一、JPA快速入门

一、关于JPA

    关于JPA是什么，在这里不做解释，感兴趣的话可以参照：

     http://blog.csdn.net/chjttony/article/details/6086298
     http://www.cnblogs.com/holbrook/archive/2012/12/30/2839842.html

二、首先而且必要的事搭建JPA环境

    1.千里之行始于jar包

    2.配置persistence.xml 

<?xml version="1.0" encoding="UTF-8"?>
<persistence version="2.0" 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">	
    <persistence-unit name="JPA002" transaction-type="RESOURCE_LOCAL">
        <class>com.tan.jpa.bean.Person</class>
        <class>com.tan.jpa.bean.Customer</class>
        <class>com.tan.jpa.bean.Order</class>
    <properties>
        <property name="javax.persistence.jdbc.driver" value="com.mysql.jdbc.Driver" />
        <property name="javax.persistence.jdbc.url" value="jdbc:mysql:///tan " />
        <property name="javax.persistence.jdbc.user" value="root" />
        <property name="javax.persistence.jdbc.password" value="123456" />

        <property name="hibernate.dialect" value="org.hibernate.dialect.MySQL5InnoDBDialect" />
        <property name="hibernate.hbm2ddl.auto" value="update" />
        <property name="hibernate.show_sql" value="true" />
        <property name="hibernate.format_sql" value="false" />

    </properties>

    </persistence-unit>
</persistence>

    3.新建各种Bean

Person实体：

/*
 * Creation : 2015年3月25日
 */
package com.tan.jpa.bean;

import java.util.Date;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.Lob;
import javax.persistence.Table;
import javax.persistence.Temporal;
import javax.persistence.TemporalType;

@Entity
@Table(name = "person")
public class Person {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @Column(name = "name", length = 10, nullable = false)
    private String name;

    @Temporal(TemporalType.DATE)
    // 指定为日期类型
    private Date birth;

    @Lob
    // 指定为处理大文本或二进制数据的映射注解
    private String info;

    public Person() {
    }

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public Date getBirth() {
        return birth;
    }

    public void setBirth(Date birth) {
        this.birth = birth;
    }

    public String getInfo() {
        return info;
    }

    public void setInfo(String info) {
        this.info = info;
    }

    public Person(String name, Date birth, String info) {
        super();
        this.name = name;
        this.birth = birth;
        this.info = info;
    }

    public Person(String name, String info) {
        super();
        this.name = name;
        this.info = info;
    }

    @Override
    public String toString() {
        return "Person [id=" + id + ", name=" + name + ", birth=" + birth + ", info=" + info + "]";
    }

}

/*
 * Creation : 2015年3月26日
 */
package com.tan.jpa.bean;

import java.util.HashSet;
import java.util.Set;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.GenerationType;
import javax.persistence.Id;
import javax.persistence.OneToMany;
import javax.persistence.Table;

@Table(name = "customer")
@Entity
public class Customer {
    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Integer id;

    @Column(name = "Last_Name")
    private String lastName;

    @OneToMany(mappedBy = "customer")
    private Set<Order> orders = new HashSet<Order>();

    public Integer getId() {
        return id;
    }

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

    public String getLastName() {
        return lastName;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public Set<Order> getOrders() {
        return orders;
    }

    public void setOrders(Set<Order> orders) {
        this.orders = orders;
    }

}

/*
 * Creation : 2015年3月26日
 */
package com.tan.jpa.bean;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.ManyToOne;
import javax.persistence.Table;

@Table(name = "orders")
@Entity
public class Order {
    @Id
    @GeneratedValue
    private Integer id;

    @Column(name = "ORDER_NAME")
    private String orderName;

    @JoinColumn(name = "CUST_ID")
    @ManyToOne
    private Customer customer;

    public Integer getId() {
        return id;
    }

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

    public String getOrderName() {
        return orderName;
    }

    public void setOrderName(String orderName) {
        this.orderName = orderName;
    }

    public Customer getCustomer() {
        return customer;
    }

    public void setCustomer(Customer customer) {
        this.customer = customer;
    }
}

/*
 * Creation : 2015年3月26日
 */
package com.jpa.test;

import java.util.Date;
import java.util.List;

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

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

import com.tan.jpa.bean.Customer;
import com.tan.jpa.bean.Order;
import com.tan.jpa.bean.Person;

public class JPATest {
    public EntityManagerFactory factory = null;
    public EntityManager em = null;

    @Before
    public void init() {
        factory = Persistence.createEntityManagerFactory("JPA002");
        em = factory.createEntityManager();
        em.getTransaction().begin();
    }

