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

@Autowired
private RestTemplate restTemplate;

// 發(fā)送 GET 請(qǐng)求
restTemplate.getForObject(...)
// 發(fā)送 POST 請(qǐng)求
restTemplate.postForObject(...)
// 自定義
restTemplate.execute(...)

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

User user = restTemplate.getForObject("http://user-service/getUser", User.class);

@Autowired
private DiscoveryClient discoveryClient;

List<ServiceInstance> instances = discoveryClient.getInstances("user-service");

@EnableFeignClients

@FeignClient("user-service")
public interface UserFeignClient {
    @GetMapping("/getUser")
    User getUser();
}

public class LoadBalancerAutoConfiguration {
	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();
}

public class LoadBalancerWebClientBuilderBeanPostProcessor implements BeanPostProcessor {
	@Override
	public Object postProcessBeforeInitialization(Object bean, String beanName)
			throws BeansException {
		if (bean instanceof WebClient.Builder) {
			if (context.findAnnotationOnBean(beanName, LoadBalanced.class) == null) {
				return bean;
			}
			((WebClient.Builder) bean).filter(exchangeFilterFunction);
		}
		return bean;
	}
}

public class LoadBalancerAutoConfiguration {
	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();
}

@Autowired
private List<RestTemplate> restTemplates = Collections.emptyList();

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

public class LoadBalancerAutoConfiguration {
	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();
	@Autowired(required = false)
	private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();
  // 第一步: 遍歷 restTemplates 集合
	@Bean
	public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
			final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
		return () -> restTemplateCustomizers.ifAvailable(customizers -> {
			for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
        // RestTemplateCustomizer#customize
				for (RestTemplateCustomizer customizer : customizers) {
					customizer.customize(restTemplate);
				}
			}
		});
	}
	@Configuration(proxyBeanMethods = false)
	@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
	static class LoadBalancerInterceptorConfig {
    // 第二步: 進(jìn)行自定義操作, 也就是把 LoadBalancerInterceptor 這個(gè)我們文章開(kāi)頭提到的攔截器設(shè)置進(jìn)去.
		@Bean
		@ConditionalOnMissingBean
		public RestTemplateCustomizer restTemplateCustomizer(
				final LoadBalancerInterceptor loadBalancerInterceptor) {
			return restTemplate -> {
				List<ClientHttpRequestInterceptor> list = new ArrayList<>(
						restTemplate.getInterceptors());
				list.add(loadBalancerInterceptor);
				restTemplate.setInterceptors(list);
			};
		}
	}

User user = restTemplate.getForObject("http://user-service/getUser", User.class);

public class RestTemplate  {
  // doExecute
  protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback,
			@Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {
    	// 創(chuàng)建請(qǐng)求對(duì)象
      // 這里最終其實(shí)通過(guò) InterceptingClientHttpRequestFactory#createRequest 方法
      // 創(chuàng)建了 InterceptingClientHttpRequest 
			ClientHttpRequest request = createRequest(url, method);
			response = request.execute();
	}
}

class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest {
  // 第一步: 執(zhí)行完父類(lèi)的 execute 方法后, 會(huì)來(lái)到這里.
	@Override
	protected final ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException {
		InterceptingRequestExecution requestExecution = new InterceptingRequestExecution();
		return requestExecution.execute(this, bufferedOutput);
	}
	private class InterceptingRequestExecution implements ClientHttpRequestExecution {
		private final Iterator<ClientHttpRequestInterceptor> iterator;
		public InterceptingRequestExecution() {
			this.iterator = interceptors.iterator();
		}
    // 第二步: 先 執(zhí)行前面設(shè)置的攔截器 LoadBalancerInterceptor 通過(guò) 服務(wù)名, + 負(fù)載均衡 , 拿到其中一個(gè)實(shí)例的請(qǐng)求地址.
    // 然后根據(jù)真實(shí)的地址, 發(fā)送 http 請(qǐng)求.
		@Override
		public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
        // 先 執(zhí)行前面設(shè)置的攔截器 LoadBalancerInterceptor 通過(guò) 服務(wù)名, + 負(fù)載均衡 , 拿到其中一個(gè)實(shí)例的請(qǐng)求地址.
				nextInterceptor.intercept(request, body, this);  
        // 然后根據(jù)真實(shí)的地址, 發(fā)送 http 請(qǐng)求.  AbstractClientHttpRequest#execute      
				return delegate.execute();
			}
		}
	}
}

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {
  // 看到這里, 我們應(yīng)該回想到一開(kāi)始說(shuō)的, @LoadBalanced 注解的相關(guān)注釋說(shuō)明.
  // 加上 @LoadBalanced 注解, 我們就能給 RestTemplate 賦予負(fù)載均衡的能力.
	private LoadBalancerClient loadBalancer;
	@Override
	public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
			final ClientHttpRequestExecution execution) throws IOException {
    // 因?yàn)槲覀兗闪?Ribbon、 所以這里 loadBalancer 就是 RibbonLoadBalancerClient
		return this.loadBalancer.execute(serviceName,
				this.requestFactory.createRequest(request, body, execution));
	}
}

/**
 * This annotation may be used on a field or parameter as a qualifier for
 * candidate beans when autowiring. It may also be used to annotate other
 * custom annotations that can then in turn be used as qualifiers.
 */
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.TYPE, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface Qualifier {
	String value() default "";
}

@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}