客戶端負載均衡Ribbon之二：Loadbalance的源碼

Load Balance負載均衡是用於解決一台機器(一個進程)無法解決所有請求而產生的一種算法。

像nginx可以使用負載均衡分配流量，ribbon為客戶端提供負載均衡，dubbo服務調用里的負載均衡等等，很多地方都使用到了負載均衡。

使用負載均衡帶來的好處很明顯：

1. 當集群里的1台或者多台服務器down的時候，剩余的沒有down的服務器可以保證服務的繼續使用
2. 使用了更多的機器保證了機器的良性使用，不會由於某一高峰時刻導致系統cpu急劇上升

負載均衡有好幾種實現策略，常見的有：

1. 隨機 (Random)
2. 輪詢 (RoundRobin)
3. 一致性哈希 (ConsistentHash)
4. 哈希 (Hash)
5. 加權（Weighted）

我們以ribbon的實現為基礎，看看其中的一些算法是如何實現的。

ribbon是一個為客戶端提供負載均衡功能的服務，它內部提供了一個叫做ILoadBalance的接口代表負載均衡器的操作，比如有添加服務器操作、選擇服務器操作、獲取所有的服務器列表、獲取可用的服務器列表等等。

還提供了一個叫做IRule的接口代表負載均衡策略：

package com.netflix.loadbalancer;

public interface IRule{
    /*
     * choose one alive server from lb.allServers or
     * lb.upServers according to key
     * 
     * @return choosen Server object. NULL is returned if none
     *  server is available 
     */

    public Server choose(Object key);
    
    public void setLoadBalancer(ILoadBalancer lb);
    
    public ILoadBalancer getLoadBalancer();    
}

IRule接口的實現類有以下幾種：

 

其中RandomRule表示隨機策略、RoundRobin表示輪詢策略、WeightedResponseTimeRule表示加權策略、BestAvailableRule表示請求數最少策略等等。

隨機策略很簡單，就是從服務器中隨機選擇一個服務器，RandomRule的實現代碼如下：

public Server choose(ILoadBalancer lb, Object key) {
    if (lb == null) {
        return null;
    }
    Server server = null;
 
    while (server == null) {
        if (Thread.interrupted()) {
            return null;
        }
        List<Server> upList = lb.getReachableServers();
        List<Server> allList = lb.getAllServers();
        int serverCount = allList.size();
        if (serverCount == 0) {
            return null;
        }
        int index = rand.nextInt(serverCount); // 使用jdk內部的Random類隨機獲取索引值index
        server = upList.get(index); // 得到服務器實例
 
        if (server == null) {
            Thread.yield();
            continue;
        }
 
        if (server.isAlive()) {
            return (server);
        }
 
        server = null;
        Thread.yield();
    }
    return server;
}

RoundRobin輪詢策略表示每次都取下一個服務器，比如一共有5台服務器，第1次取第1台，第2次取第2台，第3次取第3台，以此類推：

public Server choose(ILoadBalancer lb, Object key) {
    if (lb == null) {
        log.warn("no load balancer");
        return null;
    }
 
    Server server = null;
    int count = 0;
    while (server == null && count++ < 10) { // retry 10 次
        List<Server> reachableServers = lb.getReachableServers();
        List<Server> allServers = lb.getAllServers();
        int upCount = reachableServers.size();
        int serverCount = allServers.size();
 
        if ((upCount == 0) || (serverCount == 0)) {
            log.warn("No up servers available from load balancer: " + lb);
            return null;
        }
 
        int nextServerIndex = incrementAndGetModulo(serverCount); // incrementAndGetModulo方法內部使用nextServerCyclicCounter這個AtomicInteger屬性原子遞增對serverCount取模得到索引值
        server = allServers.get(nextServerIndex); // 得到服務器實例
 
        if (server == null) {
            Thread.yield();
            continue;
        }
 
        if (server.isAlive() && (server.isReadyToServe())) {
            return (server);
        }
 
        server = null;
    }
 
    if (count >= 10) {
        log.warn("No available alive servers after 10 tries from load balancer: "
                + lb);
    }
    return server;
}

    private int incrementAndGetModulo(int modulo) {
        for (;;) {
            int current = nextServerCyclicCounter.get();
            int next = (current + 1) % modulo;
            if (nextServerCyclicCounter.compareAndSet(current, next))
                return next;
        }
    }

