RestTemplate

Spring Cloud 提供了一个接口 DiscoveryClient ， 为 Eureka 或者 consul 等注册中心去实现， getInstances 方法获取注册的实例，一个实例对应一个工程

List<ServiceInstance> getInstances(String serviceId);

看下接口 ServiceInstance 下面两个方法 获取 主机名（IP）/ 端口

String getHost();
int getPort();

注册了实例之后，使用 阻塞式同步RestTemplate 调用 或者 异步非阻塞 WebClient 调用，RestTemplate 为每个HTTP请求创建一个线程，在响应之前一直是阻塞状态，占用系统内存资源。

多个实例 @LoadBalaced 为 RestTemplate 或者 WebClient 做负载均衡的支持，LoadBalancerInterceptor 实现 ClientHttpRequestInterceptor 接口 (intercept 方法)

ClientHttpResponse intercept(HttpRequest request, byte[] body, ClientHttpRequestExecution execution)
		throws IOException;

通过请求 URL 获取 host 然后使用 LoadBalancerClient 进行调用

public class LoadBalancerInterceptor implements ClientHttpRequestInterceptor {

	private LoadBalancerClient 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));
	}

看下 execute 方法 默认引用的是 RibbonLoadBalancerClient 通过 serviceId 和 调用的实例 ribbonServer 及 请求request 做服务调用

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);
}

配置 RestTemplate

@Configuration
public class RestTemplateConfiguration {

    @LoadBalanced
    @Bean
    public RestTemplate restTemplate(RestTemplateBuilder builder) {
        return builder
                .setConnectTimeout(Duration.ofMillis(100))
                .setReadTimeout(Duration.ofMillis(500))
                .requestFactory(this::requestFactory)
                .build();
    }
}