如何在Hibernate查询中将Integer参数设置为null？

I've got an entity:

我有一个实体：

    @Entity
    @Table(name = "smsc.security_users")
    public class User {

        @Id
        @Column(name = "id")
        private int id;

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

        @Column(name = "password")
        private String password;

        @Column(name = "enabled")
        private int enabled;

        @Column(name = "master_id", nullable = true)
        private Integer master_id;

        @Temporal(TemporalType.TIMESTAMP)
        @Column(name = "passwordExpiration")
        private Date passwordExpiration;

        public String getUsername() {
            return username;
        }
        ...
    @Entity