BestAvailableRule策略用來選取最少並發量請求的服務器：

public Server choose(Object key) {
    if (loadBalancerStats == null) {
        return super.choose(key);
    }
    List<Server> serverList = getLoadBalancer().getAllServers(); // 獲取所有的服務器列表
    int minimalConcurrentConnections = Integer.MAX_VALUE;
    long currentTime = System.currentTimeMillis();
    Server chosen = null;
    for (Server server: serverList) { // 遍歷每個服務器
        ServerStats serverStats = loadBalancerStats.getSingleServerStat(server); // 獲取各個服務器的狀態
        if (!serverStats.isCircuitBreakerTripped(currentTime)) { // 沒有觸發斷路器的話繼續執行
            int concurrentConnections = serverStats.getActiveRequestsCount(currentTime); // 獲取當前服務器的請求個數
            if (concurrentConnections < minimalConcurrentConnections) { // 比較各個服務器之間的請求數，然后選取請求數最少的服務器並放到chosen變量中
                minimalConcurrentConnections = concurrentConnections;
                chosen = server;
            }
        }
    }
    if (chosen == null) { // 如果沒有選上，調用父類ClientConfigEnabledRoundRobinRule的choose方法，也就是使用RoundRobinRule輪詢的方式進行負載均衡        
        return super.choose(key);
    } else {
        return chosen;
    }
}

加權響應時間負載均衡 （WeightedResponseTime）

區域感知輪詢負載均衡（ZoneAware）：

區域感知負載均衡內置電路跳閘邏輯，可被配置基於區域同源關系（Zone Affinity，也就是更傾向於選擇發出調用的服務所在的托管區域內，這樣可以降低延遲，節省成本）選擇目標服務實例。它監控每個區域中運行實例的行為，而且能夠實時的快速丟棄一整個區域。這樣在面對整個區域故障時，幫我們提升了彈性。

實例驗證Ribbon中的LoadBalance功能

ServerList中提供了3個instance，分別是：

compute-service:2222
compute-service:2223
compute-service:2224

然后使用不同的IRule策略查看負載均衡的實現。

package org.springframework.cloud.client.loadbalancer;

import org.springframework.beans.factory.annotation.Qualifier;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * 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 {
}

LoadBalancerAutoConfiguration.java為實現客戶端負載均衡器的自動化配置類。

package org.springframework.cloud.client.loadbalancer;



/**
 * Auto configuration for Ribbon (client side load balancing).
 *
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Will Tran
 */
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {

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

    @Bean
    public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
            final List<RestTemplateCustomizer> customizers) {
        return new SmartInitializingSingleton() {
            @Override
            public void afterSingletonsInstantiated() {
                for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
                    for (RestTemplateCustomizer customizer : customizers) {
                        customizer.customize(restTemplate);
                    }
                }
            }
        };
    }

Ribbon要實現負載均衡自動化配置需要滿足如下兩個條件：

• @ConditionalOnClass(RestTemplate.class)：RestTemplate類必須存在於當前工程的環境中。
• @ConditionalOnBean(LoadBalancerClient.class)：在spring的Bean工程中必須有LoadBalancerClient.class的實現Bean。

在自動化配置中主要做三件事：

• 創建一個LoadBalancerInterceptor的Bean，用於實現對客戶端發起請求時進行攔截，以實現客戶端負載均衡。
• 創建一個RestTemplateCustomizer的Bean，用於給RestTemplate增加LoadBalancerInterceptor攔截器。
• 維護了一個被@LoadBalanced注解修飾的RestTemplate對象列表，並在這里進行初始化，通過調用RestTemplateCustomizer的實例來給需要客戶端負載均衡的RestTemplate增加LoadBalancerInterceptor攔截器。

LoadBalancerAutoConfiguration.java里面有2個內部類，如下：

@Configuration
    @ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
    static class LoadBalancerInterceptorConfig {
        @Bean
        public LoadBalancerInterceptor ribbonInterceptor(
                LoadBalancerClient loadBalancerClient,
                LoadBalancerRequestFactory requestFactory) {
            return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
        }

