1. 概述

本文主要对 过滤器 GatewayFilter 做整体的认识。

过滤器整体类图如下 ：

是不是有点疑惑 GlobalFilter 与 GatewayFilter 的关系 ？且见本文分晓。

2. GatewyFilter

org.springframework.cloud.gateway.filter.GatewayFilter ，网关过滤器接口，代码如下 ：

public interface GatewayFilter {

	/**
	 * 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);

}

• 从接口方法可以看到，和 javax.servlet.Filter 类似。

GatewayFilter 有三种类型的子类实现，我们下面每节介绍一种。

2.1 GatewayFilterFactory 内部类

在每个 GatewayFilterFactory 实现类的 #apply(Tuple) 方法里，都声明了一个实现 GatewayFilter 的内部类，以 AddRequestHeaderGatewayFilterFactory 的代码举例子 ：

1: public class AddRequestHeaderGatewayFilterFactory implements GatewayFilterFactory {
 2: 
 3: 	@Override
 4: 	public List<String> argNames() {
 5: 		return Arrays.asList(NAME_KEY, VALUE_KEY);
 6: 	}
 7: 
 8: 	@Override
 9: 	public GatewayFilter apply(Tuple args) {
10: 		String name = args.getString(NAME_KEY);
11: 		String value = args.getString(VALUE_KEY);
12: 
13: 		return (exchange, chain) -> { // GatewayFilter  
14: 			ServerHttpRequest request = exchange.getRequest().mutate()
15: 					.header(name, value)
16: 					.build();
17: 
18: 			return chain.filter(exchange.mutate().request(request).build());
19: 		};
20: 	}
21: }

• 第 13 至 19 行 ：定义了一个 GatewayFilter 内部实现类。

在 《Spring-Cloud-Gateway 源码解析 —— 过滤器 (4.2) 之 GatewayFilterFactory 过滤器工厂》 ，我们会详细解析每个 GatewayFilterFactory 定义的GatewayFilter 内部实现类。

2.2 OrderedGatewayFilter

org.springframework.cloud.gateway.filter.OrderedGatewayFilter ，有序的网关过滤器实现类。在 FilterChain 里，过滤器数组首先会按照 order 升序排序，按照顺序过滤请求。代码如下 ：

public class OrderedGatewayFilter implements GatewayFilter, Ordered {

	private final GatewayFilter delegate;
	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);
	}

	@Override
	public int getOrder() {
		return this.order;
	}
}

• delegate 属性，委托的 GatewayFilter 。
• order 属性，顺序。
• #filter(ServerWebExchange, GatewayFilterChain) 方法，使用 delegate 过滤请求。

2.3 GatewayFilterAdapter

org.springframework.cloud.gateway.handler.FilteringWebHandler.GatewayFilterAdapter ，网关过滤器适配器。在 GatewayFilterChain 使用 GatewayFilter 过滤请求，所以通过 GatewayFilterAdapter 将 GlobalFilter 适配成 GatewayFilter 。GatewayFilterAdapter 代码如下 ：

private static class GatewayFilterAdapter implements GatewayFilter {

    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);
    }
}

• delegate 属性，委托的 GlobalFilter 。
• #filter(ServerWebExchange, GatewayFilterChain) 方法，使用 delegate 过滤请求。

在 FilteringWebHandler 初始化时，将 GlobalFilter 委托成 GatewayFilterAdapter ，代码如下 ：

1: public class FilteringWebHandler implements WebHandler {
 2: 
 3:     /**
 4:      * 全局过滤器
 5:      */
 6: 	private final List<GatewayFilter> globalFilters;
 7: 
 8: 	public FilteringWebHandler(List<GlobalFilter> globalFilters) {
 9: 		this.globalFilters = loadFilters(globalFilters);
10: 	}
11: 
12: 	private static List<GatewayFilter> loadFilters(List<GlobalFilter> filters) {
13: 		return filters.stream()
14: 				.map(filter -> {
15: 					GatewayFilterAdapter gatewayFilter = new GatewayFilterAdapter(filter);
16: 					if (filter instanceof Ordered) {
17: 						int order = ((Ordered) filter).getOrder();
18: 						return new OrderedGatewayFilter(gatewayFilter, order);
19: 					}
20: 					return gatewayFilter;
21: 				}).collect(Collectors.toList());
22: 	}
23: }

• 第 16 至 19 行 ：当 GlobalFilter 子类实现了 org.springframework.core.Ordered 接口，在委托一层 OrderedGatewayFilter 。这样 AnnotationAwareOrderComparator#sort(List) 方法好排序。
• 第 20 行 ：当 GlobalFilter 子类没有实现了 org.springframework.core.Ordered 接口，在 AnnotationAwareOrderComparator#sort(List) 排序时，顺序值为 Integer.MAX_VALUE 。
• 目前 GlobalFilter 都实现了 org.springframework.core.Ordered 接口。

3. GlobalFilter

org.springframework.cloud.gateway.filter.GlobalFilter ，全局过滤器接口，代码如下 ：

public interface GlobalFilter {

	/**
	 * 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);

}

• GlobalFilter 和 GatewayFilter 的 #filter(ServerWebExchange, GatewayFilterChain) 方法签名一致。官方说，未来的版本将作出一些调整。
  FROM 《Spring Cloud Gateway》
  This interface and usage are subject to change in future milestones
• GlobalFilter 会作用到所有的 Route 上。

GlobalFilter 实现类如下类图 ：

• RoutingFilter

• NettyRoutingFilter

• WebClientHttpRoutingFilter

• WebsocketRoutingFilter

• ForwardRoutingFilter

• 成对的 Filter

• NettyRoutingFilter / NettyRoutingFilter

• WebClientHttpRoutingFilter / WebClientWriteResponseFilter

梳理 GlobalFilter 的顺序如下 ：

TODO 【3030】

4. GatewayFilterChain

org.springframework.cloud.gateway.filter.GatewayFilterChain ，网关过滤器链接口。代码如下 ：

public interface GatewayFilterChain {

	/**
	 * 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);

}

• 从接口方法可以看到，和 javax.servlet.FilterChain 类似。

org.springframework.cloud.gateway.handler.FilteringWebHandler.GatewayFilterAdapter ，网关过滤器链默认实现类。代码如下 ：

private static class DefaultGatewayFilterChain implements GatewayFilterChain {

	private int index;
	private final List<GatewayFilter> filters;

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

	@Override
	public Mono<Void> filter(ServerWebExchange exchange) {
		if (this.index < filters.size()) {
			GatewayFilter filter = filters.get(this.index++);
			return filter.filter(exchange, this);
		} else {
			return Mono.empty(); // complete
		}
	}
}