Java Byte Code By Hand

So I thought I would look into the nitty gritty of how java real works

Notes

Magic number

cafe babe

Java version 8

Java 8 is version 52 which then decodes to

00 00 00 34

Constant Pool

this is basically the most important part to java's bytecode which allows us to reference other classes, use strings and store other variables

Access Modifiers

ACC_PUBLIC 0x0001
ACC_PRIVATE 0x0002
ACC_PROTECTED 0x0004
ACC_STATIC 0x0008
ACC_FINAL 0x0010
ACC_SUPER 0x0020
ACC_INTERFACE 0x0200
ACC_ABSTRACT 0x0400

Quick structure

Java File: CAFE BABE
Version 8: 0000 0034
Constant Pool Size of ZERO: 0000
Super Public: 0021
Unknown index of class in constant pool: 0000
Unknown index of super class in constant pool: 0000
zero interfaces: 0000
zero fields: 0000
zero methods: 0000
zero attributes: 0000

Class name must be at reference constant pool at #1

Example

07 00 02
01 00 0a 48 65 6c 6c 6f 57 6f 72 6c 64

this points #1 to #2

Constant pool:
#1 = Class #2 // JavaByHand
#2 = Utf8 JavaByHand
#3 = Class #4 // java/lang/Object
#4 = Utf8 java/lang/Object
#5 = Class #6 // java/lang/System
#6 = Utf8 java/lang/System
#7 = Class #8 // java/io/PrintStream
#8 = Utf8 java/io/PrintStream
#9 = String #10 // Hello World
#10 = Utf8 Hello World

Run program

java JavaByHand

Yes, the .class is not included

Take aways

• useage of javap
• The structure of a java class
• How variables and strings are saved and loaded in java

References

https://medium.com/@davethomas_9528/writing-hello-world-in-java-byte-code-34f75428e0adhttps://www.javaworld.com/article/2077233/bytecode-basics.htmlhttps://www.programcreek.com/2013/04/what-can-you-learn-from-a-java-helloworld-program/https://gitlab.ow2.org/