        @Bean
        @ConditionalOnMissingBean
        public RestTemplateCustomizer restTemplateCustomizer(
                final LoadBalancerInterceptor loadBalancerInterceptor) {
            return new RestTemplateCustomizer() {
                @Override
                public void customize(RestTemplate restTemplate) {
                    List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                            restTemplate.getInterceptors());
                    list.add(loadBalancerInterceptor);
                    restTemplate.setInterceptors(list);
                }
            };
        }
    }

    @Configuration
    @ConditionalOnClass(RetryTemplate.class)
    static class RetryAutoConfiguration {
        @Bean
        public RetryTemplate retryTemplate() {
            RetryTemplate template =  new RetryTemplate();
            template.setThrowLastExceptionOnExhausted(true);
            return template;
        }

        @Bean
        @ConditionalOnMissingBean
        public LoadBalancedRetryPolicyFactory loadBalancedRetryPolicyFactory() {
            return new LoadBalancedRetryPolicyFactory.NeverRetryFactory();
        }

        @Bean
        public RetryLoadBalancerInterceptor ribbonInterceptor(
                LoadBalancerClient loadBalancerClient, LoadBalancerRetryProperties properties,
                LoadBalancedRetryPolicyFactory lbRetryPolicyFactory,
                LoadBalancerRequestFactory requestFactory) {
            return new RetryLoadBalancerInterceptor(loadBalancerClient, retryTemplate(), properties,
                    lbRetryPolicyFactory, requestFactory);
        }

        @Bean
        @ConditionalOnMissingBean
        public RestTemplateCustomizer restTemplateCustomizer(
                final RetryLoadBalancerInterceptor loadBalancerInterceptor) {
            return new RestTemplateCustomizer() {
                @Override
                public void customize(RestTemplate restTemplate) {
                    List<ClientHttpRequestInterceptor> list = new ArrayList<>(
                            restTemplate.getInterceptors());
                    list.add(loadBalancerInterceptor);
                    restTemplate.setInterceptors(list);
                }
            };
        }
    }

 接下來，我們看看LoadBalancerInterceptor攔截器是如何將一個普通的RestTemplate變成客戶度負載均衡的：

/*
 * Copyright 2013-2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.client.loadbalancer;

import java.io.IOException;
import java.net.URI;

import org.springframework.http.HttpRequest;
import org.springframework.http.client.ClientHttpRequestExecution;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.http.client.ClientHttpResponse;

/**
 * @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();
        return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution)); //看這里
    }
}

LoadBalancerRequestFactory .java  

/*
 * Copyright 2017 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.client.loadbalancer;

import java.util.List;

import org.springframework.cloud.client.ServiceInstance;
import org.springframework.http.HttpRequest;
import org.springframework.http.client.ClientHttpRequestExecution;
import org.springframework.http.client.ClientHttpResponse;

/**
 * Creates {@link LoadBalancerRequest}s for {@link LoadBalancerInterceptor} and
 * {@link RetryLoadBalancerInterceptor}. Applies
 * {@link LoadBalancerRequestTransformer}s to the intercepted
 * {@link HttpRequest}.
 * 
 * @author William Tran
 *
 */
public class LoadBalancerRequestFactory {

    private LoadBalancerClient loadBalancer;
    private List<LoadBalancerRequestTransformer> transformers;

    public LoadBalancerRequestFactory(LoadBalancerClient loadBalancer,
            List<LoadBalancerRequestTransformer> transformers) {
        this.loadBalancer = loadBalancer;
        this.transformers = transformers;
    }

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

    public LoadBalancerRequest<ClientHttpResponse> createRequest(final HttpRequest request,
            final byte[] body, final ClientHttpRequestExecution execution) {
        return new LoadBalancerRequest<ClientHttpResponse>() {

            @Override
            public ClientHttpResponse apply(final ServiceInstance instance)
                    throws Exception {
                HttpRequest serviceRequest = new ServiceRequestWrapper(request, instance, loadBalancer);
                if (transformers != null) {
                    for (LoadBalancerRequestTransformer transformer : transformers) {
                        serviceRequest = transformer.transformRequest(serviceRequest, instance);
                    }
                }
                return execution.execute(serviceRequest, body);
            }

