「大话微服务」深入聊聊SpringCloud之客户端负载均衡机制

1)服务指的就是运行的节点(IP+Port)信息。

2)节点在启动时会将自身的信息注册到注册中心。

3)ServiceId是对节点运行的工程的标识。

4)InstanceId是对运行的节点的标识。

通过一个例子来捋一捋:

假设有一个和用户管理相关的工程,叫做UserManager。为了高可用,把该工程重复运行了4份,分别用um-01/um-02/um-03/um-04表示。假设它们都运行在本地,端口分别是8081/8082/8083/8084。

它们的注册信息分别是:

ServiceId=UserManager,InstanceId=um-01,IP:Port=127.0.0.1:8081

ServiceId=UserManager,InstanceId=um-02,IP:Port=127.0.0.1:8082

ServiceId=UserManager,InstanceId=um-03,IP:Port=127.0.0.1:8083

ServiceId=UserManager,InstanceId=um-04,IP:Port=127.0.0.1:8084

很显然ServiceId都是一样的,因为本来就是一个工程嘛。如果不想使用工程名的话,可以使用应用名。例如使用spring.application.name表示。

很显然InstanceId都不一样,毕竟实例之间互相独立嘛。这个实例Id通常是在注册时由注册中心自动生成,无需指定。

IP和Port肯定也不一样了,而且也无需指定,程序会自动检测运行时的IP和Port。

简言之,一个服务可以有多个实例,实例之间完全相同又互相独立,假如某个实例挂掉了,没关系,其它实例照样可以提供服务。

多多益善,非也

“单身狗”是令人可悲的,有一个更可悲的就是同时遇上两个对象,一个是你爱的,一个是爱你的。怎么办?要不一个一三五,一个二四六,周日不限号。哈哈。

“一个”会有单点问题,“多个”虽然解决了这个问题,但是又引入了另一个问题,那就是“选择综合症”。可以看到,随着事物发展,问题自然出现。真的,一切都是那么的自然。

顺便侃一侃我的看法,纯属娱乐一下。选择综合症的主要意思是指,不是担心自己选择的那个不好,而是害怕自己没有选择的那个会更好。

举个例子,假如你一不小心考了个省状元,该去清华呢还是北大呢。去清华吧,害怕万一去北大会更好呢。去北大吧,又担心万一去清华更好呢。为了避免出现以上两种情况,经过一番深思熟虑,最终去了蓝翔。哈哈。

Spring Cloud 深知选择痛苦,众口难调,所以就非常聪明地避开这些问题。那就是做出一套合理的抽象,只有规范,没有细节,每个人根据自己的口味整去吧。

抽象,用过的都说好

Spring Cloud 的目标原本就是做一块“主板”,抽象出合理的“接口”和“布线”,方便其他组件的插拔。

一个ServiceId表示一个服务,但是它可能会有多份实例,我们的请求肯定是在其中的一个实例上被执行的。所以要先能选出一个实例来,才可能会有后续的事情。

于是就有了一个简单粗暴的接口,ServiceInstanceChooser,服务实例选择器:


public interface ServiceInstanceChooser {
 /**
 * Chooses 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);
}

这个接口只能为我们选出一个实例,但这是不够的,因为我们还要执行请求啊,所以就有了进一步的接口,LoadBalancerClient,负载均衡器客户端:


public interface LoadBalancerClient extends ServiceInstanceChooser {
 /**
 * Executes 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.
 * @param <T> type of the response
 * @throws IOException in case of IO issues.
 * @return The result of the LoadBalancerRequest callback on the selected
 * ServiceInstance.
 */
 <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;
 /**
 * Executes 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.
 * @param <T> type of the response
 * @throws IOException in case of IO issues.
 * @return The result of the LoadBalancerRequest callback on the selected
 * ServiceInstance.
 */
 <T> T execute(String serviceId, ServiceInstance serviceInstance,
 LoadBalancerRequest<T> request) throws IOException;
 /**
 * Creates a proper URI with a real host and port for systems to utilize. Some systems
 * use a URI with the logical service 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 service instance to reconstruct the URI
 * @param original A URI with the host as a logical service name.
 * @return A reconstructed URI.
 */
 URI reconstructURI(ServiceInstance instance, URI original);
}

先来看第三个方法,重新构建URI,为什么要重新构建呢?其实原因很简单,下面通过例子说明。

假如我要写一个“用户查询”的地址,可是有四个服务实例呢,到底该写哪个IP和端口呢?既然不知道,那干脆就只写一个标识符(或称占位符)吧,到时让程序动态选择一个得了。

