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