本文作者：陳剛，叩丁狼高階講師。原創文章，轉載請註明出處。

本文章會通過斷點跟蹤的方式來解讀 Ribbon 原始碼 ，希望同學們把文章看完之後不僅能夠了解 Ribbon的實現原理，還能掌握原始碼解讀的方式和技巧(重要)。

回顧

回顧一下我們的 Ribbon部分內容我們當時使用TestTemplate + LoadBalanced 做了這樣的一個案例

叩丁狼教育.png

當時我們在配置類中做了如下Bean的定義去開啟了RestTemplate的負載均衡功能

//通過@LoadBalanced註解表明這個restRemplate開啟負載均衡的功能。
    //RestTemplate是spring內建的http請求封裝
    @Bean
    @LoadBalanced
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
 

然後在Consumer中去請求Producer服務（當然會同時開啟多個Producer服務）

//訪問 PRODUCER 服務
String result = 
restTemplate.getForObject("http://PRODUCER/provide?name="+name,String.class);

而我們需要達到的效果是該請求多次呼叫會從不同的Producer服務獲取到結果(根據負載均衡規則)，然而我們發的請求始終只會有一個呀，那麼它要如何才能實現服務之間的切換呼叫呢？那麼猜想一下，Ribbon是不是需要需要先攔截到我們的請求，然後根據我們定義的負載均衡演算法，然後從服務清單中去選擇合適的服務例項，然後完成呼叫呢？？？(思考一下)

那麼接下來我們就來對這樣的一個請求進行原始碼追蹤分析。

@LoadBalanced

我們先來研究一下@LoadBalanced是一個什麼東西，檢視他的原始碼如下

/**
 * Annotation to mark a RestTemplate bean to be configured to use a LoadBalancerClient
 * @author Spencer Gibb
 */
@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}

 

註釋 “Annotation to mark a RestTemplate bean to be configured to use a LoadBalancerClient” 告訴我們：@LoadBalanced標籤是用來給RestTemplate標記，以使用LoadBalancerClient（負載均衡的客戶端）來配置它。
我們繼續追蹤 LoadBalancerClient的原始碼

/**
 * Represents a client side load balancer
 * @author Spencer Gibb
 */
public interface LoadBalancerClient extends ServiceInstanceChooser {

    /**
     * execute request using a ServiceInstance from the LoadBalancer for the specified
     * service
     * @param serviceId the service id to look up the LoadBalancer
     * @param request allows implementations to execute pre and post actions such as
     * incrementing metrics
     * @return the result of the LoadBalancerRequest callback on the selected
     * ServiceInstance
     */
    <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;

    /**
     * execute request using a ServiceInstance from the LoadBalancer for the specified
     * service
     * @param serviceId the service id to look up the LoadBalancer
     * @param serviceInstance the service to execute the request to
     * @param request allows implementations to execute pre and post actions such as
     * incrementing metrics
     * @return the result of the LoadBalancerRequest callback on the selected
     * ServiceInstance
     */
    <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException;

    /**
     * Create a proper URI with a real host and port for systems to utilize.
     * Some systems use a URI with the logical serivce name as the host,
     * such as http://myservice/path/to/service.  This will replace the
     * service name with the host:port from the ServiceInstance.
     * @param instance
     * @param original a URI with the host as a logical service name
     * @return a reconstructed URI
     */
    URI reconstructURI(ServiceInstance instance, URI original);
}

註釋“Represents a client side load balancer”表示該介面它是一個客戶端負載均衡器 ，提供了幾個方法，翻譯方法上的註釋得知他們的作用大致如下
execute : 使用LoadBalancer中的ServiceInstance為指定的服務執行請求，說白了就是通過它來實現服務的請求呼叫。
reconstructURI:使用真實主機和埠建立適當的URI以供系統使用，獲取要呼叫的服務的主機和埠

並且該介面它繼承ServiceInstanceChooser介面

/**
 * Implemented by classes which use a load balancer to choose a server to
 * send a request to.
 *
 * @author Ryan Baxter
 */
public interface ServiceInstanceChooser {