于是地址就是这样的:http://UserManager/users/queryDetails/{userId},这里的UserManager其实就是ServiceId了,运行时程序会把它替换为四份实例中的一个的地址,于是就变为:http://IP:Port/users/queryDetails/{userId}。

这就是对URI的重新构建,好理解吧。这其实就是众人皆知的负载均衡的原理了。

再看前两个方法,虽然形式不同,但本质一样,都是在一个实例上执行一个请求,并返回响应。所以方法名都叫执行。具体怎么执行请求,那就要看实现类怎么实现了。

但是比执行一个请求更重要的是,如何表示一个请求?

a)如果是执行一个普通Java方法的话,需要有方法本身Method,目标对象Object,方法参数Args。

b)如果要执行的是一个HTTP请求的话,需要有请求方法Method(GET/POST等),请求地址URL,还有一些参数,如表单数据,查询字符串或头信息等。

那这里的请求应该如何表示呢?就是这个接口,LoadBalancerRequest<T>:


public interface LoadBalancerRequest<T> {
 T apply(ServiceInstance instance) throws Exception;
}

看完上面的源码后,发现只有一个apply方法,泛型参数T就表示响应。意思就是在选出的服务实例上“应用”一下,就得到了响应。这也太抽象了吧,完全看不出意图是什么。

那就再找找有没有和它相似接口,很容找到一个,LoadBalancerRequestTransformer,请求转换器:


public interface LoadBalancerRequestTransformer {
 /**
 * Order for the load balancer request tranformer.
 */
 int DEFAULT_ORDER = 0;
 HttpRequest transformRequest(HttpRequest request, ServiceInstance instance);
}

也只有一个转换请求方法,可以根据服务实例完成对HttpRequest的转换。我们发现这里的请求是基于HTTP的,也就是说微服务之间是使用HTTP协议进行调用的。那为啥是Http呢?

因为现在的微服务理论好多都是来自 Martin Fowler 的那篇微服务的文章,下面是其中一句话:

In short, the microservice architectural style is an approach to developing a single application as a suite of small services, each running in its own process and communicating with lightweight mechanisms, often an HTTP resource API.

里面写到:“使用轻量级的机制通信,通常是HTTP资源API”,即restful形式的。所以 Spring Cloud 里面通常使用RestTemplate调用其它微服务。

现在只要能创建出LoadBalancerRequest接口的实例,然后在选出的服务实例上apply一下,就可以获取到响应了。正好有一个工厂类来负责创建,它就是LoadBalancerRequestFactory:


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 instance -> {
 HttpRequest serviceRequest = new ServiceRequestWrapper(request, instance,
 this.loadBalancer);
 if (this.transformers != null) {
 for (LoadBalancerRequestTransformer transformer : this.transformers) {
 serviceRequest = transformer.transformRequest(serviceRequest,
 instance);
 }
 }
 return execution.execute(serviceRequest, body);
 };
 }
}

通过lambda表达式返回一个接口实例,在该实例上调用apply方法时会执行这个lambda表达式。

这个lambda表达式主要做三件事:

1)一是使用选出来的实例的IP和Port重新构建HttpRequest中的URI,

2)二是使用转换器对HttpRequest进行转换,

3)三是进行Http请求调用。

其中第三步其实使用的是spring-web工程里面的http客户端机制发起调用了,因为被ClientHttpRequestExecution这个类给暴露了,它的背后是抽象好的一套完善的http客户端机制。

spring-web里的http客户端模型

发送http请求,获取响应,这是很常见的功能,其实就是一个http的客户端,所以Spring框架就给实现了,位于spring-web工程里的org.springframework.http.client包下。

它的模型或者说抽象思路并不复杂,有一点需要注意,这是站在客户端的角度的,和我们经常遇到的服务器端正好相反。

Http是基于请求/响应的,所以得有个请求,ClientHttpRequest,客户端请求接口:


public interface ClientHttpRequest extends HttpRequest, HttpOutputMessage {
 /**
 * Execute this request, resulting in a {@link ClientHttpResponse} that can be read.
 * @return the response result of the execution
 * @throws IOException in case of I/O errors
 */
 ClientHttpResponse execute() throws IOException;
}

