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
1 package com.lizhou.entity.test; 2 3 import java.util.ArrayList; 4 import java.util.List; 5 6 import javax.persistence.Column; 7 import javax.persistence.Entity; 8 import javax.persistence.GeneratedValue; 9 import javax.persistence.GenerationType;10 import javax.persistence.Id;11 import javax.persistence.OneToMany;12 import javax.persistence.Table;13 14 import org.hibernate.annotations.GenericGenerator;15 16 /**17 * 部门：与员工一对多关系18 * @author bojiangzhou19 *20 */21 @Entity22 @Table(name="department")23 public class Department {24 25 @Id26 @GeneratedValue(generator="_native")27 @GenericGenerator(name="_native", strategy="native")28 private int id; //ID29 30 @Column(length=20)31 private String dname; //部门名称32 33 @OneToMany34 private List employeeList = new ArrayList<>(); //部门下的员工集合35 36 // get/set方法59 60 }
员工类：主要是第32、33行
1 package com.lizhou.entity.test; 2 3 import javax.persistence.Column; 4 import javax.persistence.Entity; 5 import javax.persistence.GeneratedValue; 6 import javax.persistence.Id; 7 import javax.persistence.ManyToOne; 8 import javax.persistence.Table; 9 10 import org.hibernate.annotations.GenericGenerator;11 12 /**13 * 员工：与部门多对一关系14 * @author bojiangzhou15 *16 */17 @Entity18 @Table(name="employee")19 public class Employee {20 21 @Id22 @GeneratedValue(generator="_native")23 @GenericGenerator(name="_native", strategy="native")24 private int id; //ID25 26 @Column(length=20)27 private String ename; //员工姓名28 29 @Column(length=20)30 private String phone; //电话31 32 @ManyToOne33 private Department department; //所属部门34 35 36 //get/set方法67 68 }
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
1 @OneToMany2 @JoinColumn(name="departmentId")3 private ListemployeeList = new ArrayList<>(); //部门下的员工集合
1 @ManyToOne//2 @JoinColumn(name="departmentId")//3 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。
1 package com.lizhou.action.test; 2 3 import org.hibernate.Session; 4 import org.hibernate.SessionFactory; 5 import org.hibernate.Transaction; 6 import org.hibernate.cfg.Configuration; 7 import org.junit.Test; 8 import org.springframework.context.ApplicationContext; 9 import org.springframework.context.support.ClassPathXmlApplicationContext;10 11 import com.lizhou.entity.test.Department;12 import com.lizhou.entity.test.Employee;13 14 /**15 * 测试类16 * @author bojiangzhou17 *18 */19 20 public class TestAction {21 22 private static SessionFactory sessionFactory = null;23 24 static {25 //读取classpath中applicationContext.xml配置文件26 ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");27 //获取session中配置的sessionFactory对象28 sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");29 }30 31 @Test32 public void testSave(){33 //创建一个部门对象34 Department d1 = new Department();35 d1.setDname("研发部");36 37 //创建两个员工对象38 Employee e1 = new Employee();39 e1.setEname("张三");40 e1.setPhone("13111111111");41 Employee e2 = new Employee();42 e2.setEname("李四");43 e2.setPhone("18523222222");44 45 //设置对象关联46 d1.getEmployeeList().add(e1);47 d1.getEmployeeList().add(e2);48 e1.setDepartment(d1);49 e2.setDepartment(d1);50 51 //获取Session52 Session session = sessionFactory.openSession();53 //开始事务54 Transaction t = session.beginTransaction();55 try {56 //添加数据57 session.save(d1);58 session.save(e1);59 session.save(e2);60 //提交事务61 t.commit();62 } catch (RuntimeException e) {63 //有异常则回滚事务64 t.rollback();65 e.printStackTrace();66 } finally {67 //关闭session68 session.close();69 }70 }71 72 73 } 执行testSave后，控制台打印如下语句：
1 Hibernate: insert into department (dname) values (?)2 Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)3 Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)4 Hibernate: update employee set departmentId=? where id=?5 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
