注解的示例：Hibernate一对一映射

今天我们将学习Hibernate中的一对一映射。我们将介绍使用注解和XML配置的Hibernate一对一映射示例。

Hibernate中的一对一映射。

大多数情况下，数据库表之间是相互关联的。关联可以有多种形式 – 一对一、一对多和多对多是在更广泛的层面上。这些可以进一步分为单向和双向映射。今天我们将介绍如何使用XML配置和注释配置来实现Hibernate的一对一映射。

Hibernate一对一映射示例数据库设置

首先，我们需要在数据库表中设置一对一映射。我们将为我们的示例创建两个表 – Transaction（交易）和Customer（顾客）。这两个表将具有一对一的映射关系。Transaction将是主表，我们将在Customer表中使用外键进行一对一映射。我将提供MySQL脚本，这是我用于本教程的数据库。如果您使用其他数据库，请确保相应地更改脚本。

-- Create Transaction Table
CREATE TABLE `Transaction` (
  `txn_id` int(11) unsigned NOT NULL AUTO_INCREMENT,
  `txn_date` date NOT NULL,
  `txn_total` decimal(10,0) NOT NULL,
  PRIMARY KEY (`txn_id`)
) ENGINE=InnoDB AUTO_INCREMENT=16 DEFAULT CHARSET=utf8;

-- Create Customer table
CREATE TABLE `Customer` (
  `txn_id` int(11) unsigned NOT NULL,
  `cust_name` varchar(20) NOT NULL DEFAULT '',
  `cust_email` varchar(20) DEFAULT NULL,
  `cust_address` varchar(50) NOT NULL DEFAULT '',
  PRIMARY KEY (`txn_id`),
  CONSTRAINT `customer_ibfk_1` FOREIGN KEY (`txn_id`) REFERENCES `Transaction` (`txn_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8;

上述表格之间的一对一映射的实体关系图（ERD）如下图所示。我们的数据库设置已经准备好了，现在让我们继续进行Hibernate一对一示例项目。

在此项目中，提供了一个Hibernate的一对一映射示例项目结构。

在你的Java IDE中创建一个简单的Maven项目，我使用的是Eclipse。我们最后的项目结构将如下图所示。首先，我们将研究基于XML的Hibernate一对一映射示例，然后我们将使用注释实现相同的内容。

Maven依赖的Hibernate

我们最终的pom.xml文件如下所示。

<project xmlns="https://maven.apache.org/POM/4.0.0" xmlns:xsi="https://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="https://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
  <modelVersion>4.0.0</modelVersion>
  <groupId>com.Olivia.hibernate</groupId>
  <artifactId>HibernateOneToOneMapping</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <dependencies>
  	<dependency>
  		<groupId>org.hibernate</groupId>
  		<artifactId>hibernate-core</artifactId>
  		<version>4.3.5.Final</version>
  	</dependency>
  	<dependency>
  		<groupId>mysql</groupId>
  		<artifactId>mysql-connector-java</artifactId>
  		<version>5.0.5</version>
  	</dependency>
  </dependencies>
</project>

依赖项仅适用于Hibernate和MySQL的Java驱动程序。请注意，我正在使用最新版本的Hibernate（4.3.5.Final）和根据我的MySQL数据库服务器版本（5.0.5）选择的MySQL Java驱动程序。Hibernate 4使用JBoss日志记录，并作为传递依赖项自动导入。您可以在项目的Maven依赖项中确认。如果您使用的是较旧版本的Hibernate，则可能需要添加slf4j依赖项。

使用Hibernate实现一对一映射模型类

反映数据库表的Hibernate一对一映射的模型类将如下所示。

package com.Olivia.hibernate.model;

import java.util.Date;

public class Txn {

	private long id;
	private Date date;
	private double total;
	private Customer customer;
	
	@Override
	public String toString(){
		return id+", "+total+", "+customer.getName()+", "+customer.getEmail()+", "+customer.getAddress();
	}
	public long getId() {
		return id;
	}
	public void setId(long id) {
		this.id = id;
	}
	public Date getDate() {
		return date;
	}
	public void setDate(Date date) {
		this.date = date;
	}
	public double getTotal() {
		return total;
	}
	public void setTotal(double total) {
		this.total = total;
	}
	public Customer getCustomer() {
		return customer;
	}
	public void setCustomer(Customer customer) {
		this.customer = customer;
	}
	