我们要把请求信息写入这个接口,以客户端的立场来看相当于“输出”,所以继承了HttpOutputMessage接口。它只有一个方法,叫执行,直接就返回响应。

所以响应就是,ClientHttpResponse,客户端响应接口:


public interface ClientHttpResponse extends HttpInputMessage, Closeable {
 /**
 * Return the HTTP status code of the response.
 * @return the HTTP status as an HttpStatus enum value
 * @throws IOException in case of I/O errors
 * @throws IllegalArgumentException in case of an unknown HTTP status code
 * @see HttpStatus#valueOf(int)
 */
 HttpStatus getStatusCode() throws IOException;
 /**
 * Return the HTTP status code (potentially non-standard and not
 * resolvable through the {@link HttpStatus} enum) as an integer.
 * @return the HTTP status as an integer
 * @throws IOException in case of I/O errors
 * @since 3.1.1
 * @see #getStatusCode()
 * @see HttpStatus#resolve(int)
 */
 int getRawStatusCode() throws IOException;
 /**
 * Return the HTTP status text of the response.
 * @return the HTTP status text
 * @throws IOException in case of I/O errors
 */
 String getStatusText() throws IOException;
 /**
 * Close this response, freeing any resources created.
 */
 @Override
 void close();
}

我们要把结果从这个接口中读出来,以客户端的立场来看相当于“输入”,所以继承了HttpInputMessage接口。

可以看到,只要能够获取“请求接口”的实例,就可以调用“执行”方法了。按照惯例,自然有相关的工厂类来创建实例了,ClientHttpRequestFactory,客户端请求工厂:


public interface ClientHttpRequestFactory {
 /**
 * Create a new {@link ClientHttpRequest} for the specified URI and HTTP method.
 * <p>The returned request can be written to, and then executed by calling
 * {@link ClientHttpRequest#execute()}.
 * @param uri the URI to create a request for
 * @param httpMethod the HTTP method to execute
 * @return the created request
 * @throws IOException in case of I/O errors
 */
 ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException;
}

它是一个工厂接口,那么它的实现类就非常重要了,只要拿到了实现类,就相当于拿到了“响应”了。

为了使整个处理过程更加灵活,又引入了老掉牙的套路,就是拦截器了,ClientHttpRequestInterceptor,客户端请求拦截器:


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;
}

拦截器和过滤器是一样的,多个过滤器构成一个过滤器链,在链的尽头会调用Servlet。多个拦截器也构成一个拦截器链,在链的尽头会真正的发起http调用。

所以需要另外一个接口,ClientHttpRequestExecution,就是为了配合实现过滤器链的:


public interface ClientHttpRequestExecution {
 /**
 * Execute the request with the given request attributes and body,
 * and return the response.
 * @param request the request, containing method, URI, and headers
 * @param body the body of the request to execute
 * @return the response
 * @throws IOException in case of I/O errors
 */
 ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException;
}

所以在工厂创建一个请求接口实例时,会把拦截器直接包含进去,InterceptingClientHttpRequestFactory,这个类是对一个真正工厂的“装饰”,目的就是为了把拦截器链添加进去:


public class InterceptingClientHttpRequestFactory extends AbstractClientHttpRequestFactoryWrapper {
 private final List<ClientHttpRequestInterceptor> interceptors;
 /**
 * Create a new instance of the {@code InterceptingClientHttpRequestFactory} with the given parameters.
 * @param requestFactory the request factory to wrap
 * @param interceptors the interceptors that are to be applied (can be {@code null})
 */
 public InterceptingClientHttpRequestFactory(ClientHttpRequestFactory requestFactory,
 @Nullable List<ClientHttpRequestInterceptor> interceptors) {
 super(requestFactory);
 this.interceptors = (interceptors != null ? interceptors : Collections.emptyList());
 }
 @Override
 protected ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod, ClientHttpRequestFactory requestFactory) {
 return new InterceptingClientHttpRequest(requestFactory, this.interceptors, uri, httpMethod);
 }
}

它的父类里包含了真正的工厂实例,那才是用来真正创建请求实例的。

还需要一个类来处理链式调用的逻辑,InterceptingRequestExecution,就是如果后面还有拦截器,就执行后面的拦截器,否则就发起http调用:


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();
 }
 }
 }

可以看它的执行方法,前面都是对拦截器的逐个迭代调用,最后是使用工厂创建了一个请求接口的实例,然后调用该实例的执行方法,返回响应。

这就是http客户端的整体情况,这只是抽象,不包括具体实现的,所以不算复杂。那么Spring也提供了四种实现:

基于 Apache HttpComponents 的实现,基于 Netty4 的实现,基于OkHttp3 的实现,还有基于JDK的 HttpURLConnection 的实现。

其实RestTemplate就是一个rest风格的http客户端,内部机制和上面完全一样,只是添加了很多好用的rest风格的便捷方法而已。

让RestTemplate支持微服务调用

从上面得知,RestTemplate就是一个rest风格的http客户端,使用它在Spring Cloud内部进行微服务调用是再合适不过了。

但是会有一个问题,微服务调用时只能指定ServiceId,IP和Port是动态发现的。RestTemplate是不具备这个功能的。但是要加上这个功能也是很简单的。

