Common mistake one:
The path configured in url-pattern does not start with /
Common mistake two:
The content of the servlet-name in the servlet-mapping tag does not correspond to the value of the servlet-name in the previous servlet tag (the idea will be automatically marked in red)
Error:
Common mistake three:
The class in the servlet-class in the servlet tag does not correspond to it
This kind of error does not report an error when it runs, and it will be wrong as soon as it is accessed
