一、编写示例
1.服务端
pom.xml
<properties>
<java.version>1.8</java.version>
<spring-cloud.version>Finchley.SR2</spring-cloud.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
</dependencies>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-dependencies</artifactId>
<version>${spring-cloud.version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-alibaba-dependencies</artifactId>
<version>0.2.1.RELEASE</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>2.提供者代码，就是一个普通的HTTP REST接口。
@SpringBootApplication
@EnableDiscoveryClient
@RestController
@Slf4j
public class ProviderApplication {
public static void main(String[] args) {
SpringApplication.run(ProviderApplication.class, args);
}
@GetMapping("provider")
public String provider(){
log.info("..................................");
return "hello,provider";
}
}3.消费端
POM
<properties>
<java.version>1.8</java.version>
<spring-cloud.version>Finchley.SR2</spring-cloud.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-actuator</artifactId>
<exclusions>
<exclusion>
<groupId>org.springframework.retry</groupId>
<artifactId>spring-retry</artifactId>
</exclusion>
</exclusions>
</dependency>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<!-- <dependency>-->
<!-- <groupId>org.springframework.retry</groupId>-->
<!-- <artifactId>spring-retry</artifactId>-->
<!-- </dependency>-->
<dependency>
<groupId>org.projectlombok</groupId>
<artifactId>lombok</artifactId>
</dependency>
<!--<dependency>-->
<!--<groupId>org.springframework.retry</groupId>-->
<!--<artifactId>spring-retry</artifactId>-->
<!--</dependency>-->
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
</dependencies>
<dependencyManagement>
<dependencies>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-dependencies</artifactId>
<version>${spring-cloud.version}</version>
<type>pom</type>
<scope>import</scope>
</dependency>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-alibaba-dependencies</artifactId>
<version>0.2.1.RELEASE</version>
<type>pom</type>
<scope>import</scope>
</dependency>
</dependencies>
</dependencyManagement>这里注意，如果不想要RetryTemplate，就去掉Retry的依赖，否则默认会引入的。
消费端代码
@SpringBootApplication
@Slf4j
@EnableDiscoveryClient
@RestController
public class ConsumeApplication {
@Autowired
RestTemplate restTemplate;
@Bean
@LoadBalanced
public RestTemplate getRestTemplate(){
HttpComponentsClientHttpRequestFactory clientHttpRequestFactory = new HttpComponentsClientHttpRequestFactory();
clientHttpRequestFactory.setConnectTimeout(5 * 1000);
clientHttpRequestFactory.setReadTimeout(5 * 1000);
return new RestTemplate(clientHttpRequestFactory);
}
public static void main(String[] args) {
SpringApplication.run(ConsumeApplication.class, args);
}
@GetMapping("test")
public String getContent(){
log.info("发起请求");
return restTemplate.getForObject("http://provider/provider",String.class);
}
}二、初始化自动配置代码分析
1.org.springframework.cloud.netflix.ribbon.RibbonAutoConfiguration，这个为RIBBON私有的
LoadBalancerClient,LoadBalancer,IRule对象的初始化。
@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, ServerIntrospectorProperties.class})
public class RibbonAutoConfiguration {
@Bean
public SpringClientFactory springClientFactory() {
SpringClientFactory factory = new SpringClientFactory();
factory.setConfigurations(this.configurations);
return factory;
}
@Bean
@ConditionalOnMissingBean(LoadBalancerClient.class)
public LoadBalancerClient loadBalancerClient() {
return new RibbonLoadBalancerClient(springClientFactory());
}
@Bean
@ConditionalOnClass(name = "org.springframework.retry.support.RetryTemplate")
@ConditionalOnMissingBean
public LoadBalancedRetryFactory loadBalancedRetryPolicyFactory(final SpringClientFactory clientFactory) {
return new RibbonLoadBalancedRetryFactory(clientFactory);
}
2.org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration为springcloud-common的对象自动初始化。
@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 loadBalancedRestTemplateInitializerDeprecated(
final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
return () -> restTemplateCustomizers.ifAvailable(customizers -> {
for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
for (RestTemplateCustomizer customizer : customizers) {
customizer.customize(restTemplate);
}
}
});
}
@Autowired(required = false)
private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();
@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 restTemplate -> {
List<ClientHttpRequestInterceptor> list = new ArrayList<>(
restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
restTemplate.setInterceptors(list);
};
}
}这里的 restTemplate只会取加了@LoadBalance注解的，因为@LoadBalance加上了@Qualifer，这个注解会有筛选过滤作用。
去除了RetryTemplate就会走默认，否则走重试balance.
