Simple JVM assembler based on the BiteScript gem for JRuby.

assembler.rb

# Simple JVM assembler using the BiteScript gem for JRuby.
# @see http://kenai.com/projects/jvmscript
require 'rubygems'
require 'bitescript'

class JVM
  class Assembler
    attr_reader :options

    def self.compile(name, options = {}, &block)
      self.new(name, options, &block).compile!
    end

    def initialize(name, options = {}, &block)
      @options = options
      @file    = BiteScript::FileBuilder.new(options[:source_file] || __FILE__)
      @class   = @file.public_class(name.to_s)
      @method  = @class.public_static_method('main', @class.void, @class.string[])

      @method.start
      if block_given?
        case block.arity
          when 1 then block.call(@method)
          else @method.instance_eval(&block)
        end
      end
    end

    def assemble(&block)
      @method.instance_eval(&block)
      self
    end

    def compile!
      assemble { returnvoid }

      @method.stop
      @file.generate do |filename, class_builder|
        File.open(filename, 'wb') do |file|
          file.write(class_builder.generate)
        end
      end
    end

    def method_missing(op, *args, &block)
      if @method.respond_to?(op)
        @method.send(op, *args, &block)
      else
        super # throws NoMethodError
      end
    end
  end
end

if __FILE__ == $0
  # JVM instruction set reference:
  # @see http://homepages.inf.ed.ac.uk/kwxm/JVM/codeByFn.html

  # A starting point:
  JVM::Assembler.compile('hello') do
    ldc "Hello, world!"
    println
  end

  # Small integer arithmetic:
  JVM::Assembler.compile('fixnum') do
    ldc 7
    ldc 6
    imul
    println(int)
  end

  # Big integer arithmetic:
  JVM::Assembler.compile('bignum') do
    BigDecimal = java.math.BigDecimal

    def ldc_bignum(num)
      new BigDecimal
      dup
      ldc num.to_s
      invokespecial BigDecimal, '<init>', [void, string]
    end

    ldc_bignum 10**100
    ldc_bignum 2**64
    invokevirtual BigDecimal, :subtract, [BigDecimal, BigDecimal]

    invokevirtual BigDecimal, :toString, [string]
    println(string)
  end
end