Ruby + Java file

gistfile1.java

//;"
/*"

code = <<-CODE.chop
class AJavaClass {
  public static void main(String[] args) {
    System.out.println("#{Random.rand(100)}");
  }
}
CODE

file = IO.read(__FILE__)
IO.write("#{__FILE__}.bak", file)
file.sub!(%r{(?<=^// BEGIN GEN\n)[\s\S]*(?=\n^// END GEN)}, code)
IO.write(__FILE__, file)

puts "Done"

=begin */

// BEGIN GEN
class AJavaClass {
  public static void main(String[] args) {
    System.out.println("38");
  }
}
// END GEN

/*
=end
#*/