Javax @NotNull annotation usage

To activate parameter validation, simply annotate the class with @Validated

import org.springframework.validation.annotation.Validated;

From The Java EE 6 Tutorial:

The Bean Validation model is supported by constraints in the form of annotations placed on a field, method, or class of a JavaBeans component, such as a managed bean.

You should place your validation of a field related to a declared bean, something like this:

@Entity
@Table(name="users")
public class BackgammonUser {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Long userId;

    @Column(name="username")
    @NotBlank
    private String userName;

    @NotBlank
    private String password;

    @NotNull
    private Boolean enabled;
}

The BackgammonUser is considered to be a bean.