A brief description of the reference service process on the dubbo consumer side

1.解析<dubbo:reference /> 

2. Get the registration center connection and subscribe to the message center

3. Assemble the invoker according to the notification information of the subscription center

4. Wrap the invoker and package RegistryDirectory --> failoverCulsterInvoker --> MockClusterInvoker

5. The proxy class that generates the invoker warpoer object

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