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SpringCloud OpenFeign使用及原理

OpenFeign使用及原理

    • 1.是什麼
    • 2.怎麼用
      • 2.1 增加依賴
      • 2.2 定義服務接口
      • 2.3 定義通路服務的控制層
      • 2.4 在程式啟動類加@EnableFeignClients
      • 2.5 建立服務提供者程式
      • 2.5 啟動程式測試
    • 3 為什麼(原理）
    • 4 一張圖總結

1.是什麼

OpenFeign是一個僞用戶端，為微服務架構下服務之間的調用提供了解決方案，之是以是僞用戶端是因為它不做任何的請求處理。Feign通過處理注解生成request，進而實作簡化HTTP API開發的目的，即開發人員可以使用注解的方式定制request api模闆，在發送http request請求之前，feign通過處理注解的方式替換掉request模闆中的參數，這種實作方式顯得更為直接、可了解。

2.怎麼用

OpenFeign使用起來很簡單，需要引入依賴，增加兩個注解@EnableFeignClients、@FeignClient。

寫一個示例示範下：

2.1 增加依賴

在消費者工程中增加openfeign的依賴：

<dependency><!--可以支援OKHTTP-->
           <groupId>org.springframework.cloud</groupId>
           <artifactId>spring-cloud-starter-openfeign</artifactId>
           <version>2.2.3.RELEASE</version>
  </dependency>

2.2 定義服務接口

在接口上增加 @FeignClient注解

@FeignClient("spring-cloud-user-service")
public interface OrderServiceFeignClient {

   @GetMapping("/user")
   String getUser();

}

2.3 定義通路服務的控制層 

@RestController
public class OrderControllerFeignClient {
   @Autowired
   OrderServiceFeignClient orderServiceFeignClient;
   @RequestMapping("/orderFeign/{id}")
   public String getOrdersById(){

       return orderServiceFeignClient.getUser();
   }
}

}

2.4 在程式啟動類加@EnableFeignClients

在消費者工程啟動程式類加上@EnableFeignClients注解，開啟Feign功能，如果啟動類和@FeignClient所在的包在同一級，可以不加basePackages 。

@EnableFeignClients(basePackages = "com.xiaohou.springcloud.demo.order")
@SpringBootApplication
public class SpringCloudOrderServiceApplication {

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

}

2.5 建立服務提供者程式

如使用者服務

（1）application.properties中設定端口和服務名稱

server.port=9091
spring.application.name=spring-cloud-user-service

（2）建立服務接口

@RestController
public class UserService {
    @Value("${server.port}")
    private int port;

    @GetMapping("/user")
    public String getUser(){
        System.out.println("port:"+port);
        return "provider port = "+port;
    }

}

2.5 啟動程式測試

服務提供者程式類至少啟動兩個，可以在Configurations 中設定，如圖：

SpringCloud OpenFeign使用及原理

消費者程式啟動，浏覽器中輸入url:http://localhost:9090/orderFeign/1

SpringCloud OpenFeign使用及原理

3 為什麼(原理）

首先來看OpenFeign隻增加了兩個注解就完成了遠端通信，是以分析原理要從這兩個注解入手：

@EnableFeignClients

SpringCloud OpenFeign使用及原理

@import之前分析SpringBoot自動裝配原理的時候說明過，再次說明下：

@Import:

（1）如果括号中的類實作了ImportSelector接口，spring容器就會執行個體化此類，并且調用其selectImports方法。

（2）如果實作了DeferredImportSelector接口，spring容器就會執行個體化此類，并且調用其selectImports方法，DeferredImportSelector是ImportSelector的子類，和ImportSelector的執行個體不同的是，DeferredImportSelector的執行個體的selectImports方法調用時機晚于ImportSelector的執行個體，要等到@Configuration注解中相關的業務全部都處理完了才會調用。

（3）如果實作了ImportBeanDefinitionRegistrar接口，spring容器就會執行個體化此類，并且調用其registerBeanDefinitions方法；

（4）如果沒有實作 ImportSelector、DeferredImportSelector、ImportBeanDefinitionRegistrar等其中的任何一個，spring容器就會執行個體化此類

FeignClientsRegistrar實作了ImportBeanDefinitionRegistrar接口，會調用registerBeanDefinitions方法

registerBeanDefinitions() ：

@Override
	public void registerBeanDefinitions(AnnotationMetadata metadata,
			BeanDefinitionRegistry registry) {
		registerDefaultConfiguration(metadata, registry);
		registerFeignClients(metadata, registry);
	}

registerDefaultConfiguration（）:

