三、JPA增删改查常用方法

前言：创建EntityManager对象，需要先创建创建EntityManagerFactory对象

方式一：直接通过persistenceUnitName创建

 String persistenceUnitName = "Jpa-1";
 EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory(persistenceUnitName);

方式二：通过persistenceUnitName和properties一起创建

 String persistenceUnitName = "Jpa-1";
 Map<String, Object> properties = new HashMap<String, Object>();
 properties.put("hibernate.show_sql", false);
 EntityManagerFactory entityManagerFactory = Persistence.createEntityManagerFactory(persistenceUnitName, properties);

2.创建一个测试增删改查的JPA项目

 package com.jtfr.jpa.test;

 import javax.persistence.EntityManager;
 import javax.persistence.EntityManagerFactory;
 import javax.persistence.EntityTransaction;
 import javax.persistence.Persistence;

 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;

 public class JpaTest {

     private EntityManagerFactory entityManagerFactory;
     private EntityManager entityManager;
     private EntityTransaction transaction;

     @Before
     public void create(){
         entityManagerFactory = Persistence.createEntityManagerFactory("Jpa-1");
         entityManager = entityManagerFactory.createEntityManager();
         transaction = entityManager.getTransaction();
         transaction.begin();
     }

     @Test
     public void testFind(){
         System.out.println("没问题");
     }

     @After
     public void end(){
         transaction.commit();
         entityManager.close();
         entityManagerFactory.close();
     }
 }

3.常用增删改查方法解释

查询方法

find方法：方法调用时，已经发出了sql进行查询

 @Test
 public void testFind(){
     CustomerJpa customerJpa = entityManager.find(CustomerJpa.class, 100);
     System.out.println("--------分割线作用：区分确实是find的时候就发出了sql语句---------");
     System.out.println(customerJpa.toString());
 }

getReference方法：方法调用时，返回的是代理对象，当代理对象使用时候才真正发送sql查询数据库，需要注意“懒加载”问题

 @Test // 类似Hibernate的load方法
 public void testGetReference(){
     CustomerJpa customerJpa = entityManager.getReference(CustomerJpa.class, 100);
     System.out.println("返回的是代理对象"+customerJpa.getClass().getName());
     System.out.println("--------getReference方法调用的时候没有发sql-------------");
     System.out.println(customerJpa.toString());
 }

persist方法：方法调用为保存一条数据

 // 类似 Hibernate 的 save 方法
 @Test
 public void testPersist(){
     CustomerJpa customerJpa = new CustomerJpa();
     customerJpa.setId(300100);
     customerJpa.setLasName("夕阳游子");
     customerJpa.setEmail("23228858@qq.com");
     customerJpa.setAge(15);
     customerJpa.setCreateTime(new Date());
     customerJpa.setBirth(new Date());
     entityManager.persist(customerJpa); // 注意不能设置 Id
 }