To use Google’s OAuth 2.0 authentication system for login, you must set up a project in the Google API Console to obtain OAuth 2.0 credentials.

	Note
	Google’s OAuth 2.0 implementation for authentication conforms to the OpenID Connect 1.0 specification and is OpenID Certified.

Follow the instructions on the OpenID Connect page, starting in the section, "Setting up OAuth 2.0".

After completing the "Obtain OAuth 2.0 credentials" instructions, you should have a new OAuth Client with credentials consisting of a Client ID and a Client Secret.

The redirect URI is the path in the application that the end-user’s user-agent is redirected back to after they have authenticated with Google and have granted access to the OAuth Client (created in the previous step) on the Consent page.

In the "Set a redirect URI" sub-section, ensure that the Authorized redirect URIs field is set to http://localhost:8080/login/oauth2/code/google.

	Tip
	The default redirect URI template is {baseUrl}/login/oauth2/code/{registrationId}. The registrationId is a unique identifier for the ClientRegistration. For our example, the registrationId is google.

Now that you have a new OAuth Client with Google, you need to configure the application to use the OAuth Client for the authentication flow. To do so:

Launch the Spring Boot 2.0 sample and go to http://localhost:8080. You are then redirected to the default auto-generated login page, which displays a link for Google.

Click on the Google link, and you are then redirected to Google for authentication.

After authenticating with your Google account credentials, the next page presented to you is the Consent screen. The Consent screen asks you to either allow or deny access to the OAuth Client you created earlier. Click Allow to authorize the OAuth Client to access your email address and basic profile information.

At this point, the OAuth Client retrieves your email address and basic profile information from the UserInfo Endpoint and establishes an authenticated session.

In a Spring Boot application, to specify which authorization server to use, simply do:

spring:
  security:
    oauth2:
      resourceserver:
        jwt:
          issuer-uri: https://idp.example.com

Where https://idp.example.com is the value contained in the iss claim for JWT tokens that the authorization server will issue. Resource Server will use this property to further self-configure, discover the authorization server’s public keys, and subsequently validate incoming JWTs.

	Note
	To use the issuer-uri property, it must also be true that https://idp.example.com/.well-known/openid-configuration is a supported endpoint for the authorization server. This endpoint is referred to as a Provider Configuration endpoint.

And that’s it!

When this property and these dependencies are used, Resource Server will automatically configure itself to validate JWT-encoded Bearer Tokens.

It achieves this through a deterministic startup process:

A consequence of this process is that the authorization server must be up and receiving requests in order for Resource Server to successfully start up.

	Note
	If the authorization server is down when Resource Server queries it (given appropriate timeouts), then startup will fail.

Once the application is started up, Resource Server will attempt to process any request containing an Authorization: Bearer header:

GET / HTTP/1.1
Authorization: Bearer some-token-value # Resource Server will process this

So long as this scheme is indicated, Resource Server will attempt to process the request according to the Bearer Token specification.

Given a well-formed JWT token, Resource Server will:

	Note
	As the authorization server makes available new keys, Spring Security will automatically rotate the keys used to validate the JWT tokens.

The resulting Authentication#getPrincipal, by default, is a Spring Security Jwt object, and Authentication#getName maps to the JWT’s sub property, if one is present.

How to Configure without Tying Resource Server startup to an authorization server’s availability

How to Configure without Spring Boot

If the authorization server doesn’t support the Provider Configuration endpoint, or if Resource Server must be able to start up independently from the authorization server, then issuer-uri can be exchanged for jwk-set-uri:

security:
  oauth2:
    resourceserver:
      jwt:
        jwk-set-uri: https://idp.example.com/.well-known/jwks.json

	Note
	The JWK Set uri is not standardized, but can typically be found in the authorization server’s documentation

Consequently, Resource Server will not ping the authorization server at startup. However, it will also no longer validate the iss claim in the JWT (since Resource Server no longer knows what the issuer value should be).

	Note
	This property can also be supplied directly on the DSL.

There are two @Bean s that Spring Boot generates on Resource Server’s behalf.

The first is a SecurityWebFilterChain that configures the app as a resource server:

@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
    http
        .authorizeExchange()
            .anyExchange().authenticated()
            .and()
        .oauth2ResourceServer()
            .jwt();
    return http.build();
}

If the application doesn’t expose a SecurityWebFilterChain bean, then Spring Boot will expose the above default one.

Replacing this is as simple as exposing the bean within the application:

@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
    http
        .authorizeExchange()
            .pathMatchers("/message/**").hasAuthority("SCOPE_message:read")
            .anyExchange().authenticated()
            .and()
        .oauth2ResourceServer()
            .jwt();
    return http.build();
}

The above requires the scope of message:read for any URL that starts with /messages/.

Methods on the oauth2ResourceServer DSL will also override or replace auto configuration.

For example, the second @Bean Spring Boot creates is a ReactiveJwtDecoder, which decodes String tokens into validated instances of Jwt:

@Bean
public ReactiveJwtDecoder jwtDecoder() {
    return ReactiveJwtDecoders.fromOidcIssuerLocation(issuerUri);
}

If the application doesn’t expose a ReactiveJwtDecoder bean, then Spring Boot will expose the above default one.

And its configuration can be overridden using jwkSetUri() or replaced using decoder().

