Creating Custom Constraints

Bean Validation defines annotations, interfaces, and classes to allow developers to create custom constraints.

Using the Built-In Constraints To Make a New Constraint

Bean Validation includes several built-in constraints that can be combined to create new, reusable constraints. This can simplify constraint definitions by allowing developers to define a custom constraint made up of several built-in constraints that may then be applied to component attributes with a single annotation.

Example 32–1 The @Email Constraint

@Pattern.List({
    @Pattern(regexp = "[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\\."
        +"[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*"
        +"@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?")
})
@Constraint(validatedBy = {})
@Documented
@Target({ElementType.METHOD,
    ElementType.FIELD,
    ElementType.ANNOTATION_TYPE,
    ElementType.CONSTRUCTOR,
    ElementType.PARAMETER})
@Retention(RetentionPolicy.RUNTIME)
public @interface Email {

    String message() default "{invalid.email}";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

    @Target({ElementType.METHOD,
        ElementType.FIELD,
        ElementType.ANNOTATION_TYPE,
        ElementType.CONSTRUCTOR,
        ElementType.PARAMETER})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @interface List {
        Email[] value();
    }
}

This custom constraint can then be applied to an attribute.

...
@Email
protected String email;
...