Examples about how to access java classes with jruby

Calculator.java

package e7r;

/**
 * Simple calculator class to be used inside jruby
 */
public class Calculator {

  /**
   * Adds two integer numbers
   * @param a First value
   * @param b Second value
   * @return The sum of two values
   */
  public int sum(int a, int b) {
    return a + b;
  }

}

calculator.rb

#
# First you must require java to gain some special powers
# Remember that this works only with jruby!
#
require 'java'

#
# Then require your jar file. Note that you can't do something like
# import SmallestJar.*. You must require each jar by yourself.
#
require 'SmallestJar'

# Some ways of consuming java classes inside jruby

#
# 1. Let's do it the hard way: using full package name every time
#    - remember to always start with a Java:: and then the package namespace
#    - some namespaces like org, com, etc. are mapped directly
#

calculator = Java::e7r.Calculator.new
puts calculator.sum(10, 20)

#
# 2. Let's do it the messy way: including the package in the current context
#

include_class Java::e7r.Calculator
calculator = Calculator.new # look mama, no namespaces
puts calculator.sum 10, 20

#
# 3. The man's way: Create a module to put the classes (via include_package)
#

module E7R
  include_package 'e7r'
end

calculator = E7R::Calculator.new
puts calculator.sum 10, 20

# EOF

smallest-jar-creation.sh

# two steps to create a jar manually

javac Calculator.java
jar cvf SmallestJar.jar Calculator.class