    /**
     * Choose a ServiceInstance from the LoadBalancer for the specified service
     * @param serviceId the service id to look up the LoadBalancer
     * @return a ServiceInstance that matches the serviceId
     */
    ServiceInstance choose(String serviceId);
}

翻譯介面上的註釋“ Implemented by classes which use a load balancer to choose a server to send a request to.”大致意思為： 使用負載均衡器選擇一個服務，然後去發起請求，而 choose方法的大致作用為：從LoadBalancer負載均衡器中為指定的服務(serviceId)選擇一個服務例項(ServiceInstance) ，其實到這裡我們大致已經明白了LoadBalancerClient的目的，就是通過choose方法選擇要呼叫的服務例項，通過reconstructURI獲取服務和主機和埠，然後通過execute執行服務的呼叫，而 RibbonLoadBalancerClient是對 LoadBalancerClient 的實現 ，他們的層級關係如下(idea中按ctrl+alt+u檢視)：

image.png

那麼LoadBalancer到底是怎麼讓RestTtemplate 實現負載均衡的呢？要揭露這個答案我們必須得跟蹤RestTemplate的的請求，斷點一下RestTemplate服務呼叫的程式碼,然後去瀏覽器請求該方法觸發斷點，看看底層是如何實現呼叫的

  @RequestMapping("/consumer")
    public String consumer(@RequestParam("name") String name){
        //訪問 PRODUCER 服務
        String result = restTemplate.getForObject("http://PRODUCER/provide?name="+name,String.class);
        return result;
    }

跟蹤 getForObject 方法進入

public <T> T getForObject(String url, Class<T> responseType, Object... uriVariables) throws RestClientException {
      RequestCallback requestCallback = acceptHeaderRequestCallback(responseType);
      HttpMessageConverterExtractor<T> responseExtractor =
                new HttpMessageConverterExtractor<>(responseType, getMessageConverters(), logger);
      return execute(url, HttpMethod.GET, requestCallback, responseExtractor, uriVariables);
    }

這裡繼續呼叫了RestTemplate.execute 方法，並且把呼叫的服務地址傳入進去，然後使用HttpMethod.GET方式進行呼叫，繼續跟蹤下去

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

        Assert.notNull(url, "'url' must not be null");
        Assert.notNull(method, "'method' must not be null");
        ClientHttpResponse response = null;
        try {
            ClientHttpRequest request = createRequest(url, method);
            if (requestCallback != null) {
                requestCallback.doWithRequest(request);
            }
            response = request.execute();
            handleResponse(url, method, response);
            if (responseExtractor != null) {
                return responseExtractor.extractData(response);
            }
            else {
                return null;
            }
        }
        ...省略程式碼...

這裡把服務地址包裝成 URI物件 ，然後呼叫 ClientHttpRequest request = createRequest(url, method); 去建立http客戶端請求物件，然後呼叫 response = request.execute();執行請求，到這裡我們依然沒有找到他是如何實現負載均衡的。我們斷點跟蹤一下 createRequest方法最終會呼叫HttpAccessor#createRequest方法，HttpAccessor是個什麼東西呢，我們在RestTemplate類中按下 ctrl+alt+u檢視他們的層級關係如下

叩丁狼教育.png

請注意看，RestTemplate繼承了InterceptingHttpAccessor(看名字像http請求的訪問攔截的意思，後續再說) 而 InterceptingHttpAccessor又繼承了HttpAccessor，而createRequest方法如下

    protected ClientHttpRequest createRequest(URI url, HttpMethod method) throws IOException {
        ClientHttpRequest request = getRequestFactory().createRequest(url, method);
        if (logger.isDebugEnabled()) {
            logger.debug("Created " + method.name() + " request for \"" + url + "\"");
        }
        return request;
    }