1.首先创建LoadBalancerRequestFactory工厂对象。
2.创建LoadBalancerInterceptor对象
3.对RestTemplate列表对象，设置拦截器属性。
三、调用分析
1.RestTemplate.getObject方法开始。这个方法会调用doExecute
public class RestTemplate extends InterceptingHttpAccessor implements RestOperations {
@Nullable
protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback,
@Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {
Assert.notNull(url, "URI is required");
Assert.notNull(method, "HttpMethod is required");
ClientHttpResponse response = null;
try {
ClientHttpRequest request = createRequest(url, method);
if (requestCallback != null) {
requestCallback.doWithRequest(request);
}
response = request.execute();
handleResponse(url, method, response);
return (responseExtractor != null ? responseExtractor.extractData(response) : 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();
}
}
}
2.createRequest方法，RestTemplate是继承于InterceptingHttpAccessor和HttpAccessor
HttpAccessor
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;
}3.getRequestFactory方法又是来源于InterceptingHttpAccessor，这里会对原有httpRequestFactory工厂包装。
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();
}
}
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);
}
}所以最后的ClientHttpRequest为InterceptingClientHttpRequest
3.接着我们回到第一步，response = request.execute();因为已经创建好request，现在开始执行。
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;
}
@Override
public HttpMethod getMethod() {
return this.method;
}
@Override
public String getMethodValue() {
return this.method.name();
}
@Override
public URI getURI() {
return this.uri;
}
@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 {
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();
}
}
}
}InterceptingClientHttpRequest的execute会执行executeInternel方法，最后调用
InterceptingRequestExecution.execute
InterceptingRequestExecution.execute这个方法先会调用各种拦截器进行处理，处理完后，再会由拦截器重新调用这个对象的execute方法，做成责任链模式。最后调用缺省的HTTP代理请求工厂生成代理对象真正发起HTTP请求。
目前的拦截器就是从注册中心获取实例地址，替换URL.
4.接着进行LoadBalancerInterceptor.interropt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();
return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
}
}
5.这里的loadBalancer就是RibbonLoadBalancerClient，所以调用这个进行URL替换。
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) {
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);
}6.接着调用RibbonLoadBalancerClient.getLoadBalancer获取负载均衡器。
7.SpringClientFactory.getLoadBalancer
return getInstance(name, ILoadBalancer.class);这个会去容器工厂自动创建获取ILoadBalancer类。
8.ribbonLoadBalancingHttpClient为实现类。
package org.springframework.cloud.netflix.ribbon.apache;
/**
* @author Spencer Gibb
*/
@Configuration
@ConditionalOnClass(name = "org.apache.http.client.HttpClient")
@ConditionalOnProperty(name = "ribbon.httpclient.enabled", matchIfMissing = true)
public class HttpClientRibbonConfiguration {
@Bean
@ConditionalOnMissingBean(AbstractLoadBalancerAwareClient.class)
@ConditionalOnMissingClass(value = "org.springframework.retry.support.RetryTemplate")
public RibbonLoadBalancingHttpClient ribbonLoadBalancingHttpClient(
IClientConfig config, ServerIntrospector serverIntrospector,
ILoadBalancer loadBalancer, RetryHandler retryHandler, CloseableHttpClient httpClient) {
RibbonLoadBalancingHttpClient client = new RibbonLoadBalancingHttpClient(httpClient, config, serverIntrospector);
client.setLoadBalancer(loadBalancer);
client.setRetryHandler(retryHandler);
Monitors.registerObject("Client_" + this.name, client);
return client;
}9.这个依赖于ribbonLoadBalancer，实际为这个类ZoneAwareLoadBalancer
package org.springframework.cloud.netflix.ribbon;
/**
* @author Dave Syer
* @author Tim Ysewyn
*/
@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({HttpClientConfiguration.class, OkHttpRibbonConfiguration.class, RestClientRibbonConfiguration.class, HttpClientRibbonConfiguration.class})
public class RibbonClientConfiguration {
@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);
}
10.上面那个类又依赖于IRULE，所以最后创建 ribbonRule，实现类为
ZoneAvoidanceRule
11.创建完RULE后，创建loadBalance
12.创建ribbonLoadBalancer
13.我们再回到第5步， @Override
public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
ILoadBalancer loadBalancer = getLoadBalancer(serviceId);
Server server = getServer(loadBalancer);，开始获取server
14.选择好server后开始执行。
15.回到LoadBalancerRequestFactory.apply
16.又回到InterceptingRequestExecution.execute
17.接下来开始替换URL
18。RibbonLoadBalancerClient.reconstructURI
19.
