JPA关系映射之one-to-many和many-to-one

one-to-many（一对多）和many-to-one（多对一）双向关联

假设部门与员工是一对多关系，反过来员工与部门就是多对一关系。

Dept.java类

 public class Dept implements java.io.Serializable {

     // Fields

     private Integer deptId;
     private String deptName;
     private Set<Emp> emps=new HashSet<Emp>();

     // Constructors

     /** default constructor */
     public Dept() {
     }

     /** full constructor */
     public Dept(String deptName) {
         this.deptName = deptName;
     }

     // Property accessors

     public Integer getDeptId() {
         return this.deptId;
     }

     public void setDeptId(Integer deptId) {
         this.deptId = deptId;
     }

     public String getDeptName() {
         return this.deptName;
     }

     public void setDeptName(String deptName) {
         this.deptName = deptName;
     }

     public Set<Emp> getEmps() {
         return emps;
     }

     public void setEmps(Set<Emp> emps) {
         this.emps = emps;
     }

 }

Emp.java类

 public class Emp implements java.io.Serializable {

     // Fields

     private Integer empNo;
     private String empName;
     private Date empBrithday;
     private Dept dept;

     // Constructors

     /** default constructor */
     public Emp() {
     }

     /** full constructor */
     public Emp(String empName, Date empBrithday) {
         this.empName = empName;
         this.empBrithday = empBrithday;
     }

     // Property accessors

     public Integer getEmpNo() {
         return this.empNo;
     }

     public void setEmpNo(Integer empNo) {
         this.empNo = empNo;
     }

     public String getEmpName() {
         return this.empName;
     }

     public void setEmpName(String empName) {
         this.empName = empName;
     }

     public Date getEmpBrithday() {
         return this.empBrithday;
     }

     public void setEmpBrithday(Date empBrithday) {
         this.empBrithday = empBrithday;
     }

     public Dept getDept() {
         return dept;
     }

     public void setDept(Dept dept) {
         this.dept = dept;
     }

 }

Dept.hbm.xml

 <hibernate-mapping>
     <class name="com.db.entity.Dept" table="dept" catalog="mydb">
         <id name="deptId" type="java.lang.Integer">
             <column name="deptId" />
             <generator class="native" />
         </id>
         <property name="deptName" type="java.lang.String">
             <column name="deptName" length="32" />
         </property>
         <set name="emps" inverse="true" cascade="all">
             <key column="deptId" not-null="true" />
             <one-to-many class="com.db.entity.Emp" />
         </set>
     </class>
 </hibernate-mapping>

Emp.hbm.xml

 <hibernate-mapping>
     <class name="com.db.entity.Emp" table="emp" catalog="mydb">
         <id name="empNo" type="java.lang.Integer">
             <column name="empNo" />
             <generator class="native" />
         </id>
         <property name="empName" type="java.lang.String">
             <column name="empName" length="20" />
         </property>
         <property name="empBrithday" type="java.util.Date">
             <column name="empBrithday"/>
         </property>
         <many-to-one name="dept" column="deptId" class="com.db.entity.Dept" not-null="true"  fetch="select" cascade="save-update,delete"/>
     </class>
 </hibernate-mapping>

hibernate.cfg.xml

 <hibernate-configuration>

     <session-factory>
         <property name="dialect">
             org.hibernate.dialect.MySQLDialect
         </property>
         <property name="connection.url">
             jdbc:mysql://localhost:3306/mydb
         </property>
         <property name="connection.username">root</property>
         <property name="connection.password">123456</property>
         <property name="connection.driver_class">
             com.mysql.jdbc.Driver
         </property>
         <property name="myeclipse.connection.profile">
             MyDBAccount
         </property>
         <mapping resource="com/db/entity/Dept.hbm.xml" />
         <mapping resource="com/db/entity/Emp.hbm.xml" />
     </session-factory>

 </hibernate-configuration>

测试用例

 public class TestOneMany {

     public static void main(String[] args) throws ParseException {
         // TODO Auto-generated method stub

         Session session=HibernateSessionFactory.getSession();
         Dept dept1=new Dept();
         dept1.setDeptName("开发部");
         Emp emp1=new Emp();
         emp1.setEmpName("王洋");
         String brithString="1999-03-05";
         SimpleDateFormat sdf=new SimpleDateFormat("yyyy-MM-dd");
         Date brithday=sdf.parse(brithString);
         emp1.setEmpBrithday(brithday);
         Emp emp2=new Emp();
         emp2.setEmpName("李林");
         brithString="2005-02-07";
         brithday=sdf.parse(brithString);
         emp2.setEmpBrithday(brithday);
         //把emp和dpt分别添加到对方的实力对象中
         dept1.getEmps().add(emp1);
         dept1.getEmps().add(emp2);
         emp1.setDept(dept1);
         emp2.setDept(dept1);
         session.beginTransaction();
         session.save(dept1);
         session.getTransaction().commit();
     }

 }

注意：

在一的set配置中声明inverse=“true”，意味着Dept不在作为主控方，将关联关系的维护工作交给关联对象Emp来完成。在保存dept1对象的时不在关心Emp类中dept属性对应的deptId列，必须由Emp自己去维护，即设置emp.setDept(dept)

上述操作完成的sql语句是：

Hibernate: insert into mydb.dept (deptName) values (?)
Hibernate: insert into mydb.emp (empName, empBrithday, deptId) values (?, ?, ?)
Hibernate: insert into mydb.emp (empName, empBrithday, deptId) values (?, ?, ?)