32. RestTemplate and FeignClient call between microservices

There are two ways to call between SpringCloud services: RestTemplate and FeignClient . No matter what the method is, he calls the http interface of the service through the REST interface, and the parameters and results are serialized and deserialized through jackson by default . Because of the interface defined by Spring MVC 's RestController , the returned data is serialized into JSON data through Jackson .

1. RestTemplate

In this way, you only need to define a RestTemplate Bean and set it to LoadBalanced .

The following example:

@Configuration
public class SomeCloudConfiguration {
@LoadBalanced
@Bean
RestTemplate restTemplate() {
    return new RestTemplate();
}
}

This way we can inject this bean to use where we need it:

public class SomeServiceClass {
@Autowired
private RestTemplate restTemplate;
public String getUserById(Long userId) {
    UserDTO results = restTemplate.getForObject("http://users/getUserDetail/" + userId, UserDTO.class);
    return results;
}
}

Other example references:

@SpringBootApplication
public class SleuthClientApplication {

    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate();
    }

    public static void main(String[] args) {
        SpringApplication.run(SleuthClientApplication.class, args);
    }
}

@RestController
class HomeController {

    private static final Log log = LogFactory.getLog(HomeController.class);
    @Autowired
    private RestTemplate restTemplate;

    private String url="http://localhost:9986";

    @RequestMapping("/service1")
    public String service1() throws Exception {
        log.info("service1");
        Thread.sleep(200L);
        String s = this.restTemplate.getForObject(url + "/service2", String.class);
        return s;
    }
}

二、FeignClient

 除了上面的方式，我们还可以用FeignClient。

@FeignClient(value = "users", path = "/users")
public interface UserCompositeService {
@RequestMapping(value = "/getUserDetail/{id}",
        method = RequestMethod.GET,
        produces = MediaType.APPLICATION_JSON_VALUE)
UserDTO getUserById(@PathVariable Long id);
}

  我们只需要使用@FeignClient定义一个接口，Spring Cloud Feign会帮我们生成一个它的实现，从相应的users服务获取数据。

  其中，@FeignClient(value = “users”， path = “/users/getUserDetail”)里面的value是服务ID，path是这一组接口的path前缀。在下面的方法定义里，就好像设置Spring MVC的接口一样，对于这个方法，它对应的URL是/users/getUserDetail/{id}。然后，在使用它的时候，就像注入一个一般的服务一样注入后使用即可：

public class SomeOtherServiceClass {
@Autowired
private UserCompositeService userService;
public void doSomething() {
    // .....
    UserDTO results = userService.getUserById(userId);
    // other operation...
}
}