Secure Your APIs: OAuth2 and JWT for Beginners

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This tutorial was written by an external contributor.

APIs are frequent targets for bad actors since they expose data and functionality. Securing them while maintaining usability is often one of the most challenging and time-consuming parts of API development. OAuth 2.0 and JSON Web Tokens (JWT) help make these processes more manageable and reliable. They allow developers to represent and verify identity and manage access by safely transmitting claims and enabling delegated authorization.

This article discusses these technologies and the most efficient ways you can use them to secure your Spring Boot-built APIs and backends. If you’re interested in a coroutine‑driven solution, a companion tutorial using Ktor is also planned and will be published soon.

OAuth2 and JWT Primer

OAuth2 and JWT(s) aren’t competing technologies. They’re complementary pieces of the puzzle, with one handling the delegation of authorization and the other serving as the compact, verifiable token format that carries secure information.

Authentication vs. Authorization

Authentication verifies identity (who you are), usually through credentials like passwords, tokens, or certificates. JWTs can carry identity information and act like a form of ID once issued. Roles and other claims within a JWT are then used for authorization.

Authorization helps control what a user has access to (what they can do). This includes the scopes or resources that they can “touch” and how those permissions are managed. In a system that uses OAuth2 and JWT, the access badge is bundled into your ID card. OAuth2 oversees and manages this process.

The Role of OAuth2

OAuth2 is a framework for delegated access. Instead of sharing passwords directly, users grant applications a token that represents their permissions. This means that your backend (acting as a Resource Server) doesn’t have to issue tokens. Instead, it trusts and validates the ones coming from the Authorization Server within OAuth2’s framework. This decoupling of duties allows you to simplify your APIs while reducing security risks and ensuring all tokens follow a clear, consistent, centralized policy.

You don’t have to worry about implementing user logins or browser redirects within your API. As far as validation and authorization are concerned, your backend or API’s job is to receive the Bearer Token, authenticate the signature, check expiration, and enforce scopes/roles.

Your API just checks badges; it’s not responsible for printing them. So how do JWTs fit into the equation?

What Is a JWT?

A JWT is a small, web-friendly piece of text (string) that securely transports information between systems. Their compactness makes them easy to pass around in HTTP headers or URLs. Each token uses Base64URL encoding, making them safe to include in query strings or headers.

JWTs are signed (and sometimes encrypted), so that recipients can verify that they weren’t tampered with. They’re also self-contained, carrying details like user ID, roles, or permissions. These elements (especially self-containment and signing) allow for stateless authentication without Session Storage. This means that you don’t need a database or cache to track active sessions. It also encourages fewer lookups and less infrastructure complexity, which reduces your system’s overhead.

JWTs have a very simple, standardized structure made up of three parts, separated by dots:

• The Header contains metadata about the token, such as the type (JWT) and the signing algorithm (HS256, RS256).
• The Payload features the claims, which are statements about the user or system (like user ID, roles, or token expiry).
• The Signature is a cryptographic signature created using the header, payload, and a secret or private key. This ensures the token has not been tampered with.

The basic structure of a JWT looks like this:

xxxxx.yyyyy.zzzzz

A real-world Base64URL-encoded token typically resembles the following:

eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9 .eyJzdWIiOiIxMjM0NTY3ODkwIiwibmFtZSI6IkpvaG4gRG9lIiwiaWF0IjoxNTE2MjM5MDIyfQ .SflKxwRJSMeKKF2QT4fwpMeJf36POk6yJV_adQssw5c

When OAuth2 meets JWT

There are four key roles in OAuth2’s implementation:

• Resource Owner: The entity (usually the user) granting access to the protected resources.
• Client: The application requesting access to the resource on behalf of the resource owner.
• Authorization Server: The server that authenticates the resource owner and issues access tokens to the client.
• Resource Server: The server hosting the protected resources, which accepts and validates tokens.

The Resource Owner grants permission (e.g., you click “Allow” when an app requests access), the Client then requests authorization from the Authorization Server, which issues an access token (JWT) if the Resource Owner approves. The Client uses this access token to access data from the Resource Server.

Note: It’s important to note that JWTs aren’t the only token format that OAuth2 can work with; it’s just the most popular because of its perks. OAuth2 can also work with Opaque Tokens, SAML Tokens, or custom token formats like Microsoft’s reference tokens or Google’s access tokens.

How to Implement OAuth2 and JWT

Imagine you’re building a simple document management system with a Kotlin and Spring-based backend that exposes a REST API. This implementation lets clients upload documents, list them, view specific ones, etc. Some potential endpoints the API can expose include:

• GET /documents: Lists all documents.
• GET /documents/{id}: View a specific document.
• POST /documents: Upload a new document.

