- 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
RestTemplate类图
- 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 - 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
image.png
它主要有四个参数
-
url请求的http 地址 -
methodhttp请求的method -
requestCallback是一个接口Request Callback -
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
