feat(ap1): enforce resource audience

backend/src/main/java/com/example/keycloakpattern/JwtDecoderConfig.java

@@ -0,0 +1,31 @@
+package com.example.keycloakpattern;
+
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.security.oauth2.core.DelegatingOAuth2TokenValidator;
+import org.springframework.security.oauth2.core.OAuth2TokenValidator;
+import org.springframework.security.oauth2.jwt.Jwt;
+import org.springframework.security.oauth2.jwt.JwtDecoder;
+import org.springframework.security.oauth2.jwt.JwtValidators;
+import org.springframework.security.oauth2.jwt.NimbusJwtDecoder;
+
+@Configuration
+public class JwtDecoderConfig {
+
+    @Bean
+    JwtDecoder jwtDecoder(
+        @Value("${spring.security.oauth2.resourceserver.jwt.issuer-uri}") String issuer,
+        @Value("${spring.security.oauth2.resourceserver.jwt.jwk-set-uri}") String jwkSetUri,
+        @Value("${security.expected-audience}") String expectedAudience
+    ) {
+        NimbusJwtDecoder decoder = NimbusJwtDecoder.withJwkSetUri(jwkSetUri).build();
+        OAuth2TokenValidator<Jwt> issuerAndTimestamp =
+            JwtValidators.createDefaultWithIssuer(issuer);
+        OAuth2TokenValidator<Jwt> audience = new AudienceValidator(expectedAudience);
+        decoder.setJwtValidator(
+            new DelegatingOAuth2TokenValidator<>(issuerAndTimestamp, audience)
+        );
+        return decoder;
+    }
+}