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title: Secure Your APIs: OAuth2 and JWT for Beginners | daily.dev
description: A beginner-friendly tutorial on securing Spring Boot APIs using OAuth2 and JWT in Kotlin. Covers the conceptual relationship between authentication and...
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# Secure Your APIs: OAuth2 and JWT for Beginners

**[JetBrains](https://daily.dev/sources/jetbrains)** · 17 min read · 5 upvotes · 0 comments

## Summary

A beginner-friendly tutorial on securing Spring Boot APIs using OAuth2 and JWT in Kotlin. Covers the conceptual relationship between authentication and authorization, JWT structure (header, payload, signature), and the four OAuth2 roles. Walks through setting up a Spring Boot resource server with JWT validation, configuring scope-based access control, extracting custom claims (roles) via a custom JwtAuthenticationConverter, using @PreAuthorize for fine-grained method-level security, and enforcing issuer and audience validation with a custom JwtDecoder.

## Full article

daily.dev links to this article rather than hosting it. Read it at the original source: <https://blog.jetbrains.com/kotlin/2026/07/secure-your-apis-oauth2-and-jwt-for-beginners>

## Questions this post answers

### How do I validate both the issuer and audience claims of a JWT in a Spring Boot resource server?

Create a custom JwtDecoder that combines a default issuer validator with a custom AudienceValidator implementing OAuth2TokenValidator<Jwt>. Build the decoder via JwtDecoders.fromIssuerLocation, wrap both validators in a DelegatingOAuth2TokenValidator, and set it on the NimbusJwtDecoder. This rejects tokens whose iss or aud claims don't match your expected values, preventing token replay across applications.

_Developers hardening Spring Boot auth flows can track OAuth2 and JWT implementation patterns on daily.dev._

### Why aren't roles automatically mapped from JWT claims into Spring Security authorities like scopes are?

Scopes are standardized under OAuth2 and OpenID Connect, so Spring Security can safely map them into authorities automatically, but roles have no universal standard for how identity providers represent them. Because role claims vary by provider, they typically appear as custom claims and require a custom JwtAuthenticationConverter to translate them into Spring's expected authority format before use.

_Anyone wiring role-based access into Spring Security can compare JWT authorization patterns on daily.dev._

### Why shouldn't I use JWTs for session storage in an API?

Using JWTs as session storage leads to token bloat, complicates revocation, and increases the risk of exposing sensitive data. Instead, JWTs should stay focused on authentication and authorization claims, paired with short-lived tokens and clear refresh and revocation strategies, while issuer, audience, signature, and expiry are always validated to keep tokens trustworthy.

_Teams designing token strategies can weigh JWT session tradeoffs alongside other API security reads on daily.dev._

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---

Tags: [#kotlin](https://daily.dev/tags/kotlin), [#spring-boot](https://daily.dev/tags/spring-boot), [#jwt](https://daily.dev/tags/jwt), [#spring-security](https://daily.dev/tags/spring-security)

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