说起负载均衡一般都会想到服务端的负载均衡，常用产品包括LBS硬件或云服务、Nginx等，都是耳熟能详的产品。
而Spring Cloud提供了让服务调用端具备负载均衡能力的Ribbon，通过和Eureka的紧密结合，不用在服务集群内再架设负载均衡服务，很大程度简化了服务集群内的架构。
具体也不想多写虚的介绍，反正哪里都能看得到相关的介绍。
直接开撸代码，通过代码来看Ribbon是如何实现的。
配置
详解：
1.RibbonAutoConfiguration配置生成RibbonLoadBalancerClient实例。
代码位置：
spring-cloud-netflix-core-1.3.5.RELEASE.jar
org.springframework.cloud.netflix.ribbon
RibbonAutoConfiguration.class
@Configuration @ConditionalOnClass({ IClient.class, RestTemplate.class, AsyncRestTemplate.class, Ribbon.class}) @RibbonClients @AutoConfigureAfter(name = "org.springframework.cloud.netflix.eureka.EurekaClientAutoConfiguration") @AutoConfigureBefore({LoadBalancerAutoConfiguration.class, AsyncLoadBalancerAutoConfiguration.class}) @EnableConfigurationProperties(RibbonEagerLoadProperties.class) public class RibbonAutoConfiguration { // 略 @Bean @ConditionalOnMissingBean(LoadBalancerClient.class) public LoadBalancerClient loadBalancerClient() { return new RibbonLoadBalancerClient(springClientFactory()); } // 略 }
先看配置条件项，RibbonAutoConfiguration配置必须在LoadBalancerAutoConfiguration配置前执行，因为在LoadBalancerAutoConfiguration配置中会使用RibbonLoadBalancerClient实例。
RibbonLoadBalancerClient继承自LoadBalancerClient接口，是负载均衡客户端，也是负载均衡策略的调用方。
2.LoadBalancerInterceptorConfig配置生成：
1).负载均衡拦截器LoadBalancerInterceptor实例
包含：
LoadBalancerClient实现类的RibbonLoadBalancerClient实例
负载均衡的请求创建工厂LoadBalancerRequestFactory:实例
2).RestTemplate自定义的RestTemplateCustomizer实例
代码位置：
spring-cloud-commons-1.2.4.RELEASE.jar
org.springframework.cloud.client.loadbalancer
LoadBalancerAutoConfiguration.class
@Configuration @ConditionalOnClass(RestTemplate.class) @ConditionalOnBean(LoadBalancerClient.class) @EnableConfigurationProperties(LoadBalancerRetryProperties.class) public class LoadBalancerAutoConfiguration { // 略 @Bean @ConditionalOnMissingBean public LoadBalancerRequestFactory loadBalancerRequestFactory( LoadBalancerClient loadBalancerClient) { return new LoadBalancerRequestFactory(loadBalancerClient, transformers); } @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); } }; } } // 略 }
先看配置条件项：
要求在项目环境中必须要有RestTemplate类。
要求必须要有LoadBalancerClient接口的实现类的实例，也就是上一步生成的RibbonLoadBalancerClient。
3.通过上面一步创建的RestTemplateCustomizer配置所有RestTemplate实例，就是将负载均衡拦截器设置给RestTemplate实例。
@Configuration @ConditionalOnClass(RestTemplate.class) @ConditionalOnBean(LoadBalancerClient.class) @EnableConfigurationProperties(LoadBalancerRetryProperties.class) public class LoadBalancerAutoConfiguration { // 略 @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); } } } }; } // 略 @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); } }; } } // 略 }
restTemplate.setInterceptors(list)这个地方就是注入负载均衡拦截器的地方LoadBalancerInterceptor。
从这个地方实际上也可以猜出来，RestTemplate可以通过注入的拦截器来构建相应的请求实现负载均衡。
也能看出来可以自定义拦截器实现其他目的。
4.RibbonClientConfiguration配置生成ZoneAwareLoadBalancer实例
代码位置：
spring-cloud-netflix-core-1.3.5.RELEASE.jar
org.springframework.cloud.netflix.ribbon
RibbonClientConfiguration.class
@SuppressWarnings("deprecation")
@Configuration
@EnableConfigurationProperties
//Order is important here, last should be the default, first should be optional
// see https://github.com/spring-cloud/spring-cloud-netflix/issues/2086#issuecomment-316281653
