Hibernate关联关系 收藏 一, 一对多的单向关联关系 首先先建立Customer和Order类,代码如下: Customer类: package mypack; import java.io.Serializable; import org.apache.commons.lang.builder.ToStringBuilder; public class Customer implements Serializable { private Long id; private String name; public Customer(String name) { this.name = name; } public Customer() { } public Long getId() { return this.id; } public void setId(Long id) { this.id = id; } public String getName() { return this.name; } public void setName(String name) { this.name = name; } public String toString() { return new ToStringBuilder(this) .append("id", getId()) .toString(); } } Order类: package mypack; import java.io.Serializable; import org.apache.commons.lang.builder.ToStringBuilder; public class Order implements Serializable { private Long id; private String orderNumber; private mypack.Customer customer; public Order(String orderNumber, mypack.Customer customer) { this.orderNumber = orderNumber; this.customer = customer; } public Order() { } public Order(mypack.Customer customer) { this.customer = customer; } public Long getId() { return this.id; } public void setId(Long id) { this.id = id; } public String getOrderNumber() { return this.orderNumber; } public void setOrderNumber(String orderNumber) { this.orderNumber = orderNumber; } public mypack.Customer getCustomer() { return this.customer; } public void setCustomer(mypack.Customer customer) { this.customer = customer; } public String toString() { return new ToStringBuilder(this) .append("id", getId()) .toString(); } } Customer和Order类的配置文件分别是Customer.hbm.xml和Order.hbm.xml Customer.hbm.xml如下: <?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 2.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd"> <hibernate-mapping > <class name="mypack.Customer" table="CUSTOMERS" > <id name="id" type="long" column="ID"> <generator class="increment"/> </id> <property name="name" type="string" > <column name="NAME" length="15" /> </property>
</class> </hibernate-mapping> Order.hbm.xml如下: <?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 2.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd"> <hibernate-mapping > <class name="mypack.Order" table="ORDERS">
<id name="id" type="long" column="ID"> <generator class="increment"/> </id>
<property name="orderNumber" type="string" > <column name="ORDER_NUMBER" length="15" /> </property> <many-to-one name="customer" column="CUSTOMER_ID" class="mypack.Customer" not-null="true" /> </class> </hibernate-mapping> 在Order.hbm.xml中<many-to-one>元素建立了Customer和ORDERS表的外键CUSTOMER_ID之间 的映射。 它包括如下属性。 ● name:设定待映射的持久化类的属性名,此处为Order类的customer属性。 ● column:设定和持久化类的属性对应的表的外键,此处为ORDERS表的外键CUSTOMER_ID。 ● class:设定持久化类的属性的类型,此处设定customer属性为Customer类。 ● not-null:如果为ture,表示customer属性不能为null,默认为false。 接下来我们持久化一个Customer对象以及和他关联的Order对象: (以下代码并非是完整代码,只是简单把逻辑列出)
Configuration config = new Configuration(); config.addClass(Customer.class) . addClass(Order.class); sessionFactory = config.buildSessionFactory(); Session session = sessionFactory.openSession(); Transaction tx = null; tx = session.beginTransaction(); Customer customer=new Customer("Jack"); session.save(customer); Order order1=new Order("Jack_Order001",customer); Order order2=new Order("Jack_Order002",customer); session.save(order1); session.save(order2); tx.commit(); 当我们要持久化一个Customer对象以及和他关联的Order对象的时候一定要先建立Customer对象并持久化它,否则当创建Order持久对象的时候会抛出异常,如果把session.save(customer) 注销将会在session.save(order1)抛出PropertyValueException异常。分析产生异常原因:在调用session.save(order1)方法之前,order1和customer对象都是临时对象,临时对象就是刚刚用new创建出来,但是还没有持久会的对象,而Hibernate不会自动持久化order1关联的customer对象,在数据库中意味着仅仅向ORDERS表中插入了一条记录,并且该记录的CUSTOMER_ID字段为null,违反了数据库完整性约束,因为不允许ORDERS表的CUSTOMER_ID字段为null。 从上面的例子可以看出当Hibernate持久化一个临时对象时,在默认情况下,它不会自动持久化所关联的其他临时对象,所以会抛出PropertyValueException异常。如果希望当Hibernate持久化Order对象时自动持久化所关联的Customer对象,可以把<many-to-one>的cascade属性设为“save-update”,cascade属性默认为“none”:
