springcloud-gateway的spi加载SlotChainBuilder问题

[jasonliu459]

问题描述：

使用springcloud-gateway+sentinel，引入

 <dependency>
     <groupId>com.alibaba.cloud</groupId>
     <artifactId>spring-cloud-starter-alibaba-sentinel</artifactId>
 </dependency>
 <dependency>
     <groupId>com.alibaba.csp</groupId>
     <artifactId>sentinel-spring-cloud-gateway-adapter</artifactId>
</dependency>

发现两者顺序对调，会影响网关流控是否生效，sentinel-spring-cloud-gateway-adapter在前会生效，在后则不会生效，经过分析源码，是由于两者都引入了

 <dependency>
   <groupId>com.alibaba.csp</groupId>
   <artifactId>sentinel-parameter-flow-control</artifactId>
 </dependency>

flow-control的SlotChainBuilder的spi是com.alibaba.csp.sentinel.slots.HotParamSlotChainBuilder
而gateway-adapter的SlotChainBuilder的spi是com.alibaba.csp.sentinel.adapter.gateway.common.slot.GatewaySlotChainBuilder，两者差了关键的：

chain.addLast(new GatewayFlowSlot());

继续分析，发现原因是

SlotChainProvider.newSlotChain()
...
slotChainBuilder = SpiLoader.loadFirstInstanceOrDefault(SlotChainBuilder.class, DefaultSlotChainBuilder.class);
...

使用了SpiLoader.loadFirstInstanceOrDefault方法，所以获取到第一个非default的builder时就直接返回了，如果gateway-adapter在后面，则GatewaySlotChainBuilder不会被加载到，这里的加载方式改用SpiLoader.loadHighestPriorityInstance并且设置Builder的优先级会好点

[cdfive]

Refer to #1216, #1306
It has been improved via #411, may try latest version 1.8.1

[jasonliu459]

。