What is Bean Validation?

In the context of Java EE (now Jakarta EE) and Spring Framework, Bean Validation is a mechanism for validating data within Java beans (POJOs - Plain Old Java Objects). It’s a declarative way to express validation constraints on the fields of your beans.

Bean Validation works through annotations that you place on fields, getter methods, or class level. These annotations define the constraints that data must satisfy. Some common constraints include:

• @NotNull: Specifies that the value cannot be null.
• @Size: Specifies the size constraints for a string, collection, or array.
• @Pattern: Specifies a regular expression pattern that the value must match.
• @Min and @Max: Specify minimum and maximum values for numeric fields.
• @Email: Specifies that the value must be a well-formed email address.

When you use Bean Validation, you can define these constraints directly on your bean classes. Then, when you pass instances of these beans to the validation framework, it automatically validates them according to the constraints you’ve defined.

For example, if you have a User class with fields like username, email, and password, you can use Bean Validation annotations to ensure that these fields meet certain criteria, such as not being empty, being a valid email format, or having a password of a certain length.

Bean Validation is particularly useful in web applications for validating user input before processing it further. It helps to ensure data integrity and maintain consistency in your application. Additionally, it promotes cleaner code by centralizing validation logic and making it easier to understand and maintain.