Possible Causes:
1. Path access error: as follows: Servlet page should be accessed instead of .jsp.
Reason: The .jsp page only forms a frame for the table, that is, the appearance; the servlet obtains the data and then transfers the data back to the .jsp for display, so it is said to access the servlet instead of the .jsp.
2. The backend code did not get the id. For example: when querying, modifying and deleting based on the id, the id needs to be obtained at the back end. If the id is not obtained, it means that the sql statement is invalid and nothing can be searched.
As follows: When querying detailed information based on id, the id needs to be obtained.
3. The variable name on the jsp page is inconsistent with the variable name in the back-end code. Need to be consistent
4. No data is stored in the list
