JASM : Java Glossary

JASM

JASM (Java Assembler). Pronounced "jazz-em", a generic term for assemblers for the JVM (Java Virtual Machine) bytecodes. It also includes postfix assemblers. There are four ways to create class files of JVM byte codes:

Write a Java program and compile it.
Write a program in some other language such as NetRexx or JPython that support the JVM and compile it.
Write a Jasmin program and assemble it, or use some other similar JASM assembler.
Generate the hex byte codes directly. You might write them to disk and load them in the conventional way with Class.forName, or leave them in RAM (Random Access Memory) and load them with a custom ClassLoader or use java.lang.ClassLoader.defineClass directly.

The easiest way to create class file on the fly is to generate Java source code then invoke the Javac or other compiler. Sometimes it is simpler to generate byte codes directly, such as a spreadsheet engine, a regex parser or a grammar parser since you can use cheating tactics like GOTO, and have precise control of the byte codes generated.

Jeroen Frijters (pronounced Yeroon Frighters) delivered a paper called On the Fly Byte Code Generation at the 1999 Colorado Software Conference. He wrote classes for creating byte codes. His email address is jeroen@sumatra.nl and his website is http://weblog.ikvm.net/.

Probably the easiest way to understand byte codes is to write sample Java code that does what you want, then disassemble it. Then it should become clear what you need to generate. Sun now has some classes for assembling byte code: see sun.reflect.ClassFileAssembler.

Key Byte Op Codes

Some of the more interesting byte codes include:

aastore	store a reference in an array, making sure the type of the reference is compatible with the run-time type of the array.
athrow	throw an exception.
checkcast	checks that a cast to another reference type is valid.
fadd	adds top two floats on the stack and replaces them with the result. The JVM does not have a separate floating point stack.
goto	At the byte code level there has to be a goto to implmement if/else branching.
iadd	adds top two ints on the stack and replaces them with the result.
iload	push the value of a local integer variable to the top of the stack.
iconst	push an integer literal to the stack.
instanceof	replaces reference on top of stack with a boolean.
invokeinterface	call a method when all you have is an interface reference to it.
invokespecial	née invokenonvirtual in JDK (Java Development Kit) 1.0. Call a private instance method, and final instance methods where you can tell ahead of time which variant of the method you want. Also used to call clinit, the code to initialise a new object prior to the constructor running. Also used to call super. someMethod, where you also know the class of the method to invoke at compile time.
invokestatic	call a static method.
invokevirtual	call a non-final instance method.
lookupswitch	used when switch case labels are not dense. Searches for a matching key and jumps.
new	allocates RAM for objects.
pop	drops the top element of the stack.
swap	exchanges the top two elements of the stack.
tableswitch	used when switch case labels are dense. Indexes into table for offset.

Richer Class Files

There are several techniques for persuading the compiler to add extra information into the class files:

Specifying -g:vars as a compiler option will result in saving the names of method parameters in LocalVariableTable_attribute structure of the generated class file.
Specifying -g as a compiler option will generate all the additional information needede by debuggers.
With @annotations.

Books

recommend book⇒Java(TM) Virtual Machine Specification, The 2nd Edition

by:

Tim Lindholm and Frank Yellin

978-0-201-43294-7

paperback

publisher:

Prentice Hall

published:

1999-04-24

Make sure you get the second edition. This book has an unusual copyright notice. You might want to read a different book if you plan to write a clean-room JVM. Read the book online.

Greyed out stores probably do not have the item in stock

recommend book⇒The Java Virtual Machine

by:

Joshua Engel

978-1-56592-194-8

paperback

publisher:

O’Reilly

published:

1997-04

Known known as the goldfish bowl book because of its cover. The book is frustrating because it spends so much time with the irrelevant Jasmin assembler and its syntax. You are interested in generating byte codes directly, not assembler. It leaves out much you must discover by experiment looking at generated class files, such as whether offsets are signed or absolute, where the base is etc. In its next revision, it should set the Jasmin aside in an appendix, and include examples and more precise documentation on the binary formats. The book is still valuable because it gives a fair bit of background exposition you will not find in the vmspec itself. You would use this book to understand the VM, then the vmspec to actually write code that generated or modified class files. Out of print.

Greyed out stores probably do not have the item in stock

recommend book⇒Programming For The Java Virtual Machine

by:

Joshua Engel

978-0-201-30972-0

paperback

publisher:

Addison-Wesley

published:

1999-07-02

From the reviews it looks like it too gets hung up on a propietary assembler called Oolong. He does provide a code-generating class and spends one chapter on using it. I think the book would have been more useful if all the examples were in code-generator code, rather than an obscure dialect of assembler.

Greyed out stores probably do not have the item in stock

recommend book⇒Inside The Java Virtual Machine

by:

Bill Venners

978-0-07-135093-8

paperback

publisher:

McGraw-Hill

published:

2000-01-06

Covers Java 2 JVM internals.
ISBN of first edition was 0-07-913248-0.
See the blurb.

Greyed out stores probably do not have the item in stock

Learning MOre

Oracle’s JVMS (Java Virtual Machine Specification): class file format specification

Oracle’s JVMS: Op Codes

Oracle’s JVMS: Java Virtual Machine Specification

Oracle’s JVMS: download

ANTLR: byte code parser
ASM Java byte code generator
BCEL (Byte Code Engineering Library)
cglib: code generation library
changes to byte codes in JDK 1.5
Class.forName
ClassLoader
disassembler
Jamaica byte code assembler
Jasmin
Java Bytecode Assembler
Java Class file format
Javac.exe
Jenesis
Jeroen Frijters’ presentation : on on-the-fly byte code generation
Jeroen Frijters’ source code : on on-the-fly byte code generation
JikesBT
JVM
ObjectWeb ASM
on the fly byte code generation and run
Oolong
Soot
spreadsheet amanuensis

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