hibernate inverse cascade详解

     Inverse和cascade是Hibernate映射中最难掌握的两个属性。两者都在对象的关联操作中发挥作用。
1.inverse属性：inverse所描述的是对象之间关联关系的维护方式。
inverse只存在于集合标记的元素中。Hibernate提供的集合元素包括<set/> <map/> <list/> <array /> <bag />
Inverse属性的作用是：是否将对集合对象的修改反映到数据库中。
inverse属性的默认值为false，表示对集合对象的修改会被反映到数据库中
inverse=”true” 表示对集合对象的修改不会被反映到数据库中。
<one-to-many>中inverse的属性的设置
例：Person中包含一个Child集合，Child包含一个Person对象。
要求：新建Person和Child对象，Child属于此Person对象
分析：在数据库中，就是新增一条Child记录
     //one-to-many的inverse的配置
    public void testInverse() {
       Qx qx = new Qx();
       qx.setQx("天山区");
       Jd jd = new Jd();
       jd.setJd("人民路");
       qx.getJds().add(jd);
       jd.setQx(qx);
       super.add(qx);
       super.add(jd);
}
配置Inverse=”false”，Hibernate的操作：
（1）Hibernate: insert into TBL_QX (qx) values (?)
（2）Hibernate: insert into TBL_JD (jd, qxid) values (?, ?)
（3）Hibernate: update TBL_JD set qxid=? where jdid=?
配置Inverse=”true”，Hibernate的操作
（1）Hibernate: insert into TBL_QX (qx) values (?)
（2）Hibernate: insert into TBL_JD (jd, qxid) values (?, ?)
可以看出：Inverse=”true”，SQL少执行一条更新操作（Java代码不会少）

<many-to-many>中inverse的属性的设置
在多对多的关系中，可以在关联的两边都不设置inverse属性（默认的false）,那么说明关联的双方都可以维护两个对象之间的关联关系。
注：在设置多对多关系的inverse属性时，不能两个对象都将其设置为true

2.cascade属性
cascade属性的作用是描述关联对象进行操作时的级联特性。因此，只有涉及到关系的元素才有cascade属性。
具有cascade属性的标记包括<many-to-one /> <one-to-one /> <any /> <set /><bag /> <idbag /> <list /> <array />
注意：<ont-to-many />和 <many-to-many />是用在集合标记内部的，所以是不需要cascade属性的。
级联操作：指当主控方执行某项操作时，是否要对被关联方也执行相同的操作。

3.inverse和cascade的区别
作用的范围不同：
     Inverse是设置在集合元素中的。
   Cascade对于所有涉及到关联的元素都有效。
   <many-to-one/><ont-to-many/>没有inverse属性，但有cascade属性
执行的策略不同
   Inverse 会首先判断集合的变化情况，然后针对变化执行相应的处理。
   Cascade 是直接对集合中每个元素执行相应的处理
执行的时机不同
     Inverse是在执行SQL语句之前判断是否要执行该SQL语句
     Cascade则在主控方发生操作时用来判断是否要进行级联操作
执行的目标不同
     Inverse对于<ont-to-many>和<many-to-many>处理方式不相同。
   对于<ont-to-many>，inverse所处理的是对被关联表进行修改操作。
   对于<many-to-many>，inverse所处理的则是中间关联表
     Cascade不会区分这两种关系的差别，所做的操作都是针对被关联的对象。

总结：
<one-to-many>中，建议inverse=”true”，由“many”方来进行关联关系的维护
<many-to-many>中，只设置其中一方inverse=”false”，或双方都不设置
Cascade，通常情况下都不会使用。特别是删除，一定要慎重。

说明约定-如：
1.表属性 :T1oo->id,name
          T2oo->id,avg,aid( 外键 ),version

2.代码过程是：Save->t1oo对象(T1oo 外键 T2oo)
        T1oo t1oo = new T1oo();
        t1oo.setName("xx");      
        t1oo.setT2ooSet(new HashSet());
        T2oo t2oo2 = new T2oo(24);
        //t2oo2.setT1oo(t1oo); (在下面的本用例表中的第3属性 )
        t1oo.getT2ooSet().add(t2oo2); (在下面的本用例表中的第2属性 )
        session.save(t1oo);

