About Jasmin

Jonathan Meyer, July '96

1.1 Introduction

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.

1.2 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
       aload_0
       invokenonvirtual java/lang/Object/<init>()V
       return
    .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
       return
    .end method

Jasmin was written as the companion to the book "Java Virtual Machine", soon to be published by O'Reilly, written by Troy Downing and myself.

1.3 Motivation for Jasmin ("why did you write Jasmin?")

We were at first reluctant to write Jasmin, in the main because we didn't want to introduce yet another syntax/file format if it could be avoided. However, for a number of reasons, we eventually decided we had no choice. Two of the reasons were:

The Java .class file format is hairy

Generating a Java class file manually is pretty fiddly. Its like creating an a.out (or .exe) file by hand.

Even using a 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.

Sun has not defined an assembler format

Unfortunately, Sun has not seen the need for a Java assembler, and has not created a standard assembler format.

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.

1.4 What can I do with Jasmin?

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

Teachers

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.

Hobbyists

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. Some people might argue that this is a good reason not to make the assembler public. I would disagree.

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