@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
    http
        .authorizeExchange()
            .anyExchange().authenticated()
            .and()
        .oauth2ResourceServer()
            .jwt()
                .jwkSetUri("https://idp.example.com/.well-known/jwks.json");
    return http.build();
}

@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
    http
        .authorizeExchange()
            .anyExchange().authenticated()
            .and()
        .oauth2ResourceServer()
            .jwt()
                .decoder(myCustomDecoder());
    return http.build();
}

@Bean
public JwtDecoder jwtDecoder() {
    return new NimbusReactiveJwtDecoder(jwkSetUri);
}

A JWT that is issued from an OAuth 2.0 Authorization Server will typically either have a scope or scp attribute, indicating the scopes (or authorities) it’s been granted, for example:

{ …​, "scope" : "messages contacts"}

When this is the case, Resource Server will attempt to coerce these scopes into a list of granted authorities, prefixing each scope with the string "SCOPE_".

This means that to protect an endpoint or method with a scope derived from a JWT, the corresponding expressions should include this prefix:

@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
    http
        .authorizeExchange()
            .mvcMatchers("/contacts/**").hasAuthority("SCOPE_contacts")
            .mvcMatchers("/messages/**").hasAuthority("SCOPE_messages")
            .anyExchange().authenticated()
            .and()
        .oauth2ResourceServer()
            .jwt();
    return http.build();
}

Or similarly with method security:

@PreAuthorize("hasAuthority('SCOPE_messages')")
public List<Message> getMessages(...) {}

@Bean
SecurityWebFilterChain springSecurityFilterChain(ServerHttpSecurity http) {
    http
        .authorizeExchange()
            .anyExchange().authenticated()
            .and()
        .oauth2ResourceServer()
            .jwt()
                .jwtAuthenticationConverter(grantedAuthoritiesExtractor());
    return http.build();
}

Converter<Jwt, Mono<AbstractAuthenticationToken>> grantedAuthoritiesExtractor() {
    GrantedAuthoritiesExtractor extractor = new GrantedAuthoritiesExtractor();
    return new ReactiveJwtAuthenticationConverterAdapter(extractor);
}

static class GrantedAuthoritiesExtractor extends JwtAuthenticationConverter {
    protected Collection<GrantedAuthorities> extractAuthorities(Jwt jwt) {
        Collection<String> authorities = (Collection<String>)
                jwt.getClaims().get("mycustomclaim");

        return authorities.stream()
                .map(SimpleGrantedAuthority::new)
                .collect(Collectors.toList());
    }
}

static class CustomAuthenticationConverter implements Converter<Jwt, Mono<AbstractAuthenticationToken>> {
    public AbstractAuthenticationToken convert(Jwt jwt) {
        return Mono.just(jwt).map(this::doConversion);
    }
}

Using minimal Spring Boot configuration, indicating the authorization server’s issuer uri, Resource Server will default to verifying the iss claim as well as the exp and nbf timestamp claims.

In circumstances where validation needs to be customized, Resource Server ships with two standard validators and also accepts custom OAuth2TokenValidator instances.

Customizing Timestamp Validation

JWT’s typically have a window of validity, with the start of the window indicated in the nbf claim and the end indicated in the exp claim.

However, every server can experience clock drift, which can cause tokens to appear expired to one server, but not to another. This can cause some implementation heartburn as the number of collaborating servers increases in a distributed system.

Resource Server uses JwtTimestampValidator to verify a token’s validity window, and it can be configured with a clockSkew to alleviate the above problem:

@Bean
ReactiveJwtDecoder jwtDecoder() {
     NimbusReactiveJwtDecoder jwtDecoder = (NimbusReactiveJwtDecoder)
             ReactiveJwtDecoders.withOidcIssuerLocation(issuerUri);

     OAuth2TokenValidator<Jwt> withClockSkew = new DelegatingOAuth2TokenValidator<>(
            new JwtTimestampValidator(Duration.ofSeconds(60)),
            new IssuerValidator(issuerUri));

     jwtDecoder.setJwtValidator(withClockSkew);

     return jwtDecoder;
}

	Note
	By default, Resource Server configures a clock skew of 30 seconds.

public class AudienceValidator implements OAuth2TokenValidator<Jwt> {
    OAuth2Error error = new OAuth2Error("invalid_token", "The required audience is missing", null);

    public OAuth2TokenValidatorResult validate(Jwt jwt) {
        if (jwt.getAudience().contains("messaging")) {
            return OAuth2TokenValidatorResult.success();
        } else {
            return OAuth2TokenValidatorResult.failure(error);
        }
    }
}

@Bean
ReactiveJwtDecoder jwtDecoder() {
    NimbusReactiveJwtDecoder jwtDecoder = (NimbusReactiveJwtDecoder)
            ReactiveJwtDecoders.withOidcIssuerLocation(issuerUri);

    OAuth2TokenValidator<Jwt> audienceValidator = new AudienceValidator();
    OAuth2TokenValidator<Jwt> withIssuer = JwtValidators.createDefaultWithIssuer(issuerUri);
    OAuth2TokenValidator<Jwt> withAudience = new DelegatingOAuth2TokenValidator<>(withIssuer, audienceValidator);

    jwtDecoder.setJwtValidator(withAudience);

    return jwtDecoder;
}