Spring Cloud的commons模块提供了一个@LoadBalanced注解，方便我们对RestTemplate添加一个LoadBalancerClient，以实现客户端负载均衡。通过源码可以发现这是一个标记注解：
/** * 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 {
}因为Spring Boot的自动配置机制，我们可以在这个包下看到一个LoadBalancerAutoConfiguration自动配置类：
/** * 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();//这里持有@LoadBalanced标记的RestTemplate实例
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
final List<RestTemplateCustomizer> customizers) {
return new SmartInitializingSingleton() {
//为restTemplate添加定制
@Override
public void afterSingletonsInstantiated() {
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")//不存在RetryTemplate时生效
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() {
//为restTemplate添加拦截器
@Override
public void customize(RestTemplate restTemplate){
List<ClientHttpRequestInterceptor> list = new ArrayList<>(restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
restTemplate.setInterceptors(list);
}
};
}
}
//以下针对classpath存在RetryTemplate.class的情况配置，先忽略
@Configuration
@ConditionalOnClass(RetryTemplate.class)
public static class RetryInterceptorAutoConfiguration {
@Bean
@ConditionalOnMissingBean
public RetryLoadBalancerInterceptor ribbonInterceptor() {
//...
}
@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);
}
};
}
}
}先忽略存在RetryTemplate的情况，我们看到添加的拦截器是LoadBalancerInterceptor:
/** * @author Spencer Gibb * @author Dave Syer * @author Ryan Baxter * @author William Tran */
public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {
private LoadBalancerClient loadBalancer;
private LoadBalancerRequestFactory requestFactory;
public LoadBalancerInterceptor(LoadBalancerClient loadBalancer, LoadBalancerRequestFactory requestFactory) {
this.loadBalancer = loadBalancer;
this.requestFactory = requestFactory;
}
public LoadBalancerInterceptor(LoadBalancerClient loadBalancer) {
// for backwards compatibility
this(loadBalancer, new LoadBalancerRequestFactory(loadBalancer));
}
@Override
public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
final ClientHttpRequestExecution execution) throws IOException {
final URI originalUri = request.getURI();
String serviceName = originalUri.getHost();
Assert.state(serviceName != null, "Request URI does not contain a valid hostname: " + originalUri);
//主要的拦截处理，就是在这里通过LoadBalancerClient执行具体的请求发送
return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
}
}LoadBalancerClient:
/** * Represents a client side load balancer * @author Spencer Gibb */
public interface LoadBalancerClient extends ServiceInstanceChooser {
<T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;
<T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException;
URI reconstructURI(ServiceInstance instance, URI original);
}
我们可以找到Spring Cloud的netflix-core模块提供了一个LoadBalancerClient的实现类RibbonLoadBalancerClient：
/** * @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);
}
//通过LoadBalancer找到service_id对应的ServiceInstance
@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;
}
protected Server getServer(String serviceId) {
return getServer(getLoadBalancer(serviceId));
}
protected Server getServer(ILoadBalancer loadBalancer) {
if (loadBalancer == null) {
return null;
}
//实际负载均衡的地方
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 {
//...
}
}
我们看到实际负载均衡的地方是netflix的ILoadBalancer来实现的。
Spring Cloud默认使用的是ZoneAwareLoadBalancer。这个解析就需要另外再开一篇。上面主要是了解了一下Spring Cloud的@LoadBalanced注解让我们可以很容易的通过ribbon实现客户端的负载均衡功能。
