文章目录

• 前言
• 1. RestTemplate请求整体过程
• 2. httpRequest 创建
• 3. doWithRequest
• 4. execute
• 5. http响应解析

前言

目前Spring RestTemplate是常用的http请求工具类，本文简单Spring RestTemplate的请求过程。

1. RestTemplate请求整体过程

接下来以ResponseEntity exchange(String url, HttpMethod method, @Nullable HttpEntity requestEntity,
ParameterizedTypeReference responseType, Object… uriVariables)方法为起点，揭开http请求过程。

public  ResponseEntity exchange(String url, HttpMethod method, @Nullable HttpEntity requestEntity,ParameterizedTypeReference responseType, Object... uriVariables) throws RestClientException {Type type = responseType.getType();RequestCallback requestCallback = httpEntityCallback(requestEntity, type);ResponseExtractor> responseExtractor = responseEntityExtractor(type);return nonNull(execute(url, method, requestCallback, responseExtractor, uriVariables));
}

http的整体流程如下所示，首先创建httpRequest，其次处理httpRequest，最重要就是执行http请求，最后就是处理http响应。

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;try {// 创建httpRequestClientHttpRequest request = createRequest(url, method);if (requestCallback != null) {// HttpEntityRequestCallback#doWithRequestrequestCallback.doWithRequest(request);}// http执行response = request.execute();// http响应处理handleResponse(url, method, response);return (responseExtractor != null ? responseExtractor.extractData(response) : null);}catch (IOException ex) {String resource = url.toString();String query = url.getRawQuery();resource = (query != null ? resource.substring(0, resource.indexOf('?')) : resource);throw new ResourceAccessException("I/O error on " + method.name() +" request for \"" + resource + "\": " + ex.getMessage(), ex);}finally {if (response != null) {// 关闭流response.close();}}
}

2. httpRequest 创建

Restemplate继承了InterceptingHttpAccessor，使用的是InterceptingClientHttpRequestFactory创建httpRequest。

public ClientHttpRequestFactory getRequestFactory() {// ClientHttpRequestInterceptor 支持拓展List interceptors = getInterceptors();if (!CollectionUtils.isEmpty(interceptors)) {ClientHttpRequestFactory factory = this.interceptingRequestFactory;if (factory == null) {// 创建默认的HttpRequestFactoryfactory = new InterceptingClientHttpRequestFactory(super.getRequestFactory(), interceptors);this.interceptingRequestFactory = factory;}return factory;}else {return super.getRequestFactory();}
}

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

3. doWithRequest

调用HttpEntityRequestCallback的doWithRequest，若body为null，主要是设置header。若body补位null，则主要是通过根据body的类型匹配HttpMessageConverter，将body序列化为httpRequest中

public void doWithRequest(ClientHttpRequest httpRequest) throws IOException {// 调用父类的doWithRequest方法super.doWithRequest(httpRequest);Object requestBody = this.requestEntity.getBody();if (requestBody == null) {HttpHeaders httpHeaders = httpRequest.getHeaders();HttpHeaders requestHeaders = this.requestEntity.getHeaders();// 将HttpEntity中的header复制到httpRequest中if (!requestHeaders.isEmpty()) {requestHeaders.forEach((key, values) -> httpHeaders.put(key, new ArrayList<>(values)));}// 设置ContentLengthif (httpHeaders.getContentLength() < 0) {httpHeaders.setContentLength(0L);}}else {Class requestBodyClass = requestBody.getClass();Type requestBodyType = (this.requestEntity instanceof RequestEntity ?((RequestEntity)this.requestEntity).getType() : requestBodyClass);HttpHeaders httpHeaders = httpRequest.getHeaders();HttpHeaders requestHeaders = this.requestEntity.getHeaders();MediaType requestContentType = requestHeaders.getContentType();// getMessageConverters()中的MessageConverters是在RestTemplate构造函数中添加的for (HttpMessageConverter messageConverter : getMessageConverters()) {// 依次轮询每个messageConverter判断是否支持写当前httpRequestBodyif (messageConverter instanceof GenericHttpMessageConverter) {GenericHttpMessageConverter