JPA 初探—配置及逆向工程增删改查案例

　   JPA全称Java Persistence API，即Java持久化API，它为Java开发人员提供了一种对象/关系映射工具来管理Java应用中的关系数据，结合其他ORM的使用（如Hibernate），能达到简化开发流程的目的，使开发者能够专注于实现自己的业务逻辑上。目前比较成熟的 JPA 框架主要包括 Jboss 的 Hibernate EntityManager、Oracle 捐献给 Eclipse 社区的 EclipseLink、Apache 的 OpenJPA 等。本文中使用EclipseLink Jpa配置实现

一、创建新数据库连接

在MyEclipse中，点击Window—>Open Perspective，选择MyEclipse Database Explorer，创建一个新的数据库连接，如图所示，本文所用是MySQL数据库。

二、创建一个示例项目

1、右击项目—>MyEclipse—>Add Jpa Capabilities，添加EclipseLink，点击下一步，如图：

2、选择刚刚创建的数据库连接，如图所示：

            3、点击完成后，会自动默认在src的META-INF中生成persistence.xml持久化文件（路径不需要修改），且persistence.xml内容如下。其中，中内容是后面通过JPA逆向工程自动添加进的实体类，此处可暂时忽略。

	
		
			org.eclipse.persistence.jpa.PersistenceProvider
		
		com.spring.jersy.jpa.hibernate.model.Country
		com.spring.jersy.jpa.hibernate.model.Province
		com.spring.jersy.jpa.hibernate.model.User
		
			
			
			
			
			
			
		
	

三、JPA逆向工程

MyEclipse提供了JPA逆向工程，能够将数据库中的表、字段等信息自动生成实体类并注解一一对应。具体流程如下：

1、进入MyEclipse Database Explorer，打开刚刚创建的“com.jdbc.mysql.Driver”连接，选择测试数据库“test”。

2、选择要逆向生成的表，右击选择“JPA Reverse Engineering”。弹出如图：

3、生成的实体类如下所示：

package com.spring.jersy.jpa.hibernate.model;

import java.util.HashSet;
import java.util.Set;
import javax.persistence.CascadeType;
import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

/**
 * Country entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table(name = "country", catalog = "test")
public class Country implements java.io.Serializable {

	// Fields

	private Integer cid;
	private String cname;
	private Set provinces = new HashSet(0);

	// Constructors

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

	/** minimal constructor */
	public Country(String cname) {
		this.cname = cname;
	}

	/** full constructor */
	public Country(String cname, Set provinces) {
		this.cname = cname;
		this.provinces = provinces;
	}

	// Property accessors
	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "cId", unique = true, nullable = false)
	public Integer getCid() {
		return this.cid;
	}

	public void setCid(Integer cid) {
		this.cid = cid;
	}

	@Column(name = "cName", nullable = false, length = 32)
	public String getCname() {
		return this.cname;
	}

	public void setCname(String cname) {
		this.cname = cname;
	}

	@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, mappedBy = "country")
	public Set getProvinces() {
		return this.provinces;
	}

	public void setProvinces(Set provinces) {
		this.provinces = provinces;
	}

}

package com.spring.jersy.jpa.hibernate.model;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import static javax.persistence.GenerationType.IDENTITY;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

/**
 * Province entity. @author MyEclipse Persistence Tools
 */
@Entity
@Table(name = "province", catalog = "test")
public class Province implements java.io.Serializable {

	// Fields

	private Short id;
	private Country country;
	private String proName;

	// Constructors

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

	/** full constructor */
	public Province(Country country, String proName) {
		this.country = country;
		this.proName = proName;
	}

	// Property accessors
	@Id
	@GeneratedValue(strategy = IDENTITY)
	@Column(name = "id", unique = true, nullable = false)
	public Short getId() {
		return this.id;
	}

	public void setId(Short id) {
		this.id = id;
	}

	@ManyToOne(fetch = FetchType.LAZY)
	@JoinColumn(name = "cId", nullable = false)
	public Country getCountry() {
		return this.country;
	}

	public void setCountry(Country country) {
		this.country = country;
	}

	@Column(name = "proName", nullable = false, length = 32)
	public String getProName() {
		return this.proName;
	}

	public void setProName(String proName) {
		this.proName = proName;
	}

}

四、JPA测试

JPA与Hibernate很相似，Hibernate依赖SessionFactory—>Session，JPA依赖EntityManagerFactory—>EntityManager实现各种操作。测试案例如下：

package com.spring.jersy.jpa.hibernate.test;

import java.util.Iterator;
import java.util.List;
import javax.persistence.EntityManager;
import javax.persistence.EntityManagerFactory;
import javax.persistence.Persistence;
import javax.persistence.Query;

import com.spring.jersy.jpa.hibernate.model.Country;

public class JpaTest {

	private static EntityManagerFactory entityFactory = Persistence
			.createEntityManagerFactory("springjersyjpa");

	public static void main(String agrs[]) {
		queryProvinceInnerJoinCountry();
	}

	/**
	 * 内连接多表关联查询
	 */
	public static void queryProvinceInnerJoinCountry() {
		EntityManager em = entityFactory.createEntityManager();
		// JPA中JPQL查询，必须为每个table定义别名
		//字段名要和实体类中属性相对应
		String jql = "select p.proName,c.cname from Province p inner join p.country c ";
		Query query = em.createQuery(jql);
		List