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

1. Introduction

Spring Boot inherits Spring's features and simplifies configuration; sometimes a module needs to call external URLs or services. This guide shows three ways to perform such calls without using Dubbo.

2. Method 1 – Raw HttpClient

Using Apache HttpClient you can build a POST request, set JSON body, add Authorization header, and parse the JSON response.

/* get方式获取入参，插入数据并发起流程 */
@RequestMapping("/submit/{documentId}")
public String submit1(@PathVariable String documentId) throws ParseException {
    Map
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();
}

/* 使用原生httpClient调用外部接口 */
public static JSONObject doPost(JSONObject date) {
    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(date.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);
    }
    throw new RuntimeException("Request failed");
}

3. Method 2 – RestTemplate

RestTemplate provides convenient methods such as getForEntity , getForObject , and postForEntity . Example of getForEntity with URI builder:

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

Example of postForEntity with JSON body and custom headers:

@PostMapping("/submit2")
public Object insertFinanceCompensation(@RequestBody JSONObject jsonObject) {
    String documentId = jsonObject.get("documentId").toString();
    return task2Service.submit(documentId);
}

4. Method 3 – Feign Client

Add the Feign starter dependency, enable Feign in the SpringBoot application, and define a Feign interface.

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

Feign client example with token header interceptor:

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

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

Controller can invoke the Feign client:

@PostMapping("/test2")
public String test2(@RequestBody TestDto testDto) {
    return feignService2.getMessage(testDto);
}

The article ends with a reminder to add proper headers and token handling when using Feign.