创建一个简单的JPA Hallo World

Love The Way You Lie 2023-09-30 11:11 5阅读 0赞

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评论 [\#][Link 1]

JPA 是 Java Persistence API，一种处理 Java 对象映射及其与关系数据库的关系的规范。这称为对象关系映射器 (ORM)。它是更底层[JDBC][]的替代（或补充） 。在追求面向 Java 的方法以及需要持久化复杂的对象图时，它最有用。

JPA 本身并不是一个实现。为此，您将需要一个持久性提供程序（参见示例）。最新 JPA 2.1 标准的当前实现是 [EclipseLink][]（也是 JPA 2.1 的参考实现，意思是“证明可以实现规范”）；[休眠][Link 2]和[数据核][Link 3]。

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元数据[\#][Link 4]

Java 对象和数据库表之间的映射是通过**持久性元数据**定义的。JPA 提供者将使用持久性元数据信息来执行正确的数据库操作。JPA 通常通过 Java 类中的注释定义元数据。

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对象关系实体架构[\#][Link 4]

实体架构由以下部分组成：

*  实体
 *  持久性单元
 *  持久化上下文
 *  实体管理器工厂
 *  实体经理

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版本 [\#][Link 5]

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你好世界 [\#][Link 6]

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 实现
2.  建立与创建数据库的连接`persistence-unit`
3.  实现实体
4.  实现 DAO（数据访问对象）来操作实体
5.  测试应用程序

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图书馆 [\#][Link 4]

使用 maven，我们需要这个依赖：

<dependencies>
 
 
 <dependency>
 <groupId>org.hibernate</groupId>
 <artifactId>hibernate-entitymanager</artifactId>
 <version>5.2.6.Final</version>
 </dependency>
 
 
 <dependency>
 <groupId>com.h2database</groupId>
 <artifactId>h2</artifactId>
 <version>1.4.193</version>
 </dependency>
 
 </dependencies>

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持久性单元 [\#][Link 4]

在资源文件夹中，我们需要创建一个名为`persistence.xml`. 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>
 
 <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"/>
 
 
 <property name="hibernate.hbm2ddl.auto" value="create-drop"/>
 
 </properties>
 </persistence-unit>

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实现一个实体 [\#][Link 4]

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;
        }
    }

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Implement a DAO [\#][Link 4]

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;
        }
    }

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测试应用程序 [\#][Link 4]

打包 it.hello.jpa.test；

公共类 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 休眠：如果存在则删除表 BIKER 休眠：如果存在则删除序列 hibernate\_sequence 休眠：创建序列 hibernate\_sequence 以 1 递增 1 开始休眠：创建表 BIKER（id bigint 不为 null，battleName varchar（255 )、beard boolean、生日日期、bikerName varchar(255)、registrationDate 时间、主键 (id)) Hibernate：更改表 BIKER 添加约束 UK\_a64ce28nywyk8wqrvfkkuapli 唯一（battleName） Hibernate：插入 BIKER（battleName、beard、birthday、bikerName、registrationDate , id) 值 (?, ?, ?, ?, ?, ?) mar 01, 2017 11:00:02 PM org.hibernate.jpa.internal.util.LogHelper logPersistenceUnitInformation INFO: HHH000204: Processing PersistenceUnitInfo \[ name: hello- jpa-pu ...\] 结果：
> 
> 测试运行：1，失败：0，错误：0，跳过：0

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安装或设置 [\#][Link 7]

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类路径要求 [\#][Link 4]

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Eclipselink [\#][Link 4]

需要包含 Eclipselink 和 JPA API。示例 Maven 依赖项：

<dependencies>
 <dependency>
 <groupId>org.eclipse.persistence</groupId>
 <artifactId>eclipselink</artifactId>
 <version>2.6.3</version>
 </dependency>
 <dependency>
 <groupId>org.eclipse.persistence</groupId>
 <artifactId>javax.persistence</artifactId>
 <version>2.1.1</version>
 </dependency>
 
 </dependencies>

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休眠 [\#][Link 4]

需要休眠核心。示例 Maven 依赖项：

<dependencies>
 <dependency>
 
 
 <groupId>org.hibernate</groupId>
 <artifactId>hibernate-core</artifactId>
 <version>5.2.1.Final</version>
 </dependency>
 <dependency>
 <groupId>org.hibernate.javax.persistence</groupId>
 <artifactId>hibernate-jpa-2.1-api</artifactId>
 <version>1.0.0</version>
 </dependency>
 
