hibernate基于注解的维护权反转：@OneToMany(mappedBy=)

背景说明：首先是SSH环境下，对象基于注解的方式映射到数据库；

昨天遇到一个比较纠结的问题，@OneToMany(mappedBy="xxx")， mappedBy属性有什么用，然后是写在哪一边？

还有一个问题是：@JoinColumn(name="xxxxx")，JoinColumn有什么用？

先贴出最初的代码：一些基本的注解，在一对多的关系上没有使用JoinColumn和mappedBy属性

部门类：主要是第33、34行

package com.lizhou.entity.test;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

import org.hibernate.annotations.GenericGenerator;

/**
* 部门：与员工一对多关系
* @author bojiangzhou
*
*/
@Entity
@Table(name="department")
public class Department {

@Id
@GeneratedValue(generator="_native")
@GenericGenerator(name="_native", strategy="native")
private int id; //ID

@Column(length=20)
private String dname; //部门名称

@OneToMany
private List<Employee> employeeList = new ArrayList<>(); //部门下的员工集合

// get/set方法
}

员工类：主要是第32、33行

package com.lizhou.entity.test;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

import org.hibernate.annotations.GenericGenerator;

/**
* 员工：与部门多对一关系
* @author bojiangzhou
*
*/
@Entity
@Table(name="employee")
public class Employee {

@Id
@GeneratedValue(generator="_native")
@GenericGenerator(name="_native", strategy="native")
private int id; //ID

@Column(length=20)
private String ename; //员工姓名

@Column(length=20)
private String phone; //电话

@ManyToOne
private Department department; //所属部门

//get/set方法
}

最初的注解配置里，在一对多的关系上，即employeeList和department没有使用JoinColumn。

看下图，employee表会自动添加一个外键列department_id，虽然关系映射上是正确了，但是有一个问题，数据库里多了一张表出来，这不是想要的结果。



解决方法：在employeeList和department字段上加上@JoinColumn注解

@OneToMany
@JoinColumn(name="departmentId")
private List<Employee> employeeList = new ArrayList<>(); //部门下的员工集合

@ManyToOne//
@JoinColumn(name="departmentId")//
private Department department; //所属部门

这样一来的话就只有两张表了，所以在一对多或者一对一的关系下，需要加上@JoinColumn来指定外键列，避免生成一张中间表。

而且经试验，多的一方(Employee)里的department必须加上@JoinColumn，Department里不加不会影响表的结构，不知道会不会有其它影响；

但是如果Employee属于多的一方，如果没有指定外键列，还是会自动生成一个department_id外键列。



接下来讨论mappedBy属性：mappedBy属性主要是针对外键而言。与之相对应的是xml中的inverse属性。

如下是测试类代码：此时还没有设置mappedBy属性，映射时，默认是都由自身维护关联关系。

package com.lizhou.action.test;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.lizhou.entity.test.Department;
import com.lizhou.entity.test.Employee;

/**
* 测试类
* @author bojiangzhou
*
*/

public class TestAction {

private static SessionFactory sessionFactory = null;

static {
//读取classpath中applicationContext.xml配置文件
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
//获取session中配置的sessionFactory对象
sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");
}

@Test
public void testSave(){
//创建一个部门对象
Department d1 = new Department();
d1.setDname("研发部");

//创建两个员工对象
Employee e1 = new Employee();
e1.setEname("张三");
e1.setPhone("13111111111");
Employee e2 = new Employee();
e2.setEname("李四");
e2.setPhone("18523222222");

//设置对象关联
d1.getEmployeeList().add(e1);
d1.getEmployeeList().add(e2);
e1.setDepartment(d1);
e2.setDepartment(d1);

//获取Session
Session session = sessionFactory.openSession();
//开始事务
Transaction t = session.beginTransaction();
try {
//添加数据
session.save(d1);
session.save(e1);
session.save(e2);
//提交事务
t.commit();
} catch (RuntimeException e) {
//有异常则回滚事务
t.rollback();
e.printStackTrace();
} finally {
//关闭session
session.close();
}
}

}

执行testSave后，控制台打印如下语句：

Hibernate: insert into department (dname) values (?)
Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)
Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)
Hibernate: update employee set departmentId=? where id=?
Hibernate: update employee set departmentId=? where id=?