在啟動配置上檢查是否有@EnableFeignClients注解，如果有該注解，則開啟包掃描，掃描被@FeignClient注解的接口

private void registerDefaultConfiguration(AnnotationMetadata metadata,
			BeanDefinitionRegistry registry) {
		Map<String, Object> defaultAttrs = metadata
				.getAnnotationAttributes(EnableFeignClients.class.getName(), true);

		if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {
			String name;
			if (metadata.hasEnclosingClass()) {
				name = "default." + metadata.getEnclosingClassName();
			}
			else {
				name = "default." + metadata.getClassName();
			}
			registerClientConfiguration(registry, name,
					defaultAttrs.get("defaultConfiguration"));
		}
	}

registerFeignClients() 方法

程式啟動後通過包掃描，當類有@FeignClient注解，将注解的資訊取出，連同類名一起取出，賦給BeanDefinitionBuilder，然後根據BeanDefinitionBuilder得到beanDefinition，最後beanDefinition注入到ioc容器中,比較特殊的是注入到容器的是一個工廠bean:FeignClientFactoryBean,源碼在registerFeignClient方法中

public void registerFeignClients(AnnotationMetadata metadata,
			BeanDefinitionRegistry registry) {
		ClassPathScanningCandidateComponentProvider scanner = getScanner();
		scanner.setResourceLoader(this.resourceLoader);

		Set<String> basePackages;

		Map<String, Object> attrs = metadata
				.getAnnotationAttributes(EnableFeignClients.class.getName());
		AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
				FeignClient.class);
		final Class<?>[] clients = attrs == null ? null
				: (Class<?>[]) attrs.get("clients");
		if (clients == null || clients.length == 0) {
			scanner.addIncludeFilter(annotationTypeFilter);
			basePackages = getBasePackages(metadata);
		}
		else {
			final Set<String> clientClasses = new HashSet<>();
			basePackages = new HashSet<>();
			for (Class<?> clazz : clients) {
				basePackages.add(ClassUtils.getPackageName(clazz));
				clientClasses.add(clazz.getCanonicalName());
			}
			AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {
				@Override
				protected boolean match(ClassMetadata metadata) {
					String cleaned = metadata.getClassName().replaceAll("\\$", ".");
					return clientClasses.contains(cleaned);
				}
			};
			scanner.addIncludeFilter(
					new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));
		}

		for (String basePackage : basePackages) {
			Set<BeanDefinition> candidateComponents = scanner
					.findCandidateComponents(basePackage);
			for (BeanDefinition candidateComponent : candidateComponents) {
				if (candidateComponent instanceof AnnotatedBeanDefinition) {
					// verify annotated class is an interface
					AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;
					AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
					Assert.isTrue(annotationMetadata.isInterface(),
							"@FeignClient can only be specified on an interface");

					Map<String, Object> attributes = annotationMetadata
							.getAnnotationAttributes(
									FeignClient.class.getCanonicalName());

					String name = getClientName(attributes);
					registerClientConfiguration(registry, name,
							attributes.get("configuration"));

					registerFeignClient(registry, annotationMetadata, attributes);
				}
			}
		}
	}

	private void registerFeignClient(BeanDefinitionRegistry registry,
			AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
		String className = annotationMetadata.getClassName();
		//将工廠bean:FeignClientFactoryBean注入到IOC容器中
		BeanDefinitionBuilder definition = BeanDefinitionBuilder
				.genericBeanDefinition(FeignClientFactoryBean.class);
		validate(attributes);
		definition.addPropertyValue("url", getUrl(attributes));
		definition.addPropertyValue("path", getPath(attributes));
		String name = getName(attributes);
		definition.addPropertyValue("name", name);
		String contextId = getContextId(attributes);
		definition.addPropertyValue("contextId", contextId);
		definition.addPropertyValue("type", className);
		definition.addPropertyValue("decode404", attributes.get("decode404"));
		definition.addPropertyValue("fallback", attributes.get("fallback"));
		definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));
		definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);

		String alias = contextId + "FeignClient";
		AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
		beanDefinition.setAttribute(FactoryBean.OBJECT_TYPE_ATTRIBUTE, className);

		// has a default, won't be null
		boolean primary = (Boolean) attributes.get("primary");

		beanDefinition.setPrimary(primary);

		String qualifier = getQualifier(attributes);
		if (StringUtils.hasText(qualifier)) {
			alias = qualifier;
		}

		BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,
				new String[] { alias });
		BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
	}

工廠bean:

在Spring容器中有一類特殊的bean叫工廠bean，普通的bean注入容器内的方式是通過調用其無參構造器建立一個對象導入在容器中，而工廠bean會通過FactoryBean的接口的getObject()方法将對象注冊在容器中。

