Hibernate_复合主键的用法和关键

复合主键在表中有多个属性共同确定一个元素，对应到实体类中就有多个属性共同构成主键属性。可以把主键属性分离出来单独成一个类，也可以不分离。

如果不分离，那实体类也是主键类，关键点是主键类要实现Serializable接口,并且重写hashCode和equals方法。

下面的例子是将复合主键分离出来单独成一个类：

StudentKey.java:

/**
 * 包含学生表格主键对应的属性，主键类
 * 
 * @author hanyuan
 * @time 2012-8-23 下午09:21:51
 */
public class StudentKey implements Serializable {
    // 学生学号
    private long studentId;
    // 学生姓名
    private String name;

    public long getStudentId() {
        return studentId;
    }

    public void setStudentId(long studentId) {
        this.studentId = studentId;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    //重写hashCode和equals方法
    @Override
    public int hashCode() {
        final int prime = 31;
        int result = 1;
        result = prime * result + ((name == null) ? 0 : name.hashCode());
        result = prime * result + (int) (studentId ^ (studentId >>> 32));
        return result;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        StudentKey other = (StudentKey) obj;
        if (name == null) {
            if (other.name != null)
                return false;
        }
        else if (!name.equals(other.name))
            return false;
        if (studentId != other.studentId)
            return false;
        return true;
    }
}

再在实体类Student中将主键类包含进来:

Student.java:

/**
 * 对应于student表的实体类，表示学生
 * 
 * @author hanyuan
 * @time 2012-8-23 下午09:20:16
 */
public class Student {
    private StudentKey studentKey;
    private int age;
    private String address;

    public StudentKey getStudentKey() {
        return studentKey;
    }

    public void setStudentKey(StudentKey studentKey) {
        this.studentKey = studentKey;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getAddress() {
        return address;
    }

    public void setAddress(String address) {
        this.address = address;
    }

}

实体类Student配置文件：

Student.hbm.xml:

<hibernate-mapping>
    <class name="com.sunflower.entity.Student" table="student">
        <!-- 复合主键 -->
        <composite-id name="studentKey" class="com.sunflower.entity.StudentKey">
            <key-property name="studentId" type="long"></key-property>
            <key-property name="name" type="string"></key-property>
        </composite-id>

        <property name="age" type="integer"></property>
        <property name="address" type="string"></property>
    </class>
</hibernate-mapping>

第４行至第７行是复合主键的配置，如果没有将主键属性分离出来单独成一个主键类，那么就不用加 class="com.sunflower.entity.StudentKey".

Hibernate_根据配置文件反向生成表格 得到：

HibernateUtil.java：

/**
 * 解析配置文件，创建session的工具类
 * 
 * @author hanyuan
 * @time 2012-8-23 下午09:42:35
 */
public class HibernateUtil {
    private static SessionFactory factory;

    static {
        Configuration cfg = null;
        // 解析配置文件
        try {
            cfg = new Configuration().configure();
        }
        catch (Exception e) {
            e.printStackTrace();
        }
        factory = cfg.buildSessionFactory();
    }

    /**
     * 打开一个session
     */
    public static Session getSession() {
        Session session = factory.openSession();
        return session;
    }

    /**
     * 关闭session
     */
    public static void closeSession(Session session) {
        if (null != session)
            session.close();
    }
}

 第２６行中，刚开始写法：Session session = factory.getCurrentSession(), 结果程序报错:session is already closed.

进行测试：

Test.java:

public class Test {
//    // 保存测试
//    public void save() {
//        Session session = HibernateUtil.getSession();
//        Transaction transaction = session.beginTransaction();
//
//        try {
//            StudentKey studentKey = new StudentKey();
//            studentKey.setStudentId(20090124);
//            studentKey.setName("任德华");
//
//            Student student = new Student();
//            student.setStudentKey(studentKey);
//            student.setAge(22);
//            student.setAddress("人民路２２号");
//            session.save(student);
//            transaction.commit();
//        }
//        catch (Exception e) {
//            if (null != transaction)
//                transaction.rollback();
//            e.printStackTrace();
//        }
//        finally {
//            HibernateUtil.closeSession(session);
//        }
//    }

    // 查询测试
    public void get() {
        Session session = HibernateUtil.getSession();
        Transaction transaction = session.beginTransaction();

        try {
            StudentKey studentKey = new StudentKey();
            studentKey.setStudentId(20090121);
            studentKey.setName("令狐聪");

            Student student = (Student) session.get(Student.class, studentKey);
            transaction.commit();
            
            System.out.println("name is:" + student.getStudentKey().getName() + "\n" + "address is:" + student.getAddress());
        }
        catch (Exception e) {
            if (null != transaction)
                transaction.rollback();
            e.printStackTrace();
        }
        finally {
            HibernateUtil.closeSession(session);
        }
    }

    public static void main(String[] args) {
        Test test = new Test();
//        test.save();
        test.get();
    }
}