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Hibernate的集合一对多与多对一
需求: 部门与员工
一个部门有多个员工; 【一对多】
多个员工,属于一个部门 【多对一】
1、javaBean
——Dept.java
package com.gqx.onetomany; import java.util.HashSet; import java.util.Set; public class Dept { private int deptId; private String deptName; //部门对应的多个员工 (一对多) private Set<Employee> emps=new HashSet<Employee>(); public int getDeptId() { return deptId; } public void setDeptId(int deptId) { this.deptId = deptId; } public String getDeptName() { return deptName; } public void setDeptName(String deptName) { this.deptName = deptName; } public Set<Employee> getEmps() { return emps; } public void setEmps(Set<Employee> emps) { this.emps = emps; } }
——Employee.java
package com.gqx.onetomany; public class Employee { private int empId; private String empName; private double salary; //员工与部门 (多对一) private Dept dept; public int getEmpId() { return empId; } public void setEmpId(int empId) { this.empId = empId; } public String getEmpName() { return empName; } public void setEmpName(String empName) { this.empName = empName; } public double getSalary() { return salary; } public void setSalary(double salary) { this.salary = salary; } public Dept getDept() { return dept; } public void setDept(Dept dept) { this.dept = dept; } }
2、一对多与多对一在映射文件中的表现
多个员工对应一个部门(Employee.hbm.xml)
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.gqx.onetomany"> <class name="Employee" table="t_Employee"> <id name="empId" > <generator class="native"></generator> </id> <property name="empName" length="20"></property> <property name="salary" type="double"></property> <!-- 多对一映射配置 Employee映射关键点 1、映射的部门属性 dept 2、映射的部门对象,对应的字段 dept_id 3、部门的类型 --> <many-to-one name="dept" column="dept_id" class="Dept"></many-to-one> </class> </hibernate-mapping>
一个部门对应多个员工(Dept.hbm.xml)
<?xml version="1.0"?> <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd"> <hibernate-mapping package="com.gqx.onetomany"> <class name="Dept" table="t_Dept" > <id name="deptId"> <generator class="native"></generator> </id> <property name="deptName" length="20"></property> <!-- 一对多关联映射配置(通过部门管理到员工) Dept 映射关键点 1、指定映射的集合属性: "emps" 2、集合属性对应的集和表 "t_employee" 3、集合表的外键字段 "t_employee中的dept_id 4、集合元素的类型 --> <set name="emps" table="t_Employee"> <key column="dept_id"></key> <one-to-many class="Employee"/> </set> </class> </hibernate-mapping>
3、写测试类进存储
package com.gqx.onetomany; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration; import org.junit.Test; public class Demo { private static SessionFactory sf; static{ sf=new Configuration(). configure(). addClass(Dept.class).addClass(Employee.class) .buildSessionFactory(); } /** * 保存:部门方【一的一方】 */ @Test public void test() { Session session=sf.openSession(); session.beginTransaction(); //部门对象 Dept dept=new Dept(); dept.setDeptName("前端组"); //员工对象 Employee emp1=new Employee(); emp1.setEmpName("张三"); Employee emp2=new Employee(); emp2.setEmpName("李四"); //关系 dept.getEmps().add(emp1); dept.getEmps().add(emp2); //保存 session.save(dept); session.save(emp1); session.save(emp2); session.getTransaction().commit(); session.close(); /* 结果 *Hibernate: insert into t_Employee (empName, salary, dept_id) values (?, ?, ?) *Hibernate: insert into t_Employee (empName, salary, dept_id) values (?, ?, ?) *Hibernate: insert into t_Dept (deptName) values (?) *Hibernate: update t_Employee set deptId=? where empId=? *Hibernate: update t_Employee set deptId=? where empId=? * *为什么会有五条sql语句? *开始的时候先插入员工,并没有部门,一旦部门加入后,通过update的语句维护两者的关系 */ } /** * 保存:员工方【多的一方】 */ @Test public void test2() { Session session=sf.openSession(); session.beginTransaction(); //部门对象 Dept dept=new Dept(); dept.setDeptName("运维组"); //员工对象 Employee emp1=new Employee(); emp1.setEmpName("王五"); Employee emp2=new Employee(); emp2.setEmpName("老刘"); //关系 emp1.setDept(dept); emp2.setDept(dept); //保存 session.save(emp1); session.save(emp2); session.save(dept); session.getTransaction().commit(); session.close(); /* 结果 *Hibernate: insert into t_Dept (deptName) values (?) *Hibernate: insert into t_Employee (empName, salary, dept_id) values (?, ?, ?) *Hibernate: insert into t_Employee (empName, salary, dept_id) values (?, ?, ?) * *如果将session.save(dept)放到两个员工的下方 结果为: * Hibernate: insert into t_Employee (empName, salary, dept_id) values (?, ?, ?) Hibernate: insert into t_Employee (empName, salary, dept_id) values (?, ?, ?) Hibernate: insert into t_Dept (deptName) values (?) Hibernate: update t_Employee set empName=?, salary=?, dept_id=? where empId=? Hibernate: update t_Employee set empName=?, salary=?, dept_id=? where empId=? */ } }
存储之后,如果要获取数据,则、
package com.gqx.onetomany; import java.util.Iterator; import java.util.List; import java.util.Set; import org.hibernate.Session; import org.hibernate.SessionFactory; import org.hibernate.Transaction; import org.hibernate.cfg.Configuration; import org.junit.Test; public class DemoGet { private static SessionFactory sf; static{ sf=new Configuration(). configure(). addClass(Dept.class).addClass(Employee.class) .buildSessionFactory(); } @Test public void test() { Session session=sf.openSession(); session.beginTransaction(); //通过部门方获取另外一方 Dept dept=(Dept)session.get(Dept.class, 1); System.out.println(dept.getDeptName()); Set<Employee> set=dept.getEmps(); Iterator<Employee> iterators=set.iterator(); for (Iterator iterator = set.iterator(); iterator.hasNext();) { Employee employee = (Employee) iterator.next(); System.out.println(employee.getEmpName()); } session.getTransaction().commit(); session.close(); } @Test public void test2() { Session session=sf.openSession(); session.beginTransaction(); //通过员工方获取另外一方 Employee employee=(Employee)session.get(Employee.class, 1); System.out.println(employee.getEmpName()); System.out.println(employee.getDept().getDeptName()); session.getTransaction().commit(); session.close(); } }
总结:
在一对多与多对一的关联关系中,保存数据最好的通过多的一方来维护关系,这样可以减少update语句的生成,从而提高hibernate的执行效率!
配置一对多与多对一,这种叫“双向关联”
只配置一对多, 叫“单项一对多”
只配置多对一, 叫“单项多对一”
注意:
配置了哪一方,哪一方才有维护关联关系的权限!
js:强大的splice
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Hibernate的集合一对多与多对一
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