Java Glossary : JASM

Last updated 2004-07-02 by Roedy Green ©1996-2004 Canadian Mind Products

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JASM

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

1. Write a Java program and compile it.
2. Write a program in some other language such as NetRexx or JPython that support the JVM and compile it.
3. Write a Jasmin program and assemble it, or use some other similar JASM assembler.
4. 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 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 jeroen.nu.

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:

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.
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	call a private instance method, and final instance methods where you can tell ahead of time which variant of the method you want.
invokestatic	call a static method.
invokevirtual	call a non-final instance method.
lookupswich	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.

ASM Java byte code generator ¤ BCEL (Jakarta Byte Code Engineering Library ¤ Class.forName ¤ ClassLoader ¤ cglib: code generation library ¤ disassembler ¤ Jasmin ¤ Java Bytecode Assembler ¤ Javac.exe ¤ Jenesis ¤ JikesBT ¤ Oolong ¤ spreadsheet amanuensis

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