RestTemplate组件：ClientHttpRequestFactory、ClientHttpRequestInterceptor、ResponseExtractor【享学Spring MVC】（上）

// @since 3.0  RestTemplate这个体系都是3.0后才有的
@FunctionalInterface
public interface ClientHttpRequestFactory { 
  // 返回一个ClientHttpRequest，这样调用其execute()方法就可以发送rest请求了~
  ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException;
}

// 在Spring5.0后被标记为过时了，被org.springframework.http.client.reactive.ClientHttpConnector所取代（但还是可用的嘛）
@Deprecated
public interface AsyncClientHttpRequestFactory {
  // AsyncClientHttpRequest#executeAsync()返回的是ListenableFuture<ClientHttpResponse>
  // 可见它的异步是通过ListenableFuture实现的
  AsyncClientHttpRequest createAsyncRequest(URI uri, HttpMethod httpMethod) throws IOException;
}

public class SimpleClientHttpRequestFactory implements ClientHttpRequestFactory, AsyncClientHttpRequestFactory {
  private static final int DEFAULT_CHUNK_SIZE = 4096;
  @Nullable
  private Proxy proxy; //java.net.Proxy
  private boolean bufferRequestBody = true; // 默认会缓冲body
  // URLConnection's connect timeout (in milliseconds).
  // 若值设置为0，表示永不超时 @see URLConnection#setConnectTimeout(int)
  private int connectTimeout = -1;
  // URLConnection#setReadTimeout(int) 
  // 超时规则同上
  private int readTimeout = -1;
  //Set if the underlying URLConnection can be set to 'output streaming' mode.
  private boolean outputStreaming = true;
  // 异步的时候需要
  @Nullable
  private AsyncListenableTaskExecutor taskExecutor;
  ... // 省略所有的set方法
  @Override
  public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
    // 打开一个HttpURLConnection
    HttpURLConnection connection = openConnection(uri.toURL(), this.proxy);
    // 设置超时时间、请求方法等一些参数到connection
    prepareConnection(connection, httpMethod.name());
    //SimpleBufferingClientHttpRequest的excute方法最终使用的是connection.connect();
    // 然后从connection中得到响应码、响应体~~~
    if (this.bufferRequestBody) {
      return new SimpleBufferingClientHttpRequest(connection, this.outputStreaming);
    } else {
      return new SimpleStreamingClientHttpRequest(connection, this.chunkSize, this.outputStreaming);
    }
  }
  // createAsyncRequest()方法略，无非就是在线程池里异步完成请求
  ...
}

public static void main(String[] args) throws IOException {
    SimpleClientHttpRequestFactory clientFactory  = new SimpleClientHttpRequestFactory();
  // ConnectTimeout只有在网络正常的情况下才有效，因此两个一般都设置
    clientFactory.setConnectTimeout(5000); //建立连接的超时时间  5秒
    clientFactory.setReadTimeout(5000); // 传递数据的超时时间（在网络抖动的情况下，这个参数很有用）
    ClientHttpRequest client = clientFactory.createRequest(URI.create("https://www.baidu.com"), HttpMethod.GET);
    // 发送请求
    ClientHttpResponse response = client.execute();
    System.out.println(response.getStatusCode()); //200 OK
    System.out.println(response.getStatusText()); // OK
    System.out.println(response.getHeaders()); //
    // 返回内容 是个InputStream
    byte[] bytes = FileCopyUtils.copyToByteArray(response.getBody());
    System.out.println(new String(bytes, StandardCharsets.UTF_8)); // 百度首页内容的html
}

<dependency>
    <groupId>org.apache.httpcomponents</groupId>
    <artifactId>httpclient</artifactId>
    <version>4.5.10</version>
</dependency>

// @since 3.1 3.1后出现的。
public class HttpComponentsClientHttpRequestFactory implements ClientHttpRequestFactory, DisposableBean {
  private HttpClient httpClient;
  @Nullable
  private RequestConfig requestConfig; // 这个配置就是可以配置超时等等乱七八糟client属性的类
  private boolean bufferRequestBody = true;
  //=========下面是构造函数们=========
  public HttpComponentsClientHttpRequestFactory() {
    // HttpClientBuilder.create().useSystemProperties().build();
    // 所有若是这里，配置超时时间可以这么来设置也可：
    // System.setProperty(”sun.net.client.defaultConnectTimeout”, “5000″);
    this.httpClient = HttpClients.createSystem();
  }
  // 当然可以把你配置好了的Client扔进来
  public HttpComponentsClientHttpRequestFactory(HttpClient httpClient) {
    this.httpClient = httpClient;
  }
  ... // 省略设置超时时间。。。等等属性的一些get/set
  // 超时信息啥的都是保存在`RequestConfig`里的
  @Override
  public ClientHttpRequest createRequest(URI uri, HttpMethod httpMethod) throws IOException {
    HttpClient client = getHttpClient(); // 拿到你指定的client）=（或者系统缺省的）
    // switch语句逻辑：HttpMethod == GET --> HttpGet HEAD --> HttpHead ...
    HttpUriRequest httpRequest = createHttpUriRequest(httpMethod, uri);
    postProcessHttpRequest(httpRequest);
    ...
  }
}