Ribbon——A ribbon is a long, narrow piece of cloth that you use for tying things together or as a decoration.

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Ribbon是一個(gè)工具類框架，不需要獨(dú)立部署。

負(fù)載均衡設(shè)備/負(fù)載均衡軟件模塊都會(huì)維護(hù)一個(gè)可用的服務(wù)清單，通過心跳檢測來剔除故障節(jié)點(diǎn)，保證清單中都是可用節(jié)點(diǎn)。

客戶端負(fù)載均衡，由客戶端節(jié)點(diǎn)維護(hù)要訪問的服務(wù)清單，服務(wù)清單來自于注冊中心。

如前所示，使用客戶端負(fù)載均衡調(diào)用分兩步：

1. 服務(wù)提供者注冊到服務(wù)中心。

2. 服務(wù)消費(fèi)者通過標(biāo)有@LoadBalanced注解的RestTemplate進(jìn)行服務(wù)調(diào)用。

在service-consumer服務(wù)中，通過調(diào)用RestTemplate的getForEntity方法，GET調(diào)用hello-service的/hello接口。

RestTemplate

GET

RestTemplate有兩類GET實(shí)現(xiàn)：getForEntity和getForObject。

getForEntity()有三個(gè)重載實(shí)現(xiàn)，均返回ResponseEntity，

// url為請求地址，responseType為響應(yīng)體body的類型，uriVariables為url參數(shù)

// uriVariables配合url中的占位符進(jìn)行動(dòng)態(tài)傳參，如:

// entity = getForEntity("http://user-serivce/user?name={1}", String.class, "John");，將John傳給參數(shù)name

public ResponseEntity getForEntity(String url, Class responseType, Object... uriVariables) throws RestClientException {

RequestCallback requestCallback = acceptHeaderRequestCallback(responseType); // new AcceptHeaderRequestCallback(responseType)

ResponseExtractor> responseExtractor = responseEntityExtractor(responseType); // new ResponseEntityResponseExtractor<>(responseType)

return nonNull(execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables));

}

// uriVariables為Map類型，key需要與url中的占位符對(duì)應(yīng)，如:

// params.put("myname", "John");

// entity = getForEntity("http://user-service/user?name={myname}", String.class, params); 將key為myname對(duì)應(yīng)的value——John傳給name

public ResponseEntity getForEntity(String url, Class responseType, Map uriVariables);

// 使用URI對(duì)象代替url和uriVariables

public ResponseEntity getForEntity(URI url, Class responseType);

// 使用：

ResponseEntity entity = restTemplate.getForEntity(url, String.class, "John");

String body = entity.getBody();

ResponseEntity entity = restTemplate.getForEntity(url, User.class, "John");

User body = entity.getBody();

getForObject()也有三個(gè)重載實(shí)現(xiàn)，傳入execute方法的不是ResponseExtractor，而是HttpMessageConverterExtractor，返回的則是對(duì)象類型，三個(gè)重載和getForEntity的三個(gè)重載關(guān)系類似：

public T getForObject(String url, Class responseType, Object... uriVariables) throws RestClientException {

RequestCallback requestCallback = acceptHeaderRequestCallback(responseType); // new AcceptHeaderRequestCallback(responseType)

HttpMessageConverterExtractor responseExtractor = new HttpMessageConverterExtractor<>(responseType, getMessageConverters(), logger);

return execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables);

}

public T getForObject(String url, Class responseType, Map uriVariables);

public T getForObject(URI url, Class responseType);

// 使用：

String result = restTemplate.getForObject(url, String.class, "John");

User user = restTempleate.getForObject(url, User.class, "John");

// 使用getForObject可以省略從response中獲取body的步驟

POST

RestTemplate有三類POST實(shí)現(xiàn)：postForEntity和postForObject，postForLocation。

postForEntity()有三個(gè)重載實(shí)現(xiàn)，均返回ResponseEntity，

// 相較于getForEntity，新增參數(shù)Object request，reqeust如果是HttpEntity對(duì)象，RestTemplate將其當(dāng)作完整的http請求對(duì)象處理，request中包含了header和body的內(nèi)容。如果request是普通對(duì)象，RestTemplate將其轉(zhuǎn)換為HttpEntity來處理，request作為body。

// if (request instanceof HttpEntity){this.requestEntity = (HttpEntity) request; }

