SpringCloud远程服务调用三种方式及原理

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

@Autowired
private RestTemplate restTemplate;

// 发送 GET 请求
restTemplate.getForObject(...)
// 发送 POST 请求
restTemplate.postForObject(...)
// 自定义
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: 遍历 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: 进行自定义操作, 也就是把 LoadBalancerInterceptor 这个我们文章开头提到的拦截器设置进去.
		@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 {
    	// 创建请求对象
      // 这里最终其实通过 InterceptingClientHttpRequestFactory#createRequest 方法
      // 创建了 InterceptingClientHttpRequest 
			ClientHttpRequest request = createRequest(url, method);
			response = request.execute();
	}
}

class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest {
  // 第一步: 执行完父类的 execute 方法后, 会来到这里.
	@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 执行前面设置的拦截器 LoadBalancerInterceptor 通过 服务名, + 负载均衡 , 拿到其中一个实例的请求地址.
    // 然后根据真实的地址, 发送 http 请求.
		@Override
		public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
        // 先 执行前面设置的拦截器 LoadBalancerInterceptor 通过 服务名, + 负载均衡 , 拿到其中一个实例的请求地址.
				nextInterceptor.intercept(request, body, this);  
        // 然后根据真实的地址, 发送 http 请求.  AbstractClientHttpRequest#execute      
				return delegate.execute();
			}
		}
	}
}

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {
  // 看到这里, 我们应该回想到一开始说的, @LoadBalanced 注解的相关注释说明.
  // 加上 @LoadBalanced 注解, 我们就能给 RestTemplate 赋予负载均衡的能力.
	private LoadBalancerClient loadBalancer;
	@Override
	public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
			final ClientHttpRequestExecution execution) throws IOException {
    // 因为我们集成了 Ribbon、 所以这里 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 {
}