@Import({OkHttpRibbonConfiguration.class, RestClientRibbonConfiguration.class, HttpClientRibbonConfiguration.class})
public class RibbonClientConfiguration {
// 略
@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);
}
// 略
}
ZoneAwareLoadBalancer继承自ILoadBalancer接口，该接口有一个方法：
/** * 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);
ZoneAwareLoadBalancer就是一个具体的负载均衡实现类，也是默认的负载均衡类，通过对chooseServer方法的实现选取某个服务实例。
拦截&请求
1.使用RestTemplate进行Get、Post等各种请求，都是通过doExecute方法实现
代码位置：
spring-web-4.3.12.RELEASE.jar
org.springframework.web.client
RestTemplate.class
public class RestTemplate extends InterceptingHttpAccessor implements RestOperations { // 略 protected <T> T doExecute(URI url, HttpMethod method, RequestCallback requestCallback, 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; } } catch (IOException ex) { String resource = url.toString(); String query = url.getRawQuery(); resource = (query != null ? resource.substring(0, resource.indexOf(‘?‘)) : resource); throw new ResourceAccessException("I/O error on " + method.name() + " request for \"" + resource + "\": " + ex.getMessage(), ex); } finally { if (response != null) { response.close(); } } } // 略 }
支持的各种http请求方法最终都是调用doExecute方法，该方法内调用创建方法创建请求实例，并执行请求得到响应对象。
2.生成请求实例创建工厂
上一步代码中，调用createRequest方法创建请求实例，这个方法是定义在父类中。
先整理出主要的继承关系：
createRequest方法实际是定义在HttpAccessor抽象类中。
public abstract class HttpAccessor { private ClientHttpRequestFactory requestFactory = new SimpleClientHttpRequestFactory(); public void setRequestFactory(ClientHttpRequestFactory requestFactory) { Assert.notNull(requestFactory, "ClientHttpRequestFactory must not be null"); this.requestFactory = requestFactory; } public ClientHttpRequestFactory getRequestFactory() { return this.requestFactory; } 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; } }
在createRequest方法中调用getRequestFactory方法获得请求实例创建工厂，实际上getRequestFactory并不是当前HttpAccessor类中定义的，而是在子类InterceptingHttpAccessor中定义的。
public abstract class InterceptingHttpAccessor extends HttpAccessor { private List<ClientHttpRequestInterceptor> interceptors = new ArrayList<ClientHttpRequestInterceptor>(); public void setInterceptors(List<ClientHttpRequestInterceptor> interceptors) { this.interceptors = interceptors; } public List<ClientHttpRequestInterceptor> getInterceptors() { return interceptors; } @Override public ClientHttpRequestFactory getRequestFactory() { ClientHttpRequestFactory delegate = super.getRequestFactory(); if (!CollectionUtils.isEmpty(getInterceptors())) { return new InterceptingClientHttpRequestFactory(delegate, getInterceptors()); } else { return delegate; } } }
在这里做了个小动作，首先还是通过HttpAccessor类创建并获得SimpleClientHttpRequestFactory工厂，这个工厂主要就是在没有拦截器的时候创建基本请求实例。
其次，在有拦截器注入的情况下，创建InterceptingClientHttpRequestFactory工厂，该工厂就是创建带拦截器的请求实例，因为注入了负载均衡拦截器，所以这里就从InterceptingClientHttpRequestFactory工厂创建。
3.通过工厂创建请求实例
创建实例就看工厂的createRequest方法。
public class InterceptingClientHttpRequestFactory extends AbstractClientHttpRequestFactoryWrapper { private final List<ClientHttpRequestInterceptor> interceptors; public InterceptingClientHttpRequestFactory(ClientHttpRequestFactory requestFactory, List<ClientHttpRequestInterceptor> interceptors) { super(requestFactory); this.interceptors = (interceptors != null ? interceptors : Collections.<ClientHttpRequestInterceptor>emptyList()); } @Override protected ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod, ClientHttpRequestFactory requestFactory) { return new InterceptingClientHttpRequest(requestFactory, this.interceptors, uri, httpMethod); } }
