ContentNegotiation内容协商机制(二)---Spring MVC内容协商实现原理及自定义配置【享学Spring MVC】（上）

// A strategy for resolving the requested media types for a request.
// @since 3.2
@FunctionalInterface
public interface ContentNegotiationStrategy {
  // @since 5.0.5
  List<MediaType> MEDIA_TYPE_ALL_LIST = Collections.singletonList(MediaType.ALL);
  // 将给定的请求解析为媒体类型列表
  // 返回的 List 首先按照 specificity 参数排序，其次按照 quality 参数排序
  // 如果请求的媒体类型不能被解析则抛出 HttpMediaTypeNotAcceptableException 异常
  List<MediaType> resolveMediaTypes(NativeWebRequest webRequest) throws HttpMediaTypeNotAcceptableException;
}

public class HeaderContentNegotiationStrategy implements ContentNegotiationStrategy {
  @Override
  public List<MediaType> resolveMediaTypes(NativeWebRequest request) throws HttpMediaTypeNotAcceptableException {
    // 我的Chrome浏览器值是：[text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8,application/signed-exchange;v=b3]
    // postman的值是：[*/*]
    String[] headerValueArray = request.getHeaderValues(HttpHeaders.ACCEPT);
    if (headerValueArray == null) {
      return MEDIA_TYPE_ALL_LIST;
    }
    List<String> headerValues = Arrays.asList(headerValueArray);
    try {
      List<MediaType> mediaTypes = MediaType.parseMediaTypes(headerValues);
      // 排序
      MediaType.sortBySpecificityAndQuality(mediaTypes);
      // 最后Chrome浏览器的List如下：
      // 0 = {MediaType@6205} "text/html"
      // 1 = {MediaType@6206} "application/xhtml+xml"
      // 2 = {MediaType@6207} "image/webp"
      // 3 = {MediaType@6208} "image/apng"
      // 4 = {MediaType@6209} "application/signed-exchange;v=b3"
      // 5 = {MediaType@6210} "application/xml;q=0.9"
      // 6 = {MediaType@6211} "*/*;q=0.8"
      return !CollectionUtils.isEmpty(mediaTypes) ? mediaTypes : MEDIA_TYPE_ALL_LIST;
    } catch (InvalidMediaTypeException ex) {
      throw new HttpMediaTypeNotAcceptableException("Could not parse 'Accept' header " + headerValues + ": " + ex.getMessage());
    }
  }
}

// @since 3.2
public interface MediaTypeFileExtensionResolver {
  // 根据指定的mediaType返回一组文件扩展名
  List<String> resolveFileExtensions(MediaType mediaType);
  // 返回该接口注册进来的所有的扩展名
  List<String> getAllFileExtensions();
}

public class MappingMediaTypeFileExtensionResolver implements MediaTypeFileExtensionResolver {
  // key是lowerCaseExtension，value是对应的mediaType
  private final ConcurrentMap<String, MediaType> mediaTypes = new ConcurrentHashMap<>(64);
  // 和上面相反，key是mediaType，value是lowerCaseExtension（显然用的是多值map）
  private final MultiValueMap<MediaType, String> fileExtensions = new LinkedMultiValueMap<>();
  // 所有的扩展名（List非set哦~）
  private final List<String> allFileExtensions = new ArrayList<>();
  ...
  public Map<String, MediaType> getMediaTypes() {
    return this.mediaTypes;
  }
  // protected 方法
  protected List<MediaType> getAllMediaTypes() {
    return new ArrayList<>(this.mediaTypes.values());
  }
  // 给extension添加一个对应的mediaType
  // 采用ConcurrentMap是为了避免出现并发情况下导致的一致性问题
  protected void addMapping(String extension, MediaType mediaType) {
    MediaType previous = this.mediaTypes.putIfAbsent(extension, mediaType);
    if (previous == null) {
      this.fileExtensions.add(mediaType, extension);
      this.allFileExtensions.add(extension);
    }
  }
  // 接口方法：拿到指定的mediaType对应的扩展名们~
  @Override
  public List<String> resolveFileExtensions(MediaType mediaType) {
    List<String> fileExtensions = this.fileExtensions.get(mediaType);
    return (fileExtensions != null ? fileExtensions : Collections.emptyList());
  }
  @Override
  public List<String> getAllFileExtensions() {
    return Collections.unmodifiableList(this.allFileExtensions);
  }
  // protected 方法：根据扩展名找到一个MediaType~（当然可能是找不到的）
  @Nullable
  protected MediaType lookupMediaType(String extension) {
    return this.mediaTypes.get(extension.toLowerCase(Locale.ENGLISH));
  }
}