這裡呼叫了 getRequestFactory()方法獲取到一個請求工廠，然後去建立 ClientHttpRequest ，而getRequestFactory()方法是在InterceptingHttpAccessor中進行復寫實現的，跟蹤interceptingHttpAccessor#getRequestFactory方法如下：

public ClientHttpRequestFactory getRequestFactory() {
        List<ClientHttpRequestInterceptor> interceptors = getInterceptors();
        if (!CollectionUtils.isEmpty(interceptors)) {
            ClientHttpRequestFactory factory = this.interceptingRequestFactory;
            if (factory == null) {
                factory = new InterceptingClientHttpRequestFactory(super.getRequestFactory(), interceptors);
                this.interceptingRequestFactory = factory;
            }
            return factory;
        }
        else {
            return super.getRequestFactory();
        }
    }

/**
     * Return the request interceptors that this accessor uses.
     * <p>The returned {@link List} is active and may get appended to.
     */
    public List<ClientHttpRequestInterceptor> getInterceptors() {
        return this.interceptors;
    }

可以看到這裡呼叫getInterceptors()方法在獲取 ClientHttpRequestInterceptor http客戶端請求攔截器 ，然後使用攔截器new 了一個 InterceptingClientHttpRequestFactory 工廠出來，他這裡用到了請求攔截器，是想幹嘛呢？？？(思考一下)
我們繼續往下執行結束完getRequestFactory() 方法我們跟蹤一下 getRequestFactory().createRequest(url, method); 的createRequest 方法

    @Override
    protected ClientHttpRequest createRequest(
URI uri, HttpMethod httpMethod, ClientHttpRequestFactory requestFactory) {
        return new InterceptingClientHttpRequest(requestFactory, this.interceptors, uri, httpMethod);
    }

沒有任何懸念，斷點跟蹤下去通過呼叫了InterceptingClientHttpRequestFactory#createRequest方法來建立 l了一個InterceptingClientHttpRequest物件，而InterceptingClientHttpRequest肯定是ClientHttpRequest的實現類

  /**
 * Wrapper for a {@link ClientHttpRequest} that has support for {@link ClientHttpRequestInterceptor}s.
 *
 * @author Arjen Poutsma
 * @since 3.1
 */
class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest {

    private final ClientHttpRequestFactory requestFactory;

    private final List<ClientHttpRequestInterceptor> interceptors;

    private HttpMethod method;

    private URI uri;


    protected InterceptingClientHttpRequest(ClientHttpRequestFactory requestFactory,
            List<ClientHttpRequestInterceptor> interceptors, URI uri, HttpMethod method) {

        this.requestFactory = requestFactory;
        this.interceptors = interceptors;
        this.method = method;
        this.uri = uri;
    }

註釋：“Wrapper for a {@link ClientHttpRequest} that has support for {@link ClientHttpRequestInterceptor}s.” 告訴我們
InterceptingClientHttpRequest他是對ClientHttpRequest做了包裝，並且ClientHttpRequestInterceptor提供支援，對HttpMethod (請求方式)和 URI（服務地址）亦有描述 ，我們看一下他的繼承體系

ok，看到這裡我們知道 RestTemplate底層是通過 ClientHttpRequestFactory工廠建立了 InterceptingClientHttpRequest 擁有可攔截功能的http客戶端請求物件，那麼他是怎麼呼叫的呢？？又是如何實現攔截的呢？？？
我們回到 org.springframework.web.client.RestTemplate#doExecute 方法中的 response = request.execute(); 程式碼繼續跟蹤，然後你就會發現，當執行request.execute方法的時候會跳轉到 org.springframework.cloud.client.loadbalancer.LoadBalancerInterceptor#intercept 攔截器方法中，看名字就能猜到這個攔截器跟負載均衡(LoadBalancer)有關,他是ClientHttpRequestInterceptor的實現，

    /**
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Ryan Baxter
 * @author William Tran
 */
public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {

    private LoadBalancerClient loadBalancer;
    private LoadBalancerRequestFactory requestFactory;

    public LoadBalancerInterceptor(LoadBalancerClient loadBalancer, LoadBalancerRequestFactory requestFactory) {
        this.loadBalancer = loadBalancer;
        this.requestFactory = requestFactory;
    }

    public LoadBalancerInterceptor(LoadBalancerClient loadBalancer) {
        // for backwards compatibility
        this(loadBalancer, new LoadBalancerRequestFactory(loadBalancer));
    }

