spring cloud gateway自定义负载均衡

Spring Cloud Gateway支持多种负载均衡策略，例如随机、轮询、权重等。如果现有的负载均衡策略不能满足你的需求，你可以自定义负载均衡策略。

首先，你需要实现`org.springframework.cloud.client.loadbalancer.reactive.LoadBalancer`接口来定义你的负载均衡策略。然后，你需要创建一个`org.springframework.cloud.gateway.filter.factory.rewrite.RewriteFunction`实例，用于将服务的URI重写为负载均衡的服务实例地址。最后，你需要将这个自定义的负载均衡策略应用到Spring Cloud Gateway的路由规则中。

以下是一个示例，展示了如何定义一个基于特定请求头的自定义负载均衡策略：

public class CustomLoadBalancer implements LoadBalancer<ServiceInstance> {

    private final String headerName;

    public CustomLoadBalancer(String headerName) {
        this.headerName = headerName;
    }

    @Override
    public Mono<Response<ServiceInstance>> choose(Request request) {
        Object headerValue = request.headers().getFirst(headerName);
        String serviceName = "my-service";
        // 根据请求头的值选择服务实例
        ServiceInstance serviceInstance = ...;
        return Mono.just(new DefaultResponse(serviceInstance));
    }
}

public class CustomLoadBalancerGatewayFilterFactory extends AbstractGatewayFilterFactory<CustomLoadBalancerGatewayFilterFactory.Config> {

    public CustomLoadBalancerGatewayFilterFactory() {
        super(Config.class);
    }

    @Override
    public GatewayFilter apply(Config config) {
        LoadBalancer<ServiceInstance> loadBalancer = new CustomLoadBalancer(config.getHeaderName());
        RewriteFunction<String, String> rewriteFunction = uri -> {
            // 将URI重写为负载均衡的服务实例地址
            ServiceInstance serviceInstance = loadBalancer.choose(Request.create("", new HttpHeaders())).block().getServer();
            return "http://" + serviceInstance.getHost() + ":" + serviceInstance.getPort() + uri;
        };
        return new RewritePathGatewayFilterFactory().apply(new RewritePathGatewayFilterFactory.Config().setRewriteFunction(rewriteFunction));
    }

    public static class Config {
        private String headerName;

        public String getHeaderName() {
            return headerName;
        }

        public void setHeaderName(String headerName) {
            this.headerName = headerName;
        }
    }
}

在上面的示例中，`CustomLoadBalancer`实现了`LoadBalancer`接口，并基于特定的请求头选择服务实例。`CustomLoadBalancerGatewayFilterFactory`则将`CustomLoadBalancer`应用到Spring Cloud Gateway的路由规则中，并将服务的URI重写为负载均衡的服务实例地址。最后，你可以在路由规则中使用`CustomLoadBalancerGatewayFilterFactory`来定义自定义的负载均衡策略。

spring:
  cloud:
    gateway:
      routes:
        - id: my-route
          uri: http://my-service
          filters:
            - CustomLoadBalancer=my-header