custom validation

hi, i'm pretty new in roo and have some trouble with validation. Here my test project:

Code:

---

project --topLevelPackage org.dischi --projectName dischi
jpa setup --database MYSQL --provider HIBERNATE
entity jpa --class ~.model.Album --testAutomatically
field string --fieldName autore --notNull
field string --fieldName titolo --notNull
field number --fieldName anno --type int
field number --fieldName prezzo --type float
web mvc setup
web mvc scaffold --class ~.web.AlbumController --backingType ~.model.Album

---

the modified Album model

Code:

---

...
@RooJavaBean
@RooToString
@RooJpaActiveRecord
public class Album {

    @NotNull
    private String autore;

    @NotNull
    private String titolo;

    private int anno;

    private float prezzo;
   
    @AssertTrue(message = "invalid year.")
    public boolean isAnnoValid() {
            if ((anno >1999) && (anno < 2021) )
                    return true;
            else
                    return false;           
    }
}

---

the model is validated but the message ("invalid year") does not appear.
where am i wrong?
Thanks in advance