LoadBalancerContext.reconstructURIWithServer
20.url替换完毕，调用默认的HTTP代理对象发起请求，返回
21 请求结束，处理消息体转换，返回数据。
21.请求完毕。
四、服务实例获取和选择分析
1.我们先回到第三单，第9步，这个依赖于ribbonLoadBalancer，实际为这个类ZoneAwareLoadBalancer，这个为负载均衡器的选择服务实例逻辑。
2.ZoneAwareLoadBalancer主要为感知分区的实例选择，我们一般用不到。目前使用DynamicServerListLoadBalancer
AbstractLoadBalancer
AbstractLoadBalancer类的定义如下：
public abstract class AbstractLoadBalancer implements ILoadBalancer {
public enum ServerGroup{
ALL,
STATUS_UP,
STATUS_NOT_UP
}
public Server chooseServer() {
return chooseServer(null);
}
public abstract List<Server> getServerList(ServerGroup serverGroup);
public abstract LoadBalancerStats getLoadBalancerStats();
}关于这个类我说以下几点：
1. AbstractLoadBalancer实现了ILoadBalancer接口，但它是一个抽象类，它里边定义了一个关于服务实例的分组枚举类，包含了三种类型的服务：ALL表示所有服务，STATUS_UP表示正常运行的服务，STATUS_NOT_UP表示下线的服务。
2. chooseServer方法毫无疑问是用来选取一个服务实例，但是要怎么选这里并没有说，我们以后在它的实现类里边寻找选取策略。
3. getServerList方法用来获取某一个分组中所有的的服务实例。
4. getLoadBalancerStats方法用来获取LoadBalancerStats对象，LoadBalancerStats对象中保存了每一个服务的所有细节信息。
BaseLoadBalancer
BaseLoadBalancer是AbstractLoadBalancer的一个实现类，源码比较长我就不贴出来的，我们在这里主要来说说BaseLoadBalancer提供了哪些功能：
1. 首先这个类中有两个List集合中放的Server对象，一个List集合用来保存所有的服务实例，还有一个List集合用来保存当前有效的服务实例。
2. BaseLoadBalancer中定义了一个IPingStrategy，用来描述服务检查策略，IPingStrategy默认实现采用了SerialPingStrategy实现，SerialPingStrategy中的pingServers方法就是遍历所有的服务实例，一个一个发送请求，查看这些服务实例是否还有效，如果网络环境不好的话，这种检查策略效率会很低，如果我们想自定义检查策略的话，可以重写SerialPingStrategy的pingServers方法。
3. 在BaseLoadBalancer的chooseServer方法中(负载均衡的核心方法)，我们发现最终调用了IRule中的choose方法来找到一个具体的服务实例，IRule是一个接口，在BaseLoadBalancer它的默认实现是RoundRobinRule类，RoundRobinRule类中采用了最常用的线性负载均衡规则，也就是所有有效的服务端轮流调用。
4. 在BaseLoadBalancer的构造方法中会启动一个PingTask，这个PingTask用来检查Server是否有效，PingTask的默认执行时间间隔为10秒。
5. markServerDown方法用来标记一个服务是否有效，标记方式为调用Server对象的setAlive方法设置isAliveFlag属性为false。
6. getReachableServers方法用来获取所有有效的服务实例列表。
7. getAllServers方法用来获取所有服务的实例列表。
8. addServers方法表示向负载均衡器中添加一个新的服务实例列表。
BaseLoadBalancer的功能大概就这么多。
DynamicServerListLoadBalancer
DynamicServerListLoadBalancer是BaseLoadBalancer的一个子类，在DynamicServerListLoadBalancer中对基础负载均衡器的功能做了进一步的扩展，我们来看看。
1. 首先DynamicServerListLoadBalancer类一开始就声明了一个变量serverListImpl，serverListImpl变量的类型是一个ServerList<T extends Server>，这里的泛型得是Server的子类，ServerList是一个接口，里边定义了两个方法：一个getInitialListOfServers用来获取初始化的服务实例清单；另一个getUpdatedListOfServers用于获取更新的服务实例清单。
2. ServerList接口有很多实现类，DynamicServerListLoadBalancer默认使用了DomainExtractingServerList类作为ServerList的实现，但是在DomainExtractingServerList的构造方法中又传入了DiscoveryEnabledNIWSServerList对象，查看源码发现最终两个清单的获取方式是由DiscoveryEnabledNIWSServerList类来提供的。
