Inside a Java .class File — Visual Guide

A complete visual walkthrough of the Java class file format — magic number, constant pool, access flags, methods, fields, attributes, and Code — with ASCII diagrams.

The 10,000-Foot View

Magic Number: 0xCAFEBABE

Constant Pool

Access Flags

Fields

Methods

The Code Attribute: Heart of Every Method

Nested Attributes

A Minimal Hello World: Full Structure

Viewing Class Files in Practice