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<hr>
<h1>Building an OAuth 2.0 Server from Scratch: A Practical Guide</h1>
<h2>Introduction</h2>
Recently, my manager assigned me the task of building an OAuth server (similar to Google/GitHub) for Amazon Seller Central. This would allow sellers to log in via Google or Facebook instead of traditional credentials.
In this guide, I’ll share my experiences, challenges, errors, and learnings. While I can’t share the production code, I have replicated a similar setup on my personal GitHub, implementing OAuth using two microservices.
Let’s dive in! Before jumping into implementation, we need to first understand OAuth 2.0 and its impact on user experience.
<hr>
<h2>OAuth 2.0 Overview</h2>
<h3>Why OAuth?</h3>
OAuth 2.0 is an industry-standard authorization protocol that enables secure access to third-party applications without exposing user credentials.
<ul>
<li>Example: Instead of storing passwords, websites offer “Login with Google” or “Login with Facebook,” allowing users to authenticate via these trusted providers.</li>
</ul>
<h3>Key Participants in OAuth</h3>
<ol>
<li>User – The individual requesting access to a resource.</li>
<li>Authorization Server – Handles authentication and issues access tokens (e.g., Google’s OAuth server).</li>
<li>Application – The service the user is trying to access (e.g., a shopping website).</li>
<li>Resource Server – Hosts protected resources (e.g., a user’s order history).</li>
</ol>
<h3>OAuth Flow</h3>
<ol>
<li>The user requests access to an application.</li>
<li>The application redirects the user to the Authorization Server.</li>
<li>After successful authentication, the Authorization Server issues an access token.</li>
<li>The user sends this token to the Application, which then requests access from the Resource Server.</li>
<li>The Resource Server verifies the token and grants access.</li>
</ol>
<img src="https://d2beiqkhq929f0.cloudfront.net/public_assets/assets/000/088/683/original/2.png?1725537820" alt="OAuth Flow">
<hr>
<h2>Hands-On Implementation</h2>
To implement OAuth, we will build two microservices using Java + Spring Boot + Spring Security + MySQL:
<ol>
<li>UserService – OAuth Server (Authentication &amp; Token Management).</li>
<li>ProductService – Application using the OAuth server for authentication.</li>
</ol>
<h3>1. Setting Up the OAuth Server (UserService)</h3>
🔗 GitHub Repo: <a href="https://github.com/Ms-Shahid/User-microservice" target="_blank" rel="noopener nofollow">UserService</a>
In production, OAuth has numerous attributes and functionalities, but for simplicity, we’ll cover only the core concepts.
<h4>Key Components in UserService</h4>
Spring provides two key beans for OAuth:
<ul>
<li><code>UserDetailsService</code> – Manages user authentication.</li>
<li><code>RegisteredClientRepository</code> – Stores OAuth client details.</li>
</ul>
Since we are using a database, we implement custom beans. We follow an MVC structure:
<h3>📌 Models (Database Entities) – Representing Database Tables</h3>
<ol>
<li><code>Authorization.java</code> – Stores OAuth authorization details, such as access tokens, refresh tokens, expiration times, and user permissions.</li>
<li><code>AuthorizationConsent.java</code> – Tracks user consent for specific OAuth clients, ensuring users have explicitly granted permissions to third-party apps.</li>
<li><code>Client.java</code> – Represents registered OAuth clients (e.g., Google, Facebook) with details like client ID, secret, redirect URIs, and authorization grant types.</li>
<li><code>CustomGrantedAuthority.java</code> – Defines user roles and permissions, ensuring the system enforces proper access control based on assigned authorities.</li>
<li><code>CustomUserDetails.java</code> – Implements <code>UserDetails</code> from Spring Security, encapsulating user authentication details such as username, password, and roles.</li>
</ol>
<hr>
<h3>📌 Repository (Data Layer) – Database Interaction Layer</h3>
<ol>
<li><code>AuthorizationRepository.java</code> – Manages OAuth token storage and retrieval, ensuring tokens can be validated, refreshed, and revoked when necessary.</li>
<li><code>AuthorizationConsentRepository.java</code> – Handles CRUD operations for user consent records, keeping track of which permissions users have approved.</li>
<li><code>ClientRepository.java</code> – Stores and retrieves OAuth client details, ensuring only registered applications can authenticate users.</li>
<li><code>JpaRegisteredClientRepository.java</code> – Implements <code>RegisteredClientRepository</code>, allowing OAuth clients to be registered and managed dynamically in the database.</li>
</ol>
<hr>
<h3>📌 Service (Business Logic) – Implementing Authentication &amp; Authorization</h3>
<ol>
<li><code>CustomUserServiceDetails.java</code> – Implements <code>UserDetailsService</code> to load user authentication details from the database, supporting login and credential verification.</li>
<li><code>JpaOAuth2AuthorizationService.java</code> – Manages OAuth token persistence and lookup, ensuring tokens are securely stored and validated when accessed.</li>
<li><code>JpaOAuth2AuthorizationConsentService.java</code> – Handles user consent approvals, storing and retrieving granted permissions for OAuth clients.</li>
</ol>
<blockquote>
This structure ensures separation of concerns, making the system modular, maintainable, and scalable! 🚀
</blockquote>
<hr>
<h3>2. Integrating OAuth in ProductService</h3>
🔗 GitHub Repo: <a href="https://github.com/Ms-Shahid/Product-microservice" target="_blank" rel="noopener nofollow">ProductService</a>
To allow ProductService to authenticate users via OAuth, we need to configure it as an OAuth Resource Server.
<h4>Dependencies</h4>
Add the OAuth dependency in <code>pom.xml</code>:
<pre><code class="language-xml">&lt;dependency&gt;
 &lt;groupId&gt;org.springframework.boot&lt;/groupId&gt;
 &lt;artifactId&gt;spring-boot-starter-oauth2-resource-server&lt;/artifactId&gt;
 &lt;version&gt;2.7.5&lt;/version&gt;
&lt;/dependency&gt;
</code></pre>
<h4>Application Properties Configuration</h4>
Set the OAuth issuer URI (UserService URL) in <code>application.properties</code>:
<pre><code class="language-properties">spring.security.oauth2.resourceserver.jwt.issuer-uri=http://localhost:1902
</code></pre>
<h4>Spring Security Configuration</h4>
We configure JWT authentication in <code>JWTSecurityConfig.java</code>:
<pre><code class="language-java">@Configuration
public class JWTSecurityConfig {

