Method Constraints are declared with annotations on the method argument. The annotation itself is custom, and it is possible to make your own.

public interface Dialer
{
    void callPhoneNumber(@PhoneNumber String phoneNo);

}

In the code above we say that we want the argument to the callPhoneNumber() method to be a valid phone number. This annotation is not built-in, so we need to declare it.

@ConstraintDeclaration
@Retention( RetentionPolicy.RUNTIME )
@Target( { ElementType.PARAMETER, ElementType.ANNOTATION_TYPE, ElementType.METHOD } )
public @interface PhoneNumber
{
}

We then need to provide the Constraint implementation.

public class PhoneNumberConstraint
        implements Constraint<PhoneNumber, String>
{
    public boolean isValid( PhoneNumber annotation, String number )
    {
        boolean validPhoneNumber = true; // check phone number format...
        return validPhoneNumber;  // return true if valid phone number.
    }
}

We also need to include the Constraint on the Composites we want to have them present.

@Constraints( PhoneNumberConstraint.class )
public interface DialerComposite extends Dialer
{
}

If a Constraint is violated, then a ConstraintViolationException is thrown. The Exception contains ALL violations found in the method invocation. Concerns can be used to catch and report these violations.

public class ParameterViolationConcern extends ConcernOf<InvocationHandler>
    implements InvocationHandler
{
    public Object invoke( Object proxy, Method method, Object[] args )
        throws Throwable
    {
        try
        {
            return next.invoke( proxy, method, args );
        }
        catch( ConstraintViolationException e )
        {
            for( ValueConstraintViolation violation : e.constraintViolations() )
            {
                String name = violation.name();
                Object value = violation.value();
                Annotation constraint = violation.constraint();
                report( name, value, constraint );
            }
            throw new IllegalArgumentException("Invalid argument(s)", e);
        }
    }

      [...snip...]

    private void report( String name, Object value, Annotation constraint )
    {
    }
}

Property Constraints are declared on the Property method.

public interface HasPhoneNumber
{
    @PhoneNumber
    Property<String> phoneNumber();
}

In this case, the Constraint associated with the phoneNumber() method, will be called before the set() method on that Property is called. If there is a constraint violation, the Exception thrown will be part of the caller, and not the composite containing the Property, so a reporting constraint on the containing Composite will not see it. If you want the containing Composite to handle the Constraint Violation, then you need to add a Concern on the Property itself, which can be done like this;

public abstract class PhoneNumberParameterViolationConcern extends ConcernOf<HasPhoneNumber>
    implements HasPhoneNumber
{
    @Concerns( CheckViolation.class )
    public abstract Property<String> phoneNumber();

    private abstract class CheckViolation extends ConcernOf<Property<String>>
        implements Property<String>
    {
        public void set( String number )
        {
            try
            {
                next.set( number );
            }
            catch( ConstraintViolationException e )
            {
                Collection<ValueConstraintViolation> violations = e.constraintViolations();
                report( violations );
            }
        }

          [...snip...]

        private void report( Collection<ValueConstraintViolation> violations )
        {
        }
    }
}