        };
    }

}

ServiceRequestWrapper.java

package org.springframework.cloud.client.loadbalancer;

import java.net.URI;

import org.springframework.cloud.client.ServiceInstance;
import org.springframework.http.HttpRequest;
import org.springframework.http.client.support.HttpRequestWrapper;

/**
 * @author Ryan Baxter
 */
public class ServiceRequestWrapper extends HttpRequestWrapper {
    private final ServiceInstance instance;
    private final LoadBalancerClient loadBalancer;

    public ServiceRequestWrapper(HttpRequest request, ServiceInstance instance,
                                 LoadBalancerClient loadBalancer) {
        super(request);
        this.instance = instance;
        this.loadBalancer = loadBalancer;
    }

    @Override
    public URI getURI() {
        URI uri = this.loadBalancer.reconstructURI(
                this.instance, getRequest().getURI());
        return uri;
    }
}

 spring cloud中對應的實現類：

package org.springframework.cloud.netflix.ribbon;


/**
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Ryan Baxter
 */
public class RibbonLoadBalancerClient implements LoadBalancerClient {

    private SpringClientFactory clientFactory;

    public RibbonLoadBalancerClient(SpringClientFactory clientFactory) {
        this.clientFactory = clientFactory;
    }

    @Override
    public URI reconstructURI(ServiceInstance instance, URI original) {
        Assert.notNull(instance, "instance can not be null");
        String serviceId = instance.getServiceId();
        RibbonLoadBalancerContext context = this.clientFactory
                .getLoadBalancerContext(serviceId);
        Server server = new Server(instance.getHost(), instance.getPort());
        IClientConfig clientConfig = clientFactory.getClientConfig(serviceId);
        ServerIntrospector serverIntrospector = serverIntrospector(serviceId);
        URI uri = RibbonUtils.updateToHttpsIfNeeded(original, clientConfig,
                serverIntrospector, server);
        return context.reconstructURIWithServer(server, uri);
    }

    @Override
    public ServiceInstance choose(String serviceId) {
        Server server = getServer(serviceId);
        if (server == null) {
            return null;
        }
        return new RibbonServer(serviceId, server, isSecure(server, serviceId),
                serverIntrospector(serviceId).getMetadata(server));
    }

    @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);
    }

    @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
        catch (IOException ex) {
            statsRecorder.recordStats(ex);
            throw ex;
        }
        catch (Exception ex) {
            statsRecorder.recordStats(ex);
            ReflectionUtils.rethrowRuntimeException(ex);
        }
        return null;
    }

    private ServerIntrospector serverIntrospector(String serviceId) {
        ServerIntrospector serverIntrospector = this.clientFactory.getInstance(serviceId,
                ServerIntrospector.class);
        if (serverIntrospector == null) {
            serverIntrospector = new DefaultServerIntrospector();
        }
        return serverIntrospector;
    }

    private boolean isSecure(Server server, String serviceId) {
        IClientConfig config = this.clientFactory.getClientConfig(serviceId);
        ServerIntrospector serverIntrospector = serverIntrospector(serviceId);
        return RibbonUtils.isSecure(config, serverIntrospector, server);
    }

    protected Server getServer(String serviceId) {
        return getServer(getLoadBalancer(serviceId));
    }

    protected Server getServer(ILoadBalancer loadBalancer) {
        if (loadBalancer == null) {
            return null;
        }
        return loadBalancer.chooseServer("default"); // TODO: better handling of key
    }

    protected ILoadBalancer getLoadBalancer(String serviceId) {
        return this.clientFactory.getLoadBalancer(serviceId);
    }

getServer方法中並沒有使用LoadBalancerClient中的choose方法，而是使用Netflix Rion自身的ILoadBalancer接口中定義的chooseServer方法。再看ILoadBalancer 接口：

package com.netflix.loadbalancer;

public interface ILoadBalancer {
    //向負載均衡器的實例列表中增加實例
    public void addServers(List<Server> newServers);

    //通過某種策略，從負載均衡器中選擇一個具體的實例    
    public Server chooseServer(Object key);
    //用來通知和標識負載均衡器中某個具體實例已經停止服務，不然負載均衡器在下一次獲取服務實例清單前都會認為服務實例均是正常服務的。
    public void markServerDown(Server server);
    
