在看SpringCloudGateWay的时候，在实现父类的GatewayFilter apply(C config)方法的时候怎么看也没看明白：

package com.quantex.scg;

import org.springframework.cloud.gateway.filter.GatewayFilter;
import org.springframework.cloud.gateway.filter.factory.AbstractNameValueGatewayFilterFactory;
import org.springframework.http.server.reactive.ServerHttpRequest;

import java.util.List;


public class WsSecurityGatewayFilterFactory extends AbstractNameValueGatewayFilterFactory {

    @Override
    public GatewayFilter apply(NameValueConfig config) {
        return (exchange, chain) -> {
            List<String> strings = exchange.getRequest().getHeaders().get(config.getName());
            if (strings!=null){
                ServerHttpRequest request = exchange.getRequest().mutate()
                        .header(config.getName(), strings.get(0))
                        .build();
//                System.out.println("-----------------------------------------");
//                System.out.println(strings.get(0));
                return chain.filter(exchange.mutate().request(request).build());
            }else{
                return chain.filter(exchange);
            }
        };
    }
}

怎么也没看明白这个函数式的写法(exchange, chain)->{} 不知道到底这exchange和chain是怎么来的，也没看到全局变量啊，我寻思着难道是继承的父类的全局变量？我就一层层找，也灭有找到，实在想不明白，我还以为是容器托管的有名字为exchange和chain的实例，想破脑袋，但是觉得不可能，后来我看了一下apply（）这个函数的返回值，是一个GateWayFilter，我就点进去看了一下这个源码如下：

package org.springframework.cloud.gateway.filter;

/*
 * Copyright 2002-2015 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import org.springframework.cloud.gateway.support.ShortcutConfigurable;
import org.springframework.web.server.ServerWebExchange;

import reactor.core.publisher.Mono;

/**
 * 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";

	/**
	 * 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);

}

结果我恍然大悟，这尼玛不是JAVA8的新操作？当接口只有一个没有实现的方法时，如果返回的是一个函数，其实就时返回了这个接口的是一个实现类的实例，对应的实现方法的逻辑就是那个函数，这里的GatewayFilter 的filter方法的实现其实就是：

(exchange, chain) -> {
            List<String> strings = exchange.getRequest().getHeaders().get(config.getName());
            if (strings!=null){
                ServerHttpRequest request = exchange.getRequest().mutate()
                        .header(config.getName(), strings.get(0))
                        .build();
//                System.out.println("-----------------------------------------");
//                System.out.println(strings.get(0));
                return chain.filter(exchange.mutate().request(request).build());
            }else{
                return chain.filter(exchange);
            }
        };

同时（exchange，chain）也刚好对上了接口里的filter的两个参数ServerWebExchange exchange, GatewayFilterChain chain

综上：这个其实就是JAVA8的函数式接口, 知道java8的特性，但是操作的少。才会出现这种知道但是看不懂的情况,其实函数式接口很简单,下面的链接3分钟就能看明白.

JAVA8特性之函数式接口-@FunctionalInterface注解https://blog.csdn.net/qq_15022971/article/details/126176275?spm=1001.2014.3001.5501