Programming in Java bytecode

Question

I'm looking to write a short program (maybe a Hello World) in Java bytecode. I just want to write the bytecode using my text editor and run it. How would I do this? Got an example? Thanks!

Answer 1

Have you considered JBE? It's based on Apache's Bytecode Engineering Library (BEL)

Answer 2

Maybe this article can get you started: Bytecode basics (a little old, but you will get the idea).

The class file format will come in handy too :D

Answer 3

You could try Jasmin!

.class public HelloWorld
.super java/lang/Object

.method public static main([Ljava/lang/String;)V
  .limit stack 3
  .limit locals 1

  getstatic      java/lang/System/out Ljava/io/PrintStream;
  ldc            "Hello World."
  invokevirtual  java/io/PrintStream/println(Ljava/lang/String;)V

  return

.end method

You compile it using:

> java -jar jasmin.jar hello.j

And then you run it like any class:

> java HelloWorld
Hello World.

Update

I see that your question mentions "without using Javac or Java". Could you clarify how you meant that statement?

Answer 4

Byte code is written as actual bytes, which are not normally easily editable by a normal text editor.

This means you will need something that converts a textual representation to binary. A reasonable place to start would be an assembler like Jasmin. http://en.wikipedia.org/wiki/Jasmin_(Java_assembler)