Hibernate教程-使用注解在Hibernate中实现一对多映射

在本节中，我们将使用注解来执行一对多关联，以映射持久化类的List对象。

这里，我们使用论坛的场景，其中一个问题有多个答案。

在这种情况下，一个问题可以有多个答案，每个答案可能有自己的信息，因此我们在持久化类中使用了包含Answer类引用的List来表示答案的集合。

使用注解的一对多映射示例

1) 创建持久化类

这个持久化类定义了类的属性，包括List。

Question.java

package cn.javatiku;

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

@Entity
@Table(name="q5991")
public class Question {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE)
    private int id;
    private String qname;

    @OneToMany(cascade = CascadeType.ALL)
    @JoinColumn(name="qid")
    @OrderColumn(name="type")
    private List<Answer> answers;

    public int getId() {
        return id;
    }

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

    public String getQname() {
        return qname;
    }

    public void setQname(String qname) {
        this.qname = qname;
    }

    public List<Answer> getAnswers() {
        return answers;
    }

    public void setAnswers(List<Answer> answers) {
        this.answers = answers;
    }
}

Answer.java

package cn.javatiku;

import javax.persistence.*;

@Entity
@Table(name="ans5991")
public class Answer {

    @Id
    @GeneratedValue(strategy=GenerationType.TABLE)
    private int id;
    private String answername;
    private String postedBy;

    public int getId() {
        return id;
    }

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

    public String getAnswername() {
        return answername;
    }

    public void setAnswername(String answername) {
        this.answername = answername;
    }

    public String getPostedBy() {
        return postedBy;
    }

    public void setPostedBy(String postedBy) {
        this.postedBy = postedBy;
    }
}

2) 在pom.xml文件中添加项目信息和配置

打开pom.xml文件并点击source。现在，在<dependencies>...</dependencies>标签之间添加以下依赖项。

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-core</artifactId>
    <version>5.3.1.Final</version>
</dependency>

<dependency>
    <groupId>com.oracle</groupId>
    <artifactId>ojdbc14</artifactId>
    <version>10.2.0.4.0</version>
</dependency>

3) 创建配置文件

此文件包含有关数据库和映射文件的信息。

<?xml version='1.0' encoding='UTF-8'?>
<!DOCTYPE hibernate-configuration PUBLIC 
    "-//Hibernate/Hibernate Configuration DTD 5.3//EN" 
    "http://hibernate.sourceforge.net/hibernate-configuration-5.3.dtd">

<hibernate-configuration>
    <session-factory>
        <property name="hbm2ddl.auto">update</property>
        <property name="dialect">org.hibernate.dialect.Oracle9Dialect</property>
        <property name="connection.url">jdbc:oracle:thin:@localhost:1521:xe</property>
        <property name="connection.username">system</property>
        <property name="connection.password">jtp</property>
        <property name="connection.driver_class">oracle.jdbc.driver.OracleDriver</property>
        <mapping class="cn.javatiku.Question"/>
    </session-factory>
</hibernate-configuration>

4) 创建存储数据的类

在这个类中，我们存储了Question类的数据。

package cn.javatiku;

import java.util.ArrayList;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

public class StoreData {
    public static void main(String[] args) {
        StandardServiceRegistry ssr = new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();
        Metadata meta = new MetadataSources(ssr).getMetadataBuilder().build();

        SessionFactory factory = meta.getSessionFactoryBuilder().build();
        Session session = factory.openSession();

        Transaction t = session.beginTransaction();

        Answer ans1 = new Answer();
        ans1.setAnswername("Java is a programming language");
        ans1.setPostedBy("Ravi Malik");

        Answer ans2 = new Answer();
        ans2.setAnswername("Java is a platform");
        ans2.setPostedBy("Sudhir Kumar");

        Answer ans3 = new Answer();
        ans3.setAnswername("Servlet is an Interface");
        ans3.setPostedBy("Jai Kumar");

        Answer ans4 = new Answer();
        ans4.setAnswername("Servlet is an API");
        ans4.setPostedBy("Arun");

        ArrayList<Answer> list1 = new ArrayList<Answer>();
        list1.add(ans1);
        list1.add(ans2);

        ArrayList<Answer> list2 = new ArrayList<Answer>();
        list2.add(ans3);
        list2.add(ans4);

        Question question1 = new Question();
        question1.setQname("What is Java?");
        question1.setAnswers(list1);

        Question question2 = new Question();
        question2.setQname("What is Servlet?");
        question2.setAnswers(list2);

        session.persist(question1);
        session.persist(question2);

        t.commit();
        session.close();
        System.out.println("success");
    }
}

输出

如何获取List的数据

这里，我们使用HQL来获取Question类的所有记录，包括答案。在这种情况下，它从功能依赖的两个表中获取数据。

package cn.javatiku;

import java.util.*;
import javax.persistence.TypedQuery;
import org.hibernate.*;
import org.hibernate.boot.Metadata;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

public class FetchData {
    public static void main(String[] args) {
        StandardServiceRegistry ssr = new StandardServiceRegistryBuilder().configure("hibernate.cfg.xml").build();
        Metadata meta = new MetadataSources(ssr).getMetadataBuilder().build();

        SessionFactory factory = meta.getSessionFactoryBuilder().build();
        Session session = factory.openSession();

        TypedQuery query = session.createQuery("from Question");
        List<Question> list = query.getResultList();

        Iterator<Question> itr = list.iterator();
        while (itr.hasNext()) {
            Question q = itr.next();
            System.out.println("Question Name: " + q.getQname());

            // 打印答案
            List<Answer> list2 = q.getAnswers();
            Iterator<Answer> itr2 = list2.iterator();
            while (itr2.hasNext()) {
                Answer a = itr2.next();
                System.out.println(a.getAnswername() + ": " + a.getPostedBy());
            }
        }
        session.close();
        System.out.println("success");
    }
}

输出