JPA中的多对多和联合主键映射

完整版见https://jadyer.github.io/2011/02/11/jpa-composite-pk-mapping/

下面的Java Project，示例的是多对多映射

首先是学生的实体Bean

package com.jadyer.model;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.JoinColumn;import javax.persistence.JoinTable;import javax.persistence.ManyToMany;/** * 学生的实体Bean * @see 三个属性的setter和getter略 */@Entitypublic class Student {@Id @GeneratedValueprivate Integer id;@Column(length=10, nullable=false)private String name;//@JoinTable(name="student_teacher") //这种情况下中间表的字段名默认为students_id和teachers_id//@JoinTable注解的inverseJoinColumns属性指的是被维护端的外键，joinColumns用来定义关联表中维护端的外键@ManyToMany(cascade=CascadeType.REFRESH)@JoinTable(name="student_teacher", joinColumns=@JoinColumn(name="student_id"), inverseJoinColumns=@JoinColumn(name="teacher_id"))private Set<Teacher> teachers = new HashSet<Teacher>();public Student(){}public Student(String name) {this.name = name;}/** * 建立学生跟老师的关系 */public void addTeacher(Teacher teacher){this.teachers.add(teacher);}/** * 解除学生跟老师的关系 */public void removeTeacher(Teacher teacher){if(this.teachers.contains(teacher)){this.teachers.remove(teacher);}}}

然后是教师的实体Bean

package com.jadyer.model;import java.util.HashSet;import java.util.Set;import javax.persistence.CascadeType;import javax.persistence.Column;import javax.persistence.Entity;import javax.persistence.GeneratedValue;import javax.persistence.Id;import javax.persistence.ManyToMany;/** * 教师的实体Bean * @see 三个属性的setter和getter略 */@Entitypublic class Teacher {@Id @GeneratedValueprivate Integer id;@Column(length=10, nullable=false)private String name;//双向多对多是一种对等关系，所以我们可以人为的决定关系维护端@ManyToMany(mappedBy="teachers", cascade=CascadeType.REFRESH)private Set<Student> students = new HashSet<Student>();public Teacher(){}public Teacher(String name) {this.name = name;}//用来判断两个对象是否相同，所以要重写hashCode()和equals()方法@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((id == null) ? 0 : id.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;final Teacher other = (Teacher) obj;if (id == null) {if (other.id != null)return false;} else if (!id.equals(other.id))return false;return true;}}

用到的位于src下的META-INF目录下的persistence.xml文件

<?xml version="1.0" encoding="UTF-8"?><persistence xmlns="http://java.sun.com/xml/ns/persistence"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://java.sun.com/xml/ns/persistencehttp://java.sun.com/xml/ns/persistence/persistence_1_0.xsd"version="1.0"><persistence-unit name="JPAManyToManyDemo" transaction-type="RESOURCE_LOCAL"><properties><property name="hibernate.dialect" value="org.hibernate.dialect.OracleDialect" /><property name="hibernate.show_sql" value="true"/><property name="hibernate.format_sql" value="true"/><property name="hibernate.hbm2ddl.auto" value="update" /><property name="hibernate.connection.driver_class" value="oracle.jdbc.OracleDriver" /><property name="hibernate.connection.username" value="scott" /><property name="hibernate.connection.password" value="jadyer" /><property name="hibernate.connection.url" value="jdbc:oracle:thin:@127.0.0.1:1521:jadyer" /></properties></persistence-unit></persistence>