就是new了个InterceptingClientHttpRequest实例，并且把拦截器、基本请求实例创建工厂注进去。
4.请求实例调用配置阶段注入的负载均衡拦截器的拦截方法intercept
可从第1步看出，创建完请求实例后，通过执行请求实例的execute方法执行请求。
ClientHttpRequest request = createRequest(url, method); if (requestCallback != null) { requestCallback.doWithRequest(request); } response = request.execute();
实际请求实例是InterceptingClientHttpRequest，execute实际是在它的父类中。
类定义位置：
spring-web-4.3.12.RELEASE.jar
org.springframework.http.client
InterceptingClientHttpRequest.class
看一下它们的继承关系。
在execute方法中实际调用了子类实现的executeInternal方法。
public abstract class AbstractClientHttpRequest implements ClientHttpRequest { private final HttpHeaders headers = new HttpHeaders(); private boolean executed = false; @Override public final HttpHeaders getHeaders() { return (this.executed ? HttpHeaders.readOnlyHttpHeaders(this.headers) : this.headers); } @Override public final OutputStream getBody() throws IOException { assertNotExecuted(); return getBodyInternal(this.headers); } @Override public final ClientHttpResponse execute() throws IOException { assertNotExecuted(); ClientHttpResponse result = executeInternal(this.headers); this.executed = true; return result; } protected void assertNotExecuted() { Assert.state(!this.executed, "ClientHttpRequest already executed"); } protected abstract OutputStream getBodyInternal(HttpHeaders headers) throws IOException; protected abstract ClientHttpResponse executeInternal(HttpHeaders headers) throws IOException; }
其实就是InterceptingClientHttpRequest类的executeInternal方法，其中，又调用了一个执行器InterceptingRequestExecution的execute，通关判断如果有拦截器注入进来过，就调用拦截器的intercept方法。
这里的拦截器实际上就是在配置阶段注入进RestTemplate实例的负载均衡拦截器LoadBalancerInterceptor实例，可参考上面配置阶段的第2步。
class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest { // 略 @Override protected final ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException { InterceptingRequestExecution requestExecution = new InterceptingRequestExecution(); return requestExecution.execute(this, bufferedOutput); } 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 { 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(); } } } }
5.负载均衡拦截器调用负载均衡客户端
在负载均衡拦截器LoadBalancerInterceptor类的intercept方法中，又调用了负载均衡客户端LoadBalancerClient实现类的execute方法。
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)); } }
在配置阶段的第1步，可以看到实现类是RibbonLoadBalancerClient。
6.负载均衡客户端调用负载均衡策略选取目标服务实例并发起请求
在RibbonLoadBalancerClient的第一个execute方法以及getServer方法中可以看到，实际上是通过ILoadBalancer的负载均衡器实现类作的chooseServer方法选取一个服务，交给接下来的请求对象发起一个请求。
这里的负载均衡实现类默认是ZoneAwareLoadBalancer区域感知负载均衡器实例，其内部通过均衡策略选择一个服务。
ZoneAwareLoadBalancer的创建可以参考配置阶段的第4步。
public class RibbonLoadBalancerClient implements LoadBalancerClient { @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; } // 略 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); } 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; } // 略 } }
代码撸完，总结下。
普通使用RestTemplate请求其他服务时，内部使用的就是常规的http请求实例发送请求。
为RestTemplate增加了@LoanBalanced 注解后，实际上通过配置，为RestTemplate注入负载均衡拦截器，让负载均衡器选择根据其对应的策略选择合适的服务后，再发送请求。
End