You want to restrict access so that only authenticated users can view or upload documents, but you don’t want to manage passwords in your backend. You also don’t have to maintain sessions or deal with login forms.

Prerequisites

If you want to follow along, you’ll need:

• IntelliJ IDEA
• JDK 17+
• Google Cloud Console
• A basic understanding of Kotlin, Spring Boot, and Spring Security

All the code used in this tutorial is available on GitHub repository.

Initial Application Setup

To start, run IntelliJ IDEA and create a new project (File > New > Project):

Select Spring Boot under the Generators section on the left panel. Give your project a name (like doc-manager), select Kotlin as the Language, Gradle – Kotlin as the Type, 17 as the Java version, Jar as the packaging, and Properties as the configuration. Leave all other properties in their default state and then click Next.

On the next screen, select dependencies for your project. Make sure you’re using the latest stable version of Spring Boot (4.0.3 at the time of writing) and then use the search bar to find and add the following dependencies:

• Spring Security
• OAuth2 Authorization Server
• OAuth2 Resource Server
• Spring Web

Once that’s done, click Create.

After your project’s done importing and loading, expand your project, scroll down, and find the application.properties file under the resources folder (src > main > resources). Add the following lines to it:

spring.application.name=doc-manager-kotlin-demo spring.security.oauth2.resourceserver.jwt.public-key-location=classpath:public.pem

In most cases, you’d specify an Authorization Server (issuer-uri) here. But to keep things simple, you won’t be using a real Authorization Server for this part of the implementation (this will come in later). So you need to supply your application with a public key to verify signed tokens. You can generate your own publickey.pem using OpenSSL or use the ones provided in this project’s resources folder. Make sure to save and store the private.pem. You’ll need it for JWT generation. 

Configure Your Resource Server

Create a resource controller for your endpoint:

// Insert Your Package Name Here + .controller import org.springframework.web.bind.annotation.GetMapping import org.springframework.web.bind.annotation.RequestMapping import org.springframework.web.bind.annotation.RestController @RestController @RequestMapping("/api") class ResourceController { @GetMapping("/fetchDocuments") fun fetchDocumentsEndpoint(): String { return "Here are your documents" } }

For now, the ResourceController class contains only one endpoint.

Next, create a security configuration for your Resource Server:

// Insert Your Package Name Here + .config import org.springframework.context.annotation.Bean import org.springframework.context.annotation.Configuration import org.springframework.http.HttpMethod import org.springframework.security.config.annotation.web.builders.HttpSecurity import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity import org.springframework.security.config.http.SessionCreationPolicy import org.springframework.security.web.SecurityFilterChain @Configuration @EnableWebSecurity class OAuth2ResourceServerSecurityConfiguration { @Bean @Throws(Exception::class) fun securityFilterChain(http: HttpSecurity): SecurityFilterChain = http .httpBasic { it.disable() } .formLogin { it.disable() } // Disables Spring's default form-based login .csrf { it.disable() } .authorizeHttpRequests { it.requestMatchers(HttpMethod.GET, "/api/fetchDocuments").hasAuthority("SCOPE_read:documents") // Verifies that client has read access it.anyRequest().authenticated() } .oauth2ResourceServer { // Enables JWT‑based authentication for an OAuth2 Resource Server. it.jwt { } } .sessionManagement { it.sessionCreationPolicy(SessionCreationPolicy.STATELESS) } .build() }

If you’ve worked with Spring Security in Java before, you’ll likely notice how clean the Kotlin DSL looks in comparison. References to OAuth2LoginConfigurer, wrapping lambdas in Customizer, or even annotations like @Throws(Exception::class) aren’t strictly necessary (unless you’re working with a mix of Java and Kotlin). Kotlin’s DSL trims that away and lets you express the rules directly.

Now, generate the JWT using the private key (found in the private.pem). Make sure to encode it using the RS256 and that the claims are set and formatted correctly:

Run your Spring Boot application and then initiate an authenticated request to the /fetchDocuments API endpoint with your generated JWT as the bearer token:

GET http://localhost:8080/api/fetchDocuments Bearer Token <JWT>

If it works as it should, you should see “Here are your documents” as a response. This implementation enables you to simulate a client sending a request with a Bearer token (JWT). Upon receiving the token, your Resource Server (backend) checks the expiry date and signature using the details in your application’s properties file. It also looks for the read:documents scope before granting access to the fetchDocument endpoint.

