jpa入门

JPA(java persistent api) 是sun公司推出的一个对象/关系（ORM）标准，常见的orm厂商有Hibernate, Toplink,jdo等，这些产品都有各自的实现方式，sun 公司为了统一ORM 持久化技术提高代码的灵活性（面向接口编程）推出了这一标准，这套标准只定义了一些接口，具体实现有各个厂商自己决定(类似于jdbc标准,sun提供接口，各个数据库厂商提供具体实现）。下面举例讲述以Hibernate实现JPA的常见操作。

先直接写个简单的例子，从整体上大致了解下JPA的书写流程。

 

1. 定义一个简单的JavaBean

   package com.zt.bean;
   
   import javax.persistence.Column;
   import javax.persistence.Entity;
   import javax.persistence.GeneratedValue;
   import javax.persistence.Id;
   import javax.persistence.Table;
   
   @Entity
   @Table(name="tb_person")
   public class Person {
   	private int id;
   	private String name;
   	@Id
   	@GeneratedValue
   	public int getId() {
   		return id;
   	}
   	public void setId(int id) {
   		this.id = id;
   	}
   	@Column(name="name")
   	public String getName() {
   		return name;
   	}
   	public void setName(String name) {
   		this.name = name;
   	}
   }
   

    
2. JPA 需要在类路径的META-INF有persistence.xml文件，在src目录下新建META-INF目录，然后新建persistence.xml文件：persistence-unit是一系列持久化javabean的集合name 要和 Persistence.createEntityManagerFactory("mysqljpa")的名字一致。 

   <?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_2_0.xsd"
                version="2.0">
       <persistence-unit name="mysqljpa" transaction-type="RESOURCE_LOCAL">
           <properties>
               <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5Dialect"/>
               <property name="hibernate.connection.driver_class" value="com.mysql.jdbc.Driver"/>
               <property name="hibernate.connection.username" value="root"/>
               <property name="hibernate.connection.password" value="root"/>
               <property name="hibernate.connection.url" value="jdbc:mysql://localhost:3306/hibernate?useUnicode=true&amp;characterEncoding=UTF-8"/>
               <property name="hibernate.hbm2ddl.auto" value="update"/>
               <property name="hibernate.show_sql" value="true"/>
               <property name="hibernate.format_sql" value="true"/>
           </properties>
       </persistence-unit>
   </persistence>
   

    
3. 导入jar包（附件可下载）
4. 测试代码
   

   @Test
   	public void testPersistent() {
   		EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory("mysqljpa");
   		
   		EntityManager entityManager = entityManagerFactory.createEntityManager();
   		EntityTransaction transaction = entityManager.getTransaction();
   		transaction.begin();
   		
   		Person person = new Person();
   		person.setName("zhangsan");
   		entityManager.persist(person);
   		transaction.commit();
   		entityManager.close();
   		entityManagerFactory.close();
   	}