1 package com.lizhou.action.test; 2 3 import org.hibernate.Session; 4 import org.hibernate.SessionFactory; 5 import org.hibernate.Transaction; 6 import org.hibernate.cfg.Configuration; 7 import org.junit.Test; 8 import org.springframework.context.ApplicationContext; 9 import org.springframework.context.support.ClassPathXmlApplicationContext;10 11 import com.lizhou.entity.test.Department;12 import com.lizhou.entity.test.Employee;13 14 /**15 * 测试类16 * @author bojiangzhou17 *18 */19 20 public class TestAction {21 22 private static SessionFactory sessionFactory = null;23 24 static {25 //读取classpath中applicationContext.xml配置文件26 ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");27 //获取session中配置的sessionFactory对象28 sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");29 }30 31 @Test32 public void testSave(){33 //创建一个部门对象34 Department d1 = new Department();35 d1.setDname("研发部");36 37 //创建两个员工对象38 Employee e1 = new Employee();39 e1.setEname("张三");40 e1.setPhone("13111111111");41 Employee e2 = new Employee();42 e2.setEname("李四");43 e2.setPhone("18523222222");44 45 //设置对象关联46 // d1.getEmployeeList().add(e1);47 // d1.getEmployeeList().add(e2);48 e1.setDepartment(d1);49 e2.setDepartment(d1);50 51 //获取Session52 Session session = sessionFactory.openSession();53 //开始事务54 Transaction t = session.beginTransaction();55 try {56 //添加数据57 session.save(d1);58 session.save(e1);59 session.save(e2);60 //提交事务61 t.commit();62 } catch (RuntimeException e) {63 //有异常则回滚事务64 t.rollback();65 e.printStackTrace();66 } finally {67 //关闭session68 session.close();69 }70 }71 72 73 } 1 Hibernate: insert into department (dname) values (?)2 Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)3 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
1 package com.lizhou.entity.test; 2 3 import java.util.ArrayList; 4 import java.util.List; 5 6 import javax.persistence.CascadeType; 7 import javax.persistence.Column; 8 import javax.persistence.Entity; 9 import javax.persistence.FetchType;10 import javax.persistence.GeneratedValue;11 import javax.persistence.GenerationType;12 import javax.persistence.Id;13 import javax.persistence.JoinColumn;14 import javax.persistence.OneToMany;15 import javax.persistence.Table;16 17 import org.hibernate.annotations.GenericGenerator;18 19 /**20 * 部门：与员工一对多关系21 * @author bojiangzhou22 *23 */24 @Entity25 @Table(name="department")26 public class Department {27 28 @Id29 @GeneratedValue(generator="_native")30 @GenericGenerator(name="_native", strategy="native")31 private int id; //ID32 33 @Column(length=20)34 private String dname; //部门名称35 36 @OneToMany(mappedBy="department")37 private List employeeList = new ArrayList<>(); //部门下的员工集合38 39 // get/set方法62 63 } 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
1 package com.lizhou.action.test; 2 3 import org.hibernate.Session; 4 import org.hibernate.SessionFactory; 5 import org.hibernate.Transaction; 6 import org.hibernate.cfg.Configuration; 7 import org.junit.Test; 8 import org.springframework.context.ApplicationContext; 9 import org.springframework.context.support.ClassPathXmlApplicationContext;10 11 import com.lizhou.entity.test.Department;12 import com.lizhou.entity.test.Employee;13 14 /**15 * 测试类16 * @author bojiangzhou17 *18 */19 20 public class TestAction {21 22 private static SessionFactory sessionFactory = null;23 24 static {25 //读取classpath中applicationContext.xml配置文件26 ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");27 //获取session中配置的sessionFactory对象28 sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");29 }30 31 @Test32 public void testSave(){33 //创建一个部门对象34 Department d1 = new Department();35 d1.setDname("研发部");36 37 //创建两个员工对象38 Employee e1 = new Employee();39 e1.setEname("张三");40 e1.setPhone("13111111111");41 Employee e2 = new Employee();42 e2.setEname("李四");43 e2.setPhone("18523222222");44 45 //设置对象关联46 d1.getEmployeeList().add(e1);47 d1.getEmployeeList().add(e2);48 e1.setDepartment(d1);49 e2.setDepartment(d1);50 51 //获取Session52 Session session = sessionFactory.openSession();53 //开始事务54 Transaction t = session.beginTransaction();55 try {56 //添加数据57 session.save(d1);58 session.save(e1);59 session.save(e2);60 //提交事务61 t.commit();62 } catch (RuntimeException e) {63 //有异常则回滚事务64 t.rollback();65 e.printStackTrace();66 } finally {67 //关闭session68 session.close();69 }70 }71 72 73 } 控制台打印的语句：只有三条插入语句，没有更新语句了