最后是使用的JUnit4单元测试类ManyToManyTest.java

package com.jadyer.junit;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;import com.jadyer.model.Student;import com.jadyer.model.Teacher;public class ManyToManyTest {/** * 添加学生和老师 */@Testpublic void save(){EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPAManyToManyDemo");EntityManager em = factory.createEntityManager();em.getTransaction().begin();em.persist(new Student("令狐冲"));em.persist(new Teacher("风清扬"));em.getTransaction().commit();em.close();factory.close();}/** * 建立学生跟老师的关系 */@Testpublic void buildTS(){EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPAManyToManyDemo");EntityManager em = factory.createEntityManager();em.getTransaction().begin();Student student = em.find(Student.class, 3);student.addTeacher(em.getReference(Teacher.class, 4));em.getTransaction().commit();em.close();factory.close();}/** * 解除学生跟老师的关系 */@Testpublic void removeTS(){EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPAManyToManyDemo");EntityManager em = factory.createEntityManager();em.getTransaction().begin();Student student = em.find(Student.class, 3);student.removeTeacher(em.getReference(Teacher.class, 4));em.getTransaction().commit();em.close();factory.close();}/** * 删除老师 */@Testpublic void deleteTeacher(){EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPAManyToManyDemo");EntityManager em = factory.createEntityManager();em.getTransaction().begin();Student student = em.find(Student.class, 1);Teacher teacher = em.getReference(Teacher.class, 2);student.removeTeacher(teacher);//Teacher是关系被维护端，所以它没有权利直接更新外键，所以必须在解除关系之后再删除Teacherem.remove(em.getReference(Teacher.class, 2));em.getTransaction().commit();em.close();factory.close();}/** * 删除学生 */@Testpublic void deleteStudent(){EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPAManyToManyDemo");EntityManager em = factory.createEntityManager();em.getTransaction().begin();Student student = em.getReference(Student.class, 1);//Student是关系维护端，有权利更新外键，所以能够直接删除em.remove(student);em.getTransaction().commit();em.close();factory.close();}}

上面就是多对多映射的示例代码

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下面是联合主键映射的示例代码

下面的Java Project，演示联合主键映射

首先是飞机航线的复合主键类

package com.jadyer.model;import java.io.Serializable;import javax.persistence.Column;import javax.persistence.Embeddable;/** * 飞机航线的复合主键类 * @see 通常复合主键类的类名尾部都带有PK字样 * @see 在复合主键类中通常只定义用作主键的字段 * @see 作为复合主键类，以下三点是必须要遵守的JPA规范 * @see 1.必须提供一个public的无参构造函数 * @see 2.必须实现序列化接口 * @see 3.必须重写hashCode()和equals()方法，其中以复合主键类的字段作为判断依据 * @see ------------------------------------------------------------------------ * @see 两个数据的setter和getter略 */@SuppressWarnings("serial")@Embeddable //即嵌入注解，它是用来告诉JPA，我们只是使用复合主键类里面的属性，作为实体的持久化字段public class AirLinePK implements Serializable {//飞机航线在国际上是有标准的，通常都会采用三字码来代表出发城市和目的城市//所谓三字码就是三个英文字母。比如北京的三字码是PEK，广州是CAN，上海是SHA//所以这里字段的长度只需要设成3位就可以@Column(length=3)private String startCity;@Column(length=3)private String endCity;public AirLinePK(){}public AirLinePK(String startCity, String endCity) {this.startCity = startCity;this.endCity = endCity;}@Overridepublic int hashCode() {final int prime = 31;int result = 1;result = prime * result + ((endCity == null) ? 0 : endCity.hashCode());result = prime * result+ ((startCity == null) ? 0 : startCity.hashCode());return result;}@Overridepublic boolean equals(Object obj) {if (this == obj)return true;if (obj == null)return false;if (getClass() != obj.getClass())return false;final AirLinePK other = (AirLinePK) obj;if (endCity == null) {if (other.endCity != null)return false;} else if (!endCity.equals(other.endCity))return false;if (startCity == null) {if (other.startCity != null)return false;} else if (!startCity.equals(other.startCity))return false;return true;}}

然后是飞机航线的实体Bean

package com.jadyer.model;import javax.persistence.Column;import javax.persistence.EmbeddedId;import javax.persistence.Entity;/** * 飞机航线的实体Bean * @see 两个属性的setter和getter略 */@Entitypublic class AirLine {@EmbeddedId //用于标注该属性为实体的标识符，该注解专门用于复合主键类private AirLinePK id;@Column(length=20)private String name;public AirLine(){}public AirLine(String startCity, String endCity, String name){this.id = new AirLinePK(startCity, endCity);this.name = name;}}

这里用到的persistence.xml与上面多对多映射中用到的，相同，故略去

最后是用到的JUnit4单元测试类CompositePKTest.java

package com.jadyer.junit;import javax.persistence.EntityManager;import javax.persistence.EntityManagerFactory;import javax.persistence.Persistence;import org.junit.Test;import com.jadyer.model.AirLine;public class CompositePKTest {@Testpublic void save(){EntityManagerFactory factory = Persistence.createEntityManagerFactory("JPACompositePKDemo");EntityManager em = factory.createEntityManager();em.getTransaction().begin();em.persist(new AirLine("PEK", "SHA", "北京飞往上海"));em.getTransaction().commit();em.close();factory.close();}}