 </dependencies>

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数据核 [\#][Link 4]

需要 datanucleus-core、datanucleus-api-jpa 和 datanucleus-rdbms（使用 RDBMS 数据存储时）。示例 Maven 依赖项：

<dependencies>
 <dependency>
 <groupId>org.datanucleus</groupId>
 <artifactId>datanucleus-core</artifactId>
 <version>5.0.0-release</version>
 </dependency>
 <dependency>
 <groupId>org.datanucleus</groupId>
 <artifactId>datanucleus-api-jpa</artifactId>
 <version>5.0.0-release</version>
 </dependency>
 <dependency>
 <groupId>org.datanucleus</groupId>
 <artifactId>datanucleus-rdbms</artifactId>
 <version>5.0.0-release</version>
 </dependency>
 <dependency>
 <groupId>org.datanucleus</groupId>
 <artifactId>javax.persistence</artifactId>
 <version>2.1.2</version>
 </dependency>
 
 </dependencies>

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配置详情 [\#][Link 4]

JPA 需要使用文件*persistence.xml*，位于`META-INF` CLASSPATH 的根目录下。此文件包含 JPA 可以操作的可用持久性单元的定义。

JPA 还允许使用映射配置文件*orm.xml*，该文件也位于`META-INF`. 此映射文件用于配置类如何映射到数据存储，并且是在 JPA 实体类本身中使用 Java 注释的替代/补充。

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最小的 persistence.xml 示例 [\#][Link 8]

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Hibernate（和嵌入式 H2 DB） [\#][Link 4]

<?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_1.xsd"
 version="2.1">
 
 <persistence-unit name="persistenceUnit">
 <provider>org.hibernate.ejb.HibernatePersistence</provider>
 
 <class>my.application.entities.MyEntity</class>
 
 <properties>
 <property name="javax.persistence.jdbc.driver" value="org.h2.Driver" /> 
 <property name="javax.persistence.jdbc.url" value="jdbc:h2:data/myDB.db" /> 
 <property name="javax.persistence.jdbc.user" value="sa" /> 
 
 
 <property name="javax.persistence.schema-generation.database.action" value="drop-and-create"/>
 
 <property name="hibernate.dialect" value="org.hibernate.dialect.H2Dialect"/>
 <property name="hibernate.flushMode" value="FLUSH_AUTO" /> 
 </properties>
 </persistence-unit>
 </persistence>

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Eclipselink（和嵌入式 H2 DB） [\#][Link 4]

<?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_1.xsd"
 version="2.1">
 
 <persistence-unit name="persistenceUnit">
 <provider>org.eclipse.persistence.jpa.PersistenceProvider</provider>
 
 <class>my.application.entities.MyEntity</class>
 
 <properties>
 <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/>
 <property name="javax.persistence.jdbc.url" value="jdbc:h2:data/myDB.db"/>
 <property name="javax.persistence.jdbc.user" value="sa"/>
 
 
 <property name="javax.persistence.schema-generation.database.action" value="drop-and-create-tables" />
 </properties>
 </persistence-unit>
 
 </persistence>

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DataNucleus（和嵌入式 H2 DB） [\#][Link 4]

<?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_1.xsd"
 version="2.1">
 
 <persistence-unit name="persistenceUnit">
 <provider>org.datanucleus.api.jpa.PersistenceProviderImpl</provider>
 
 <class>my.application.entities.MyEntity</class>
 
 <properties>
 <property name="javax.persistence.jdbc.driver" value="org.h2.Driver"/>
 <property name="javax.persistence.jdbc.url" value="jdbc:h2:data/myDB.db"/>
 <property name="javax.persistence.jdbc.user" value="sa"/>
 
 
 <property name="javax.persistence.schema-generation.database.action" value="drop-and-create-tables" />
 </properties>
 </persistence-unit>
 
 </persistence>

[Link 1]: https://riptutorial.com/jpa#remarks
[JDBC]: https://riptutorial.com/jdbc/topic/1685/getting-started-with-jdbc
[EclipseLink]: https://www.eclipse.org/eclipselink/
[Link 2]: http://hibernate.org/
[Link 3]: http://www.datanucleus.org/
[Link 4]: https://riptutorial.com/jpa#undefined
[Link 5]: https://riptutorial.com/jpa#versions
[Link 6]: https://riptutorial.com/jpa#hello-world
[Link 7]: https://riptutorial.com/jpa#installation-or-setup
[Link 8]: https://riptutorial.com/jpa#minimal-persistence-xml-example