RoutePredicateHandlerMapping#getHandlerInternal
webflux的入口是DispatcherHandler，他这里会调用HandlerMapping的getHandler方法。AbstractHandlerMapping实现了HandlerMapping接口，他有个抽象方法getHandlerInternal需要子类实现。
RoutePredicateHandlerMapping继承了AbstractHandlerMapping，所以我们的重点就是他的getHandlerInternal方法。这里面就是用来处理是否有对应的Route。
这个方法，其实主要调用lookupRoute方法。
protected Mono> getHandlerInternal(ServerWebExchange exchange) {
// don’t handle requests on management port if set and different than server port
if (this.managementPortType == DIFFERENT && this.managementPort != null
&& exchange.getRequest().getURI().getPort() == this.managementPort) {
return Mono.empty();
}
exchange.getAttributes().put(GATEWAY_HANDLER_MAPPER_ATTR, getSimpleName());
return lookupRoute(exchange)
// .log(“route-predicate-hand