为什么加上@LoadBalanced，RestTemplate就有负载均衡的能力呢？源码分析：

RestTemplate拦截器

首先看RestTemplate类，继承了InterceptingHttpAccessor，代码中有个类型为ClientHttpRequestInterceptor拦截器集合，Ribbon的功能就是在这扩展的。

public class RestTemplate extends InterceptingHttpAccessor implements RestOperations {......
}

public abstract class InterceptingHttpAccessor extends HttpAccessor {private final List interceptors = new ArrayList();......
}

拦截器注入

Interceptor在哪里注入给RestTemplate的？看LoadBalancerAutoConfiguration这个类，该类重点关注这个静态内部类LoadBalancerInterceptorConfig，其往spring注入了LoadBalancerInterceptor这个Bean，实现了上面所讲的ClientHttpRequestInterceptor接口，并赋值给RestTemplate。

@Configuration(proxyBeanMethods = false
)
@ConditionalOnClass({RestTemplate.class})
@ConditionalOnBean({LoadBalancerClient.class})
@EnableConfigurationProperties({LoadBalancerRetryProperties.class})
public class LoadBalancerAutoConfiguration {......@ConditionalOnMissingClass({"org.springframework.retry.support.RetryTemplate"})static class LoadBalancerInterceptorConfig {LoadBalancerInterceptorConfig() {}@Beanpublic LoadBalancerInterceptor ribbonInterceptor(LoadBalancerClient loadBalancerClient, LoadBalancerRequestFactory requestFactory) {return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);}@Bean@ConditionalOnMissingBeanpublic RestTemplateCustomizer restTemplateCustomizer(final LoadBalancerInterceptor loadBalancerInterceptor) {return (restTemplate) -> {List list = new ArrayList(restTemplate.getInterceptors());list.add(loadBalancerInterceptor);restTemplate.setInterceptors(list);};}}
}

RestTemplate执行过程

接着看RestTemplate请求执行的过程，在执行doExecute()时获取一个ClientHttpRequest->InterceptingClientHttpRequest，调用request.execute()时会做两件事

1. 遍历intercept，对请求的URL进行处理，这里的intercept也就时步骤2的LoadBalancerInterceptor。
2. 对处理后的URL进行真正的HTTP请求。

protected  T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback, @Nullable ResponseExtractor responseExtractor) throws RestClientException {Assert.notNull(url, "URI is required");Assert.notNull(method, "HttpMethod is required");ClientHttpResponse response = null;Object var14;try {//InterceptingClientHttpRequestClientHttpRequest request = this.createRequest(url, method);......response = request.execute();......} catch (IOException var12) {......} finally {......}return var14;}

class InterceptingClientHttpRequest extends AbstractBufferingClientHttpRequest {......protected final ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException {InterceptingClientHttpRequest.InterceptingRequestExecution requestExecution = new InterceptingClientHttpRequest.InterceptingRequestExecution();return requestExecution.execute(this, bufferedOutput);}private class InterceptingRequestExecution implements ClientHttpRequestExecution {......public ClientHttpResponse execute(HttpRequest request, byte[] body) throws IOException {//这里执行LoadBalancerInterceptorif (this.iterator.hasNext()) {ClientHttpRequestInterceptor nextInterceptor = (ClientHttpRequestInterceptor)this.iterator.next();return nextInterceptor.intercept(request, body, this);} else {//拦截器处理完后执行真正的HTTP请求......ClientHttpRequest delegate = InterceptingClientHttpRequest.this.requestFactory.createRequest(request.getURI(), method);......return delegate.execute();}}}
}

Ribbon拦截器处理

4.intercept是如何处理URL的？调用intercept时有个loadBalancer，其实现为RibbonLoadBalancerClient，在执行loadBalancer的execute时将URL映射成对应的ip:port，期间包括负载策略、重试策略、服务列表的获取等功能。

//拦截器的处理
public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {......public ClientHttpResponse intercept(final HttpRequest request, final byte[] body, final ClientHttpRequestExecution execution) throws IOException {......return (ClientHttpResponse)this.loadBalancer.execute(serviceName, this.requestFactory.createRequest(request, body, execution));}
}

public class RibbonLoadBalancerClient implements LoadBalancerClient {......public  T execute(String serviceId, LoadBalancerRequest request, Object hint) throws IOException {//获取“service-provider”的服务列表ILoadBalancer loadBalancer = this.getLoadBalancer(serviceId);//通过负载策略选择具体的服务Server server = this.getServer(loadBalancer, hint);if (server == null) {throw new IllegalStateException("No instances available for " + serviceId);} else {RibbonLoadBalancerClient.RibbonServer ribbonServer = new RibbonLoadBalancerClient.RibbonServer(serviceId, server, this.isSecure(server, serviceId), this.serverIntrospector(serviceId).getMetadata(server));return this.execute(serviceId, (ServiceInstance)ribbonServer, (LoadBalancerRequest)request);}}protected Server getServer(ILoadBalancer loadBalancer, Object hint) {//这里是BaseLoadBalancerreturn loadBalancer == null ? null : loadBalancer.chooseServer(hint != null ? hint : "default");}......
}

public class BaseLoadBalancer extends AbstractLoadBalancer implementsPrimeConnections.PrimeConnectionListener, IClientConfigAware {//默认的负载策略private final static IRule DEFAULT_RULE = new RoundRobinRule();protected IRule rule = DEFAULT_RULE;public BaseLoadBalancer() {this.name = DEFAULT_NAME;this.ping = null;setRule(DEFAULT_RULE);setupPingTask();lbStats = new LoadBalancerStats(DEFAULT_NAME);}//添加服务public void addServer(Server newServer) {...}//设置服务列表public void setServersList(List lsrv) {......}//获取服务列表public List getServerList(boolean availableOnly) {return (availableOnly ? getReachableServers() : getAllServers());} //根据负载策略选择具体的服务public Server chooseServer(Object key) {if (counter == null) {counter = createCounter();}counter.increment();if (rule == null) {return null;} else {try {return rule.choose(key);} catch (Exception e) {logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", name, key, e);return null;}}}
}

简单提一嘴，Ribbon在2019年停止维护，2020年cloud版本删除了Ribbon的依赖由Loadbalancer顶替负载均衡，个人感觉跟Ribbon一样。