Writing your own JRuby extension. Part I: BasicLibraryService.

Note: not code in this post, but you can see the code in Github. Follow the links!

Writing a JRuby extension is very easy, but there are almost not post out there about it. As far as I know, there is only one, Ola’s. It is a really good tutorial indeed, but it lacks some details that might be not-that-easy to solve. Please, take some time to read it and, if some details are different, do follow Ola’s way.

Everything’s ready now, so let’s start talking about BasicLibraryService. If you take a look at Nokogiri4J sourcecode, in ext/java/nokogiri folder, you will see a NokogiriService.java file. NokogiriService implements BasicLibraryService. This interface consists only of the method basicLoad which receives a Ruby object.

We will use this method to define classes and methods in the Ruby world. For defining a module, defineModule method is used with the name of the module. After that, modules and classes under that module can be defined easily by using the methods defineModuleUnder, which takes the name as parameter, and defineClassUnder, which takes the name and few parameters more. Let’s dive into it.

defineClassUnder needs three arguments. The first one is the class’ name. The second, is the parent class. If you have defined it previously, just passed it,  otherwise use RubyObject by calling the method getObject on the Ruby instance. The third parameter is an ObjectAllocator. ObjectAllocators returns intances of the classes in Java world. When instantiating Nokogiri::XML::Comment in Ruby world, JRuby will ask the ObjectAllocator for an instance of the Java class. It passes a Ruby object and the RubyClass being instantiated to the allocate method in the ObjectAllocator (more on RubyClass in following posts).

Finally, we will need to define some methods. Easiest way is by using the defineAnnotatedMethods. It takes a Java class as parameter. For knowing what this method does, you need to know a bit more about @JRubyMethod annotation (more on it in following post, have you realized the “Part I” in the title?). As you define methods, you may need to undefine some in a subclass. So easy! Use the undefineMethod method, which takes the name of the method as parameter (surprisingly, it undefines a method by redefining it!).

Next time, Implementing your first class.

2 thoughts on “Writing your own JRuby extension. Part I: BasicLibraryService.”

  1. I have been going through Nokogiri code and about to take on another extension project. This is very helpful. Thanks for posting your experience.

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