<many-to-one name="customer" column="CUSTOMER_ID" class="mypack.Customer" cascade=“save-update” not-null="true" /> 这样就能够在Hibernate持久化Order对象时自动持久化所关联的Customer对象了。 二, 一对多的双向关联关系 我们依然使用前面的Customer和Order的例子,由于是双向关联,Order类代码如上面一样,由于Customer类中有Order的关联对象,因此Customer类改为如下: package mypack; import java.io.Serializable; import java.util.Set; import org.apache.commons.lang.builder.ToStringBuilder; public class Customer implements Serializable { private Long id; private String name; private Set orders; public Customer(String name, Set orders) { this.name = name; this.orders = orders; } public Customer() { } public Customer(Set orders) { this.orders = orders; } public Long getId() { return this.id; } public void setId(Long id) { this.id = id; } public String getName() { return this.name; } public void setName(String name) { this.name = name; } public Set getOrders() { return this.orders; } public void setOrders(Set orders) { this.orders = orders; } public String toString() { return new ToStringBuilder(this) .append("id", getId()) .toString(); } } 如何在映射文件中映射集合类型的order属性呢?由于在CUSTOMERS表中没有直接与order属性对应的字段,因此不能用<property>元素来映射order属性,而是要使用<set>元素: <?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 2.0//EN" "http://hibernate.sourceforge.net/hibernate-mapping-2.0.dtd"> <hibernate-mapping > <class name="mypack.Customer" table="CUSTOMERS" > <id name="id" type="long" column="ID"> <generator class="increment"/> </id> <property name="name" type="string" > <column name="NAME" length="15" /> </property> <set name="orders" cascade="save-update"
>
<key column="CUSTOMER_ID" /> <one-to-many class="mypack.Order" /> </set> </class> </hibernate-mapping> <set>元素还包含两个子元素:<key>和<one-to-many>,<one-to-many>元素设定所关联的持久化类,此处为Order类,<key>元素设定与所关联的持久化对应的表的外键,此处为ORDERS表的CUSTOMERS_ID字段。 它包括如下属性: ● <set>元素表明Customer类的orders属性为java.util.Set集合类型。 ● <one-to-many>表明orders集合中存放的是一组Order对象。 ● <key>属性表明ORDERS表通过外键CUSTOMERS_ID参照CUSTOMERS表。 ● cascade 属性取值为"save-update",表明当保存或更新Customer对象时,会级联保存或更新orders集合中的所有Order对象。 接下来我们持久化一个Customer对象以及和他关联的Order对象: Session session = sessionFactory.openSession(); Transaction tx = null; tx = session.beginTransaction(); Customer customer=new Customer("Tom",new HashSet()); Order order=new Order(); order.setOrderNumber("Tom_Order001"); order.setCustomer(customer); customer.getOrders().add(order); session.save(customer); tx.commit(); 当<set>元素的cascade属性为"save-update"时,Hibernate在持久化Customer对象时,会自动持久化关联的Order对象。 如果是加载持久化对象,然后再建立关联关系,那又该怎么做呢?如下所示: Session session = sessionFactory.openSession(); Transaction tx = null; tx = session.beginTransaction(); Customer customer=(Customer)session.load(Customer.class,new Long(2)); Order order=(Order)session.load(Order.class,new Long(2)); order.setCustomer(customer); customer.getOrders().add(order); tx.commit(); Hibernate会自动清理缓存中的所有持久化对象,按照持久化对象状态改变来同步更新数据库,Hibernate在清理以上Customer对象和Order对象时执行了以下两条SQL语句: update ORDERS set ORDER_NUMBER='Jack_Order001',CUSTOMER_ID=2 where ID=2; update ORDERS set CUSTOMER_ID=2 where ID=2; 尽管只是修改了ORDERS表的一条记录,但是以上SQL语句表明Hibernate执行了两次update操作这是因为HIbernate根据内存中持久化对象的状态变化来决定需要执行的SQL语句,order.setCustomer(customer)执行的相应SQL语句为: update ORDERS set ORDER_NUMBER='Jack_Order001',CUSTOMER_ID=2 where ID=2; customer.getOrders().add(order)执行的相应SQL语句为: update ORDERS set CUSTOMER_ID=2 where ID=2; 重复执行对于的SQL语句会影响应用程序的性能,那么解决的办法是把<set>元素的inverse属性设置为true,默认为false: <set name="orders" cascade="save-update" inverse="ture" >
<key column="CUSTOMER_ID" /> <one-to-many class="mypack.Order" /> </set> 因此在映射一对多双向关联关系时,应该在"one"方把inverse属性设为“ture”,这样可以提高性能。 |
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转载自svsesunyuan.iteye.com/blog/1026018
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