@NotNull: Checks whether the value is not null, disregarding the content

@NotEmpty: Checks whether the value is not null nor empty. If it has just empty spaces, it will allow it as not empty.

@NotBlank: Checks whether the value is not null nor empty, trimming the value first. It means that, it won’t allow just empty spaces.

Here are a few examples:

1. String name = null;
   @NotNull: false
   @NotEmpty: false
   @NotBlank: false

2. String name = "";
   @NotNull:true
   @NotEmpty: false
   @NotBlank: false

3. String name = " ";
   @NotNull:true
   @NotEmpty:true
   @NotBlank: false

4. String name = "Great answer!";
   @NotNull:true
   @NotEmpty:true
   @NotBlank:true