// else if (requestBody != null) { this.requestEntity = new HttpEntity<>(request); }

// else { this.requestEntity = HttpEntity.EMPTY; }

// 使用：

// User user = new User("didi", 30);

// entity = postForEntity("http://user-serivce/user", user, String.class, "John");，將John傳給參數(shù)name

public ResponseEntity postForEntity(String url, Object request, Class responseType, Object... uriVariables) throws RestClientException {

RequestCallback requestCallback = httpEntityCallback(request, responseType); // new HttpEntityRequestCallback(requestBody, responseType)

ResponseExtractor> responseExtractor = responseEntityExtractor(responseType); // new ResponseEntityResonseExtracor<>(responseType)

return nonNull(execute(url, HttpMethod.POST, requestCallback, responseExtractor, uriVariables));

}

public ResponseEntity postForEntity(String url, Object request, Class responseType, Map uriVariables);

public ResponseEntity postForEntity(URI url, Object request, Class responseType);

postForObject()也有三個(gè)重載實(shí)現(xiàn)，傳入execute方法的不是ResponseExtractor，而是HttpMessageConverterExtractor，返回的則是對(duì)象類型，三個(gè)重載和postForEntity的三個(gè)重載關(guān)系類似：

public T postForObject(String url, Object request, Class responseType, Object... uriVariables) throws RestClientException {

RequestCallback requestCallback = httpEntityCallback(request, responseType);

HttpMessageConverterExtractor responseExtractor = new HttpMessageConverterExtractor<>(responseType, getMessageConverters(), logger);

return execute(url, HttpMethod.POST, requestCallback, responseExtractor, uriVariables);

}

public T postForObject(String url, Object request, Class responseType, Map uriVariables);

public T postForObject(URI url, Object request, Class responseType);

postForLocation()用于發(fā)送post請求，返回新資源的URI，有三個(gè)重載實(shí)現(xiàn)，均返回URI對(duì)象，

public URI postForLocation(String url, Object request, Object... uriVariables) throws RestClientException {

RequestCallback requestCallback = httpEntityCallback(request); // new HttpEntityRequestCallback(request, null)

HttpHeaders headers = execute(url, HttpMethod.POST, requestCallback, headersExtractor(), uriVariables); // new HeadersExtractor()

return (headers != null ? headers.getLocation() : null);

}

public URI postForLocation(String url, Object request, Map uriVariables);

public URI postForLocation(URI url, Object ruquest);

execute

RestTemplate中，不同的請求方式，最終會(huì)調(diào)用到execute的三個(gè)重載實(shí)現(xiàn)上來

// ------------------------------------

public T execute(String url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor responseExtractor, Object... uriVariables) {

URI expanded = getUriTemplateHandler().expand(url, uriVariables);

return doExecute(expanded, method, requestCallback, responseExtractor);

}

public T execute(String url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor responseExtractor, Map uriVariables);

public T execute(URI url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor responseExtractor);

execute()的三個(gè)重載實(shí)現(xiàn)，都會(huì)調(diào)用doExecute()方法，去執(zhí)行請求

protected T doExecute(URI url, HttpMethod method, RequestCallback requestCallback, ResponseExtractor responseExtractor) throws RestClientException {

ClientHttpResponse response = null;

ClientHttpRequest request = createRequest(url, method);

if (requestCallback != null) { requestCallback.doWithRequest(request); }

response = request.execute();// 此處執(zhí)行前會(huì)被攔截

handleResponse(url, method, response);

return (responseExtractor != null ? responseExtractor.extractData(response) : null);

}

doExecute()方法接收的參數(shù)中，有RequestCallback和ResponseExtractor。

RequestCallback

AcceptHeaderRequestCallback implements RequestCallback // AcceptHeaderRequestCallback用于GET請求

HttpEntityRequestCallback extends AcceptHeaderRequestCallback // HttpEntityRequestCallback 用于POST、PUT等請求

ResponseEntity

// ResponseEntity擴(kuò)展自HttpEntity，增加了http的status(http請求狀態(tài)碼)

package org.springframework.http;

public class ResponseEntity extends HttpEntity {

private final Object status; // status為int或HttpStatus類型

// getter/setter...

}

// HttpEntity表示http的request或response的entity，包含headers(http請求的頭信息)和body(http請求的請求體)

package org.springframework.http;

public class HttpEntity {

private final HttpHeaders headers;

private final T body;

// getter/setter...

