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

// @since 4.1.5  它是一个访问权限是default的类，是对其它ClientHttpResponse的一个包装
class MessageBodyClientHttpResponseWrapper implements ClientHttpResponse {
  private final ClientHttpResponse response;
  // java.io.PushbackInputStream
  @Nullable
  private PushbackInputStream pushbackInputStream;
  // 判断相应里是否有body体
  // 若响应码是1xx 或者是204；或者getHeaders().getContentLength() == 0 那就返回false  否则返回true
  public boolean hasMessageBody() throws IOException {
    HttpStatus status = HttpStatus.resolve(getRawStatusCode());
    if (status != null && (status.is1xxInformational() || status == HttpStatus.NO_CONTENT || status == HttpStatus.NOT_MODIFIED)) {
      return false;
    }
    if (getHeaders().getContentLength() == 0) {
      return false;
    }
    return true;
  }
  // 上面是完全格局状态码（ContentLength）来判断是否有body体的~~~这里会根据流来判断
  // 如果response.getBody() == null,返回true
  // 若流里有内容，最终就用new PushbackInputStream(body)包装起来~~~
  public boolean hasEmptyMessageBody() throws IOException {
    ...
  }
  ...  // 其余接口方法都委托~
  @Override
  public InputStream getBody() throws IOException {
    return (this.pushbackInputStream != null ? this.pushbackInputStream : this.response.getBody());
  }
}

// @since 3.0 泛型T：the data type
public class HttpMessageConverterExtractor<T> implements ResponseExtractor<T> {
  // java.lang.reflect.Type
  private final Type responseType;
  // 这个泛型也是T，表示数据的Class嘛~
  // 该calss有可能就是上面的responseType
  @Nullable
  private final Class<T> responseClass;
  // 重要：用于消息解析的转换器
  private final List<HttpMessageConverter<?>> messageConverters;
  ... // 省略构造函数
  // 从ClientHttpResponse 里提取值
  @Override
  @SuppressWarnings({"unchecked", "rawtypes", "resource"})
  public T extractData(ClientHttpResponse response) throws IOException {
    MessageBodyClientHttpResponseWrapper responseWrapper = new MessageBodyClientHttpResponseWrapper(response);
    // 若没有消息体（状态码不对 或者 消息体为空都被认为是木有）
    if (!responseWrapper.hasMessageBody() || responseWrapper.hasEmptyMessageBody()) {
      return null;
    }
    // content-type若响应头header里没有指定，那默认是它MediaType.APPLICATION_OCTET_STREAM
    MediaType contentType = getContentType(responseWrapper);
    // 遍历所有的messageConverters，根据contentType 来选则一个消息转换器
    // 最终return messageConverter.read((Class) this.responseClass, responseWrapper)
    ...
  }
}

RestTemplate：
  // 提取为`ResponseEntity`  最终委托给HttpMessageConverterExtractor完成的
  private class ResponseEntityResponseExtractor<T> implements ResponseExtractor<ResponseEntity<T>> {
    @Nullable
    private final HttpMessageConverterExtractor<T> delegate;
    public ResponseEntityResponseExtractor(@Nullable Type responseType) {
      // 显然：只有请求的返回值不为null 才有意义~
      if (responseType != null && Void.class != responseType) {
        this.delegate = new HttpMessageConverterExtractor<>(responseType, getMessageConverters(), logger);
      } else {
        this.delegate = null;
      }
    }
    // 数据提取。都是交给`delegate.extractData(response)`做了，然后new一个ResponseEntity出来包装进去
    // 若木有返回值（delegate=null），那就是一个`ResponseEntity`实例，body为null
    @Override
    public ResponseEntity<T> extractData(ClientHttpResponse response) throws IOException {
      if (this.delegate != null) {
        T body = this.delegate.extractData(response);
        return ResponseEntity.status(response.getRawStatusCode()).headers(response.getHeaders()).body(body);
      }
      else {
        return ResponseEntity.status(response.getRawStatusCode()).headers(response.getHeaders()).build();
      }
    }
  }
  // 提取请求头
  private static class HeadersExtractor implements ResponseExtractor<HttpHeaders> {
    @Override
    public HttpHeaders extractData(ClientHttpResponse response) {
      return response.getHeaders();
    }
  }

// @since 4.2 出现较晚  
// @see RestTemplate#setUriTemplateHandler(UriTemplateHandler)
public interface UriTemplateHandler {
  URI expand(String uriTemplate, Map<String, ?> uriVariables);
  URI expand(String uriTemplate, Object... uriVariables);
}