什么是网关过滤器？怎样实现自定义过滤器？

网关登录校验，需要经过客户端到网关再到服务器，过滤器是网关的一部分，过滤器内部可以包含两部分逻辑，分别是pre和post，分别会在请求路由到微服务之前和之后执行。

当所有Filter的pre逻辑都依次顺序执行通过后，请求才会被路由到微服务，否则会被拦截，后续过滤器不再执行。微服务返回结果后，再倒序执行Filter的post逻辑。

Netty路由过滤器负责将请求转发到微服务，当微服务返回结果后进入Filter的post阶段。

网关过滤器有两种，分别是：

GatewayFilter：路由过滤器，作用范围灵活，可以是任意指定的路由。

GlobalFilter：全局过滤器，作用范围是所有路由。

两种过滤器的过滤方法签名完全一致：

public interface GatewayFilter extends ShortcutConfigurable {
Name key.
String NAME_KEY - "name";
Value kkey.
String VALUE_KEY = "value";
Process the Web request and (optionally/ delegate to the next WebFi Lter
through the given GatewayFilterChain.
Params: exchange – the current server exchange
chain - provides a way to delegate to the next fiter
Retums: Mono <Void> to indicate when request processing is complete
MonocVoid> fiLter(ServerwebExchange exchange, GatewayFilterChain chain);

public interface GlobalFilter {
Process the Web request and (optionally) delegate to the next WebFiLter
through the glven GatewayFilterChain.
Params: exchange - the current server exchange
chain - provides a way to delegate to the next filter
Returns: Mono<Voi d> to indicate when request processing is complete
Mono<Void> filter(ServerwebExchange exchange, GatewayFilterChain chain);

@Component
public class PrintAnyGatewayFilterFactory extends AbstractGatewayFilterFactory<Config> {

    @Override
    public GatewayFilter apply(Config config) {
        return new GatewayFilter() {
            @Override
            public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
                // 获取config值
                       String a = config.getA();
                String b = config.getB();
                String c = config.getC();
                // 编写过滤器逻辑
                System.out.println("a = " + a);
                System.out.println("b = " + b);
                System.out.println("c = " + c);                
                // 放行
                       return chain.filter(exchange);
            }
        };
    }
}

@Component
public class PrintAnyGlobalFilter implements GlobalFilter, Ordered {
    @Override
    public Mono<Void> filter(ServerWebExchange exchange, GatewayFilterChain chain) {
        // 编写过滤器逻辑
        System.out.println("GlobalFilter 执行了。");
        // 放行
        return chain.filter(exchange);
    }

    @Override
    public int getOrder() {
        // 过滤器执行顺序，值越小，优先级越高
        return 0;
    }
}