asm_test.rb

require 'java'
require 'rubygems'
require 'mvn:org.jruby.extras:jnr-x86asm'
require 'ffi'
require 'udis86'

Asm = Java::jnr.x86asm.Asm
PM = Java::com.kenai.jffi.PageManager

# Create a simple native function that returns 0xdeadbeef
asm = Java::jnr.x86asm.Assembler.new Asm::X86_64
asm.mov Asm::rax, Asm::imm(0xdeadbeef)
asm.ret

# Get some native memory to write the code into
pg = PM.instance.allocate_pages(1, PM::PROT_READ | PM::PROT_WRITE)
buf = java.nio.ByteBuffer.allocate(asm.code_size)
asm.reloc_code(buf, pg)
buf.flip

code = FFI::Pointer.new(pg)
code.put_bytes(0, String.from_java_bytes(buf.array), 0, asm.code_size)

# Make the page executable (and non-writable, since some OS require that)
PM.instance.protect_pages(pg, 1, PM::PROT_READ | PM::PROT_EXEC)

puts "disassembly:"
FFI::UDis86::UD.create(:mode => 64, :syntax => :intel, :buffer => code.get_bytes(0, asm.code_size)).disas do |insn|
  puts "\t#{insn}"
end
puts

# Wire up the function via FFI, and call it
fptr = FFI::Function.new :ulong, [ ], code
puts "function returns: #{fptr.call.to_s(16)}"

Running it

$ env LD_LIBRARY_PATH=/opt/local/lib jruby -J-d64 asm_test.rb 
disassembly:
	mov rax, 0xdeadbeef
	ret 

function returns: deadbeef