}

package com.Olivia.hibernate.model;

public class Customer {

	private long id;
	private String name;
	private String email;
	private String address;
	
	private Txn txn;
	
	public long getId() {
		return id;
	}
	public void setId(long id) {
		this.id = id;
	}
	public String getName() {
		return name;
	}
	public void setName(String name) {
		this.name = name;
	}
	public String getEmail() {
		return email;
	}
	public void setEmail(String email) {
		this.email = email;
	}
	public String getAddress() {
		return address;
	}
	public void setAddress(String address) {
		this.address = address;
	}
	public Txn getTxn() {
		return txn;
	}
	public void setTxn(Txn txn) {
		this.txn = txn;
	}
	
}

由于我们使用基于XML的配置进行映射，以上的模型类是简单的POJO类或者Java Beans，具有getter-setter方法。我将使用Txn作为类名，以免与Hibernate的API中的Transaction类混淆。

一对一映射配置的 Hibernate 配置

让我们为Txn和Customer表创建一对一映射的Hibernate配置文件。txn.hbm.xml

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
	"https://hibernate.org/dtd/hibernate-mapping-3.0.dtd">
<hibernate-mapping>
	<class name="com.Olivia.hibernate.model.Txn" table="TRANSACTION" >
		<id name="id" type="long">
			<column name="txn_id" />
			<generator class="identity" />
		</id>
		<property name="date" type="date">
			<column name="txn_date" />
		</property>
		<property name="total" type="double">
			<column name="txn_total" />
		</property>
		<one-to-one name="customer" class="com.Olivia.hibernate.model.Customer"
			cascade="save-update" />
	</class>
	
</hibernate-mapping>

需要注意的重要一点是客户属性中关于 hibernate 一对一元素的部分。customer.hbm.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN" 
"https://hibernate.org/dtd/hibernate-mapping-3.0.dtd" >
<hibernate-mapping>

	<class name="com.Olivia.hibernate.model.Customer" table="CUSTOMER">
		<id name="id" type="long">
			<column name="txn_id" />
			<generator class="foreign">
				<param name="property">txn</param>
			</generator>
		</id>
		<one-to-one name="txn" class="com.Olivia.hibernate.model.Txn"
			constrained="true"></one-to-one>

		<property name="name" type="string">
			<column name="cust_name"></column>
		</property>
		<property name="email" type="string">
			<column name="cust_email"></column>
		</property>
		<property name="address" type="string">
			<column name="cust_address"></column>
		</property>
	</class>

</hibernate-mapping>

生成器类“foreign”是用于实现Hibernate外键的重要部分。

Hibernate配置文件

这是一个用于基于XML的Hibernate映射配置的Hibernate配置文件。hibernate.cfg.xml

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
		"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
		"https://hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.password">pankaj123</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost/TestDB</property>
        <property name="hibernate.connection.username">pankaj</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        
        <property name="hibernate.current_session_context_class">thread</property>
        <property name="hibernate.show_sql">true</property>
        
        <mapping resource="txn.hbm.xml"/>
        <mapping resource="customer.hbm.xml"/>
    </session-factory>
</hibernate-configuration>

Hibernate配置文件很简单，它包含了数据库连接属性和Hibernate映射资源。

Hibernate会话工厂工具

这是创建Hibernate SessionFactory实例的实用类。

package com.Olivia.hibernate.util;

import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;

public class HibernateUtil {

	private static SessionFactory sessionFactory;
	
	private static SessionFactory buildSessionFactory() {
        try {
            // Create the SessionFactory from hibernate.cfg.xml
        	Configuration configuration = new Configuration();
        	configuration.configure("hibernate.cfg.xml");
        	System.out.println("Hibernate Configuration loaded");
        	
        	ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties()).build();
        	System.out.println("Hibernate serviceRegistry created");
        	
        	SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        	
            return sessionFactory;
        }
        catch (Throwable ex) {
            System.err.println("Initial SessionFactory creation failed." + ex);
            ex.printStackTrace();
            throw new ExceptionInInitializerError(ex);
        }
    }
	
	public static SessionFactory getSessionFactory() {
		if(sessionFactory == null) sessionFactory = buildSessionFactory();
        return sessionFactory;
    }
}