3. DomainExtractingServerList类中的obtainServersViaDiscovery方法是用来发现服务实例并获取的，obtainServersViaDiscovery方法的主要逻辑是这样：首先依靠EurekaClient从服务注册中心获取到具体的服务实例InstanceInfo列表，然后对这个列表进行遍历，将状态为UP的实例转换成DiscoveryEnabledServer对象并放到一个集合中，最后将这个集合返回。
4. DynamicServerListLoadBalancer中还定义了一个ServerListUpdater.UpdateAction类型的服务更新器，Spring Cloud提供了两种服务更新策略：一种是PollingServerListUpdater，表示定时更新；另一种是EurekaNotificationServerListUpdater表示由Eureka的事件监听来驱动服务列表的更新操作，默认的实现策略是第一种，即定时更新，定时的方式很简单，创建Runnable，调用DynamicServerListLoadBalancer中updateAction对象的doUpdate方法，Runnable延迟启动时间为1秒，重复周期为30秒。
5. 在更新服务清单的时候，调用了我们在第一点提到的getUpdatedListOfServers方法，拿到实例清单之后，又调用了一个过滤器中的方法进行过滤。过滤器的类型有好几种，默认是DefaultNIWSServerListFilter，这是一个继承自ZoneAffinityServerListFilter的过滤器，具有区域感知功能。即它会对服务提供者所处的Zone和服务消费者所处的Zone进行比较，过滤掉哪些不是同一个区域的实例。
综上，DynamicServerListLoadBalancer主要是实现了服务实例清单在运行期间的动态更新能力，同时提供了对服务实例清单的过滤功能。
ZoneAwareLoadBalancer
ZoneAwareLoadBalancer是DynamicServerListLoadBalancer的子类，ZoneAwareLoadBalancer的出现主要是为了弥补DynamicServerListLoadBalancer的不足。由于DynamicServerListLoadBalancer中并没有重写chooseServer方法，所以DynamicServerListLoadBalancer中负责均衡的策略依然是我们在BaseLoadBalancer中分析出来的线性轮询策略，这种策略不具备区域感知功能，这样当需要跨区域调用时，可能会产生高延迟。ZoneAwareLoadBalancer重写了setServerListForZones方法，该方法在其父类中的功能主要是根据区域Zone分组的实例列表，为负载均衡器中的LoadBalancerStats对象创建ZoneStats并存入集合中，ZoneStats是一个用来存储每个Zone的状态和统计信息。重写之后的setServerListForZones方法主要做了两件事：一件是调用getLoadBalancer方法来创建负载均衡器，同时创建服务选择策略；另一件是对Zone区域中的实例清单进行检查，如果对应的Zone下已经没有实例了，则将Zone区域的实例列表清空，防止节点选择时出现异常。
2.我们来看调用堆栈分析
首先回到第9步，自动配置生成
package org.springframework.cloud.netflix.ribbon;
/**
* @author Dave Syer
* @author Tim Ysewyn
*/
@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({HttpClientConfiguration.class, OkHttpRibbonConfiguration.class, RestClientRibbonConfiguration.class, HttpClientRibbonConfiguration.class})
public class RibbonClientConfiguration {
@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);
}从这里可以看到，会依赖自动注入ServerList,IRule等。
package org.springframework.cloud.alibaba.nacos.ribbon;
import com.netflix.client.config.IClientConfig;
import com.netflix.loadbalancer.ServerList;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
/**
* integrated Ribbon by default
* @author xiaojing
*/
@Configuration
@ConditionalOnRibbonNacos
public class NacosRibbonClientConfiguration {
@Bean
@ConditionalOnMissingBean
public ServerList<?> ribbonServerList(IClientConfig config) {
NacosServerList serverList = new NacosServerList();
serverList.initWithNiwsConfig(config);
return serverList;
}
}因为我们使用的是NACOS,所以会注入NacosServerList.
