I have encountered this error several times, and then sorted out three reasons (there may be other reasons, but all I have encountered so far are these three)
The first is that the implementation class of the service layer does not have a return method.
This kind of error cannot be seen when the service is started. There is no error message. The
default is null, so there may be omissions and forget to write.
The second is in the interface path There is no / { id } in
it, which is also an easy-to-find error report.
The third type is a problem with the dynamic SQL statement of the Mapper file.
The Mapper file error report is an error report that is easy to occur for people who do not have a deep understanding of the database, field names, mappings and data type wrapper classes. Here is a brief example