FactoryBean.java源碼,getObject調用getTarget()方法

@Override
	public Object getObject() throws Exception {
		return getTarget();
	}

	/**
	 * @param <T> the target type of the Feign client
	 * @return a {@link Feign} client created with the specified data and the context
	 * information
	 */
	<T> T getTarget() {
		FeignContext context = this.applicationContext.getBean(FeignContext.class);
		Feign.Builder builder = feign(context);

		if (!StringUtils.hasText(this.url)) {
			if (!this.name.startsWith("http")) {
				this.url = "http://" + this.name;
			}
			else {
				this.url = this.name;
			}
			this.url += cleanPath();
			return (T) loadBalance(builder, context,
					new HardCodedTarget<>(this.type, this.name, this.url));
		}
		if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
			this.url = "http://" + this.url;
		}
		String url = this.url + cleanPath();
		Client client = getOptional(context, Client.class);
		if (client != null) {
			if (client instanceof LoadBalancerFeignClient) {
				// not load balancing because we have a url,
				// but ribbon is on the classpath, so unwrap
				client = ((LoadBalancerFeignClient) client).getDelegate();
			}
			if (client instanceof FeignBlockingLoadBalancerClient) {
				// not load balancing because we have a url,
				// but Spring Cloud LoadBalancer is on the classpath, so unwrap
				client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
			}
			builder.client(client);
		}
		Targeter targeter = get(context, Targeter.class);
		return (T) targeter.target(this, builder, context,
				new HardCodedTarget<>(this.type, this.name, url));
	}

進入loadBalance()方法

protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
			HardCodedTarget<T> target) {
		Client client = getOptional(context, Client.class);
		if (client != null) {
			builder.client(client);
			Targeter targeter = get(context, Targeter.class);
			return targeter.target(this, builder, context, target);
		}

		throw new IllegalStateException(
				"No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-netflix-ribbon?");
	}

點選targeter.target()方法，最終進入ReflectiveFeign.newInstance方法

@Override
  public <T> T newInstance(Target<T> target) {
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
    Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
    List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();

    for (Method method : target.type().getMethods()) {
      if (method.getDeclaringClass() == Object.class) {
        continue;
      } else if (Util.isDefault(method)) {
        DefaultMethodHandler handler = new DefaultMethodHandler(method);
        defaultMethodHandlers.add(handler);
        methodToHandler.put(method, handler);
      } else {
        methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
      }
    }
    InvocationHandler handler = factory.create(target, methodToHandler);
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
        new Class<?>[] {target.type()}, handler);

    for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
      defaultMethodHandler.bindTo(proxy);
    }
    return proxy;
  }

InvocationHandler：處理被攔截的方法，也就是本示例中的getOrdersById

SpringCloud OpenFeign使用及原理

進入invoke方法，SynchronousMethodHandler#invoke方法

當FeignClient的方法被攔截會根據參數生成RequestTemplate對象，該對象就是http請求的模闆

@Override
  public Object invoke(Object[] argv) throws Throwable {
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    Options options = findOptions(argv);
    Retryer retryer = this.retryer.clone();
    while (true) {
      try {
        return executeAndDecode(template, options);
      } catch (RetryableException e) {
        try {
          retryer.continueOrPropagate(e);
        } catch (RetryableException th) {
          Throwable cause = th.getCause();
          if (propagationPolicy == UNWRAP && cause != null) {
            throw cause;
          } else {
            throw th;
          }
        }
        if (logLevel != Logger.Level.NONE) {
          logger.logRetry(metadata.configKey(), logLevel);
        }
        continue;
      }
    }
  }

executeAndDecode()方法，該方法是通RequestTemplate生成Request請求對象，然後根據用client擷取response。

進入executeAndDecode方法：

Object executeAndDecode(RequestTemplate template, Options options) throws Throwable {
    Request request = targetRequest(template);

    if (logLevel != Logger.Level.NONE) {
      logger.logRequest(metadata.configKey(), logLevel, request);
    }

    Response response;
    long start = System.nanoTime();
    try {
      response = client.execute(request, options);
      // ensure the request is set. TODO: remove in Feign 12
      response = response.toBuilder()
          .request(request)
          .requestTemplate(template)
          .build();
   //代碼省略
  }

LoadBalancerFeignClient#execute方法()

@Override
	public Response execute(Request request, Request.Options options) throws IOException {
		try {
			URI asUri = URI.create(request.url());
			String clientName = asUri.getHost();
			URI uriWithoutHost = cleanUrl(request.url(), clientName);
			FeignLoadBalancer.RibbonRequest ribbonRequest = new FeignLoadBalancer.RibbonRequest(
					this.delegate, request, uriWithoutHost);

			IClientConfig requestConfig = getClientConfig(options, clientName);
			return lbClient(clientName)
					.executeWithLoadBalancer(ribbonRequest, requestConfig).toResponse();
		}
		catch (ClientException e) {
			IOException io = findIOException(e);
			if (io != null) {
				throw io;
			}
			throw new RuntimeException(e);
		}
	}

再往下就是結合ribbon的負載均衡算法來實作了，不深究了。

4 一張圖總結

SpringCloud OpenFeign使用及原理

說明：這張圖不是我畫的，是上課老師畫的，源碼分析也是根據這張圖一點點看代碼明白的，圖檔更清晰直覺。

參考文章：https://cloud.tencent.com/developer/article/1009212

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