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jasmin/jasmin-2.4/docs/about.html
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<html>
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<head>
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<title>About Jasmin</title>
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<link href="style.css" rel="stylesheet" type="text/css">
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</head>
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<body>
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<table>
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<tr><td width=550>
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<center>
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<p><img src=jasmin_icon.jpg></p>
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<p>
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<div class="h1">ABOUT JASMIN</div>
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Jonathan Meyer, July 1996
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</p>
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</center>
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<h1>Introduction</h1>
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This document tries to answer some questions you might have
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about Jasmin. In particular, several people have asked me what
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Jasmin is, why they might use Jasmin, and why I wrote it in the
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first place. I've tried to give some answers to these questions
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below.<p>
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<h1>Jasmin Assembler</h1>
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Jasmin is a Java Assembler Interface. It takes ASCII descriptions for Java
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classes, written in a simple assembler-like syntax using the Java Virtual
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Machine instructions set. It converts them into binary Java class files
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suitable for loading into a Java interpreter.<p>
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To give you a flavor, here is the Jasmin assembly code for HelloWorld:<p>
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<pre>
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.class public HelloWorld
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.super java/lang/Object
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;
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; standard initializer (calls java.lang.Object's initializer)
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;
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.method public <init>()V
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aload_0
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invokenonvirtual java/lang/Object/<init>()V
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return
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.end method
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;
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; main() - prints out Hello World
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;
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.method public static main([Ljava/lang/String;)V
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.limit stack 2 ; up to two items can be pushed
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; push System.out onto the stack
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getstatic java/lang/System/out Ljava/io/PrintStream;
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; push a string onto the stack
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ldc "Hello World!"
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; call the PrintStream.println() method.
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invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
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; done
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return
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.end method
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</pre>
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<p>
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Jasmin was originally created as a companion to the book "Java Virtual Machine",
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written by Jon Meyer and Troy Downing and published by O'Reilly Associates. The
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book is now out of print. Jasmin survives as a SourceForge Open Source project.
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</p>
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<h1>Motivation for Jasmin</h1>
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<p>
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Jasmin was written because, at the time that we wrote the Java Virtual Machine
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book for O'Reilly, Sun had not published an assembler format for the
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Java virtual machine.
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</p>
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<p>
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Generating a binary Java .class file is pretty fiddly. Its like
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creating an a.out (or .exe) file by hand. Even using a Java package like
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JAS (a Java API for creating class files, used internally by Jasmin and written by KB Sriram), you
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need to know a lot about the philosophy of the Java Virtual
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Machine before you can write something at the Virtual
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Machine level and generate a Java class. <p>
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We wanted something that made it very easy for a student or programmer
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to explore the Java Virtual Machine, or write a new language
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which targets the VM, without getting into the details of constant
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pool indices, attribute tables, and so on.<p>
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<p>
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Creating a Java assembler seemed like a good solution.
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</p>
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<p>
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Unfortunately, Sun has not seen the need to define a standard Java
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assembler format, and has not released tools perform Java assembly.
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</p>
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<p>Sun does provide a javap program which can print the assembly code
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in a class file. However, the javap output is inappropriate for
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use as an assembler format. It is designed to be read by a person,
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not to be parsed by an assembler, so it has a number of
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omissions and drawbacks. </p>
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<p>
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Internally, Sun has a Java assembler tool. In hindsight, the syntax used by their internal tool is nicer than
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the Jasmin syntax. However, to my knowledge, their tool is still not widely available, nor is it a formally
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supported part of the Sun JDK.
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</p>
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<h1>Update on Jasmin Today (2004) </h1>
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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
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been ported and cloned multiple times. For better or worse, Jasmin remains the original Java assembler.
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<p>
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[As an interesting comparison, Microsoft .NET shipped out-of-box with an
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assembler, a disassembler, a standard IL assembly format, and built-in libraries
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for code-genning (generating classes on the fly). It would be great if Sun was
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as comprehensive in their support of the JVM].
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</p>
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<h1>What can I do with Jasmin?</h1>
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To give you some ideas, below are some theoretical Jasmin users/uses.<p>
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<h3>Teachers</h3>
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If you are teaching a compilers course, you could have students
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write a compiler which generates Jasmin assembly files,
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and then assembles those files into Java class files. Then you
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can integrate the advantages of the Virtual Machine (portability,
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the verifier, an object model...) into your courseware.<p>
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<h3>Hobbyists</h3>
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Jasmin lets you poke around in Java at the VM level, so that
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you can gain a real understanding of how Java works and
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what the Virtual Machine is like.<p>
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<h3>System Implementors</h3>
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If you are implementing a Java runtime system, Jasmin is
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an essential tool for generating test classes.<p>
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<h3>Advanced Programmers</h3>
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You could use Jasmin to write a critical class or method by
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hand (e.g. if you think that Java isn't doing things
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as well as it could). <p>
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Alternatively, you could create a syntax extension to the
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Java language which uses Jasmin (or JAS). <p>
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<h3>Language Implementors</h3>
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If you want to create an implementation of your
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favorite programming language which targets the
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Virtual Machine, Jasmin may be a simpler approach than
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writing a Java class file generator. This is especially
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true if your compiler is implemented in something other
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than Java, since you can create Java class files easily
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without having to get involved in the details of the
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binary file format.<p>
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<h3>Security Wizards</h3>
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Sun's claim that the Java class verifier protects you from
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hostile programs is a pretty strong one. Jasmin lets you create
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'hostile' class files and see if a Java implementation is really as
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secure as it should be. <p>
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<hr><address>Copyright (c) Jonathan Meyer, July 1996</address>
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<hr>
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<a href="http://jasmin.sourceforge.net">Jasmin Home</a> |
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<a href="http://www.cybergrain.com/">Jon Meyer's Home</a>
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