IRule使用的是RoundRobinRule.
最后生成ZoneAwareLoadBalancer
3.接着进入到ZoneAwareLoadBalancer的构造，这里主要看DynamicServerListLoadBalancer
重点在restOfInit方法。
package com.netflix.loadbalancer;
/**
* A LoadBalancer that has the capabilities to obtain the candidate list of
* servers using a dynamic source. i.e. The list of servers can potentially be
* changed at Runtime. It also contains facilities wherein the list of servers
* can be passed through a Filter criteria to filter out servers that do not
* meet the desired criteria.
*
* @author stonse
*
*/
public class DynamicServerListLoadBalancer<T extends Server> extends BaseLoadBalancer {
private static final Logger LOGGER = LoggerFactory.getLogger(DynamicServerListLoadBalancer.class);
boolean isSecure = false;
boolean useTunnel = false;
// to keep track of modification of server lists
protected AtomicBoolean serverListUpdateInProgress = new AtomicBoolean(false);
volatile ServerList<T> serverListImpl;
volatile ServerListFilter<T> filter;
protected final ServerListUpdater.UpdateAction updateAction = new ServerListUpdater.UpdateAction() {
@Override
public void doUpdate() {
updateListOfServers();
}
};
protected volatile ServerListUpdater serverListUpdater;
@Deprecated
public DynamicServerListLoadBalancer(IClientConfig clientConfig, IRule rule, IPing ping,
ServerList<T> serverList, ServerListFilter<T> filter) {
this(
clientConfig,
rule,
ping,
serverList,
filter,
new PollingServerListUpdater()
);
}
public DynamicServerListLoadBalancer(IClientConfig clientConfig, IRule rule, IPing ping,
ServerList<T> serverList, ServerListFilter<T> filter,
ServerListUpdater serverListUpdater) {
super(clientConfig, rule, ping);
this.serverListImpl = serverList;
this.filter = filter;
this.serverListUpdater = serverListUpdater;
if (filter instanceof AbstractServerListFilter) {
((AbstractServerListFilter) filter).setLoadBalancerStats(getLoadBalancerStats());
}
restOfInit(clientConfig);
}
void restOfInit(IClientConfig clientConfig) {
boolean primeConnection = this.isEnablePrimingConnections();
// turn this off to avoid duplicated asynchronous priming done in BaseLoadBalancer.setServerList()
this.setEnablePrimingConnections(false);
enableAndInitLearnNewServersFeature();
updateListOfServers();
if (primeConnection && this.getPrimeConnections() != null) {
this.getPrimeConnections()
.primeConnections(getReachableServers());
}
this.setEnablePrimingConnections(primeConnection);
LOGGER.info("DynamicServerListLoadBalancer for client {} initialized: {}", clientConfig.getClientName(), this.toString());
}
这个ServerListUpdater就是一个定时任务更新服务列表。
具体的动作为updateAction，即为DynamicServerListLoadBalancer的updateAction
protected final ServerListUpdater.UpdateAction updateAction = new ServerListUpdater.UpdateAction() {
@Override
public void doUpdate() {
updateListOfServers();
}
};
@VisibleForTesting
public void updateListOfServers() {
List<T> servers = new ArrayList<T>();
if (serverListImpl != null) {
servers = serverListImpl.getUpdatedListOfServers();
LOGGER.debug("List of Servers for {} obtained from Discovery client: {}",
getIdentifier(), servers);
if (filter != null) {
servers = filter.getFilteredListOfServers(servers);
LOGGER.debug("Filtered List of Servers for {} obtained from Discovery client: {}",
getIdentifier(), servers);
}
}
updateAllServerList(servers);
}最后也是updateAllServerList去获取服务列表。
serverListImpl.getUpdatedListOfServers();就是从NacosServerList服务注册中心获取实例列表。
获取完后setServersList去设置服务列表。
至此，服务列表就获取到allServer,upServer了。
4.然后使用IRULE策略来选择服务，IRULE就会从绑定的LOADBALANCER取出所有的服务列表，然后去选择一个。
到这里选择完毕。