1 Hibernate: insert into department (dname) values (?)2 Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)3 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！！^_^
1 package com.lizhou.entity.test; 2 3 import java.util.ArrayList; 4 import java.util.List; 5 6 import javax.persistence.CascadeType; 7 import javax.persistence.Column; 8 import javax.persistence.Entity; 9 import javax.persistence.FetchType;10 import javax.persistence.GeneratedValue;11 import javax.persistence.GenerationType;12 import javax.persistence.Id;13 import javax.persistence.JoinColumn;14 import javax.persistence.OneToMany;15 import javax.persistence.Table;16 17 import org.hibernate.annotations.GenericGenerator;18 19 /**20 * 部门：与员工一对多关系21 * @author bojiangzhou22 *23 */24 @Entity25 @Table(name="department")26 public class Department {27 28 @Id29 @GeneratedValue(generator="_native")30 @GenericGenerator(name="_native", strategy="native")31 private int id; //ID32 33 @Column(length=20)34 private String dname; //部门名称35 36 @OneToMany(mappedBy="department")37 @JoinColumn(name="departmentId")38 private List employeeList = new ArrayList<>(); //部门下的员工集合39 40 // set/get 方法63 64 } 抛出如下异常：
1 java.lang.ExceptionInInitializerError 2 at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method) 3 at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source) 4 at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source) 5 at java.lang.reflect.Constructor.newInstance(Unknown Source) 6 at org.junit.runners.BlockJUnit4ClassRunner.createTest(BlockJUnit4ClassRunner.java:217) 7 at org.junit.runners.BlockJUnit4ClassRunner$1.runReflectiveCall(BlockJUnit4ClassRunner.java:266) 8 at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12) 9 at org.junit.runners.BlockJUnit4ClassRunner.methodBlock(BlockJUnit4ClassRunner.java:263)10 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)11 at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)12 at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)13 at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)14 at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)15 at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)16 at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)17 at org.junit.runners.ParentRunner.run(ParentRunner.java:363)18 at org.eclipse.jdt.internal.junit4.runner.JUnit4TestReference.run(JUnit4TestReference.java:86)19 at org.eclipse.jdt.internal.junit.runner.TestExecution.run(TestExecution.java:38)20 at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:459)21 at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.runTests(RemoteTestRunner.java:675)22 at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.run(RemoteTestRunner.java:382)23 at org.eclipse.jdt.internal.junit.runner.RemoteTestRunner.main(RemoteTestRunner.java:192)24 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.employeeList25 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1553)26 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:539)27 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:475)28 at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:302)29 at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)30 at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:298)31 at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:193)32 at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:684)33 at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:760)34 at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:482)35 at org.springframework.context.support.ClassPathXmlApplicationContext.