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This document tries to answer some questions you might have about Jasmin. In particular, several people have asked me what Jasmin is, why they might use Jasmin, and why I wrote it in the first place. I've tried to give some answers to these questions below.

What is Jasmin?

Jasmin is a Java Assembler Interface. It takes ASCII descriptions for Java classes, written in a simple assembler-like syntax using the Java Virtual Machine instructions set. It converts them into binary Java class files suitable for loading into a Java interpreter.

To give you a flavor, here is the Jasmin assembly code for HelloWorld:

.class public HelloWorld
.super java/lang/Object

; standard initializer (calls java.lang.Object's initializer)
.method public <init>()V
    invokenonvirtual java/lang/Object/<init>()V
.end method

; main() - prints out Hello World
.method public static main([Ljava/lang/String;)V
    .limit stack 2   ; up to two items can be pushed

    ; push System.out onto the stack
    getstatic java/lang/System/out Ljava/io/PrintStream;

    ; push a string onto the stack
    ldc "Hello World!"

    ; call the PrintStream.println() method.
    invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V

    ; done
.end method

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. The book is now out of print. Jasmin survives as a SourceForge Open Source project.

Motivation for Jasmin

Jasmin was written because, at the time that we wrote the Java Virtual Machine book for O'Reilly, Sun had not published an assembler format for the Java virtual machine.

Generating a binary Java .class file is pretty fiddly. Its like creating an a.out (or .exe) file by hand. Even using a Java package like JAS (a Java API for creating class files, used internally by Jasmin and written by KB Sriram), you need to know a lot about the philosophy of the Java Virtual Machine before you can write something at the Virtual Machine level and generate a Java class.

We wanted something that made it very easy for a student or programmer to explore the Java Virtual Machine, or write a new language which targets the VM, without getting into the details of constant pool indices, attribute tables, and so on.

Creating a Java assembler seemed like a good solution.

Unfortunately, Sun has not seen the need to define a standard Java assembler format, and has not released tools perform Java assembly.

Sun does provide a javap program which can print the assembly code in a class file. However, the javap output is inappropriate for use as an assembler format. It is designed to be read by a person, not to be parsed by an assembler, so it has a number of omissions and drawbacks.

Internally, Sun has a Java assembler tool. In hindsight, the syntax used by their internal tool is nicer than the Jasmin syntax. However, to my knowledge, their tool is still not widely available, nor is it a formally supported part of the Sun JDK.

Update on Jasmin Today (2004)

Since Jasmin was written, it has become the de-facto standard assembly format for Java. It is used in dozens of compiler classes throughout the world, and has been ported and cloned multiple times. For better or worse, Jasmin remains the original Java assembler.

[As an interesting comparison, Microsoft .NET shipped out-of-box with an assembler, a disassembler, a standard IL assembly format, and built-in libraries for code-genning (generating classes on the fly). It would be great if Sun was as comprehensive in their support of the JVM].

What can I do with Jasmin?

To give you some ideas, below are some theoretical Jasmin users/uses.


If you are teaching a compilers course, you could have students write a compiler which generates Jasmin assembly files, and then assembles those files into Java class files. Then you can integrate the advantages of the Virtual Machine (portability, the verifier, an object model...) into your courseware.


Jasmin lets you poke around in Java at the VM level, so that you can gain a real understanding of how Java works and what the Virtual Machine is like.

System Implementors

If you are implementing a Java runtime system, Jasmin is an essential tool for generating test classes.

Advanced Programmers

You could use Jasmin to write a critical class or method by hand (e.g. if you think that Java isn't doing things as well as it could).

Alternatively, you could create a syntax extension to the Java language which uses Jasmin (or JAS).

Language Implementors

If you want to create an implementation of your favorite programming language which targets the Virtual Machine, Jasmin may be a simpler approach than writing a Java class file generator. This is especially true if your compiler is implemented in something other than Java, since you can create Java class files easily without having to get involved in the details of the binary file format.

Security Wizards

Sun's claim that the Java class verifier protects you from hostile programs is a pretty strong one. Jasmin lets you create 'hostile' class files and see if a Java implementation is really as secure as it should be.

Copyright (c) Jonathan Meyer, July 1996