    @Override
    public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
            final ClientHttpRequestExecution execution) throws IOException {
        final URI originalUri = request.getURI();
        String serviceName = originalUri.getHost();
        Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
        return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
    }
}

繼續跟蹤一下 ClientHttpRequestInterceptor 介面

/**
 * Intercepts client-side HTTP requests. Implementations of this interface can be
 * {@linkplain org.springframework.web.client.RestTemplate#setInterceptors registered}
 * with the {@link org.springframework.web.client.RestTemplate RestTemplate},
 * as to modify the outgoing {@link ClientHttpRequest} and/or the incoming
 * {@link ClientHttpResponse}.
 *
 * <p>The main entry point for interceptors is
 * {@link #intercept(HttpRequest, byte[], ClientHttpRequestExecution)}.
 *
 * @author Arjen Poutsma
 * @since 3.1
 */
@FunctionalInterface
public interface ClientHttpRequestInterceptor {
    /**
     * Intercept the given request, and return a response. The given
     * {@link ClientHttpRequestExecution} allows the interceptor to pass on the
     * request and response to the next entity in the chain.
     * <p>A typical implementation of this method would follow the following pattern:
     * <ol>
     * <li>Examine the {@linkplain HttpRequest request} and body</li>
     * <li>Optionally {@linkplain org.springframework.http.client.support.HttpRequestWrapper
     * wrap} the request to filter HTTP attributes.</li>
     * <li>Optionally modify the body of the request.</li>
     * <li><strong>Either</strong>
     * <ul>
     * <li>execute the request using
     * {@link ClientHttpRequestExecution#execute(org.springframework.http.HttpRequest, byte[])},</li>
     * <strong>or</strong>
     * <li>do not execute the request to block the execution altogether.</li>
     * </ul>
     * <li>Optionally wrap the response to filter HTTP attributes.</li>
     * </ol>
     * @param request the request, containing method, URI, and headers
     * @param body the body of the request
     * @param execution the request execution
     * @return the response
     * @throws IOException in case of I/O errors
     */
    ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution)
            throws IOException;

翻譯註釋“Intercepts client-side HTTP requests. Implementations of this interface can be” 我們知道 ClientHttpRequestInterceptor就是對http請求的攔截到這裡，我們的請求已經被攔截到，貌似離真相越來越近了，我們仔細分析一下 LoadBalancerInterceptor 的 intercept方法

@Override
    public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
            final ClientHttpRequestExecution execution) throws IOException {
        final URI originalUri = request.getURI();
        String serviceName = originalUri.getHost();
        Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
        return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
    }

這裡通過 request(之前建立的InterceptingClientHttpRequest) 物件獲取到URI呼叫的服務地址 ，然後獲取到 呼叫的服務名字 serviceName ，然後呼叫org.springframework.cloud.client.loadbalancer.LoadBalancerClient.execute去執行 ，LoadBalancerClient是什麼，不就是最開始我們分析的負載均衡器客戶端嘛？？？他使用的是RibbonLoadBalancerClient實現類
繼續跟蹤下去 RibbonLoadBalancerClient.execute

    @Override
    public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
        ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
        Server server = getServer(loadBalancer);
        if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        }
        RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server,
                serviceId), serverIntrospector(serviceId).getMetadata(server));

        return execute(serviceId, ribbonServer, request);
    }

ILoadBalancer loadBalancer = getLoadBalancer(serviceId);通過服務id獲取負載均衡器 ,跟蹤一下 ILoadBalancer原始碼

/**
 * Interface that defines the operations for a software loadbalancer. A typical
 * loadbalancer minimally need a set of servers to loadbalance for, a method to
 * mark a particular server to be out of rotation and a call that will choose a
 * server from the existing list of server.
 * 
 * @author stonse
 * 
 */
public interface ILoadBalancer {

    /**
     * Initial list of servers.
     * This API also serves to add additional ones at a later time
     * The same logical server (host:port) could essentially be added multiple times
     * (helpful in cases where you want to give more "weightage" perhaps ..)
     * 
     * @param newServers new servers to add
     */
    public void addServers(List<Server> newServers);
    
