Using java libraries to fill a XLS file from Ruby - works both with MRI and jruby

rjb_loader.rb

if !(RUBY_PLATFORM =~ /java/)
  require 'rjb-loader'
  RjbLoader.before_load do |config|
    # This code changes the JVM classpath, so it has to run BEFORE loading Rjb.
    Dir[File.join(Rails.root , '/resources/*.jar')].each do |path|
      config.classpath << File::PATH_SEPARATOR + File.expand_path(path)
      config.java_options = ['-Xms128M', '-Xmx256M']
    end
  end

  RjbLoader.after_load do |config|
    FILE_CLASS = Rjb::import('java.io.FileOutputStream')
    WORKBOOK_CLASS = Rjb::import('org.apache.poi.hssf.usermodel.HSSFWorkbook')
    POIFS_CLASS = Rjb::import('org.apache.poi.poifs.filesystem.POIFSFileSystem')
    Rjb::import('org.apache.poi.hssf.usermodel.HSSFCreationHelper')
    Rjb::import('org.apache.poi.hssf.usermodel.HSSFFormulaEvaluator')
    CELL_REFERENCE_CLASS = Rjb::import('org.apache.poi.hssf.util.CellReference')
    CELL_CLASS = Rjb::import('org.apache.poi.hssf.usermodel.HSSFCell')
    FILE_INPUT_CLASS = Rjb::import('java.io.FileInputStream')
    FILE_OUTPUT_CLASS = Rjb::import('java.io.FileOutputStream')
  end
else
  apache_poi_path = File.join(Rails.root , '/resources/poi-3.9-20121203.jar')
  require 'java'
  require apache_poi_path
  FILE_CLASS = java.io.FileOutputStream
  WORKBOOK_CLASS = org.apache.poi.hssf.usermodel.HSSFWorkbook
  POIFS_CLASS = org.apache.poi.poifs.filesystem.POIFSFileSystem
  CELL_REFERENCE_CLASS = org.apache.poi.hssf.util.CellReference
  CELL_CLASS = org.apache.poi.hssf.usermodel.HSSFCell
  FILE_INPUT_CLASS = java.io.FileInputStream
  FILE_OUTPUT_CLASS = java.io.FileOutputStream
end

write_xls.rb

_docs = []
_orders = self.payment_orders.all
_orders.delete_if { |po| po.send_by_post? }
done = (_orders.size == 0)
sheet_no = 1

@file_input = FILE_INPUT_CLASS.new(File.join(Rails.root, "/resources/cscs-payment-new.xls"))
@fs = POIFS_CLASS.new(@file_input)
@book = WORKBOOK_CLASS.new(@fs)

@sheet = @book.getSheetAt(0)

while !done do
  0.upto(49) do |i|
    order = _orders[(sheet_no-1)*50+i]
    if order then
      _default_address = order.sales_order_product.shipping_address
      @row = @sheet.getRow(20+i)
      @row ||= @sheet.createRow(20+i)

      @cell = @row.getCell(2)
      @cell ||= @row.createCell(2)
      @cell.setCellValue("#{order.contact.title} #{order.contact.full_name}")
      @cell = @row.getCell(3)
      @cell ||= @row.createCell(2)
      @cell.setCellValue("#{order.contact.nino}")
      @cell = @row.getCell(4)
      @cell ||= @row.createCell(2)
      @cell.setCellValue("#{order.contact.formatted_date_of_birth}")
    else
      done = true
    end
  end
    

  if RUBY_PLATFORM =~ /java/
    @file_output = java.io.FileOutputStream.new(File.join(Rails.root, 'tmp', "cscs-payment-new-#{self.id}.xls"))
  else
    @file_output = FILE_OUTPUT_CLASS.new(File.join(Rails.root, 'tmp', "cscs-payment-new-#{self.id}.xls"))
  end
  @book.write(@file_output)
  @file_output.close
  _doc = open("#{Rails.root}/tmp/cscs-payment-new-#{self.id}.xls").read

  _docs << {:data => _doc, :filename => "supportline_applications_#{Date.today.to_s}_new_#{sheet_no}.xls"}
  sheet_no += 1
end

_docs