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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!

Corey Stevens
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3 Answers3

46

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?

Adam Paynter
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    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
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    @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
  • @Viele: Yes, this should be JVM independent (so long as the JVM you're interested in conforms to the JVM specification). – Adam Paynter Jun 30 '10 at 16:28
  • @Corey, how is this different from using a plan `.java` file and `javac` ? – OscarRyz Jun 30 '10 at 17:42
  • Wow!! I've been working for a while on bytecode. Why haven't I heard of this? – H-H Jul 01 '10 at 08:24
  • Jasmin is ok for ordinary use, but it doesn't support a lot of the more obscure things you can do in classfiles. If you want a heavy duty assembler see my answer. – Antimony May 30 '13 at 03:36
11

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.

Antimony
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9

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.

cdmckay
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Thorbjørn Ravn Andersen
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