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 (?, ?, ?)