, you're running the tests inside a real server. If you've looked into the JSP specification, you'll know that the tag lifecycle is a tricky business because of the way that JSP containers can optionally reuse and pool tag handler instances. By writing your tests as JSP pages, you instantly get all this framework for free, meaning that you can accurately test the behaviour on your desired target platform. Also, if you're aiming for cross-container compatibility, this is as simple as (in theory!) just dropping the WAR file containing the tests into a new container.

Matt wants an Ant/JUnit solution and I can see why this is desirable. I'm far more likely to run the tests if I don't have to keep starting up the JSP container. Unfortunately, there aren't many ways around this without exploring avenues such as mock objects. However, TagUnit does provide an Ant task that can be used to initiate and collect the results of running the JSP-based tests. The basics of this are documented, although this document is being expanded as we speak ... isn't it Sam? ;-) As for whether there are HTML versions of the docs ... not yet, but there might be one day.

I should probably add a disclaimer at this point, and highlight the fact that I created the TagUnit project. Clearly I'm biased, but I do think that it provides an improved way to test custom tags. Whether it's better ... that's for you to decide.

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