Hibernate one-to-many cascade探究
对于Hibernate的one-to-many关联关系中，合理使用cascade属性能够显著减少代码量，减轻程序员的工作量。
一。Husband
package com.hibernate;
import java.util.Set;
/**
* Created by IntelliJ IDEA.
* User: Zhong Gang
* Date: 9/7/11
* Time: 2:22 PM
*/
public class Husband {
private Integer id;
private String name;
private Set<Wife> wifes;
public Husband() {
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Set<Wife> getWifes() {
return wifes;
}
public void setWifes(Set<Wife> wifes) {
this.wifes = wifes;
}
}
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping default-access="field">
<class name="com.hibernate.Husband" table="husband" dynamic-insert="true" dynamic-update="true">
<id name="id" column="id" type="java.lang.Integer">
<generator class="native"/>
</id>
<property name="name" column="name" type="java.lang.String"/>
<set name="wifes">
<key column="husbandid"/>
<one-to-many class="com.hibernate.Wife"/>
</set>
</class>
</hibernate-mapping>
二。Wife
package com.hibernate;
/**
* Created by IntelliJ IDEA.
* User: Zhong Gang
* Date: 9/7/11
* Time: 2:22 PM
*/
public class Wife {
private Integer id;
private String name;
private Husband husband;
public Wife() {
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Husband getHusband() {
return husband;
}
public void setHusband(Husband husband) {
this.husband = husband;
}
}
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping default-access="field">
<class name="com.hibernate.Wife" table="wife" dynamic-insert="true" dynamic-update="true">
<id name="id" column="id" type="java.lang.Integer">
<generator class="native"/>
</id>
<property name="name" column="name" type="java.lang.String"/>
<many-to-one name="husband" column="husbandid" class="com.hibernate.Husband" cascade="save-update"/>
</class>
</hibernate-mapping>
三。db.properties and testDatasource.xml
db.url=jdbc:mysql://localhost:3306/dream db.driver=com.mysql.jdbc.Driver db.username=root db.password=root hibernate.dialect=org.hibernate.dialect.MySQL5Dialect hibernate.show_sql=true hibernate.hbm2ddl.auto=update hibernate.jdbc.batch_size=100
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:tx="http://www.springframework.org/schema/tx"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-2.5.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.0.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd"
default-autowire="byName">
<context:property-placeholder location="classpath:db.properties"/>
<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource"
destroy-method="close">
<property name="driverClassName" value="${db.driver}"/>
<property name="url" value="${db.url}"/>
<property name="username" value="${db.username}"/>
<property name="password" value="${db.password}"/>
</bean>
<bean id="sessionFactory" class="org.springframework.orm.hibernate3.LocalSessionFactoryBean">
<property name="dataSource" ref="dataSource"/>
<property name="mappingLocations">
<list>
<value>classpath:/hibernate_mappings/*</value>
</list>
</property>
<property name="hibernateProperties">
<props>
<prop key="hibernate.dialect">${hibernate.dialect}</prop>
<prop key="hibernate.show_sql">${hibernate.show_sql}</prop>
<prop key="hibernate.hbm2ddl.auto">${hibernate.hbm2ddl.auto}</prop>
<prop key="hibernate.jdbc.batch_size">${hibernate.jdbc.batch_size}</prop>
</props>
</property>
</bean>
<bean id="hibernateTemplate" class="org.springframework.orm.hibernate3.HibernateTemplate">
<property name="sessionFactory" ref="sessionFactory"/>
</bean>
<bean id="transactionManager" class="org.springframework.orm.hibernate3.HibernateTransactionManager">
<property name="sessionFactory" ref="sessionFactory"/>
</bean>
<tx:advice id="txAdvice" transaction-manager="transactionManager">
<tx:attributes>
<tx:method name="*" propagation="REQUIRED"/>
</tx:attributes>
</tx:advice>
<aop:config>
<aop:pointcut id="service" expression="execution(* com.hibernate..*.*(..))"/>
<aop:advisor advice-ref="txAdvice" pointcut-ref="service"/>
</aop:config>
</beans>
四。HibernateCascadeTest
package com.hibernate;
import junit.framework.TestCase;
import org.hibernate.SessionFactory;
import org.hibernate.classic.Session;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import java.util.HashSet;
import java.util.Set;
/**
* Created by IntelliJ IDEA.
* User: Zhong Gang
* Date: 9/7/11
* Time: 2:43 PM
*/
public class HibernateCascadeTest extends TestCase {
public void testDelete() throws Exception {
ApplicationContext context = new ClassPathXmlApplicationContext("testDatasource.xml");
SessionFactory sessionFactory = (SessionFactory) context.getBean("sessionFactory");
Session session = sessionFactory.openSession();
Husband husband = new Husband();
husband.setName("Husband");
Wife wife1 = new Wife();
wife1.setName("Wife 1");
wife1.setHusband(husband);
Wife wife2 = new Wife();
wife2.setName("Wife 2");
wife2.setHusband(husband);
Set<Wife> wifes = new HashSet<Wife>();
wifes.add(wife1);
wifes.add(wife2);
husband.setWifes(wifes);
session.saveOrUpdate(wife1);
session.saveOrUpdate(wife2);
session.flush();
session.delete(wife1);
session.flush();
}
}
理解cascade级联操作：当对实体进行CRUD操作时，对其配置文件中的配置有cascade的属性的实体是否也应该执行该操作。如当对Husband实体进行保存时，如果此时在Husband.hbm.xml配置文件中，对Wife的实体配置cascade="save-update"，表明当对Husband进行保存或修改时，也会引起对与Husband实体相关的Wife的实体的保存或修改操作。cascade有这样几种常用值，save-update,delete,all等。delete表明只有进行删除操作，才会引起级联删除操作，而all则表明对实体进行CRUD操作时，会引起cascade的实体的CRUD操作。具体配置哪种值，可根据项目中的实际需要，具体需求，具体分析。
那么对于一个one-to-many的实体关联关系，应该将cascade配置在一方还是多方呢？我的建议是配置在一方。试想，对于上述举例，如果在Husband.hbm.xml中的wifes属性上配置 cascade="all"，那么当添加或修改Husband实体时，会相应地引起对于Wife的级联添加或修改操作，而如果删除Husband实体，因为该实体在数据库中已不存在，那么再在Wife实体中的Husband维护这样的一对多关系，完全是没有任何必要的。而如果在Wife.hbm.xml中的husband属性上配置cascade="all"，那么当保存或修改Wife实体时，会级联保存或修改Husband实体，而当删除Wife实体时，会级联删除Husband实体，这样Hibernate是会报错的，原因很简单，一个Husband关联着多个Wife,当删除其中一个Wife时，就去删除Husband，那么其它的Wife的Husband不就找不到对应的实体了嘛，所以在多方配置cascade="all"或cascade="all-delete-orphan",cascade="delete"，不仅导致系统错误，在现实需求中来说也是绝对不允许的。