主键生成策略采用sequence，但没有指定具体的sequence的时候，Hibernate会访问一个默认的sequence，是hibernate_sequence，我们也需要在数据库中建立这个sequence
xml文件配置，采用默认seq
<id name="pkId" type="java.math.BigDecimal">
<column name="PK_ID" precision="22" scale="0" />
<generator class="sequence" />
</id>
采用指定eeq
<generator class="sequence" >
<param name="sequence">seq_t_user</param>
</generator>
注解配置seq
@SequenceGenerator (name = "SEQ", sequenceName = "SEQ_T_CMP_CITY", allocationSize = 1)
@Id
@GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "SEQ")
@Column(name = "id")
public Integer getId() {
return id ;
}