只需在中间某个环节拦截住执行流程,把ServiceId替换为动态发现的IP和Port,不就搞定了嘛。这种能力自然非拦截器莫属了。

所以Spring Cloud就专门实现了一个拦截器,LoadBalancerInterceptor,负载均衡器拦截器:


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,
 this.requestFactory.createRequest(request, body, execution));
 }
}

其实就是在一个负载均衡器上执行一个负载均衡请求,这个负载均衡请求实例,虽然是工厂创建出来的,其实就是一个lambda表达式,就起一个包装作用。

为了将这个拦截器应用到RestTemplate上,特意定义了这个接口,RestTemplateCustomizer,RestTemplate自定制器:


public interface RestTemplateCustomizer {
 void customize(RestTemplate restTemplate);
}

再来看这个具体的应用过程,是一个@Configuration类,LoadBalancerInterceptorConfig,拦截器配置:


@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 restTemplate -> {
 List<ClientHttpRequestInterceptor> list = new ArrayList<>(
 restTemplate.getInterceptors());
 list.add(loadBalancerInterceptor);
 restTemplate.setInterceptors(list);
 };
 }
 }

首先把RestTemplate里的所有拦截器都取出来,然后把负载均衡器拦截器添加到最后,最后再设置回RestTemplate中。

意思就是,在即将发起请求的上一步中,重新构建URI,把ServiceId替换为动态发现的真正的IP和Port,很好理解吧。

至此,RestTemplate就具备了客户端负载均衡的能力了。但是RestTemplate一开始并不是为Spring Cloud而生的,所以需要区分一下,哪些RestTemplate具备微服务调用的能力,哪些不具备。

为此,Spring Cloud专门定义了一个注解,@LoadBalanced,来为RestTemplate开启这种能力:


@Target({ ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}

只要在注册RestTemplate时标上这个注解,那么这个RestTemplate就可以用来进行微服务调用了,

Netflix的ribbon组件

Spring Cloud的客户端负载均衡机制只是一套抽象。ribbon具有客户端负载均衡能力,但是和Spring Cloud完全没有关系。

要想把ribbon纳入Spring Cloud体系,就必须写代码实现这套抽象,把真正要做的事情委托给ribbon去做。可以理解为在Spring Cloud的机制和ribbon之间架起一座桥梁。

为了便于从整体上把握,使用RestTemplate把所有的知识点串一下,会变的比较清晰一些,也更容易理解。

当把@LoadBalanced注解标记在RestTemplate上时,表明需要开启微服务调用的能力。因为RestTemplate本质是基于spring-web里的http客户端实现的,所以只能按照这个客户端的机制去扩展。

所以此时会自动为RestTemplate再添加一个额外的拦截器,LoadBalancerInterceptor,这个拦截器就把我们拉入到Spring Cloud的范畴里了,因为它需要依赖两个类,LoadBalancerClient和LoadBalancerRequestFactory。

LoadBalancerClient是一个接口,需要有人实现它,稍后再说。LoadBalancerRequestFactory本身就是一个类,它使用Spring Cloud客户端负载均衡的理论把一个HttpRequest进行包装和预处理,最终委托给http客户端去发起真正的http调用。

所以我们刚刚从http客户端机制里被拉入到Spring Cloud客户端负载均衡理论里,又立马被拉回到http客户端机制里了。其实就是借助于RestTemplate把Spring Cloud的客户端负载均衡理论和http客户端机制相结合起来。

那么在Spring Cloud客户端负载均衡理论里需要实现一个接口,就是刚刚提到的LoadBalancerClient。在spring-web的http客户端机制里需要实现两个接口(其实好几个呢),ClientHttpRequest和ClientHttpRequestFactory。

因此在基于ribbon的实现(spring-cloud-netflix-ribbon)里,一定能找到这三个接口的实现类。

LoadBalancerClient接口的实现类是RibbonLoadBalancerClient,它的所有功能都是依赖于ribbon自身的能力来提供的。其实相当于一个适配器了。


