RestTemplate的使用和原理你都烂熟于胸了吗？【享学Spring MVC】(下)

RestTemplate：
  @Override
  public <T> ResponseEntity<T> exchange(String url, HttpMethod method, @Nullable HttpEntity<?> requestEntity, Class<T> responseType, Map<String, ?> uriVariables) throws RestClientException {
    // 把请求体适配为HttpEntity
    RequestCallback requestCallback = httpEntityCallback(requestEntity, responseType);
    // 消息提取器使用ResponseEntityResponseExtractor
    ResponseExtractor<ResponseEntity<T>> responseExtractor = responseEntityExtractor(responseType);
    // 从上两个部分就能看到：exchange方法的入参、出参都是非常通用的~~~
    return nonNull(execute(url, method, requestCallback, responseExtractor, uriVariables));
  }
  // ParameterizedTypeReference参数化类型，用于处理泛型
  // 上面的responseType就是个Class。这里是个参数化类型~~~~~
  @Override
  public <T> ResponseEntity<T> exchange(String url, HttpMethod method, @Nullable HttpEntity<?> requestEntity, ParameterizedTypeReference<T> responseType, Object... uriVariables) throws RestClientException {
    Type type = responseType.getType();
    RequestCallback requestCallback = httpEntityCallback(requestEntity, type);
    ResponseExtractor<ResponseEntity<T>> responseExtractor = responseEntityExtractor(type);
    return nonNull(execute(url, method, requestCallback, responseExtractor, uriVariables));
  }
  // 这个方法就非常精简了，让调用者自己去构造RequestEntity，里面是包含了请求的URL和方法等信息的
  @Override
  public <T> ResponseEntity<T> exchange(RequestEntity<?> requestEntity, Class<T> responseType) throws RestClientException {
    RequestCallback requestCallback = httpEntityCallback(requestEntity, responseType);
    ResponseExtractor<ResponseEntity<T>> responseExtractor = responseEntityExtractor(responseType);
    return nonNull(doExecute(requestEntity.getUrl(), requestEntity.getMethod(), requestCallback, responseExtractor));
  }

RestTemplate：
  // 3个execute方法。最终调用的都是doExecute方法
  // 它做的一件事：使用UriTemplateHandler把URL的参数填进去~~~
  // 底层使用的是我上文介绍的`UriComponentsBuilder`，还是比较简单的
  @Override
  @Nullable
  public <T> T execute(String url, HttpMethod method, @Nullable RequestCallback requestCallback, @Nullable ResponseExtractor<T> responseExtractor, Object... uriVariables) throws RestClientException {
    URI expanded = getUriTemplateHandler().expand(url, uriVariables);
    return doExecute(expanded, method, requestCallback, responseExtractor);
  }
doExecute方法：
  @Nullable
  protected <T> T doExecute(URI url, @Nullable HttpMethod method, @Nullable RequestCallback requestCallback, @Nullable ResponseExtractor<T> responseExtractor) throws RestClientException {
    ClientHttpResponse response = null;
    ClientHttpRequest request = createRequest(url, method);
    // 如果有回调，那就先回调处理一下子请求
    if (requestCallback != null) {
      requestCallback.doWithRequest(request);
    }
    // 真正意义上的发送请求。
    // 请注意：如果这里的request是`InterceptingClientHttpRequest`，那就回执行拦截器的intercept方法哦~~~
    // 至于什么时候是InterceptingClientHttpRequest呢？这个上面有讲的
    response = request.execute();
    // 处理结果（若有错误，那就抛出异常~~~）
    handleResponse(url, method, response);
    // 请求正常。那就使用返回值提取器responseExtractor提取出内容即可了~~~
    return (responseExtractor != null ? responseExtractor.extractData(response) : null);
    ...
    // 关闭响应（ClientHttpResponse继承了Closeable接口）
    finally {
      if (response != null) {
        response.close();
      }
    }
  }

public static void main(String[] args) throws IOException {
    RestTemplate restTemplate = new RestTemplate();
    String pageHtml = restTemplate.getForObject("http://www.baidu.com", String.class);
    System.out.println(pageHtml); // 百度首页的html...
}

StringHttpMessageConverter：
  @Override
  protected String readInternal(Class<? extends String> clazz, HttpInputMessage inputMessage) throws IOException {
    // 从响应头的contentType里提取（若是application/json,那默认也是urf-8）
    // 若没有指定编码，就取值getDefaultCharset。比如本处访问百度，就取值默认值`ISO-8859-1`对body体进行编码的~
    Charset charset = getContentTypeCharset(inputMessage.getHeaders().getContentType());
    return StreamUtils.copyToString(inputMessage.getBody(), charset);
  }