    /**
     * Choose a server from load balancer.
     * 
     * @param key An object that the load balancer may use to determine which server to return. null if 
     *         the load balancer does not use this parameter.
     * @return server chosen
     */
    public Server chooseServer(Object key);
    
    /**
     * To be called by the clients of the load balancer to notify that a Server is down
     * else, the LB will think its still Alive until the next Ping cycle - potentially
     * (assuming that the LB Impl does a ping)
     * 
     * @param server Server to mark as down
     */
    public void markServerDown(Server server);
    
    /**
     * @deprecated 2016-01-20 This method is deprecated in favor of the
     * cleaner {@link #getReachableServers} (equivalent to availableOnly=true)
     * and {@link #getAllServers} API (equivalent to availableOnly=false).
     *
     * Get the current list of servers.
     *
     * @param availableOnly if true, only live and available servers should be returned
     */
    @Deprecated
    public List<Server> getServerList(boolean availableOnly);

    /**
     * @return Only the servers that are up and reachable.
     */
    public List<Server> getReachableServers();

    /**
     * @return All known servers, both reachable and unreachable.
     */
    public List<Server> getAllServers();
}

翻譯註釋“Interface that defines the operations for a software loadbalancer.”它定義軟負載均衡器操作的介面規範
addServers：初始化服務列表
chooseServer：選擇一個服務
markServerDown：標記服務下線，從服務列表中移除
getReachableServers：獲取可訪問的服務
getAllServers：獲取所有的服務

而預設情況下這裡預設使用的是 ZoneAwareLoadBalancer,斷點跟蹤或者檢視RibbonClientConfiguration配置類的ribbonLoadBalancer方法得知

    @Bean
    @ConditionalOnMissingBean
    public ILoadBalancer ribbonLoadBalancer(IClientConfig config,
            ServerList<Server> serverList, ServerListFilter<Server> serverListFilter,
            IRule rule, IPing ping, ServerListUpdater serverListUpdater) {
        if (this.propertiesFactory.isSet(ILoadBalancer.class, name)) {
            return this.propertiesFactory.get(ILoadBalancer.class, config, name);
        }
        return new ZoneAwareLoadBalancer<>(config, rule, ping, serverList,
                serverListFilter, serverListUpdater);
    }

那麼我們繼續跟蹤 RibbonLoadBalancerClient.execute中的 getServer(loadBalancer);程式碼這裡是在根據負載均衡器選擇將要執行呼叫的服務,跟蹤下去
com.netflix.loadbalancer.ZoneAwareLoadBalancer#chooseServer

   @Override
    public Server chooseServer(Object key) {
        if (!ENABLED.get() || getLoadBalancerStats().getAvailableZones().size() <= 1) {
            logger.debug("Zone aware logic disabled or there is only one zone");
            return super.chooseServer(key);
        }
        Server server = null;
        try {
            LoadBalancerStats lbStats = getLoadBalancerStats();
            Map<String, ZoneSnapshot> zoneSnapshot = ZoneAvoidanceRule.createSnapshot(lbStats);
            logger.debug("Zone snapshots: {}", zoneSnapshot);
            if (triggeringLoad == null) {
                triggeringLoad = DynamicPropertyFactory.getInstance().getDoubleProperty(
                        "ZoneAwareNIWSDiscov

這裡的key接受了一個預設值"default",因為我們的可用的區域只有一個，所以這裡呼叫了super.chooseServer即com.netflix.loadbalancer.BaseLoadBalancer#chooseServer

 public Server chooseServer(Object key) {
        if (counter == null) {
            counter = createCounter();
        }
        counter.increment();
        if (rule == null) {
            return null;
        } else {
            try {
                return rule.choose(key);
            } catch (Exception e) {
                logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", name, key, e);
                return null;
            }
        }
    }

繼續跟蹤 rule.choose(key); 這裡呼叫了 com.netflix.loadbalancer.PredicateBasedRule#choose 的服務選擇方法，