}

@LoadBalanced

在服務(wù)消費(fèi)者中，給RestTemplate添加了@LoadBalanced注解，根據(jù)注釋，該注解用于標(biāo)記RestTemplate使用LoadBalancerClient來配置，即客戶端負(fù)載均衡器。

// Annotation to mark a RestTemplate bean to be configured to use a LoadBalancerClient.

public @interface LoadBalanced {}

LoadBalancerClient

客戶端負(fù)載均衡器，具有如下能力：

// Represents a client-side load balancer. 即客戶端負(fù)載均衡器

interface LoadBalancerClient extends ServiceInstanceChooser {

// 使用serviceId服務(wù)執(zhí)行request請求

T execute(String serviceId, LoadBalancerRequest request) throws IOException;

T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest request) throws IOException;

// 將邏輯服務(wù)名http://myservice/path/to/service 替換為host:port的形式

URI reconstructURI(ServiceInstance instance, URI original);

}

// 選擇一個(gè)server用來發(fā)送請求的實(shí)現(xiàn)接口

interface ServiceInstanceChooser {

// 根據(jù)serviceId，從負(fù)載均衡器選擇一個(gè)服務(wù)實(shí)例ServiceInstance

ServiceInstance choose(String serviceId);

}

LoadBalancerClient有一個(gè)實(shí)現(xiàn)類RibbonLoadBalancerClient。

在RestTemplate的doExecute()方法中，調(diào)用request.execute()之前，會(huì)被LoadBalancerInterceptor攔截。該攔截器中有一個(gè)LoadBalancerClient實(shí)例，此外該攔截器在LoadBalancerAutoConfiguration中被創(chuàng)建。而LoadBalancerAutoConfiguration有兩個(gè)特殊的注解@ConditionalOnClass(RestTemplate.class)和@ConditionalOnBean(LoadBalancerClient.class)，且注釋明確說明LoadBalancerAutoConfiguration為Ribbon的自動(dòng)化配置類。

spring-cloud-commons的loadbalancer包中的配置類，以2.1.2為例

LoadBalancerAutoConfiguration：創(chuàng)建LoadBalancerInterceptor、創(chuàng)建RestTemplateCustomizer(匿名內(nèi)部類)、創(chuàng)建LoadBalancerRequestFactory、創(chuàng)建SmartInitializingSingleton(匿名內(nèi)部類)

AsyncLoadBalancerAutoConfiguration：針對(duì)AsyncRestTemplate做的類似配置

spring-cloud-netflix-ribbon中的幾個(gè)配置類，以2.1.2為例

RibbonClientConfiguration：創(chuàng)建IClientConfig、創(chuàng)建IRule，創(chuàng)建IPing，創(chuàng)建ServerList，創(chuàng)建ServerListUpdater，創(chuàng)建ILoadBalancer(使用ZoneAwareLoadBalancer實(shí)現(xiàn))，創(chuàng)建ServerListFilter、創(chuàng)建RibbonLoadBalancerContext，創(chuàng)建RetryHandler，創(chuàng)建ServerIntrospector

RibbonAutoConfiguration：創(chuàng)建HasFeatures，創(chuàng)建SpringClientFactory，創(chuàng)建LoadBalancerClient(使用RibbonLoadBalancerClient實(shí)現(xiàn))，創(chuàng)建LoadBalancedRetryFactory(使用RibbonLoadBalancedRetryFactory實(shí)現(xiàn))，創(chuàng)建PropertiesFactory，創(chuàng)建RibbonApplicationContextInitializer，創(chuàng)建RestTemplateCustomizer(使用匿名內(nèi)部類)，創(chuàng)建RibbonClientHttpRequestFactory

RestCilentRibbonConfiguration：創(chuàng)建RestClient

LoadBalancerAutoConfiguration

Ribbon的自動(dòng)化配置類：

@Configuration

@ConditionalOnClass(RestTemplate.class) // 需要RestTemplate類在classpath中

@ConditionalOnBean(LoadBalancerClient.class) // 需要LoadBalancerClient的實(shí)現(xiàn)Bean在BeanFactory中

@EnableConfigurationProperties(LoadBalancerRetryProperties.class)

class LoadBalancerAutoConfiguration {

@LoadBalanced

@Autowired(required = false)

