Hi Toan,
The code you are using has a dependency with the Community Commons module, available in the Marketplace. Actually, the StringSplit java action is provided in this module already and is ready to use:
So you don't have to write your own Java action, which I would recommend to improve maintainability.
If you really want to solve the errors in the Java code, you should remove any references to community commons (starting with the import) and introduce your own entity to return the list of split items. As this requires hard-coding the entity name and attribute name, it's prone to errors when renaming the entity or attribute. Therefore, I recommend to use the CommunityCommons solution.
Good luck!
Johan