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-34f75428e0ad https://www.javaworld.com/article/2077233/bytecode-basics.html https://www.programcreek.com/2013/04/what-can-you-learn-from-a-java-helloworld-program/ https://gitlab.ow2.org/