1.@NotNull: cannot be null, but can be an empty string
2.@NotEmpty: cannot be null, and the length must be greater than 0
3.@NotBlank: Can only be used on String, cannot be null, cannot be an empty string (remove the leading and trailing spaces), and after calling trim(), the length must be greater than 0
4.@length(min=**, max=**): set the minimum length and maximum length
5.@Email: Verify email format