// 工程中注冊的RestTemplate的Bean會(huì)在此被加載

private List restTemplates = Collections.emptyList();

@Autowired(required = false)

private List transformers = Collections.emptyList();

// 創(chuàng)建SmartInitializingSingleton的Bean，負(fù)責(zé)用每個(gè)Customizer去修飾每個(gè)RestTemplate

@Bean

public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(

ObjectProvider> restTemplateCustomizers) {

return () -> restTemplateCustomizers.ifAvailable(customizers -> {

for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {

for (RestTemplateCustomizer customizer : customizers) {

customizer.customize(restTemplate);

}

}

});

}

@Configuration

@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")

static class LoadBalancerInterceptorConfig {

// 創(chuàng)建攔截器Bean，入?yún)榭蛻舳素?fù)載均衡器

@Bean

public LoadBalancerInterceptor ribbonInterceptor(LoadBalancerClient loadBalancerClient, LoadBalancerRequestFactory requestFactory) {

return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);

}

// 創(chuàng)建一個(gè)RestTemplateCustomizer的Bean，負(fù)責(zé)將負(fù)載均衡攔截器加到入?yún)estTemplate的攔截器列表中，添加方式為get、add、set

@Bean

@ConditionalOnMissingBean

public RestTemplateCustomizer restTemplateCustomizer(final LoadBalancerInterceptor loadBalancerInterceptor) {

return restTemplate -> {

List list = new ArrayList<>(restTemplate.getInterceptors());

list.add(loadBalancerInterceptor);

restTemplate.setInterceptors(list);

}

}

}

}

LoadBalancerInterceptor

負(fù)載均衡攔截器，用于在請求最終執(zhí)行前進(jìn)行攔截，在攔截器的intercept()方法中，首先從request中獲取服務(wù)名稱serviceName，然后調(diào)用request工廠的createRequest()方法，創(chuàng)建一個(gè)負(fù)載均衡的request——LoadBalancerRequest實(shí)例，最后將其連同serviceName一起作為LoadBalancerClient的execute()方法的入?yún)ⅰ?/p>

class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {

LoadBalancerClient loadBalancer;

LoadBalancerRequestFactory requestFactory;

public ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) {

// 此處調(diào)用request的getURI方法，

final URI originalUri = request.getURI();

String serviceName = originalUri.getHost();

return this.loadBalancer.execute(serviceName, this.requestFactory.createRequest(request, body, execution));

}

}

LoadBalancerRequestFactory——?jiǎng)?chuàng)建LoadBalancerRequest的工廠

public LoadBalancerRequest createRequest(HttpRequest request, byte[] body, ClientHttpRequestExecution execution) {

// 返回LoadBalancerRequest的匿名內(nèi)部類

return instance -> {

// 該LoadBalancerRequest的匿名內(nèi)部類實(shí)現(xiàn)，先創(chuàng)建一個(gè)ServiceRequestWrapper的request，然后調(diào)用execution的execute方法執(zhí)行請求

HttpRequest serviceRequest = new ServiceRequestWrapper(request, instance, this.loadBalancer);

return execution.execute(serviceRequest, body);

};

}

LoadBalancerRequest

LoadBalancerRequest使用該接口的apply()的方法，為request添加處理動(dòng)作

interface LoadBalancerRequest {

T apply(ServiceInstance instance);

}

ServiceRequestWrapper

ServiceRequestWrapper繼承自HttpRequestWrapper，HttpRequestWrapper對(duì)外提供了獲取一個(gè)request的method、URI、headers、methodValue等信息的方法。

ServiceRequestWrapper改寫了默認(rèn)的getURI()方法，使用客戶端負(fù)載均衡器LoadBalancerClient的重構(gòu)URI的方法，將入?yún)equest的URI進(jìn)行重構(gòu)，其具體實(shí)現(xiàn)在LoadBalancerClient的實(shí)現(xiàn)類RibbonLoadBalancerClient中。

class ServiceRequestWrapper extends HttpRequestWrapper {

private final ServiceInstance instance;

private final LoadBalancerClient loadBalancer;

public ServiceRequestWrapper(HttpRequest request, ServiceInstance instance, LoadBalancerClient loadBalancer) {}

@Override

public URI getURI() {

// 調(diào)用loadBalancer的reconstructURI方法，進(jìn)行URI重構(gòu)，改寫成host:port的形式，具體實(shí)現(xiàn)在RibbonLoadBalancerClient中

return this.loadBalancer.reconstructURI(this.instance, getRequest().getURI());