就这样吧，我们来编写一个测试程序，测试基于Hibernate一对一映射的XML配置。

XML Configuration程序进行Hibernate一对一映射测试。

在一对一映射示例测试程序中，首先我们将创建一个 Txn 对象并保存它。一旦它保存到数据库中，我们将使用生成的标识符来检索 Txn 对象并打印出来。

package com.Olivia.hibernate.main;

import java.util.Date;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

import com.Olivia.hibernate.model.Customer;
import com.Olivia.hibernate.model.Txn;
import com.Olivia.hibernate.util.HibernateUtil;

public class HibernateOneToOneMain {

	public static void main(String[] args) {
		
		Txn txn = buildDemoTransaction();
		
		SessionFactory sessionFactory = null;
		Session session = null;
		Transaction tx = null;
		try{
		//Get Session
		sessionFactory = HibernateUtil.getSessionFactory();
		session = sessionFactory.getCurrentSession();
		System.out.println("Session created");
		//start transaction
		tx = session.beginTransaction();
		//Save the Model object
		session.save(txn);
		//Commit transaction
		tx.commit();
		System.out.println("Transaction ID="+txn.getId());
		
		//Get Saved Trasaction Data
		printTransactionData(txn.getId(), sessionFactory);
		
		}catch(Exception e){
			System.out.println("Exception occured. "+e.getMessage());
			e.printStackTrace();
		}finally{
			if(!sessionFactory.isClosed()){
				System.out.println("Closing SessionFactory");
				sessionFactory.close();
			}
		}
	}

	private static void printTransactionData(long id, SessionFactory sessionFactory) {
		Session session = null;
		Transaction tx = null;
		try{
			//Get Session
			sessionFactory = HibernateUtil.getSessionFactory();
			session = sessionFactory.getCurrentSession();
			//start transaction
			tx = session.beginTransaction();
			//Save the Model object
			Txn txn = (Txn) session.get(Txn.class, id);
			//Commit transaction
			tx.commit();
			System.out.println("Transaction Details=\n"+txn);
			
			}catch(Exception e){
				System.out.println("Exception occured. "+e.getMessage());
				e.printStackTrace();
			}
	}

	private static Txn buildDemoTransaction() {
		Txn txn = new Txn();
		txn.setDate(new Date());
		txn.setTotal(100);
		
		Customer cust = new Customer();
		cust.setAddress("Bangalore, India");
		cust.setEmail("pankaj@gmail.com");
		cust.setName("Pankaj Kumar");
		
		txn.setCustomer(cust);
		
		cust.setTxn(txn);
		return txn;
	}

}

现在当我们在Hibernate测试程序中运行以上一对一的映射时，我们得到以下输出结果。

Hibernate Configuration loaded
Hibernate serviceRegistry created
Session created
Hibernate: insert into TRANSACTION (txn_date, txn_total) values (?, ?)
Hibernate: insert into CUSTOMER (cust_name, cust_email, cust_address, txn_id) values (?, ?, ?, ?)
Transaction ID=19
Hibernate: select txn0_.txn_id as txn_id1_1_0_, txn0_.txn_date as txn_date2_1_0_, txn0_.txn_total as txn_tota3_1_0_, 
customer1_.txn_id as txn_id1_0_1_, customer1_.cust_name as cust_nam2_0_1_, customer1_.cust_email as cust_ema3_0_1_, 
customer1_.cust_address as cust_add4_0_1_ from TRANSACTION txn0_ left outer join CUSTOMER customer1_ on 
txn0_.txn_id=customer1_.txn_id where txn0_.txn_id=?
Transaction Details=
19, 100.0, Pankaj Kumar, pankaj@gmail.com, Bangalore, India
Closing SessionFactory

