Understanding OAuth2 Flow with a Complete Java Servlet Demo (Step-by-Step)

OAuth2 is everywhere. “Login with Google” “Continue with GitHub” “Sign in with Microsoft” We use it daily—but when it comes to explaining how it actually works, things quickly get confusing. Most tutorials either: Explain only the theory ❌ Or show isolated code without context ❌ Very few connect the full flow end-to-end . 🎯 What This Article Does Differently In this article, we will: Break down the 4 core actors Walk through the entire OAuth2 flow Map each step to working Java servlet code Build a complete runnable demo 🧠 The Key Idea (Read This First) OAuth2 is not about authentication . It is about delegating access . Instead of giving your username/password to an application, you allow it to access your data using a token issued by a trusted server . 🔷 Actors in OAuth2 This framework involves four key roles: 1. Resource Owner (User) The user who owns the data Grants or denies access 2. Client (Application) The application requesting access to user data 3. Authorization Server Authen