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);
 URI uri;
 Server server;
 if (instance instanceof RibbonServer) {
 RibbonServer ribbonServer = (RibbonServer) instance;
 server = ribbonServer.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);
 }
 return context.reconstructURIWithServer(server, uri);
 }
 @Override
 public ServiceInstance choose(String serviceId) {
 return choose(serviceId, null);
 }
 /**
 * New: Select a server using a 'key'.
 * @param serviceId of the service to choose an instance for
 * @param hint to specify the service instance
 * @return the selected {@link ServiceInstance}
 */
 public ServiceInstance choose(String serviceId, Object hint) {
 Server server = getServer(getLoadBalancer(serviceId), hint);
 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 {
 return execute(serviceId, request, null);
 }
 /**
 * New: Execute a request by selecting server using a 'key'. The hint will have to be
 * the last parameter to not mess with the `execute(serviceId, ServiceInstance,
 * request)` method. This somewhat breaks the fluent coding style when using a lambda
 * to define the LoadBalancerRequest.
 * @param <T> returned request execution result type
 * @param serviceId id of the service to execute the request to
 * @param request to be executed
 * @param hint used to choose appropriate {@link Server} instance
 * @return request execution result
 * @throws IOException executing the request may result in an {@link IOException}
 */
 public <T> T execute(String serviceId, LoadBalancerRequest<T> request, Object hint)
 throws IOException {
 ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
 Server server = getServer(loadBalancer, hint);
 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;
 }
}

ClientHttpRequest接口的实现类是RibbonHttpRequest,它也是使用ribbon的RestClient(com.netflix.niws.client.http.RestClient)客户端发起的请求。


public class RibbonHttpRequest extends AbstractClientHttpRequest {
 private HttpRequest.Builder builder;
 private URI uri;
 private HttpRequest.Verb verb;
 private RestClient client;
 private IClientConfig config;
 private ByteArrayOutputStream outputStream = null;
 public RibbonHttpRequest(URI uri, HttpRequest.Verb verb, RestClient client,
 IClientConfig config) {
 this.uri = uri;
 this.verb = verb;
 this.client = client;
 this.config = config;
 this.builder = HttpRequest.newBuilder().uri(uri).verb(verb);
 }
 @Override
 public HttpMethod getMethod() {
 return HttpMethod.valueOf(verb.name());
 }
 @Override
 public String getMethodValue() {
 return getMethod().name();
 }
 @Override
 public URI getURI() {
 return uri;
 }
 @Override
 protected OutputStream getBodyInternal(HttpHeaders headers) throws IOException {
 if (outputStream == null) {
 outputStream = new ByteArrayOutputStream();
 }
 return outputStream;
 }
 @Override
 protected ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException {
 try {
 addHeaders(headers);
 if (outputStream != null) {
 outputStream.close();
 builder.entity(outputStream.toByteArray());
 }
 HttpRequest request = builder.build();
 HttpResponse response = client.executeWithLoadBalancer(request, config);
 return new RibbonHttpResponse(response);
 }
 catch (Exception e) {
 throw new IOException(e);
 }
 }
 private void addHeaders(HttpHeaders headers) {
 for (String name : headers.keySet()) {
 // apache http RequestContent pukes if there is a body and
 // the dynamic headers are already present
 if (isDynamic(name) && outputStream != null) {
 continue;
 }
 // Don't add content-length if the output stream is null. The RibbonClient
 // does this for us.
 if (name.equals("Content-Length") && outputStream == null) {
 continue;
 }
 List<String> values = headers.get(name);
 for (String value : values) {
 builder.header(name, value);
 }
 }
 }
 private boolean isDynamic(String name) {
 return "Content-Length".equalsIgnoreCase(name)
 || "Transfer-Encoding".equalsIgnoreCase(name);
 }
}

ClientHttpRequestFactory接口的实现类是RibbonClientHttpRequestFactory,它来负责创建出上面这个请求的实例。


public class RibbonClientHttpRequestFactory implements ClientHttpRequestFactory {
 private final SpringClientFactory clientFactory;
 public RibbonClientHttpRequestFactory(SpringClientFactory clientFactory) {
 this.clientFactory = clientFactory;
 }
 @Override
 @SuppressWarnings("deprecation")
 public ClientHttpRequest createRequest(URI originalUri, HttpMethod httpMethod)
 throws IOException {
 String serviceId = originalUri.getHost();
 if (serviceId == null) {
 throw new IOException(
 "Invalid hostname in the URI [" + originalUri.toASCIIString() + "]");
 }
 IClientConfig clientConfig = this.clientFactory.getClientConfig(serviceId);
 RestClient client = this.clientFactory.getClient(serviceId, RestClient.class);
 HttpRequest.Verb verb = HttpRequest.Verb.valueOf(httpMethod.name());
 return new RibbonHttpRequest(originalUri, verb, client, clientConfig);
 }
}

最后,需要说明的是,即使是ribbon发起的http请求调用,底层使用的还是HttpComponents组件库或OkHttp组件库。

其实这里面充满了封装/适配,代码写的绕来绕去,有些乱啊。

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