RestTemplate的概述
spring 架构中，RestTemplate是客户端http请求的核心类，它使和服务端的请求更加简单，同时强化了RestFul的原理，它会处理http请求的连接，使应用代码只需提供url和参数，解析结果就可以了。
RestTemplate默认是依赖于标准sdk 工具去建立连接，你可以通过setRequestFactory方法用其它的http库来替换，比如apache httpComponents,Netty 和OkHttp

RestTemplate的类图

1. HttpAccessor 类
   RestTemplate的基类，是一个抽象类，提供了一些公共的属性，如 ClientHttpRequestFactory,ClientHttpRequestFactory 是clientHttpRequest的工厂类，通过 createRequest 来创建http请求的方法
2. InterceptingHttpAccessor 类
   这个是 HttpAccessor的一个抽象子类，多增加了 请求拦截器相关的属性，ClientHttpRequestInterceptor,而这个ClientHttpRequestInterceptor是 spring cloud中各种服务治理的关键，如客户端负载均衡，链路跟踪等

• RestTemplate的doExecute 方法

该方法是RestTemplate的核心方法

它主要有四个参数

1. url 请求的http 地址
2. method http请求的method
3. requestCallback 是一个接口Request Callback
4. responseExtractor 也是一个接口 ResponseExtractor
   我们来设想下，如果我们自己设计一个封装http请求的组件，一定也会遵守开闭原则，开放出可以让用户自定义的接口，那第三，第四个参数就是这个作用的。
   来看下，spring 里默认是怎么做的,首先看下这两个接口的定义

public interface RequestCallback {

 /**
 * Gets called by {@link RestTemplate#execute} with an opened {@code ClientHttpRequest}.
 * Does not need to care about closing the request or about handling errors:
 * this will all be handled by the {@code RestTemplate}.
 * @param request the active HTTP request
 * @throws IOException in case of I/O errors
 */
 void doWithRequest(ClientHttpRequest request) throws IOException;

}

public interface ResponseExtractor<T> {

 /**
 * Extract data from the given {@code ClientHttpResponse} and return it.
 * @param response the HTTP response
 * @return the extracted data
 * @throws IOException in case of I/O errors
 */
 T extractData(ClientHttpResponse response) throws IOException;

}

很明显，这两个接口分别是在请求前，ClientHttpRequest请求前会作回调，而在请求结束后，又会调用ResponseExtractor去作请求结果解析的动作
再来看下方法createRequest

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;
 }

而 getRequestFactory()在 InterceptingHttpAccessor 中被重载了

@Override
 public ClientHttpRequestFactory getRequestFactory() {
 ClientHttpRequestFactory delegate = super.getRequestFactory();
 if (!CollectionUtils.isEmpty(getInterceptors())) {
 return new InterceptingClientHttpRequestFactory(delegate, getInterceptors());
 }
 else {
 return delegate;
 }
 }

如果有设置拦截器，则返回的是一个InterceptingClientHttpRequestFactory类，而这个类创建的是

@Override
 protected ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod, ClientHttpRequestFactory requestFactory) {
 return new InterceptingClientHttpRequest(requestFactory, this.interceptors, uri, httpMethod);
 }

Resttemplate 调用 的ClientHttpRequest.execute() 方法，其实就是InterceptingClientHttpRequest的方法，再来看下这个类的实现吧

@Override
 protected final ClientHttpResponse executeInternal(HttpHeaders headers, byte[] bufferedOutput) throws IOException {
 InterceptingRequestExecution requestExecution = new InterceptingRequestExecution();
 return requestExecution.execute(this, bufferedOutput);
 }

最终是通过InterceptingRequestExecution类来完成最终的调用

@Override
 public ClientHttpResponse execute(HttpRequest request, final 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) {
 if (delegate instanceof StreamingHttpOutputMessage) {
 StreamingHttpOutputMessage streamingOutputMessage = (StreamingHttpOutputMessage) delegate;
 streamingOutputMessage.setBody(new StreamingHttpOutputMessage.Body() {
 @Override
 public void writeTo(final OutputStream outputStream) throws IOException {
 StreamUtils.copy(body, outputStream);
 }
 });
 }
 else {
 StreamUtils.copy(body, delegate.getBody());
 }
 }
 return delegate.execute();
 }
 }

从上面的代码可以很清晰的看到是怎么调用ClientHttpRequestInterceptor的