Handling Advanced Patterns and Validations

In a document management API (and most complex systems), simple scope checks aren’t enough. They can grant coarse permissions, but they often fail to capture the nuance of real-world access control. To address this, the system must separate token validation (ensuring the JWT is authentic) from business authorization (deciding what actions a user can perform).

Scopes alone can’t enforce ownership or hierarchical rules, and they don’t capture organizational roles. That’s why you need a combination of scope and role-based access, where administrators can access all features, while lower-level users are granted only a few. By layering roles, scopes, and resource checks, the API achieves fine-grained, context-aware authorization that balances security with usability.

Hardcoding security decisions in such systems should be avoided at all costs. Practices like embedding role checks or scope logic directly into controller methods may seem convenient at first, but it introduces significant risks as your system grows. A developer might forget to update one of these hardcoded checks when business requirements change, leaving certain endpoints exposed or inconsistent. Hardcoding also undermines separations of concerns. Security decisions should be modeled in a dedicated layer, not mixed into business logic.

Using Custom Claim Extraction and Spring Security’s PreAuthorize

Like most token formats, JWTs can carry custom claims in their payloads. A JWT with custom claims for roles and permissions would look something like this:  

{ "iss": "https://myapp.com/auth", "sub": "mdu", "iat": 1773754406, "exp": 1773840838, "scope": "read:documents", "roles": ["admin", "editor"], "permissions": ["documents:read:all", "documents:write:own"] }

Spring handles authority mapping for scopes out of the box and provides a hasRole function. However, roles aren’t automatically extracted from JWTs because there is no universal standard for how identity providers represent them. Scopes are standardized in OAuth2 and OpenID Connect, so Spring can safely map them into authorities. Roles often appear under custom claims and require a custom converter to translate them into Spring’s expected format before they can be used effectively.

Let’s say you want to authenticate and authorize based on roles and scope. Navigate to your security config and add the following function:

@Bean fun jwtAuthenticationConverter(): JwtAuthenticationConverter { val converter = JwtAuthenticationConverter() converter.setJwtGrantedAuthoritiesConverter { jwt -> val authorities = mutableListOf<GrantedAuthority>() // Map scopes val scopes = (jwt.claims["scope"] as? String)?.split(" ") ?: emptyList() authorities.addAll(scopes.map { SimpleGrantedAuthority("SCOPE_$it") }) // Map roles val roles = jwt.claims["roles"] as? Collection<*> ?: emptyList<Any>() authorities.addAll(roles.map { SimpleGrantedAuthority("ROLE_$it") }) // Map permissions val permissions = jwt.claims["permissions"] as? Collection<*> ?: emptyList<Any>() authorities.addAll(permissions.map { SimpleGrantedAuthority(it.toString()) }) authorities } return converter }

This changes the behaviour of the JwtAuthenticationConverter so that it no longer relies solely on Spring Security’s default scope mapping. Instead, it explicitly maps both scopes and roles from the JWT into Spring authorities. If you mapped only roles, then Spring Security would ignore the scope claim entirely.

Kotlin ensures the safe extraction of custom claims thanks to its null-safety. For instance, take a look at the scope mapping section of the code. The safe call operator (?.) ensures that if jwt.claims["scope"] is null, the chain stops gracefully instead of throwing a NullPointerException. The safe cast operator (as? String) attempts to convert the value returned from the jwt.claims["scope"] operation into a String from an Any? (could be anything or null). If the safe cast operator fails, it returns null instead of throwing a ClassCastException. This allows for type-safe conversions that won’t interrupt or break your code. The Elvis operator (?:) provides a fallback value when the left-hand side is null. So if the role is missing for whatever reason, the function returns an empty list as a default value.

The tricky part is adding validations for all these claims. If you were checking these claims individually, you could use the hasRole function for roles, and hasAuthority for scopes and permissions. One way to chain these validations together would be to use the access function. Here, you’ll use [Spring’s Method Security](Spring @EnableMethodSecurity Annotation | Baeldung) (@PreAuthorize) because it offers a more fine-grained and cleaner approach.

Return to your Security Config file and place the @EnableMethodSecurity(prePostEnabled = true) above the class definition:

... import org.springframework.security.config.annotation.method.configuration.EnableMethodSecurity @Configuration @EnableWebSecurity @EnableMethodSecurity(prePostEnabled = true) class OAuth2ResourceServerSecurityConfiguration { class SecurityConfig( ... 

You can keep your security filter chain as is for now. Navigate to your resource controller, and add the @PreAuthorize annotation to it:

... @GetMapping("/fetchDocuments") @PreAuthorize("hasRole('admin') and hasAuthority('documents:read:all')") fun fetchDocumentsEndpoint(): String { return "Here are your documents" } ...

Note: You’ll need to import the PreAuthorize annotation for this to work.