// @since 3.2 它是个协商策略抽象实现，同时也有了扩展名+MediaType对应关系的能力
public abstract class AbstractMappingContentNegotiationStrategy extends MappingMediaTypeFileExtensionResolver implements ContentNegotiationStrategy {
  // Whether to only use the registered mappings to look up file extensions,
  // or also to use dynamic resolution (e.g. via {@link MediaTypeFactory}.
  // org.springframework.http.MediaTypeFactory是Spring5.0提供的一个工厂类
  // 它会读取/org/springframework/http/mime.types这个文件，里面有记录着对应关系
  private boolean useRegisteredExtensionsOnly = false;
  // Whether to ignore requests with unknown file extension. Setting this to
  // 默认false：若认识不认识的扩展名，抛出异常：HttpMediaTypeNotAcceptableException
  private boolean ignoreUnknownExtensions = false;
  // 唯一构造函数
  public AbstractMappingContentNegotiationStrategy(@Nullable Map<String, MediaType> mediaTypes) {
    super(mediaTypes);
  }
  // 实现策略接口方法
  @Override
  public List<MediaType> resolveMediaTypes(NativeWebRequest webRequest) throws HttpMediaTypeNotAcceptableException {
    // getMediaTypeKey：抽象方法(让子类把扩展名这个key提供出来)
    return resolveMediaTypeKey(webRequest, getMediaTypeKey(webRequest));
  }
  public List<MediaType> resolveMediaTypeKey(NativeWebRequest webRequest, @Nullable String key) throws HttpMediaTypeNotAcceptableException {
    if (StringUtils.hasText(key)) {
      // 调用父类方法：根据key去查找出一个MediaType出来
      MediaType mediaType = lookupMediaType(key); 
      // 找到了就return就成（handleMatch是protected的空方法~~~  子类目前没有实现的）
      if (mediaType != null) {
        handleMatch(key, mediaType); // 回调
        return Collections.singletonList(mediaType);
      }
      // 若没有对应的MediaType，交给handleNoMatch处理（默认是抛出异常，见下面）
      // 注意：handleNoMatch如果通过工厂找到了，那就addMapping()保存起来（相当于注册上去）
      mediaType = handleNoMatch(webRequest, key);
      if (mediaType != null) {
        addMapping(key, mediaType);
        return Collections.singletonList(mediaType);
      }
    }
    return MEDIA_TYPE_ALL_LIST; // 默认值：所有
  }
  // 此方法子类ServletPathExtensionContentNegotiationStrategy有复写
  @Nullable
  protected MediaType handleNoMatch(NativeWebRequest request, String key) throws HttpMediaTypeNotAcceptableException {
    // 若不是仅仅从注册里的拿，那就再去MediaTypeFactory里看看~~~  找到了就返回
    if (!isUseRegisteredExtensionsOnly()) {
      Optional<MediaType> mediaType = MediaTypeFactory.getMediaType("file." + key);
      if (mediaType.isPresent()) {
        return mediaType.get();
      }
    }
    // 忽略找不到，返回null吧  否则抛出异常：HttpMediaTypeNotAcceptableException
    if (isIgnoreUnknownExtensions()) {
      return null;
    }
    throw new HttpMediaTypeNotAcceptableException(getAllMediaTypes());
  }
}