Understanding the Differences and Proper Use of @NotNull, @NotBlank, and @NotEmpty

When using Bean Validation, the author discovered that validations were being skipped, prompting a review of the @NotNull usage and leading to a clarification of the differences among @NotNull, @NotBlank, and @NotEmpty.

The three annotations belong to the javax.validation.constraints package. They allow developers to replace repetitive manual checks with declarative constraints, making the code more concise.

@NotNull

Applicable primarily to primitive wrapper types such as Long, Integer, and Double. When placed on a String, the value must not be null, but an empty string ("") is permitted.

@NotBlank

Designed for String fields. The annotated value cannot be null and, after trim(), its length must be greater than zero; therefore both null and an empty string are rejected.

@NotEmpty

Can be used on String, Collection, array, Map, and similar containers. The value must not be null and its size or length must be greater than zero.

To activate these constraints, the method parameter must also be annotated with @Validated; otherwise the annotations have no effect.

The article concludes with an invitation for readers to comment and share their experiences.