详解SpringCloud Gateway之过滤器GatewayFilter

/**
 * 网关过滤链表接口
 * 用于过滤器的链式调用
 * Contract to allow a {@link WebFilter} to delegate to the next in the chain.
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
public interface GatewayFilterChain {

  /**
   * 链表启动调用入口方法
   * Delegate to the next {@code WebFilter} in the chain.
   * @param exchange the current server exchange
   * @return {@code Mono<Void>} to indicate when request handling is complete
   */
  Mono<Void> filter(ServerWebExchange exchange);

}

  /**
   * 网关过滤的链表，用于过滤器的链式调用
   * 过滤器链表接口的默认实现，
   * 包含2个构建函数：
   * 1.集合参数构建用于初始化吧构建链表
   * 2. index，parent参数用于构建当前执行过滤对应的下次执行的链表 
   */
  private static class DefaultGatewayFilterChain implements GatewayFilterChain {

    /**
     * 当前过滤执行过滤器在集合中索引
     */
    private final int index;
    /**
     * 过滤器集合
     */
    private final List<GatewayFilter> filters;

    public DefaultGatewayFilterChain(List<GatewayFilter> filters) {
      this.filters = filters;
      this.index = 0;
    }

    /**
     * 构建
     * @param parent 上一个执行过滤器对应的FilterChain
     * @param index 当前要执行过滤器的索引
     */
    private DefaultGatewayFilterChain(DefaultGatewayFilterChain parent, int index) {
      this.filters = parent.getFilters();
      this.index = index;
    }

    public List<GatewayFilter> getFilters() {
      return filters;
    }

    /**
     * @param exchange the current server exchange
     * @return
     */
    @Override
    public Mono<Void> filter(ServerWebExchange exchange) {
      return Mono.defer(() -> {
        if (this.index < filters.size()) {
          //获取当前索引的过滤器
          GatewayFilter filter = filters.get(this.index);
          //构建当前索引的下一个过滤器的FilterChain
          DefaultGatewayFilterChain chain = new DefaultGatewayFilterChain(this, this.index + 1);
          //调用过滤器的filter方法执行过滤器
          return filter.filter(exchange, chain);
        } else {
          //当前索引大于等于过滤集合大小，标识所有链表都已执行完毕，返回空
          return Mono.empty(); // complete
        }
      });
    }
  }

/**
 * 网关路由过滤器，
 * Contract for interception-style, chained processing of Web requests that may
 * be used to implement cross-cutting, application-agnostic requirements such
 * as security, timeouts, and others. Specific to a Gateway
 *
 * Copied from WebFilter
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
public interface GatewayFilter extends ShortcutConfigurable {

  String NAME_KEY = "name";
  String VALUE_KEY = "value";

  /**
   * 过滤器执行方法
   * Process the Web request and (optionally) delegate to the next
   * {@code WebFilter} through the given {@link GatewayFilterChain}.
   * @param exchange the current server exchange
   * @param chain provides a way to delegate to the next filter
   * @return {@code Mono<Void>} to indicate when request processing is complete
   */
  Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain);

}

/**
 * 排序的网关路由过滤器，用于包装真实的网关过滤器，已达到过滤器可排序
 * @author Spencer Gibb
 */
public class OrderedGatewayFilter implements GatewayFilter, Ordered {

  //目标过滤器
  private final GatewayFilter delegate;
  //排序字段
  private final int order;

  public OrderedGatewayFilter(GatewayFilter delegate, int order) {
    this.delegate = delegate;
    this.order = order;
  }

  @Override
  public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
    return this.delegate.filter(exchange, chain);
  }
}  

  /**
   * 全局过滤器的包装类，将全局路由包装成统一的网关过滤器
   */
  private static class GatewayFilterAdapter implements GatewayFilter {

    /**
     * 全局过滤器
     */
    private final GlobalFilter delegate;

    public GatewayFilterAdapter(GlobalFilter delegate) {
      this.delegate = delegate;
    }

    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
      return this.delegate.filter(exchange, chain);
    }
  }

    /**
     * 全局过滤器，用户通过HttpClient转发请求
     * @param httpClient
     * @param headersFilters
     * @return
     */
    @Bean
    public NettyRoutingFilter routingFilter(HttpClient httpClient,
                        ObjectProvider<List<HttpHeadersFilter>> headersFilters) {
      return new NettyRoutingFilter(httpClient, headersFilters);
    }

    /**
     * 全局的过滤器，用户将HttpClient客户端转发请求的响应写入到原始的请求响应中
     * @param properties
     * @return
     */
    @Bean
    public NettyWriteResponseFilter nettyWriteResponseFilter(GatewayProperties properties) {
      return new NettyWriteResponseFilter(properties.getStreamingMediaTypes());
    }

  /**
   * 全局过滤器，用于在通过负载均衡客户端选择服务实例信息
   * @param client
   * @return
   */
  @Bean
  @ConditionalOnBean(LoadBalancerClient.class)
  public LoadBalancerClientFilter loadBalancerClientFilter(LoadBalancerClient client) {
    return new LoadBalancerClientFilter(client);
  }

  /**
   * 包装加载全局的过滤器，将全局过滤器包装成GatewayFilter
   * @param filters
   * @return
   */
  private static List<GatewayFilter> loadFilters(List<GlobalFilter> filters) {
    return filters.stream()
        .map(filter -> {
          //将所有的全局过滤器包装成网关过滤器
          GatewayFilterAdapter gatewayFilter = new GatewayFilterAdapter(filter);
          //判断全局过滤器是否实现了可排序接口
          if (filter instanceof Ordered) {
            int order = ((Ordered) filter).getOrder();
            //包装成可排序的网关过滤器
            return new OrderedGatewayFilter(gatewayFilter, order);
          }
          return gatewayFilter;
        }).collect(Collectors.toList());
  }
  @Override
  public Mono<Void> handle(ServerWebExchange exchange) {
    //获取请求上下文设置的路由实例
    Route route = exchange.getRequiredAttribute(GATEWAY_ROUTE_ATTR);
    //获取路由定义下的网关过滤器集合
    List<GatewayFilter> gatewayFilters = route.getFilters();

    //组合全局的过滤器与路由配置的过滤器
    List<GatewayFilter> combined = new ArrayList<>(this.globalFilters);
    //添加路由配置过滤器到集合尾部
    combined.addAll(gatewayFilters);
    //对过滤器进行排序
    //TODO: needed or cached?
    AnnotationAwareOrderComparator.sort(combined);

    logger.debug("Sorted gatewayFilterFactories: "+ combined);
    //创建过滤器链表对其进行链式调用
    return new DefaultGatewayFilterChain(combined).filter(exchange);
  }