Hello World

让我们看看创建一个简单的 Hallo World 的所有基本组件。

1. 定义我们将使用的 JPA 2.1 的实现
2. 建立与数据库的连接，创建 persistence-unit
3. 实现实体
4. 实现 DAO（数据访问对象）以操作实体
5. 测试应用程序

库

使用 maven，我们需要这种依赖：

<dependencies>

    <!-- JPA is a spec, I'll use the implementation with HIBERNATE -->
    <dependency>
        <groupId>org.hibernate</groupId>
        <artifactId>hibernate-entitymanager</artifactId>
        <version>5.2.6.Final</version>
    </dependency>

    <!-- JDBC Driver, use in memory DB -->
    <dependency>
        <groupId>com.h2database</groupId>
        <artifactId>h2</artifactId>
        <version>1.4.193</version>
    </dependency>

</dependencies>

持久单位

在 resources 文件夹中，我们需要创建一个名为 persistence.xml 的文件。定义它的最简单方法是这样的：

<persistence-unit name="hello-jpa-pu" transaction-type="RESOURCE_LOCAL">
    <provider>org.hibernate.jpa.HibernatePersistenceProvider</provider>

    <properties>
        <!-- ~ = relative to current user home directory -->
        <property name="javax.persistence.jdbc.url" value="jdbc:h2:./test.db"/>
        <property name="javax.persistence.jdbc.user" value=""/>
        <property name="javax.persistence.jdbc.password" value=""/>
        <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/>
        <property name="hibernate.dialect" value="org.hibernate.dialect.H2Dialect"/>
        <property name="hibernate.show_sql" value="true"/>

        <!-- This create automatically the DDL of the database's table -->
        <property name="hibernate.hbm2ddl.auto" value="create-drop"/>

    </properties>
</persistence-unit>

实施实体

我创建了一个类 Biker：

package it.hello.jpa.entities;

import javax.persistence.*;
import java.io.Serializable;
import java.util.Date;
import java.util.List;

@Entity
@Table(name = "BIKER")
public class Biker implements Serializable {

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    private Long id;

    @Column(name = "bikerName")
    private String name;

    @Column(unique = true, updatable = false)
    private String battleName;

    private Boolean beard;

    @Temporal(TemporalType.DATE)
    private Date birthday;

    @Temporal(TemporalType.TIME)
    private Date registrationDate;

    @Transient // --> this annotiation make the field transient only for JPA
    private String criminalRecord; 

    public Long getId() {
        return this.id;
    }

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

    public String getName() {
        return this.name;
    }

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

    public String getBattleName() {
        return battleName;
    }

    public void setBattleName(String battleName) {
        this.battleName = battleName;
    }

    public Boolean getBeard() {
        return this.beard;
    }

    public void setBeard(Boolean beard) {
        this.beard = beard;
    }

    public Date getBirthday() {
        return birthday;
    }

    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }

    public Date getRegistrationDate() {
        return registrationDate;
    }

    public void setRegistrationDate(Date registrationDate) {
        this.registrationDate = registrationDate;
    }

    public String getCriminalRecord() {
        return criminalRecord;
    }

    public void setCriminalRecord(String criminalRecord) {
        this.criminalRecord = criminalRecord;
    }
}

实施 DAO

package it.hello.jpa.business;

import it.hello.jpa.entities.Biker;

import javax.persistence.EntityManager;
import java.util.List;

public class MotorcycleRally {

    public Biker saveBiker(Biker biker) {
        EntityManager em = EntityManagerUtil.getEntityManager();
        em.getTransaction().begin();
        em.persist(biker);
        em.getTransaction().commit();
        return biker;
    }

}

EntityManagerUtil 是一个单例：

package it.hello.jpa.utils;

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

public class EntityManagerUtil {

    // USE THE SAME NAME IN persistence.xml!
    public static final String PERSISTENCE_UNIT_NAME = "hello-jpa-pu";

    private static EntityManager entityManager;

    private EntityManagerUtil() {
    }

    public static EntityManager getEntityManager() {
        if (entityManager == null) {
            // the same in persistence.xml
            EntityManagerFactory emFactory = Persistence.createEntityManagerFactory(PERSISTENCE_UNIT_NAME);

            return emFactory.createEntityManager();
        }
        return entityManager;
    }
}

测试应用程序

包 it.hello.jpa.test;

公共课 TestJpa {

@Test
public void insertBiker() {
    MotorcycleRally crud = new MotorcycleRally();

    Biker biker = new Biker();
    biker.setName("Manuel");
    biker.setBeard(false);

    biker = crud.saveBiker(biker);

    Assert.assertEquals(biker.getId(), Long.valueOf(1L));
}

}

输出将是：

运行 it.hello.jpa.test.TestJpa Hibernate：drop table BIKER 如果存在 Hibernate：drop sequence if if hibernate_sequence Hibernate：创建序列 hibernate_sequence 以 1 增量开始 1 Hibernate：create table BIKER（id bigint not null，battleName varchar(255) ），胡子布尔，生日日期，bikerName varchar(255)，registrationDate 时间，主键（id））Hibernate：alter table BIKER add constraint UK_a64ce28nywyk8wqrvfkkuapli unique(battleName)Hibernate：插入 BIKER（battleName，胡子，生日，bikerName，registrationDate） ，id）值（？，？，？，？，？，？）mar 01,2017 11:00:02 PM org.hibernate.jpa.internal.util.LogHelper logPersistenceUnitInformation INFO：HHH000204：处理 PersistenceUnitInfo [name：hello- jpa-pu …]结果：

测试运行：1，失败：0，错误：0，跳过：0