你可以看到，它运行正常，我们能够使用事务ID从两个表中检索数据。查看Hibernate内部使用的SQL来获取数据时，它使用了连接操作来从两个表中获取数据。

在中国的本地化翻译中，一对一映射注解会被翻译为 “Hibernate一对一映射注解”。

在上面的部分中，我们看到了如何使用基于XML的配置来进行Hibernate的一对一映射，现在让我们看看如何使用JPA和Hibernate注解来实现同样的功能。

Hibernate 配置文件

hibernate-annotation.cfg.xml翻译为中文

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-configuration PUBLIC
		"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
		"https://hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.password">pankaj123</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost/TestDB</property>
        <property name="hibernate.connection.username">pankaj</property>
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        
        <property name="hibernate.current_session_context_class">thread</property>
        <property name="hibernate.show_sql">true</property>
        
        <mapping class="com.Olivia.hibernate.model.Txn1"/>
        <mapping class="com.Olivia.hibernate.model.Customer1"/>
    </session-factory>
</hibernate-configuration>

Hibernate的配置非常简单，就像你可以看到的那样，我有两个带有注解的模型类 – Txn1和Customer1，我们将使用它们。

Hibernate一对一映射注释示例模型类

就一对一映射的注解配置而言，模型类是最重要的部分。让我们看看我们的模型类如何。

package com.Olivia.hibernate.model;

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.OneToOne;
import javax.persistence.Table;

import org.hibernate.annotations.Cascade;

@Entity
@Table(name="TRANSACTION")
public class Txn1 {

	@Id
	@GeneratedValue(strategy=GenerationType.IDENTITY)
	@Column(name="txn_id")
	private long id;
	
	@Column(name="txn_date")
	private Date date;
	
	@Column(name="txn_total")
	private double total;
	
	@OneToOne(mappedBy="txn")
	@Cascade(value=org.hibernate.annotations.CascadeType.SAVE_UPDATE)
	private Customer1 customer;
	
	@Override
	public String toString(){
		return id+", "+total+", "+customer.getName()+", "+customer.getEmail()+", "+customer.getAddress();
	}

        //Getter-Setter methods, omitted for clarity 
}

请注意，大部分的注解都是来自于Java持久化API，因为Hibernate提供了它的实现。然而，如果我们需要进行级联操作，我们需要使用Hibernate的注解org.hibernate.annotations.Cascade和枚举org.hibernate.annotations.CascadeType。

package com.Olivia.hibernate.model;

import javax.persistence.Column;
import javax.persistence.Entity;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.OneToOne;
import javax.persistence.PrimaryKeyJoinColumn;
import javax.persistence.Table;

import org.hibernate.annotations.GenericGenerator;
import org.hibernate.annotations.Parameter;

@Entity
@Table(name="CUSTOMER")
public class Customer1 {

	@Id
	@Column(name="txn_id", unique=true, nullable=false)
	@GeneratedValue(generator="gen")
	@GenericGenerator(name="gen", strategy="foreign", parameters={@Parameter(name="property", value="txn")})
	private long id;
	
	@Column(name="cust_name")
	private String name;
	
	@Column(name="cust_email")
	private String email;
	
	@Column(name="cust_address")
	private String address;
	
	@OneToOne
	@PrimaryKeyJoinColumn
	private Txn1 txn;

        //Getter-Setter methods
}

请注意，我们需要添加@GenericGenerator，这样就可以使用事务中的id而不是生成一个新的id。

Hibernate SessionFactory 实用工具类

创建SessionFactory与我们提供的Hibernate映射方式无关。我们用于创建SessionFactory的实用类如下所示。

package com.Olivia.hibernate.util;

import org.hibernate.SessionFactory;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.Configuration;
import org.hibernate.service.ServiceRegistry;

public class HibernateAnnotationUtil {

	private static SessionFactory sessionFactory;
	
	private static SessionFactory buildSessionFactory() {
        try {
            // Create the SessionFactory from hibernate-annotation.cfg.xml
        	Configuration configuration = new Configuration();
        	configuration.configure("hibernate-annotation.cfg.xml");
        	System.out.println("Hibernate Annotation Configuration loaded");
        	
        	ServiceRegistry serviceRegistry = new StandardServiceRegistryBuilder().applySettings(configuration.getProperties()).build();
        	System.out.println("Hibernate Annotation serviceRegistry created");
        	
