Refactor a clean and testable dubbo

Refactor a clean and testable dubbo, to unify the code style:
1. use guard clause #10267
2. no more than 2 depth if else
3. no more than 2 depth loop
4. avoid too big method
5. use final
6. remove useless pieces
7. use meaningful names
8. testcase coverage
9. small pieces and steps

Please add other things.