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When you're calling a secured API from a Spring Boot application, you'll often need a Bearer Token obtained via OAuth2 client credentials flow. This guide walks you through the entire setup:

1. Add dependencies
2. Configure your OAuth2 client
3. Fetch the token using RestTemplate
4. Attach it to your API requests

🧱 1. Add Required Dependencies

Maven

xml

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-security</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-oauth2-client</artifactId>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
</dependency>

⚙️ 2. Configure OAuth2 Client

application.yml

yaml

spring:
  security:
    oauth2:
      client:
        registration:
          my-client:
            client-id: your-client-id
            client-secret: your-client-secret
            authorization-grant-type: client_credentials
            scope: read,write
        provider:
          my-provider:
            token-uri: https://auth-server.com/oauth/token

🔐 3. Generate Bearer Token Programmatically

java

import org.springframework.http.*;
import org.springframework.stereotype.Service;
import org.springframework.util.LinkedMultiValueMap;
import org.springframework.util.MultiValueMap;
import org.springframework.web.client.RestTemplate;
import org.springframework.core.ParameterizedTypeReference;

import java.util.Map;

@Service
public class OAuth2TokenService {

    private final RestTemplate restTemplate = new RestTemplate();

    public String getBearerToken() {
        String tokenUri = "https://auth-server.com/oauth/token";
        String clientId = "your-client-id";
        String clientSecret = "your-client-secret";

        // Create headers
        HttpHeaders headers = new HttpHeaders();
        headers.setContentType(MediaType.APPLICATION_FORM_URLENCODED);
        headers.setBasicAuth(clientId, clientSecret);

        // Create body
        MultiValueMap<String, String> body = new LinkedMultiValueMap<>();
        body.add("grant_type", "client_credentials");

        HttpEntity<MultiValueMap<String, String>> request = new HttpEntity<>(body, headers);

        ResponseEntity<Map<String, String>> response = restTemplate.exchange(
            tokenUri,
            HttpMethod.POST,
            request,
            new ParameterizedTypeReference<>() {}
        );

        return response.getBody().get("access_token");
    }
}

🌐 4. Make API Call with Bearer Token

java

import org.springframework.http.*;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;

@Service
public class ApiService {

    private final RestTemplate restTemplate = new RestTemplate();
    private final OAuth2TokenService tokenService;

    public ApiService(OAuth2TokenService tokenService) {
        this.tokenService = tokenService;
    }

    public String callProtectedApi() {
        String url = "https://api.example.com/resource";
        String token = tokenService.getBearerToken();

        HttpHeaders headers = new HttpHeaders();
        headers.setBearerAuth(token);

        HttpEntity<String> request = new HttpEntity<>(headers);

        ResponseEntity<String> response = restTemplate.exchange(url, HttpMethod.GET, request, String.class);

        return response.getBody();
    }
}

✅ Summary

• Uses Client Credentials OAuth2 flow
• Token fetched manually using RestTemplate
• Included in subsequent API requests via Authorization: Bearer <token>
• Easily extensible for retry logic or caching tokens

🚀 Bonus Tips:

• Want to automatically manage tokens? Use WebClient with Spring Security's OAuth2AuthorizedClientManager.
• If you're calling multiple services, consider caching the token to avoid fetching it for every request.
• Add logging for token failures and API responses for better observability.