 @Bean
 public SecurityFilterChain filterChain(HttpSecurity http) throws Exception {
 http
 .authorizeHttpRequests(authorize -&gt; authorize
 .requestMatchers(&quot;products/{id}&quot;)
 .authenticated()
 )
 .oauth2ResourceServer((oauth2) -&gt; oauth2.jwt(Customizer.withDefaults()));
 return http.build();
 }
}
</code></pre>
<hr>
<h2>Final Thoughts</h2>
That’s it! We’ve successfully built an OAuth server and integrated it with a microservice for authentication. 🚀
✅ Key Takeaways: 
✔️ Understanding OAuth 2.0 and its real-world use cases. 
✔️ Implementing OAuth in Java + Spring Boot + MySQL. 
✔️ Creating an authentication server (UserService) and integrating it into another service (ProductService).
References:
<ol>
<li><a href="https://github.com/Ms-Shahid/User-microservice" target="_blank" rel="noopener nofollow">UserService</a></li>
<li><a href="https://github.com/Ms-Shahid/Product-microservice" target="_blank" rel="noopener nofollow">ProductService</a></li>
</ol>
<hr>
<h3>What’s Next?</h3>
Try deploying this on AWS/GCP and implementing token refresh mechanisms. Let me know your thoughts or if you hit any roadblocks! 🚀

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Learn to build an OAuth 2.0 server using Java, Spring Boot, Spring Security, and MySQL. This guide covers setting up an OAuth server for authentication, creating microservices, and integrating the OAuth server with a resource server for secured access. Key components, repository layers, and service implementations are discussed for a modular, scalable system.

Understanding & Implementing OAuth Mechanism

<a href="https://app.daily.dev/ms_shahid" data-mention-id="2fo5Y3jCo4umFwJpIxq5y" data-mention-username="ms_shahid" translate="no">@ms_shahid</a> Are u from scaler??