This ensures that only admins with read-all permissions can access the fetchDocuments endpoint. You can create more endpoints, like getDocument and deleteDocument to test the combination of your roles and permissions. The @PreAuthorize annotation helps you avoid embedding role checks or scope logic directly inside controller methods (for example, writing if (user.hasRole("admin")) { ... } in the body of a controller). Alternatively, you can perform your role checks in your filter chain and your scope and permission checks on the method level.

Strengthening Token Trust: Issuer and Audience Enforcement

Under most normal circumstances, you’d supply Spring Security with an issuer URI in your application properties file. Then Spring would do the work of finding the Provider Configuration or Authorization Server Metadata and using them to decode your JWT. But as you learned here, these can be bypassed when you’re using custom-generated keys.

Regardless of whether you’ve configured an issuer URI or not, it’s important to explicitly verify the issuer (iss) in your code to ensure that every incoming token actually claims the same issuer and prevent token replay across apps. This adds defense in depth and makes your security posture clear in code. Likewise, audience (aud) ensures that the token is meant for your API, not for some other application. When both the issuer and the audience are checked, it prevents tokens from other apps or environments from being accepted by your API.

To validate these claims, you’ll need to create a custom JwtDecoder. But, because Spring doesn’t have a dedicated OAuth2TokenValidator for its audience, you’ll need to create one. Re-open your Security Configuration file and add the following class (nested):

class AudienceValidator(private val audience: String) : OAuth2TokenValidator<Jwt> { override fun validate(token: Jwt): OAuth2TokenValidatorResult = if (token.audience.contains(audience)) { OAuth2TokenValidatorResult.success() } else { OAuth2TokenValidatorResult.failure(OAuth2Error("invalid_token", "The required audience is missing", null)) } }

Warning: Don’t forget to import all necessary classes and interfaces

Then, add the following method:

@Bean fun jwtDecoder(): JwtDecoder { val issuer = "https://myapp.com/auth" // Replace with your own official issuer URI val audience = "http://localhost:8080/api/" val decoder = JwtDecoders.fromIssuerLocation<NimbusJwtDecoder>(issuer) // Add audience validation val audienceValidator = AudienceValidator(audience) val issuerValidator = JwtValidators.createDefaultWithIssuer(issuer) val validator = DelegatingOAuth2TokenValidator(listOf(issuerValidator, audienceValidator)) (decoder as NimbusJwtDecoder).setJwtValidator(validator) return decoder }

This function builds a custom JwtDecoder that enforces stricter validation on incoming JWTs. It starts by creating a decoder from the configured issuer, then defines two validators: one to ensure the token’s aud claim matches the expected audience, and another to ensure the iss claim matches the trusted issuer. These validators are combined into a DelegatingOAuth2TokenValidator and applied to the decoder, so that only tokens issued by the correct identity provider and intended for your application are accepted.

Note: If you need an Authorization Server (issuer) to test this flow, you can use a local or mock server like mock-oauth2-server. It also supports custom JWT generation.

Add the validation to your security filter chain:

@Bean @Throws(Exception::class) fun securityFilterChain(http: HttpSecurity): SecurityFilterChain = http .httpBasic { it.disable() } .formLogin { it.disable() } .csrf { it.disable() } .authorizeHttpRequests { it.requestMatchers("/api/fetchDocuments").hasAuthority("SCOPE_read:documents") it.anyRequest().authenticated() } .oauth2ResourceServer { it.jwt { jwt -> jwt.jwtAuthenticationConverter(jwtAuthenticationConverter()) jwt.decoder(jwtDecoder()) // Add custom JwtDecoder } } .sessionManagement { it.sessionCreationPolicy(SessionCreationPolicy.STATELESS) } .build()

This allows the strict enforcement of the rules by the backend, never leaving it up to frontend logic to authorize or validate sensitive information.

What’s Next?

Strong security requires fine-grained control and layered safeguards beyond basic authentication. Use short-lived tokens with clear refresh and revocation strategies to limit exposure and prevent compromised tokens from persisting. Avoid using JWTs for session storage, as this leads to token bloat, complicates revocation, and increases the risk of exposing sensitive data. Instead, keep JWTs focused on authentication and authorization claims, and enforce validation of issuer, audience, signature, and expiry to ensure tokens are trustworthy and intended for your application.

Ultimately, securing Spring Boot APIs with OAuth2 and JWT depends on careful design, explicit configuration, and a clear understanding of how tokens, scopes, and identities are validated and enforced. Kotlin complements this by promoting null safety, immutability, and concise configuration, helping reduce misconfigurations and overlooked edge cases.