/**
 * A rule which delegates the server filtering logic to an instance of {@link AbstractServerPredicate}.
 * After filtering, a server is returned from filtered list in a round robin fashion.
 * 
 * 
 * @author awang
 *
 */
public abstract class PredicateBasedRule extends ClientConfigEnabledRoundRobinRule {
   
    /**
     * Method that provides an instance of {@link AbstractServerPredicate} to be used by this class.
     * 
     */
    public abstract AbstractServerPredicate getPredicate();
        
    /**
     * Get a server by calling {@link AbstractServerPredicate#chooseRandomlyAfterFiltering(java.util.List, Object)}.
     * The performance for this method is O(n) where n is number of servers to be filtered.
     */
    @Override
    public Server choose(Object key) {
        ILoadBalancer lb = getLoadBalancer();
        Optional<Server> server = getPredicate().chooseRoundRobinAfterFiltering(lb.getAllServers(), key);
        if (server.isPresent()) {
            return server.get();
        } else {
            return null;
        }       
    }
}

PredicateBasedRule本身是一個抽象策略，繼承自ClientConfigEnabledRoundRobinRule ,而ClientConfigEnabledRoundRobinRule是一個實現了輪詢策略的客戶端配置

/**
RoundRobinRule :輪詢
 * This class essentially contains the RoundRobinRule class defined in the
 * loadbalancer package
 * 
 * @author stonse
 * 
 */
public class ClientConfigEnabledRoundRobinRule extends AbstractLoadBalancerRule {
    //輪詢策略
    RoundRobinRule roundRobinRule = new RoundRobinRule();

繼續跟蹤chooseRoundRobinAfterFiltering方法

 /**                                                                                                                                         
  * Choose a server in a round robin fashion after the predicate filters a given list of servers and load balancer key.                      
  */                                                                                                                                         
 public Optional<Server> chooseRoundRobinAfterFiltering(List<Server> servers, Object loadBalancerKey) {                                      
     List<Server> eligible = getEligibleServers(servers, loadBalancerKey);                                                                   
     if (eligible.size() == 0) {                                                                                                             
         return Optional.absent();                                                                                                           
     }                                                                                                                                       
     return Optional.of(eligible.get(incrementAndGetModulo(eligible.size())));                                                               
 }                                                                                                                                           
                                                                                                                                             

翻譯方法註釋“ Choose a server in a round robin fashion after the predicate filters a given list of servers and load balancer key.”告訴我們這裡是把過濾後的服務進行輪詢選擇 ，再看程式碼 List<Server> eligible獲取到合格的伺服器 ，incrementAndGetModulo就是在以輪詢的方式獲取到服務的索引。
繼續往下回到 RibbonLoadBalancerClient#execute方法

    @Override
    public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
        ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
        Server server = getServer(loadBalancer);
        if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        }
        RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server,
                serviceId), serverIntrospector(serviceId).getMetadata(server));

        return execute(serviceId, ribbonServer, request);
    }

這裡拿到要呼叫的服務的例項的後，把服務名，服務例項等資訊包裝到 RibbonServer物件中

public static class RibbonServer implements ServiceInstance {
        private final String serviceId;
        private final Server server;
        private final boolean secure;
        private Map<String, String> metadata;

        public RibbonServer(String serviceId, Server server) {
            this(serviceId, server, false, Collections.<String, String> emptyMap());
        }

        public RibbonServer(String serviceId, Server server, boolean secure,
                Map<String, String> metadata) {
            this.serviceId = serviceId;
            this.server = server;
            this.secure = secure;
            this.metadata = metadata;
        }
.....

public interface ServiceInstance {

    /**
     * @return the service id as registered.
     */
    String getServiceId();

    /**
     * @return the hostname of the registered ServiceInstance
     */
    String getHost();

    /**
     * @return the port of the registered ServiceInstance
     */
    int getPort();

    /**
     * @return if the port of the registered ServiceInstance is https or not
     */
    boolean isSecure();

    /**
     * @return the service uri address
     */
    URI getUri();

    /**
     * @return the key value pair metadata associated with the service instance
     */
    Map<String, String> getMetadata();

