Notes when submitting blank fields through HTTP

If a text field is blank, most browsers submit an empty parameter. However, if a selection box
Is blank, and the browser does not submit anything at all. The application must be determined when the browser does not submit the selection box,
It must be false or null. In practice, this applies only to the selection box, but the specification allows the browser to ignore any blank fields.

When writing a servlet that receives an HTTP request, you must determine whether the data submitted in the selection box is null.
It is just a judgment on whether it is empty.