Wednesday, December 5, 2012

Reflections on writing an Eclipse plugin

For a personal project I'm working on, the language choice is C++. Now that's probably going to make a lot of readers cringe because C++ has some bad rep, especially with the black magic voodoo that goes by the name of Templates. However due to the requirements of the app, C++ was a good fit. It's not too bad if you apply SOLID principles, design patterns, and above all good naming conventions. IMO a major contributor to C++'s bad reputation is the extremely poor names people use (the STL doesn't help either). We can see the porting back to C++ of good design concepts, most notably in the tools. Libraries like Qt make it almost like having a JDK at ones fingertips (but with a decent UI system). Did you know you can do TDD in C++?

I use the Eclipse CDT for my IDE which when coupled with the Autotools plugin makes it really easy to edit and build my code with most of the features I get out of the Java tools in Eclipse. You don't get everything, but it does a good job.

One thing I was really missing was a way to generate test classes quickly. I use the Google C++ testing framework as it's the best I've found that matches JUnit or TestNG. It's a bit clunky given the state of Java test tooling, it's style is very much on par with JUnit 3, but given C++ doesn't have annotations, it does really well with some macro magic. One can still use BDD style testing with a few compromises to get your code to compile. What I was missing was a "Create test class" option to keep my productivity from getting bogged down in making sure my #includes were correct.

I've been wanting to write an Eclipse plugin for a while, so I thought I'd use this itch as a chance. Turned out it was a lot easier than I had thought.

I downloaded the RCP version of Eclipse as it comes with all the development tools for making plugins. I personally like to keep my Eclipse installs separate for different purposes. About the only common plugin I use across all of them is my Mercurial plugin. I installed the CDT on top of the RCP so that I had access to all the CDT JARs since I was developing a CDT focused plugin.

Starting on writing the plugin was really simple. Of course I googled around first and came across a few helpful links:

The thing that made the plugin development easy IMHO was the plugin configuration editor. I was dreading writing XML and getting it wrong (try debugging that), or writing properties to configure the build (and finding reference documentation to do what I want). Thankfully I didn't get any obscure errors, the editor did it all. Took the fear out of the project. Coupled with some extra helpful documentation on plugin terminology and what not to do; I got my first wizard up and running very quickly.

Since I was making a CDT based plugin, I needed to set my API baseline. This helps the compiler figure out if you've been doing naughty things. This is a real strength, because by letting you know when you've violated API boundaries, you're able to future proof your plugin against changes to internals; or at least tell (via your plugin conf) that you're willing to risk the consequences.

Since I was wanting to create a special instance of the "New Class" wizard option, I subclassed the relevant wizards and wizard pages classes. The one big frustration was that protected methods in these classes used private members, or private inner classes. Which meant that to override the behaviour (but still keep parent behaviour) some nasty reflection hacks were needed. I think the design moral there is that if you are going to allow your methods to be overridden make the data accessible (most likely through protected accessors). Either that or mark the methods as final so that the compiler lets you know that "you can't do that Dave". Unfortunately these CDT classes violate the whole Open Closed principle. Other than that it was actually pretty easy to debug my way through the "New Class" wizard to get an idea of how I could create a specialised class geared towards being a test class and writing the behaviour.

The bulk of the code was written on the train going to/from the YOW 2012 conference so hopefully that conveys just how easy it was once I got going. It's a credit to the PDT guys that banging out a plugin is that easy; in terms of the infrastructure code required to hook it in; I mainly had to work on application logic which is how it should be.

The final result is the Google Testing Framework Generator, so if you can make use of it, please download it and make suggestions/patches. I have a few other extra ideas of code this plugin can generate, but for now I'm just going to TDD my way faster through some new features for my C++ project.

Monday, December 3, 2012

Spring 3 MVC naming gotcha

As the title suggests, I got bitten by a naming issue in Spring 3 (3.0.5) MVC. I wasted a fair amount of time on it, so I don't want to do it again. The relevant material pertaining to this issue may be in the Spring reference documentation however hopefully this blog post will be more concise for anybody else who has had this problem.

In my MVC app I have a ProductsSearcher class that has two dependencies that are injected via the constructor. In my Spring config XML I have the following snippets:

<context:component-scan base-package="org.myproject" />

<bean class="org.myproject.ProductsSearcher"> ... </bean>
The component-scan tag is needed to get Spring to process the @Controller annotation to get the bean registered as a MVC controller However when boot strapping the application context the deployment failed
ERROR [DispatcherServlet] Context initialization failed
org.springframework.beans.factory.BeanCreationException: Error
creating bean with name 
DefaultAnnotationHandlerMapping#0': Initialization of bean
failed; nested exception is org.springframework.beans.
factory.BeanCreationException: Error creating bean with
name 'productsSearcher' ... Instantiation of bean failed;
nested exception is org.springframework.beans.
BeanInstantiationException: Could not instantiate bean class [org.
myproject.ProductsSearcher]: No default constructor
found; nested exception is java.lang.NoSuchMethodException: org.
So Spring was trying to instantiate an instance of my class via reflection and borking because there was no default constructor on the class. Why would it do that when I have a bean definition in the config?

I wasn't willing to change the constructor because, good design dictates that if the class can't function without a dependency then the dependency must be injected at construction time; otherwise the object will be in an illegal state (not "fully formed").

I thought it might be something to do with the component scanning as component scanning also involves processing configuration annotations (a good discussion of the XML tags that deal with annotation processing can be found on Stack Overflow.).

The problem lies in the name/id (or lack thereof) of the bean instance. Component scanning does involve Spring creating an instance of the stereotype (ie: my @Controller) but the instance was created with a given name. Given the good old Java Bean naming conventions the name is productsSearcher. By default whenever I create stereotyped bean config I give the bean this style of name. This time I forgot :( :(. Adding in the id fixed the deployment issue

<bean id="productsSearcher" class="org.myproject.ProductsSearcher"> ... </bean>
... </bean>
So my bean definition overrides the default and the correct bean definition is used and the bean is instantiated.

Given that most of us would name our beans using the Java Beans naming convention (since as Spring users it's been beaten into us) this problem had never occurred to me. If anybody has any insight into the workings of the Spring mechanisms behind component scanning please offer up your wisdom in the comments. For the rest of us, remember to name your stereotyped beans properly.