3.本例表使用：
T1oo.hbm.xml ->
        <set name="t2ooSet" inverse="true" cascade = "all" > (在下面的本用例表中的第1，2属性 )
            <key column="aid"/>
            <one-to-many class="T2oo"/>
        </set>
T2oo.hbm.xml ->
        <many-to-one name="t1oo" column="aid" class="T1oo" cascade="all" /> (在下面的本用例表中的第1，2属性 )
结合上面说明得表为：
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true | all   |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       | all   |          |          |
+-------+-------+-------+----------+----------+

4.执行 Hibernate语句 ：
    Hibernate: insert into t1oo (name, id) values (?, ?)
    Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
5.结果为：
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 2 | 24 | NULL |       0 | //主要就是看 aid属性
+----+-----+------+---------+
1 row in set (0.00 sec)


mysql> select * from t1oo; // 因为 T1oo总是可以 Save 下面就不再提了
+----+------+
| id | name |
+----+------+
| 2 | xx   |
+----+------+
1 row in set (0.00 sec)

可改项：
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | 可改4 | 可改2 |          |          |
+-------+-------+-------+           |   可改1 |
| t2oo |       | 可改3 |           |          |
+-------+-------+-------+----------+----------+


主本:
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true | all   |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       | all   |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 |    1 |       0 |
+----+-----+------+---------+
1 row in set (0.00 sec)


改1
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true | all   |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       | all   |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 | NULL|       0 |
+----+-----+------+---------+
1 row in set (0.00 sec)

改2
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true |       |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       | all   |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Empty set (0.00 sec)

改2 改1
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true |       |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       | all   |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Empty set (0.00 sec)


改3
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true | all   |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       |       |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 |    1 |       0 |
+----+-----+------+---------+
1 row in set (0.00 sec)

改3 改2
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true |       |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       |       |    |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Empty set (0.00 sec)


改3 改1
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true | all   |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       |       |    |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 | NULL |       0 |
+----+-----+------+---------+
1 row in set (0.00 sec)

改3 改2 改1
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | true |       |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       |       |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Empty set (0.00 sec)

改4
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false | all   |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       | all   |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
Hibernate: update t2oo set aid=? where id=?
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 |    1 |       0 |
+----+-----+------+---------+
1 row in set (0.02 sec)

改4 改1
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false | all   |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       | all   |    |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
Hibernate: update t2oo set aid=? where id=?
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 |    1 |       0 |
+----+-----+------+---------+
1 row in set (0.00 sec)

改4 改2
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false | |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       | all   |    |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: update t2oo set aid=? where id=?
Empty set (0.00 sec)

改4 改3
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false | all   |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       |       |    |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
Hibernate: update t2oo set aid=? where id=?
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 |    1 |       0 |
+----+-----+------+---------+
1 row in set (0.00 sec)

改4 改3 改2
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false |       |          |          |
+-------+-------+-------+ true    | true    |
| t2oo |       |       |    |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: update t2oo set aid=? where id=?
Empty set (0.00 sec)

改4 改1 改2
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false |       |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       | all   |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: update t2oo set aid=? where id=?
Empty set (0.00 sec)


改4 改1 改3
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false | all |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       |       |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: insert into t2oo (version, avg, aid, id) values (?, ?, ?, ?)
Hibernate: update t2oo set aid=? where id=?
mysql> select * from t2oo;
+----+-----+------+---------+
| id | avg | aid | version |
+----+-----+------+---------+
| 1 | 24 |    1 |       0 |
+----+-----+------+---------+
1 row in set (0.02 sec)

改4 改3 改2 改1
+-------+-------+-------+----------+----------+
|hbn.xml|inverse|cascade|t1oo->t2oo|t2oo->t1oo|
+-------+-------+-------+----------+----------+
| t1oo | false |       |          |          |
+-------+-------+-------+ true    | false   |
| t2oo |       |       |          |          |
+-------+-------+-------+----------+----------+
Hibernate: insert into t1oo (name, id) values (?, ?)
Hibernate: update t2oo set aid=? where id=?
Empty set (0.00 sec)