可以看到多了两条update语句，这是因为两边都维护关系，先插入的部门，再插入员工，插入员工时，已经设置好外键了，但部门方也维护关系，会再执行一次更新操作，为员工设置外键，这样就导致多出了两条update语句，这里是有性能损耗的。

一种解决办法是：将第46、47行去掉，即对象上部门不关联员工

package com.lizhou.action.test;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.lizhou.entity.test.Department;
import com.lizhou.entity.test.Employee;

/**
* 测试类
* @author bojiangzhou
*
*/

public class TestAction {

private static SessionFactory sessionFactory = null;

static {
//读取classpath中applicationContext.xml配置文件
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
//获取session中配置的sessionFactory对象
sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");
}

@Test
public void testSave(){
//创建一个部门对象
Department d1 = new Department();
d1.setDname("研发部");

//创建两个员工对象
Employee e1 = new Employee();
e1.setEname("张三");
e1.setPhone("13111111111");
Employee e2 = new Employee();
e2.setEname("李四");
e2.setPhone("18523222222");

//设置对象关联
//        d1.getEmployeeList().add(e1);
//        d1.getEmployeeList().add(e2);
e1.setDepartment(d1);
e2.setDepartment(d1);

//获取Session
Session session = sessionFactory.openSession();
//开始事务
Transaction t = session.beginTransaction();
try {
//添加数据
session.save(d1);
session.save(e1);
session.save(e2);
//提交事务
t.commit();
} catch (RuntimeException e) {
//有异常则回滚事务
t.rollback();
e.printStackTrace();
} finally {
//关闭session
session.close();
}
}

}

Hibernate: insert into department (dname) values (?)
Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)
Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)

这样部门方就不会去维护外键关系了。但是有一个问题，对象上就没有关联了，我们要做的是对象上要互相关联，数据库方面只让一方去维护关系即可。

对象上如果不关联，因为部门和员工添加到数据库后，是持久化状态，存在于session缓存中，那session操作缓存中这几个对象时，部门就没有关联员工了，那么就还得再查询一次数据库，这不是想要的结果。

这时就要用到mappedBy属性了。

在一的一方配置@OneToMany(mappedBy="department")，将维护权交由多的一方来维护；

那为什么不让多的一方交出维护权，让一的一方来维护呢？上面的实验也表明了如果让一的一方来维护，始终都会多出两条update语句，因为外键是在多的这一方的，所以维护权应该交由多的一方。

部门类的配置：第36行和第37行的配置，部门部门交出维护权利，让对方来维护

package com.lizhou.entity.test;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

import org.hibernate.annotations.GenericGenerator;

/**
* 部门：与员工一对多关系
* @author bojiangzhou
*
*/
@Entity
@Table(name="department")
public class Department {

@Id
@GeneratedValue(generator="_native")
@GenericGenerator(name="_native", strategy="native")
private int id; //ID

@Column(length=20)
private String dname; //部门名称

@OneToMany(mappedBy="department")
private List<Employee> employeeList = new ArrayList<>(); //部门下的员工集合

// get/set方法
}

员工类的配置不变。

调用testSave时，部门和员工再对象上依然是关联的：第46-49行

package com.lizhou.action.test;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.lizhou.entity.test.Department;
import com.lizhou.entity.test.Employee;

/**
* 测试类
* @author bojiangzhou
*
*/

public class TestAction {

private static SessionFactory sessionFactory = null;

static {
//读取classpath中applicationContext.xml配置文件
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
//获取session中配置的sessionFactory对象
sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");
}

@Test
public void testSave(){
//创建一个部门对象
Department d1 = new Department();
d1.setDname("研发部");

//创建两个员工对象
Employee e1 = new Employee();
e1.setEname("张三");
e1.setPhone("13111111111");
Employee e2 = new Employee();
e2.setEname("李四");
e2.setPhone("18523222222");

//设置对象关联
d1.getEmployeeList().add(e1);
d1.getEmployeeList().add(e2);
e1.setDepartment(d1);
e2.setDepartment(d1);

//获取Session
Session session = sessionFactory.openSession();
//开始事务
Transaction t = session.beginTransaction();
try {
//添加数据
session.save(d1);
session.save(e1);
session.save(e2);
//提交事务
t.commit();
} catch (RuntimeException e) {
//有异常则回滚事务
t.rollback();
e.printStackTrace();
} finally {
//关闭session
session.close();
}
}

}

控制台打印的语句：只有三条插入语句，没有更新语句了

Hibernate: insert into department (dname) values (?)
Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)
Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)

这里遇到一个问题：如果配置mappedBy属性的同时加上@JoinColumn会抛出异常，所以不能同时使用@JoinColumn和mappedBy；因为@JoinColumn本身就是自己来维护外键，和mappedBy冲突了。--->>>不知道这样理解正确否！！^_^

package com.lizhou.entity.test;

import java.util.ArrayList;
import java.util.List;

import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;
import javax.persistence.Table;

import org.hibernate.annotations.GenericGenerator;

/**
* 部门：与员工一对多关系
* @author bojiangzhou
*
*/
@Entity
@Table(name="department")
public class Department {