    /**
     * @return the scheme of the instance
     */
    default String getScheme() {
        return null;
    }
}

RibbonServer 實現了 ServiceInstance介面， ServiceInstance本身就是對服務例項的規範，有獲取服務id,主機，埠等方法。

然後繼續呼叫 excute方法執行請求

@Override
    public <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException {
        Server server = null;
        if(serviceInstance instanceof RibbonServer) {
            server = ((RibbonServer)serviceInstance).getServer();
        }
        if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        }

        RibbonLoadBalancerContext context = this.clientFactory
                .getLoadBalancerContext(serviceId);
        RibbonStatsRecorder statsRecorder = new RibbonStatsRecorder(context, server);

        try {
            T returnVal = request.apply(serviceInstance);
            statsRecorder.recordStats(returnVal);
            return returnVal;
        }
        // catch IOException and rethrow so RestTemplate behaves correctly
        ......

看程式碼 T returnVal = request.apply(serviceInstance);
這裡最終會呼叫InterceptingClientHttpRequest.InterceptingRequestExecution的execute方法，傳入服務例項，執行請求，拿到返回結果。

private class InterceptingRequestExecution implements ClientHttpRequestExecution {

        private final Iterator<ClientHttpRequestInterceptor> iterator;

        public InterceptingRequestExecution() {
            this.iterator = interceptors.iterator();
        }

        @Override
        public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {
            if (this.iterator.hasNext()) {
                ClientHttpRequestInterceptor nextInterceptor = this.iterator.next();
                return nextInterceptor.intercept(request, body, this);
            }
            else {
                HttpMethod method = request.getMethod();
                Assert.state(method != null, "No standard HTTP method");
                ClientHttpRequest delegate = requestFactory.createRequest(request.getURI(), method);
                request.getHeaders().forEach((key, value) -> delegate.getHeaders().addAll(key, value));
                if (body.length > 0) {
                    if (delegate instanceof StreamingHttpOutputMessage) {
                        StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) delegate;
                        streamingOutputMessage.setBody(outputStream -> StreamUtils.copy(body, outputStream));
                    }
                    else {
                        StreamUtils.copy(body, delegate.getBody());
                    }
                }
                return delegate.execute();
            }
        }
    }

跟蹤 requestFactory.createRequest方法

@Override
    public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
        HttpURLConnection connection = openConnection(uri.toURL(), this.proxy);
        prepareConnection(connection, httpMethod.name());

        if (this.bufferRequestBody) {
            return new SimpleBufferingClientHttpRequest(connection, this.outputStreaming);
        }
        else {
            return new SimpleStreamingClientHttpRequest(connection, this.chunkSize, this.outputStreaming);
        }
    }
.....

    protected HttpURLConnection openConnection(URL url, @Nullable Proxy proxy) throws IOException {
        URLConnection urlConnection = (proxy != null ? url.openConnection(proxy) : url.openConnection());
        if (!HttpURLConnection.class.isInstance(urlConnection)) {
            throw new IllegalStateException("HttpURLConnection required for [" + url + "] but got: " + urlConnection);
        }
        return (HttpURLConnection) urlConnection;
    }

到這裡我們看到他是通過 URLConnection 來呼叫遠端服務的。。。
好吧總結一下，這個遠端服務呼叫的背後到底做了哪些事情呢？？

[email protected]開啟了RibbonLoadBalancerClient負載均衡支援

2.RestTemplate對服務的地址(Uri),主機(host)，埠(port)等做了一些描述，然後建立了 InterceptingClientHttpRequest http請求的客戶端物件，用來執行請求用，

3.當呼叫RestTemplate發起請求時會被 LoadBalancerInterceptor請求攔截器給攔截到

4.攔截器中使用了 RibbonLoadBalancerClient執行請求，然後根據服務id獲取了負載均衡器，預設 ZoneAwareLoadBalancer

5.然後負載均衡器進行服務的選擇，預設使用了輪詢策略

6.拿到服務例項後呼叫 InterceptingClientHttpRequest 完成服務呼叫請求，獲取返回結果。

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