   //獲取正常服務列表
    public List<Server> getReachableServers();

    //所有已知實例列表
    public List<Server> getAllServers();

 再看實現類，BaseLoadBalancer類是實現了基礎的負載均衡，而DynamicServerListLoadBalancer和ZoneAwareLoadBalancer在負載均衡基礎上做了一些功能的擴展。

 

 那么Spring cloud在整合Ribbon的時候采用的哪個具體實現，可以看RibbonClientConfiguration配置類中的代碼片段如下，采用的是ZoneAwareLoadBalancer來實現負載均衡器。

    @Bean
    @ConditionalOnMissingBean
    public ILoadBalancer ribbonLoadBalancer(IClientConfig config,
            ServerList<Server> serverList, ServerListFilter<Server> serverListFilter,
            IRule rule, IPing ping) {
        if (this.propertiesFactory.isSet(ILoadBalancer.class, name)) {
            return this.propertiesFactory.get(ILoadBalancer.class, config, name);
        }
        ZoneAwareLoadBalancer<Server> balancer = LoadBalancerBuilder.newBuilder()
                .withClientConfig(config).withRule(rule).withPing(ping)
                .withServerListFilter(serverListFilter).withDynamicServerList(serverList)
                .buildDynamicServerListLoadBalancer();
        return balancer;
    }

 在回到主方法RibbonLoadBalancerClient.execute()

LoadBalancerClient的execute()
-->1、ZoneAwareLoadBalancer.chooseServer()獲取了負載均衡策略分配到的服務實例對象Server
-->2、將Server對象封裝成RibbonService實例
-->3、調用LoadBalancerRequest的apply()
-->4-1、在apply()中，先將request包裝成ServiceRequestWrapper，在Wrapper中拼接URI
-->4-2、拼接URI中，調用RibbonLoadBalancerClient.reconstructURI()
-->4-3、拼接URI中，調用RibbonLoadBalancerContext.reconstructURIWithServer()
-->4-4、拼接URI中，調用RibbonLoadBalancerContext.reconstructURIWithServer()
-->5、攔截器調用

-->6、執行完成后，Ribbon還通過RibbonStatsRecorder對象對服務的請求進行了記錄

    @Override
    public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
        ILoadBalancer loadBalancer = getLoadBalancer(serviceId); //在springcloud中是ZoneAwareLoadBalancer實例
        Server server = getServer(loadBalancer); //1、ZoneAwareLoadBalancer獲取了負載均衡策略分配到的服務實例對象Server
        if (server == null) {
            throw new IllegalStateException("No instances available for " + serviceId);
        }
        //2、將Server實例封裝成RibbonService實例
        RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server,
                serviceId), serverIntrospector(serviceId).getMetadata(server));  

        return execute(serviceId, ribbonServer, request);
    }

    @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);  //3、request看LoadBalancerRequestFactory中的createRequest()方法返回的匿名類
            statsRecorder.recordStats(returnVal);
            return returnVal;
        }
        // catch IOException and rethrow so RestTemplate behaves correctly
        catch (IOException ex) {
            statsRecorder.recordStats(ex);
            throw ex;
        }
        catch (Exception ex) {
            statsRecorder.recordStats(ex);
            ReflectionUtils.rethrowRuntimeException(ex);
        }
        return null;
    }

 

    public LoadBalancerRequest<ClientHttpResponse> createRequest(final HttpRequest request,
            final byte[] body, final ClientHttpRequestExecution execution) {
        return new LoadBalancerRequest<ClientHttpResponse>() {

            @Override
            public ClientHttpResponse apply(final ServiceInstance instance)
                    throws Exception {
                HttpRequest serviceRequest = new ServiceRequestWrapper(request, instance, loadBalancer);  //4-1、中拼接URI，見下面的ServiceRequestWrapper中的getURI()
                if (transformers != null) {
                    for (LoadBalancerRequestTransformer transformer : transformers) {
                        serviceRequest = transformer.transformRequest(serviceRequest, instance);
                    }
                }
                return execution.execute(serviceRequest, body);  //5、攔截器調用見InterceptingClientHttpRequest的內部類InterceptingRequestExecution.execute()
            }

