hibernate的inverse作用

举例如下
Customer类：
Java代码
public class Customer {
private int id;
private String name;
private Set orders = new HashSet();
……}
public class Customer {
private int id;
private String name;
private Set orders = new HashSet();
……}

即Customer类具有一个set集合属性orders，其中Order是一个普通的类：
Java代码
public class Order {
private int id;
private String orderName;
……}
public class Order {
private int id;
private String orderName;
……}

数据库中表的结构：
Java代码
t_customer: 两个字段：id name
t_order: 三个字段：id orderName customerid
t_customer: 两个字段：id name
t_order: 三个字段：id orderName customerid

Customer类的映射文件：Customer.hbm.xml (Order类的映射文件忽略)
Java代码
<hibernate-mapping>
<class name=”test.Customer” table=”t_customer” lazy=”false”>
<id name=”id”>
<generator class=”native”/>
</id>
<property name=”name”/>
<set name=”orders” cascade=”save-update” lazy=”false”>
<key column=”customerid”/>
<one-to-many class=”test.Order”/>
</set>
</class>
</hibernate-mapping>
<hibernate-mapping>
<class name=”test.Customer” table=”t_customer” lazy=”false”>
<id name=”id”>
<generator class=”native”/>
</id>
<property name=”name”/>
<set name=”orders” cascade=”save-update” lazy=”false”>
<key column=”customerid”/>
<one-to-many class=”test.Order”/>
</set>
</class>
</hibernate-mapping>

执行如下代码：
Java代码
Set orders = new HashSet();

Order o1 = new Order();
o1.setOrderName(“o1″);
Order o2 = new Order();
o2.setOrderName(“o2″);
orders.add(o1);
orders.add(o2);

Customer c = new Customer();
c.setName(“aaa”);
c.setOrders(orders);

session.save(c);
Set orders = new HashSet();

Order o1 = new Order();
o1.setOrderName(“o1″);
Order o2 = new Order();
o2.setOrderName(“o2″);
orders.add(o1);
orders.add(o2);

Customer c = new Customer();
c.setName(“aaa”);
c.setOrders(orders);

session.save(c);

此时Hibernate发出的sql语句如下：
Java代码
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName) values (?)
Hibernate: insert into t_order (orderName) values (?)
Hibernate: update t_order set customerid=? where id=?
Hibernate: update t_order set customerid=? where id=?
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName) values (?)
Hibernate: insert into t_order (orderName) values (?)
Hibernate: update t_order set customerid=? where id=?
Hibernate: update t_order set customerid=? where id=?

查看数据库：
Java代码
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 1
2 o2 1
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 1
2 o2 1

保存Customer对象时，首先发出insert into t_customer (name) values (?)语句将c同步到数据库，由于在<set>映射中设置cascade=”save-update”，所以会同时保存orders集合中的Order类型的o1,o2对象（如果没有这个设置，即cascade=”save-update”），那么Hibenrate不会自动保存orders集合中的对象，那么在更新时将会抛出如下异常：
Java代码
Hibernate: insert into t_customer (name) values (?)
Hibernate: update t_order set customerid=? where id=?
org.hibernate.TransientObjectException: test.Order
……
Hibernate: insert into t_customer (name) values (?)
Hibernate: update t_order set customerid=? where id=?
org.hibernate.TransientObjectException: test.Order
……

抛出这一异常的原因是：<set>映射默认”inverse=fasle”即由Customer对象作为主控方，那么它要负责关联的维护工作，在这里也就是负责更新t_order表中的customerid字段的值，但由于未设置cascade=”save-update”，所以orders集合中的对象不会在保存customer时自动保存，因此会抛出异常(如果未设置，需要手动保存)。
现在设置cascade=”save-update”，同时设置inverse=”true”,即：
Java代码
……
<set name=”orders” cascade=”save-update” inverse=”true” lazy=”false”>
<key column=”customerid”/>
<one-to-many class=”test.Order”/>
</set>
……
<set name=”orders” cascade=”save-update” inverse=”true” lazy=”false”>
<key column=”customerid”/>
<one-to-many class=”test.Order”/>
</set>
……

同样执行上述代码，发出如下语句：
Java代码
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName) values (?)
Hibernate: insert into t_order (orderName) values (?)
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName) values (?)
Hibernate: insert into t_order (orderName) values (?)

相比上一次执行，少了两条update语句，查看数据库：
Java代码
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 NULL
2 o2 NULL
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 NULL
2 o2 NULL

发现t_order表中customerid的值为NULL,这是由于设置了inverse=”true”，它意味着
Customer不再作为主控方，而将关联关系的维护工作交给关联对象Orders来完成。在保存Customer时，Customer不在关心Orders的customerid属性，必须由Order自己去维护，即设置order.setCustomer(customer);
如果需要通过Order来维护关联关系，那么这个关联关系转换成双向关联。
修改Order类代码：
Java代码
public class Order {
private int id;
private String orderName;
private Customer customer;
……
}
public class Order {
private int id;
private String orderName;
private Customer customer;
……}

Order.hbm.xml：
Java代码
<hibernate-mapping>
<class name=”test.Order” table=”t_order”>
<id name=”id”>
<generator class=”native”/>
</id>
<property name=”orderName”/>
<many-to-one name=”customer” column=”customerid”/>
</class>
</hibernate-mapping>
<hibernate-mapping>
<class name=”test.Order” table=”t_order”>
<id name=”id”>
<generator class=”native”/>
</id>
<property name=”orderName”/>
<many-to-one name=”customer” column=”customerid”/>
</class>
</hibernate-mapping>

此时数据库中表的结构不会变化。
再次执行上述代码，发出如下sql语句：
Java代码
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)

发现在保存Order对象时为customerid字段赋值，因为Order对象中拥有Customer属性，对应customerid字段，查看数据库表：
Java代码
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 NULL
2 o2 NULL
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 NULL
2 o2 NULL

发现customerid的值仍为NULL,因为在上述代码中并未设置Order对象的Customer属性值，由于设置了inverse=”true”，所以Order对象需要维护关联关系，所以必须进行设置，即
order.setCustomer(customer);
修改上述代码为：
Java代码
……Customer c = new Customer();

Set orders = new HashSet();
Order o1 = new Order();
o1.setOrderName(“o1″);
o1.setCustomer(c);
Order o2 = new Order();
o2.setOrderName(“o2″);
o2.setCustomer(c);
orders.add(o1);
orders.add(o2);

c.setName(“aaa”);
c.setOrders(orders);

session.save(c);
……

……Customer c = new Customer();

Set orders = new HashSet();
Order o1 = new Order();
o1.setOrderName(“o1″);
o1.setCustomer(c);
Order o2 = new Order();
o2.setOrderName(“o2″);
o2.setCustomer(c);
orders.add(o1);
orders.add(o2);

c.setName(“aaa”);
c.setOrders(orders);

session.save(c);
……

执行上述代码，发出如下语句：
Java代码
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)
Hibernate: insert into t_customer (name) values (?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)
Hibernate: insert into t_order (orderName, customerid) values (?, ?)

查看数据库：
Java代码
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 1
2 o2 1
t_customer : t_order:

id | name id | orderName | customerid
1 aaa 1 o1 1
2 o2 1

发现已经设置了customerid的值。
在一对多关联中，在多的一方设置inverse=”true”,有助于性能的改善。通过上述分析可以发现少了update语句。