        	SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);
        	
            return sessionFactory;
        }
        catch (Throwable ex) {
            System.err.println("Initial SessionFactory creation failed." + ex);
            ex.printStackTrace();
            throw new ExceptionInInitializerError(ex);
        }
    }
	
	public static SessionFactory getSessionFactory() {
		if(sessionFactory == null) sessionFactory = buildSessionFactory();
        return sessionFactory;
    }
}

Hibernate一对一映射注释示例测试程序

这是一个简单的测试程序，用于我们的Hibernate一对一映射注解示例。

package com.Olivia.hibernate.main;

import java.util.Date;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

import com.Olivia.hibernate.model.Customer1;
import com.Olivia.hibernate.model.Txn1;
import com.Olivia.hibernate.util.HibernateAnnotationUtil;

public class HibernateOneToOneAnnotationMain {

	public static void main(String[] args) {
		
		Txn1 txn = buildDemoTransaction();
		
		SessionFactory sessionFactory = null;
		Session session = null;
		Transaction tx = null;
		try{
		//Get Session
		sessionFactory = HibernateAnnotationUtil.getSessionFactory();
		session = sessionFactory.getCurrentSession();
		System.out.println("Session created using annotations configuration");
		//start transaction
		tx = session.beginTransaction();
		//Save the Model object
		session.save(txn);
		//Commit transaction
		tx.commit();
		System.out.println("Annotation Example. Transaction ID="+txn.getId());
		
		//Get Saved Trasaction Data
		printTransactionData(txn.getId(), sessionFactory);
		}catch(Exception e){
			System.out.println("Exception occured. "+e.getMessage());
			e.printStackTrace();
		}finally{
			if(sessionFactory != null && !sessionFactory.isClosed()){
				System.out.println("Closing SessionFactory");
				sessionFactory.close();
			}
		}
	}

	private static void printTransactionData(long id, SessionFactory sessionFactory) {
		Session session = null;
		Transaction tx = null;
		try{
			//Get Session
			sessionFactory = HibernateAnnotationUtil.getSessionFactory();
			session = sessionFactory.getCurrentSession();
			//start transaction
			tx = session.beginTransaction();
			//Save the Model object
			Txn1 txn = (Txn1) session.get(Txn1.class, id);
			//Commit transaction
			tx.commit();
			System.out.println("Annotation Example. Transaction Details=\n"+txn);
			
			}catch(Exception e){
				System.out.println("Exception occured. "+e.getMessage());
				e.printStackTrace();
			}
	}

	private static Txn1 buildDemoTransaction() {
		Txn1 txn = new Txn1();
		txn.setDate(new Date());
		txn.setTotal(100);
		
		Customer1 cust = new Customer1();
		cust.setAddress("San Jose, USA");
		cust.setEmail("pankaj@yahoo.com");
		cust.setName("Pankaj Kr");
		
		txn.setCustomer(cust);
		
		cust.setTxn(txn);
		return txn;
	}

}

在执行以上程序时，这是输出片段。

Hibernate Annotation Configuration loaded
Hibernate Annotation serviceRegistry created
Session created using annotations configuration
Hibernate: insert into TRANSACTION (txn_date, txn_total) values (?, ?)
Hibernate: insert into CUSTOMER (cust_address, cust_email, cust_name, txn_id) values (?, ?, ?, ?)
Annotation Example. Transaction ID=20
Hibernate: select txn1x0_.txn_id as txn_id1_1_0_, txn1x0_.txn_date as txn_date2_1_0_, txn1x0_.txn_total as txn_tota3_1_0_, 
customer1x1_.txn_id as txn_id1_0_1_, customer1x1_.cust_address as cust_add2_0_1_, customer1x1_.cust_email as cust_ema3_0_1_, 
customer1x1_.cust_name as cust_nam4_0_1_ from TRANSACTION txn1x0_ left outer join CUSTOMER customer1x1_ on 
txn1x0_.txn_id=customer1x1_.txn_id where txn1x0_.txn_id=?
Annotation Example. Transaction Details=
20, 100.0, Pankaj Kr, pankaj@yahoo.com, San Jose, USA
Closing SessionFactory

请注意，输出结果与Hibernate一对一的基于XML的配置类似。至此，Hibernate一对一映射示例介绍完毕，您可以从下方链接下载最终项目并进行实践，以便更多学习。