Calling Java from JRuby

Current JRuby is lack of detail documentation with regard to Java integration, so in certain cases it take me and my colleague a while to figure out how to do.
String to/from Java byte's array
In our recent application, we need to pass image from Ruby to image processing method written in Java. The Java method accept byte array and has result as byte array also. So we need convert Ruby String to Java byte[] and back.
The require 'java' add two methods to Ruby String class

String#to_java_bytes # instance method return Java byte[] 
String::from_java_bytes # class method return Ruby string from Java byte[]

The below code snippet demonstrate it.

require 'java'
include_class 'net.csetech.sq.image.ImageHelper'

File.open('duongpl.jpg') do |f|
  f.binmode
  @original = f.read
end

@reduced_j = ImageHelper.reduce_to_fix_size(@original.to_java_bytes,15000)
@reduced_r = String.from_java_bytes(@reduced_j)

File.open('duongpl-15k.jpg','w') do |f|
  f.binmode
  f.write(@reduced_r)
end

Ruby Array to Java Array
Other example, that we may need is to convert Ruby Array to Java Array. The require 'java' add method Array::to_java(Symbol), that convert Ruby Array to Java Array, where Symbol represent Java Class of the Array's elements. If it is omitted, the result is Java Object[].

['a','b'].to_array(:String) # return Java String[]
['a','b'].to_array # return java Object[]

More about method to_array is on JRuby Cookbook