Data Validation Overview

What is Validation?

• Validation ensures data integrity by asserting conditions against data.
• Examples:
  • Is a value required?
  • What is the maximum length of a string?
  • Is a given date valid?
• Often referred to as “defensive programming” or “trust but verify.”
• An essential but often overlooked step in software development.

When to Validate?

• Validate data early and often!
• Validation should occur at every exchange:
  • UI level with user-friendly feedback.
  • RESTful API level before reaching the service layer.
  • Before persisting data into a database.
  • Database constraints also enforce validation.
• Early validation prevents difficult error handling later.

Java Bean Validation

• Java Bean Validation is a standardized Java API for validation.
• Provides a structured way to validate data and handle errors.
• More elegant than writing custom validation logic.
• Bean Validation is an API, requiring an implementation.
• Fun Fact: Gunnar Morling, founder of MapStruct, leads the Bean Validation API and contributes to Hibernate Validator.

Jakarta Bean Validation 3.0

• Released in July 2020.
• Renamed from “Bean Validation” to “Jakarta Bean Validation.”
• The primary change from 2.0 to 3.0 was package renaming:
  • 2.0: javax.validation
  • 3.0: jakarta.validation
• Supported in Spring Framework 6.x+.
• Hibernate Validator 7.x+ is a common implementation.

Built-In Constraint Definitions

• @Null - Checks if value is null.
• @NotNull - Ensures value is not null.
• @AssertTrue - Value must be true.
• @AssertFalse - Value must be false.
• @Min(value) - Checks if number is equal to or greater than value.
• @Max(value) - Checks if number is equal to or less than value.
• @Size(min, max) - Checks if a string or collection size is within limits.
• @Pattern(regex) - Validates against a regular expression.
• @NotEmpty - Ensures string/collection is neither null nor empty.
• @Email - Validates email format.
• And many more…

Hibernate Validator Constraints

• @CreditCardNumber - Validates credit card format.
• @Currency - Ensures valid currency amount.
• @ISBN - Checks for valid ISBN.
• @Range(min, max) - Validates if number falls within range.
• @URL - Ensures a valid URL format.
• @UniqueElements - Ensures a collection has unique elements.
• Several additional domain-specific constraints exist.

Validation and Spring Framework

• Spring Framework integrates seamlessly with Bean Validation.
• Can be used in controllers, services, and Spring-managed components.
• Spring MVC automatically returns 400 Bad Request for validation failures.
• Spring Data JPA enforces validation at persistence level.

Spring Boot and Validation

• Spring Boot auto-configures validation if an implementation is on the classpath.
• If only the API is present (without an implementation), validation annotations exist but do nothing.
• Spring Boot 2.3+ requires manually adding the validation starter dependency.
• Example:

  <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-validation</artifactId>
  </dependency>