@Id
@GeneratedValue(generator="_native")
@GenericGenerator(name="_native", strategy="native")
private int id; //ID

@Column(length=20)
private String dname; //部门名称

@OneToMany(mappedBy="department")
@JoinColumn(name="departmentId")
private List<Employee> employeeList = new ArrayList<>(); //部门下的员工集合

// set/get 方法
}

抛出如下异常：

java.lang.ExceptionInInitializerError
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:217)
at org.junit.runners.BlockJUnit4ClassRunner$1.runReflectiveCall(BlockJUnit4ClassRunner.java:266)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:263)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)
at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)
at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)
Caused by: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sessionFactory' defined in class path resource [applicationContext.xml]: Invocation of init method failed; nested exception is org.hibernate.AnnotationException: Associations marked as mappedBy must not define database mappings like @JoinTable or @JoinColumn: com.lizhou.entity.test.Department.employeeList
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1553)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:539)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:475)
at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)
at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)
at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)
at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)
at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:684)
at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:760)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:482)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
at com.lizhou.action.test.TestAction.<clinit>(TestAction.java:26)
... 22 more
Caused by: org.hibernate.AnnotationException: Associations marked as mappedBy must not define database mappings like @JoinTable or @JoinColumn: com.lizhou.entity.test.Department.employeeList
at org.hibernate.cfg.annotations.CollectionBinder.bind(CollectionBinder.java:493)
at org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:2156)
at org.hibernate.cfg.AnnotationBinder.processIdPropertiesIfNotAlready(AnnotationBinder.java:963)
at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:796)
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processAnnotatedClassesQueue(Configuration.java:3788)
at org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:3742)
at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1410)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1844)
at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1928)
at org.springframework.orm.hibernate4.LocalSessionFactoryBuilder.buildSessionFactory(LocalSessionFactoryBuilder.java:343)
at org.springframework.orm.hibernate4.LocalSessionFactoryBean.buildSessionFactory(LocalSessionFactoryBean.java:431)
at org.springframework.orm.hibernate4.LocalSessionFactoryBean.afterPropertiesSet(LocalSessionFactoryBean.java:416)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1612)
at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1549)
... 34 more

还有一点说明下：

如果将第57行代码移到第59行后面，即先保存员工，再保存部门，会多出四条update语句

package com.lizhou.action.test;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.junit.Test;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import com.lizhou.entity.test.Department;
import com.lizhou.entity.test.Employee;

/**
* 测试类
* @author bojiangzhou
*
*/

public class TestAction {

private static SessionFactory sessionFactory = null;

static {
//读取classpath中applicationContext.xml配置文件
ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");
//获取session中配置的sessionFactory对象
sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");
}

@Test
public void testSave(){
//创建一个部门对象
Department d1 = new Department();
d1.setDname("研发部");

//创建两个员工对象
Employee e1 = new Employee();
e1.setEname("张三");
e1.setPhone("13111111111");
Employee e2 = new Employee();
e2.setEname("李四");
e2.setPhone("18523222222");

//设置对象关联
d1.getEmployeeList().add(e1);
d1.getEmployeeList().add(e2);
e1.setDepartment(d1);
e2.setDepartment(d1);

//获取Session
Session session = sessionFactory.openSession();
//开始事务
Transaction t = session.beginTransaction();
try {
//添加数据
session.save(e1);
session.save(e2);
session.save(d1);
//提交事务
t.commit();
} catch (RuntimeException e) {
//有异常则回滚事务
t.rollback();
e.printStackTrace();
} finally {
//关闭session
session.close();
}
}

}

Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)
Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)
Hibernate: insert into department (dname) values (?)
Hibernate: update employee set departmentId=?, ename=?, phone=? where id=?
Hibernate: update employee set departmentId=?, ename=?, phone=? where id=?
Hibernate: update employee set departmentId=? where id=?
Hibernate: update employee set departmentId=? where id=?

很明显，在插入员工时，还没有部门的信息，等插入部门的时候，员工方会维护外键关系，更新外键；而部门方也会维护一次，所以多了四条语句。所以在添加数据的时候先保存一的一方，再保存多的一方。

总结：mappedBy属性跟xml配置文件里的inverse一样。在一对多或一对一的关系映射中，如果不表明mappedBy属性，默认是由本方维护外键。但如果两方都由本方来维护的话，会多出一些update语句，性能有一定的损耗。

解决的办法就是在一的一方配置上mappedBy属性，将维护权交给多的一方来维护，就不会有update语句了。

至于为何要将维护权交给多的一方，可以这样考虑：要想一个国家的领导人记住所有人民的名字是不可能的，但可以让所有人民记住领导人的名字！

注意，配了mappedBy属性后，不要再有@JoinColumn，会冲突！

OK!!!