}

}

RibbonLoadBalancerClient

RibbonLoadBalancerClient實(shí)現(xiàn)了LoadBalancerClient和ServiceInstanceChooser中的execute、reconstructURI、choose方法，完成了請求執(zhí)行、URI重構(gòu)和選擇服務(wù)實(shí)例的任務(wù)，execute

class RibbonLoadBalancerClient implements LoadBalancerClient {

SpringClientFactory clientFactory;

@Override

public URI reconstructURI(ServiceInstance instance, URI original) {

String serviceId = instance.getServiceId();

RibbonLoadBalancerContext context = this.clientFactory.getLoadBalancerContext(serviceId);

URI uri; Server server;

if (instance instanceof RibbonServer) {

server = ((RibbonServer) instance).getServer();

uri = updateToSecureConnectionIfNeeded(original, ribbonServer);

} else {

server = new Server(instance.getScheme(), instance.getHost(), instance.getPort());

IClientConfig clientConfig = clientFactory.getClientConfig(serviceId);

ServerIntrospector serverIntrospector = serverIntrospector(serviceId);

uri = updateToSecureConnectionIfNeeded(original, clientConfig, serverIntrospector, server);

}

// 用server中的host、port等替換原始uri

return context.reconstructURIWithServer(server, uri);

}

public ServiceInstance choose(String serviceId, Object hint) {

// 先調(diào)用getLoadBalancer方法，根據(jù)serviceId，獲取一個(gè)ILoadBalancer

// 然后調(diào)用getServer(ILoadBalancer loadBalancer, Object hint)，使用loadBalancer選擇一個(gè)Server，hint默認(rèn)為"default"

Server server = getServer(getLoadBalancer(serviceId), hint);

if (server == null) { return null; }

// 用入?yún)erviceId、選擇的Server，構(gòu)造一個(gè)RibbonServer

return new RibbonServer(serviceId, server, isSecure(server, serviceId), serverIntrospector(serviceId).getMetadata(server));

}

public T execute(String serviceId, LoadBalancerRequest request, Object hint) {

// 首先選擇一個(gè)RibbonServer，該部分流程與choose相同

Server server = getServer(getLoadBalancer(serviceId), hint);

RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server, serviceId), serverIntrospector(serviceId).getMetadata(server));

// 調(diào)用execute的重載實(shí)現(xiàn)，執(zhí)行請求

return execute(serviceId, ribbonServer, request);

}

public T execute(String serviceId, ServiceInstance serviceIntance, LoadBalancerRequest request) {

Server server = null;

if (serviceInstance instanceof RibbonServer) {

server = ((RibbonServer) serviceInstance).getServer();

}

RibbonLoadBalancerContext context = this.clientFactory.getLoadBalancerContext(serviceId);

RibbonStatsRecorder statsRecorder = new RibbonStatsRecorder(context, server);

try { 無錫做人流手術(shù)多少錢 http://www.ytsg029.com/

// 調(diào)用apply，向服務(wù)實(shí)例發(fā)起請求

T returnVal = request.apply(serviceInstance);

statsRecorder.recordStats(returnVal);

return returnVal;

} catch (IOException ex) {

statsRecorder.recordStats(ex); throw ex;

} catch (Exception ex) {

statsRecorder.recordStats(ex); throw ex;

}

return null;

}

// Ribbon 實(shí)現(xiàn)了ServiceInstance接口，即服務(wù)實(shí)例接口

public static class RibbonServer implements ServiceInstance {

private final String serviceId;

private final Server server;

private final boolean secure;

private Map metadata;

// @Override方法

}

}

ILoadBalancer

在RibbonLoadBalancerClient的choose()和execute()方法中，都是通過調(diào)用ILoadBalancer的chooseServer()方法，來選擇一個(gè)服務(wù)實(shí)例Server的，該ILoadBalancer接口是由Ribbon定義的。

