Calling External APIs in Spring Boot: HttpClient, RestTemplate, and Feign

Spring Boot inherits Spring's core features and further simplifies application setup; when a module needs to call external interfaces, three Dubbo‑free methods are demonstrated.

Method 1: Raw HttpClient request

@RequestMapping("/submit/{documentId}")
public String submit1(@PathVariable String documentId) throws ParseException {
    Map<String,Object> map = task2Service.getMap(documentId);
    String jsonStr = JSON.toJSONString(map, SerializerFeature.WRITE_MAP_NULL_FEATURES, SerializerFeature.QuoteFieldNames);
    JSONObject jsonObject = JSON.parseObject(jsonStr);
    JSONObject sr = task2Service.doPost(jsonObject);
    return sr.toString();
}

public static JSONObject doPost(JSONObject data) {
    String assessToken = "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzUxMiJ9...";
    CloseableHttpClient client = HttpClients.createDefault();
    String url = "http://39.103.201.110:30661/xdap-open/open/process/v1/submit";
    HttpPost post = new HttpPost(url);
    StringEntity s = new StringEntity(data.toString());
    s.setContentType("application/json");
    s.setContentEncoding("UTF-8");
    post.setEntity(s);
    post.addHeader("Authorization", "Bearer " + assessToken);
    HttpResponse res = client.execute(post);
    if (res.getStatusLine().getStatusCode() == HttpStatus.SC_OK) {
        String result = EntityUtils.toString(res.getEntity());
        return JSONObject.parseObject(result);
    }
    return null;
}

This code builds a JSON payload, adds an Authorization header with a Bearer token, and sends a POST request using Apache HttpClient.

Method 2: RestTemplate

RestTemplate offers convenient getForEntity, getForObject, and postForEntity methods. Example usages:

// GET using URI
RestTemplate restTemplate = new RestTemplate();
UriComponents uriComponents = UriComponentsBuilder.fromUriString("http://USER-SERVICE/user?name={name}")
    .build()
    .expand("dodo")
    .encode();
URI uri = uriComponents.toUri();
ResponseEntity<String> response = restTemplate.getForEntity(uri, String.class);

// GET with URL variables
Map<String,Object> params = new HashMap<>();
params.put("name", "dada");
ResponseEntity<String> response2 = restTemplate.getForEntity("http://USER-SERVICE/user?name={name}", String.class, params);

// POST using Entity
HttpHeaders headers = new HttpHeaders();
headers.add(HttpHeaders.AUTHORIZATION, "Bearer " + assessToken);
headers.setContentType(MediaType.APPLICATION_JSON);
HttpEntity<Map> entity = new HttpEntity<>(map, headers);
String url = "http://39.103.201.110:30661/xdap-open/open/process/v1/submit";
ResponseEntity<String> postResponse = restTemplate.postForEntity(url, entity, String.class);

These snippets show how to perform GET requests with URI or variable substitution and how to send a JSON body with custom headers using POST.

Method 3: Feign client

Add the Feign starter dependency, enable Feign in the main class, and define a client interface.

<!-- Maven dependency -->
<dependency>
    <groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-feign</artifactId>
    <version>1.2.2.RELEASE</version>
</dependency>

// Main application
@SpringBootApplication
@EnableFeignClients
public class MobilecardApplication {
    public static void main(String[] args) {
        SpringApplication.run(MobilecardApplication.class, args);
    }
}

// Feign client interface
@FeignClient(url = "${outSide.url}", name = "service2")
public interface FeignService2 {
    @RequestMapping(value = "/custom/outSide", method = RequestMethod.POST)
    @ResponseBody
    String getMessage(@Valid @RequestBody TestDto testDto);
}

// Controller using Feign
@Autowired
FeignService2 feignService2;
@PostMapping("/test2")
public String test2(@RequestBody TestDto testDto) {
    return feignService2.getMessage(testDto);
}

To attach tokens or other headers, implement a RequestInterceptor:

@Configuration
public class FeignConfig implements RequestInterceptor {
    @Override
    public void apply(RequestTemplate requestTemplate) {
        requestTemplate.header("token", "eyJ0eXAiOiJKV1QiLCJhbGciOiJIUzUxMiJ9...");
    }
}

Finally, a Feign client that uses this configuration can be defined similarly, and the controller can call it to send requests with the token automatically included.

These three techniques give developers flexible options for integrating external HTTP services into Spring Boot applications.