Validation

Mateu enforces standard Java Bean Validation annotations automatically. Rules defined in your ViewModel are reflected in the UI without any additional configuration.

Basic annotations

@NotBlank
String name;

@NotNull
Status status;

@Email
String contactEmail;

@Size(min = 2, max = 200)
String description;

@Min(0)
int stockCount;

Mateu uses these annotations to:

display validation errors next to the affected field
prevent form submission when validation fails
keep backend and UI validation consistent with a single source of truth

Triggering validation from an action

By default, actions run regardless of validation state. To require valid input before an action executes, use @Action(validationRequired = true):

@Button
@Action(validationRequired = true)
public URI create() {
    String id = productRepository.create(name, status);
    return URI.create("/products/" + id);
}

If any field fails validation, the method is not called. Errors appear next to the affected fields.

Full example

@UI("/products/new")
public class NewProductForm {

    @NotBlank(message = "Name is required")
    String name;

    @NotNull(message = "Status is required")
    @Stereotype(FieldStereotype.radio)
    Status status;

    @Size(max = 500)
    @Stereotype(FieldStereotype.textarea)
    String description;

    @Min(value = 0, message = "Stock cannot be negative")
    int stockCount;

    @Button
    @Action(validationRequired = true)
    public List<?> save() {
        String id = productRepository.create(name, status, description, stockCount);
        return List.of(
            new Message("Product created"),
            URI.create("/products/" + id)
        );
    }

}

Why this matters

In traditional architectures, validation is duplicated:

frontend validation (JavaScript)
backend validation (Java)

With Mateu, validation is defined once in Java and applied in both places. There is no risk of the two diverging.

Custom validators

Standard Bean Validation custom constraints work as expected. Any @Constraint annotation on a field is picked up automatically.

@ValidProductName   // custom constraint
String name;