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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!

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5 Answers

up vote 19 down vote accepted

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?

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3  
This post makes me want to fake the work I'm doing today and tinker around with Jasmin. :-) – corsiKa Jun 30 '10 at 14:47
+1 Jasmin is what came to my mind and I couldn't remember the name. It was featured in a book that explains JVM internals. I Forget the name of the book too, sigh... – bakkal Jun 30 '10 at 14:48
@Bakkal: According to the link: "Jasmin was originally created as a companion to the book "Java Virtual Machine", written by Jon Meyer and Troy Downing and published by O'Reilly Associates." – Adam Paynter Jun 30 '10 at 14:50
by "without using Javac or Java," I just meant that I want to write the code using bytecode. Thanks for the info! – Corey Stevens Jun 30 '10 at 15:06
wow~ This is super cool. is this JVM independent as well? can this run on a blackberry jvm? – Viele Jun 30 '10 at 15:36
show 3 more comments
up vote 4 down vote

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

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typo: it's BCEL. Otherwise great editor – H-H Jul 1 '10 at 8:25
up vote 3 down vote

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

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+1 for the edit – Lord Torgamus Jun 30 '10 at 16:26
up vote 4 down vote

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)

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up vote 0 down vote

I've created a new Java bytecode assembler that is backwards compatible with Jasmin but also adds lots of new features and simplifies the syntax slightly.

Here's an example of how you might write a Hello World program.

.class public hello
.super java/lang/Object

.method public static main : ([Ljava/lang/String;)V
    .limit stack 10
    .limit locals 10

    getstatic java/lang/System out Ljava/io/PrintStream;
    ldc "Hello World!"
    invokevirtual java/io/PrintStream println (Ljava/lang/Object;)V
    return
.end method

I've also written a tutorial on bytecode assembly. It currently only covers Hello, World, but I can continue it if there is interest.

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