    @After
    public void close() {
        em.getTransaction().commit();
        em.close();
        factory.close();
    }

    // 数据的持久化
    @Test
    public void save() {
        em.persist(new Person("ccc", new Date(), "ccc"));
        System.out.println(" insert data success!");
    }

    @Test
    public void find() {
        // find - get
        // getReference - load
        Person p2 = em.find(Person.class, 2);
        Person p1 = em.getReference(Person.class, 1);
        System.out.println(p1);
        System.out.println(p2);

        // Person [id=1, name=tzq, birth=2015-03-26, info=123]
        // Person [id=2, name=tan, birth=2015-03-26, info=tanzhengqiang]

    }

    /*
     * merge和saveOrUpdate区别： saveOrUpdate后的对象会纳入session的管理，对象的状态会跟数据库同步，再次查询该对象会直接从session中取， merge后的对 象不会纳入session的管理，再次查询该对象还是会从数据库中取。
     */
    @Test
    public void update() {
        Person person = em.find(Person.class, 5);
        System.out.println(person);
        person.setName("ddd");
        person.setInfo("update");
        Person person2 = em.merge(person);

        System.out.println("update success !");
        System.out.println(person.getId() == person2.getId()); // true
    }

    @Test
    public void delete() {
        em.remove(em.find(Person.class, 7));
        System.out.println("delete success !");
    }

    @Test
    // ×
    public void testmerge() {
        Person p = new Person();
        p.setId(6);
        p.setName("xiaoming");
        p.setBirth(new Date());
        p.setInfo("java developer");
        // 无法从临时状态中获取对应的记录，报告：空指针异常 ×
        Person p2 = em.find(Person.class, 6);
        System.out.println(p2.getName());

        em.merge(p);
        System.out.println(p2.getName());

        System.out.println(p == p2);
    }

    @Test
    public void testmerge2() {
        Person p = new Person();
        p.setId(6);
        p.setName("xiaoming");
        p.setBirth(new Date());
        p.setInfo("java developer");
        Person p2 = em.merge(p);

        System.out.println(p == p2); // false
    }

    /*
     * 一对多：有
     */
    @Test
    public void testOneToMany() {
        Customer customer = new Customer();
        customer.setLastName("cust1");

        Order order1 = new Order();
        order1.setOrderName("order1");

        Order order2 = new Order();
        order2.setOrderName("order2");

        order1.setCustomer(customer);
        order2.setCustomer(customer);

        customer.getOrders().add(order1);
        customer.getOrders().add(order2);

        em.persist(customer);
        em.persist(order1);
        em.persist(order2);

    }

    /**
     * jpql测试
     */
    @Test
    public void testJPQL() {
        // 注意JPQL操作的事对象，莫不可以将Person写成表中的person
        String jpql = "SELECT p FROM Person p WHERE p.name LIKE ?1";
        Query query = em.createQuery(jpql);

        query.setParameter(1, "%t%");
        List<Person> stus = query.getResultList();
        System.out.println(stus);
        // [Person [id=1, name=tzq, birth=2015-03-26, info=123], Person [id=2, name=tan, birth=2015-03-26, info=tanzhengqiang]]

    }

    /*
     * 查询部分属性
     */
    @Test
    public void testPartlyProperties() {
        String jpql = "SELECT new Person( p.name,p.info) FROM Person p";
        Query query = em.createQuery(jpql);

        List<Person> stus = query.getResultList();
        for (Person p : stus) {
            System.out.println(p.getName());
        }
    }

    /*
     * 设置命名参数/查询ID>1的所有对象 共有6个，大于1的有5个
     */
    @Test
    public void testNamedParameter() {
        String jpql = "SELECT new Person( p.name,p.info) FROM Person p WHERE p.id>:id";
        Query query = em.createQuery(jpql).setParameter("id", 1);

        List stus = query.getResultList();
        System.out.println(stus.size());// 5
    }

    /*
     * 子查询
     */
    @Test
    public void testSubQuery() {
        String jpql = "SELECT s FROM Person s WHERE s.id = (SELECT min(p.id) FROM Person p)";
        Query query = em.createQuery(jpql);

        Person person = (Person) query.getSingleResult();
        System.out.println(person.getName());// tzq
    }

}

      好了，大概的内容就是这样，不过一般会和Spring/SpringData进行集成，这样它的优越性更加明细。下一节将探讨和springData集成示例，欢迎板砖飞来~~

一、JPA快速入门

原文：http://blog.csdn.net/u010834071/article/details/44678947