JPA中的多对多和联合主键映射

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
 */
@Entity
public class Student {
	@Id @GeneratedValue
	private 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
 */
@Entity
public class Teacher {
	@Id @GeneratedValue
	private 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
	@Override
	public int hashCode() {
		final int prime = 31;
		int result = 1;
		result = prime * result + ((id == null) ? 0 : id.hashCode());
		return result;
	}
	
	@Override
	public 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/persistence
	http://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
	 */
	@Test
	public 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
	 */
	@Test
	public 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
	 */
	@Test
	public 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
	 */
	@Test
	public 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Teacher
		em.remove(em.getReference(Teacher.class, 2));
		em.getTransaction().commit();
		em.close();
		factory.close();
	}
	
	/**
	 * 删除学生
	 */
	@Test
	public 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

---

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;
	}

	@Override
	public 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;
	}
	@Override
	public 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
 */
@Entity
public 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 {
	@Test
	public 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();
	}
}