JRuby on Rails: Using custom Java classes in your Rails app

Question

I just started with JRuby on Rails and absolutely love it. I know how to use current classes within the Java API in my Rails app, but if I wanted to create a new custom class written in purely Java code, how would I be able to use it in my Rails app?

For example, let's say I created Dog.java:

class Dog {
  private String name;

  public Dog() {
    name = "Fido";
  }

  public String getName() {
    return name;
  }
}

How would I be able to create a new Dog object (Dog.new) in my Rails app? I need to put the Dog.java or Dog.class file somewhere, and then call some form of "import" to import it into my Rails app. I have no idea where this should go in the directory structure nor do I know where and how I should tell my app how to include it.

Answer 1

You'll need a couple things.

1. Compile the class.

   mkdir classes
   javac -d classes src/Dog.java
   

2. Add classes to the classpath in your Rails application (an initializer for example).

   require 'java'
   $CLASSPATH << File.join(Rails.root, "classes")
   

3. Import the class.

   java_import Java::Dog
   

If you want to make a war file of your Rails app with Warbler, you could also add the classes directory to the war file using the config.dirs option in config/warble.rb, and the Dog class will be available without having to add to $CLASSPATH because of the Java convention that WEB-INF/classes be added to the classpath in a Java web application.