In Hibernate Validator 4.1+, what is the difference between @NotNull, @NotEmpty, and @NotBlank?

@NotNull: The CharSequence, Collection, Map or Array object is not null, but can be empty.
@NotEmpty: The CharSequence, Collection, Map or Array object is not null and size > 0.
@NotBlank: The string is not null and the trimmed length is greater than zero.

To help you understand, let's look into how these constraints are defined and carried out (I'm using version 4.1):

1. The @NotNull constraint is defined as:

   @Constraint(validatedBy = {NotNullValidator.class})  
   

   This class has an isValid method defined as:

   public boolean isValid(Object object, ConstraintValidatorContext constraintValidatorContext) {
    return object != null;  
   }
   

2. The @NotEmpty constraint is defined as:

   @NotNull  
   @Size(min = 1)    
   

   So this constraint uses the @NotNull constraint above, and @Size whose definition differs based on the object but should be self explanitory.

3. Finally, the @NotBlank constraint is defined as:

   @NotNull  
   @Constraint(validatedBy = {NotBlankValidator.class})        
   

   So this constraint also uses the @NotNull constraint, but also constrains with the NotBlankValidator class. This class has an isValid method defined as:

   if ( charSequence == null ) {  //curious 
     return true;   
   }   
   return charSequence.toString().trim().length() > 0;  
   

   Interestingly, this method returns true if the string is null, but false if and only if the length of the trimmed string is 0. It's ok that it returns true if it's null because, as I mentioned, the @NotEmpty definition also requires @NotNull.

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

I liked the explanation in the below link: http://www.itprogrammingtutorials.com/2015/java/hibernate/hibernate-validator-diff-notblank-notempty/

@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.

So, if you want to validate that a field is not null but also that it doesn’t has just empty spaces, but text, you should use @NotBlank.

1. @NotNull: a constrained CharSequence, Collection, Map, or Array is valid as long as it’s not null, but it can be empty
2. @NotEmpty: a constrained CharSequence, Collection, Map, or Array is valid as long as it’s not null and its size/length is greater than zero.
3. @NotBlank: a constrained String is valid as long as it’s not null and the trimmed length is greater than zero.