        };
    }

    @Override
    public URI getURI() {
        URI uri = this.loadBalancer.reconstructURI(
                this.instance, getRequest().getURI()); //4-2、reconstructURI被RibbonLoadBalancerClient重載，看RibbonLoadBalancerClient.reconstructURI()
        return uri;
    }

    @Override
    public URI reconstructURI(ServiceInstance instance, URI original) {
        Assert.notNull(instance, "instance can not be null");
        String serviceId = instance.getServiceId();
        RibbonLoadBalancerContext context = this.clientFactory
                .getLoadBalancerContext(serviceId);
        Server server = new Server(instance.getHost(), instance.getPort());
        IClientConfig clientConfig = clientFactory.getClientConfig(serviceId);
        ServerIntrospector serverIntrospector = serverIntrospector(serviceId);
        URI uri = RibbonUtils.updateToHttpsIfNeeded(original, clientConfig,
                serverIntrospector, server);
        return context.reconstructURIWithServer(server, uri);//4-3、構建服務實例的URI，RibbonLoadBalancerContext.reconstructURIWithServer()見下面的LoadBalancerContext.reconstructURIWithServer()
    }

    public URI reconstructURIWithServer(Server server, URI original) {//4-4、LoadBalancerContext的reconstructURIWithServer()
        String host = server.getHost();
        int port = server .getPort();
        if (host.equals(original.getHost()) 
                && port == original.getPort()) {
            return original;
        }
        String scheme = original.getScheme();
        if (scheme == null) {
            scheme = deriveSchemeAndPortFromPartialUri(original).first();
        }

        try {
            StringBuilder sb = new StringBuilder();
            sb.append(scheme).append("://");
            if (!Strings.isNullOrEmpty(original.getRawUserInfo())) {
                sb.append(original.getRawUserInfo()).append("@");
            }
            sb.append(host);
            if (port >= 0) {
                sb.append(":").append(port);
            }
            sb.append(original.getRawPath());
            if (!Strings.isNullOrEmpty(original.getRawQuery())) {
                sb.append("?").append(original.getRawQuery());
            }
            if (!Strings.isNullOrEmpty(original.getRawFragment())) {
                sb.append("#").append(original.getRawFragment());
            }
            URI newURI = new URI(sb.toString());
            return newURI;            
        } catch (URISyntaxException e) {
            throw new RuntimeException(e);
        }
    }

        @Override
        public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {//5、攔截器調用，InterceptingClientHttpRequest的內部類InterceptingRequestExecution.execute()
            if (this.iterator.hasNext()) {
                ClientHttpRequestInterceptor nextInterceptor = this.iterator.next();
                return nextInterceptor.intercept(request, body, this);
            }
            else {
                ClientHttpRequest delegate = requestFactory.createRequest(request.getURI(), request.getMethod());
                for (Map.Entry<String, List<String>> entry : request.getHeaders().entrySet()) {
                    List<String> values = entry.getValue();
                    for (String value : values) {
                        delegate.getHeaders().add(entry.getKey(), value);
                    }
                }
                if (body.length > 0) {
                    StreamUtils.copy(body, delegate.getBody());
                }
                return delegate.execute();
            }
        }

public class RibbonStatsRecorder {

    private RibbonLoadBalancerContext context;
    private ServerStats serverStats;
    private Stopwatch tracer;

    public RibbonStatsRecorder(RibbonLoadBalancerContext context, Server server) {
        this.context = context;
        if (server != null) {
            serverStats = context.getServerStats(server);
            context.noteOpenConnection(serverStats);
            tracer = context.getExecuteTracer().start();
        }
    }

    public void recordStats(Object entity) {
        this.recordStats(entity, null);
    }

    public void recordStats(Throwable t) {
        this.recordStats(null, t);
    }

    protected void recordStats(Object entity, Throwable exception) {
        if (this.tracer != null && this.serverStats != null) {
            this.tracer.stop();
            long duration = this.tracer.getDuration(TimeUnit.MILLISECONDS);
            this.context.noteRequestCompletion(serverStats, entity, exception, duration, null/* errorHandler */);
        }
    }
}