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
}