(ClassPathXmlApplicationContext.java:139)36 at org.springframework.context.support.ClassPathXmlApplicationContext. (ClassPathXmlApplicationContext.java:83)37 at com.lizhou.action.test.TestAction. (TestAction.java:26)38 ... 22 more39 Caused by: org.hibernate.AnnotationException: Associations marked as mappedBy must not define database mappings like @JoinTable or @JoinColumn: com.lizhou.entity.test.Department.employeeList40 at org.hibernate.cfg.annotations.CollectionBinder.bind(CollectionBinder.java:493)41 at org.hibernate.cfg.AnnotationBinder.processElementAnnotations(AnnotationBinder.java:2156)42 at org.hibernate.cfg.AnnotationBinder.processIdPropertiesIfNotAlready(AnnotationBinder.java:963)43 at org.hibernate.cfg.AnnotationBinder.bindClass(AnnotationBinder.java:796)44 at org.hibernate.cfg.Configuration$MetadataSourceQueue.processAnnotatedClassesQueue(Configuration.java:3788)45 at org.hibernate.cfg.Configuration$MetadataSourceQueue.processMetadata(Configuration.java:3742)46 at org.hibernate.cfg.Configuration.secondPassCompile(Configuration.java:1410)47 at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1844)48 at org.hibernate.cfg.Configuration.buildSessionFactory(Configuration.java:1928)49 at org.springframework.orm.hibernate4.LocalSessionFactoryBuilder.buildSessionFactory(LocalSessionFactoryBuilder.java:343)50 at org.springframework.orm.hibernate4.LocalSessionFactoryBean.buildSessionFactory(LocalSessionFactoryBean.java:431)51 at org.springframework.orm.hibernate4.LocalSessionFactoryBean.afterPropertiesSet(LocalSessionFactoryBean.java:416)52 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.invokeInitMethods(AbstractAutowireCapableBeanFactory.java:1612)53 at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1549)54 ... 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
1 package com.lizhou.action.test; 2 3 import org.hibernate.Session; 4 import org.hibernate.SessionFactory; 5 import org.hibernate.Transaction; 6 import org.hibernate.cfg.Configuration; 7 import org.junit.Test; 8 import org.springframework.context.ApplicationContext; 9 import org.springframework.context.support.ClassPathXmlApplicationContext;10 11 import com.lizhou.entity.test.Department;12 import com.lizhou.entity.test.Employee;13 14 /**15 * 测试类16 * @author bojiangzhou17 *18 */19 20 public class TestAction {21 22 private static SessionFactory sessionFactory = null;23 24 static {25 //读取classpath中applicationContext.xml配置文件26 ApplicationContext applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");27 //获取session中配置的sessionFactory对象28 sessionFactory = (SessionFactory) applicationContext.getBean("sessionFactory");29 }30 31 @Test32 public void testSave(){33 //创建一个部门对象34 Department d1 = new Department();35 d1.setDname("研发部");36 37 //创建两个员工对象38 Employee e1 = new Employee();39 e1.setEname("张三");40 e1.setPhone("13111111111");41 Employee e2 = new Employee();42 e2.setEname("李四");43 e2.setPhone("18523222222");44 45 //设置对象关联46 d1.getEmployeeList().add(e1);47 d1.getEmployeeList().add(e2);48 e1.setDepartment(d1);49 e2.setDepartment(d1);50 51 //获取Session52 Session session = sessionFactory.openSession();53 //开始事务54 Transaction t = session.beginTransaction();55 try {56 //添加数据57 session.save(e1);58 session.save(e2);59 session.save(d1);60 //提交事务61 t.commit();62 } catch (RuntimeException e) {63 //有异常则回滚事务64 t.rollback();65 e.printStackTrace();66 } finally {67 //关闭session68 session.close();69 }70 }71 72 73 } 1 Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)2 Hibernate: insert into employee (departmentId, ename, phone) values (?, ?, ?)3 Hibernate: insert into department (dname) values (?)4 Hibernate: update employee set departmentId=?, ename=?, phone=? where id=?5 Hibernate: update employee set departmentId=?, ename=?, phone=? where id=?6 Hibernate: update employee set departmentId=? where id=?7 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!!!