在ILoadBalancer接口中，定義了軟件負(fù)載均衡器的操作：一個(gè)服務(wù)實(shí)例的集合、標(biāo)記一個(gè)服務(wù)停止、選擇服務(wù)

package com.netflix.loadbalancer;

interface ILoadBalancer {

void addServers(List newServers); // 初始化、后續(xù)添加服務(wù)列表

Server chooseServer(Object key); // 從負(fù)載均衡器選擇一個(gè)服務(wù)實(shí)例

void markServerDown(Server server); // 標(biāo)記并通知某個(gè)服務(wù)實(shí)例已經(jīng)停止

List getReachableServers(); // up/reachable狀態(tài)的服務(wù)實(shí)例，可以提供正常服務(wù)

List getAllServers(); // 所有已知的服務(wù)實(shí)例，reachable/unreachable都包括

}

其中的Server，代表一個(gè)服務(wù)端節(jié)點(diǎn)，包含了一個(gè)服務(wù)的基本信息：host、port、scheme、id、zone、元數(shù)據(jù)等等。

在RibbonLoadBalancerClient的choose()和execute()方法中，通過getLoadBalancer()方法，來根據(jù)serviceId獲取ILoadBalancer的實(shí)例，然后將其包裝成RibbonServer。

配置類RibbonClientConfiguration創(chuàng)建ILoadBalancer時(shí)如果配置文件里有配置，則使用配置的實(shí)現(xiàn)，否則默認(rèn)使用ZoneAwareLoadBalancer實(shí)現(xiàn)。

ClientHttpRequestExecution

RibbonLoadBalancerClient的execute()方法中，調(diào)用了入?yún)oadBalancerRequest的apply方法，execute()方法在LoadBlancerInterceptor的intercept方法中調(diào)用，并傳入LoadBalancerRequestFactory.createRequest創(chuàng)建的LoadBalancerRequest實(shí)現(xiàn)，其實(shí)現(xiàn)中最終使用ClientHttpRequestExecution的execute方法執(zhí)行請求。

總結(jié)

使用時(shí)，注冊一個(gè)使用@LoadBalanced注解修飾的RestTemplate，在需要發(fā)起請求的地方調(diào)用RestTemplate的相應(yīng)的請求方法，最終調(diào)用到其doExecute方法。

@LoadBalanced注解關(guān)聯(lián)了LoadBalancerClient。

配置類LoadBalancerAutoConfiguration注冊了如下Bean：

SmartInitializingSingleton：遍歷restTemplates、遍歷customizers，并customizer.customize(restTemplate)

LoadBalancerRequestFactory：使用LoadBalancerClient構(gòu)造

LoadBalancerInterceptor：使用LoadBalancerClient和LoadBalancerRequestFactory創(chuàng)建

RestTemplateCustomizer：使用LoadBalancerInterceptor構(gòu)造一個(gè)匿名類，將注冊的LoadBalancerInterceptor添加進(jìn)restTemplate的interceptors列表中

攔截器LoadBalancerInterceptor的intercept方法從原始請求中獲取URI，然后使用LoadBalancerClient的execute方法執(zhí)行請求，接收兩個(gè)參數(shù)：serviceName即host和請求工廠創(chuàng)建的request

請求工廠LoadBalancerRequestFactory的createRequest方法，由原始請求創(chuàng)建一個(gè)LoadBalancerRequest的匿名實(shí)現(xiàn)

負(fù)載均衡請求LoadBalancerRequest接口只有apply方法，其匿名實(shí)現(xiàn)創(chuàng)建HttpRequest的實(shí)現(xiàn)類ServcieRequestWrapper的實(shí)例，然后由ClientHttpRequestExecution的execute方法執(zhí)行請求，返回響應(yīng)ClientHttpResponse

ServiceRequestWrapper重寫了父類HttpRequestWrapper的getURI方法，返回LoadBalancerClient的reconstructURI方法重構(gòu)的URI

ClientHttpRequestExecution的實(shí)現(xiàn)類是InterceptingClientHttpRequest的內(nèi)部類IntercpetingRequestExecution，其execute方法遍歷interceptors，如果有攔截器，就執(zhí)行攔截方法，如果沒有了，就執(zhí)行請求。

在4中，LoadBalancerClient的execute方法執(zhí)行請求，其實(shí)現(xiàn)類是RibbonLoadBalancerClient。execute 先通過serviceId獲取ILoadBalancer，然后調(diào)用ILoadBalancer的chooseServer方法，選擇一個(gè)Server，并將之轉(zhuǎn)換成RibbonServer，RibbonServer是ServiceInstance的子類，最后調(diào)用LoadBalancerRequest的apply方法，執(zhí)行請求，返回響應(yīng)。