集合Map多对多映射（使用xml文件）

我们可以使用set，bag，map等来映射多对多关系。在这里，我们将使用map来进行多对多映射。在这种情况下，将创建三个表。

多对多映射示例

我们需要创建以下文件来映射map元素。首先创建一个项目：ternaryobject，它们分别如下 -

Question.java
User.java
question.hbm.xml
user.hbm.xml
hibernate.cfg.xml
MainTest.java
FetchTest.java

项目：ternaryobject的目录结构如下图所示 -

集合Map多对多映射（使用xml文件）

下面我们看看每个文件的代码。

文件：Question.java

package com.yiibai;

import java.util.Map;

public class Question {
    private int id;
    private String name;
    private Map<String, User> answers;

    public Question() {
    }

    public Question(String name, Map<String, User> answers) {
        super();
        this.name = name;
        this.answers = answers;
    }

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public Map<String, User> getAnswers() {
        return answers;
    }

    public void setAnswers(Map<String, User> answers) {
        this.answers = answers;
    }

}

文件：User.java

package com.yiibai;

public class User {
    private int id;
    private String username, email, country;

    public User() {
    }

    public User(String username, String email, String country) {
        super();
        this.username = username;
        this.email = email;
        this.country = country;
    }

    public int getId() {
        return id;
    }

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

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getCountry() {
        return country;
    }

    public void setCountry(String country) {
        this.country = country;
    }

}

文件：question.hbm.xml

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

<hibernate-mapping>
    <class name="com.yiibai.Question" table="question_m2m">
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="name"></property>

        <map name="answers" table="answer_m2m" cascade="all">
            <key column="questionid"></key>
            <index column="answer" type="string"></index>
            <many-to-many class="com.yiibai.User" column="userid"></many-to-many>
        </map>
    </class>

</hibernate-mapping>

文件：user.hbm.xml

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

<hibernate-mapping>
    <class name="com.yiibai.User" table="user_m2m">
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="username"></property>
        <property name="email"></property>
        <property name="country"></property>
    </class>

</hibernate-mapping>

文件：hibernate.cfg.xml

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

<hibernate-mapping>
    <class name="com.yiibai.Question" table="question_m2m">
        <id name="id">
            <generator class="native"></generator>
        </id>
        <property name="name"></property>

        <map name="answers" table="answer_m2m" cascade="all">
            <key column="questionid"></key>
            <index column="answer" type="string"></index>
            <many-to-many class="com.yiibai.User" column="userid"></many-to-many>
        </map>
    </class>

</hibernate-mapping>

文件：MainTest.java

package com.yiibai;

import java.util.HashMap;
import org.hibernate.*;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.*;

public class MainTest {
    public static void main(String[] args) {
        // 在5.1.0版本中，hibernate则采用如下新方式获取：
        // 1. 配置类型安全的准服务注册类，这是当前应用的单例对象，不作修改，所以声明为final
        // 在configure("cfg/hibernate.cfg.xml")方法中，如果不指定资源路径，默认在类路径下寻找名为hibernate.cfg.xml的文件
        final StandardServiceRegistry registry = new StandardServiceRegistryBuilder()
                .configure("hibernate.cfg.xml").build();
        // 2. 根据服务注册类创建一个元数据资源集，同时构建元数据并生成应用一般唯一的的session工厂
        SessionFactory sessionFactory = new MetadataSources(registry)
                .buildMetadata().buildSessionFactory();

        /**** 上面是配置准备，下面开始我们的数据库操作 ******/
        Session session = sessionFactory.openSession();// 从会话工厂获取一个session

        // creating transaction object
        Transaction t = session.beginTransaction();

        HashMap<String, User> map1 = new HashMap<String, User>();
        map1.put("java is a programming language", new User("张小哥",
                "user2@gmail.com", "usa"));
        map1.put("java is a platform", new User("王达叔",
                "user1@gmail.com", "China"));

        HashMap<String, User> map2 = new HashMap<String, User>();
        map2.put("servlet technology is a server side programming", new User(
                "John Milton", "john.su@gmail.com", "usa"));
        map2.put("Servlet is an Interface", new User("Ashok Kumar",
                "as-top@gmail.com", "China"));

        Question question1 = new Question("Java是什么?", map1);
        Question question2 = new Question("Servlet是什么?", map2);

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

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

文件：FetchTest.java

package com.yiibai;

import java.util.*;
import org.hibernate.*;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;
import org.hibernate.cfg.*;

public class FetchTest {
    public static void main(String[] args) {
        // 在5.1.0版本中，hibernate则采用如下新方式获取：
        // 1. 配置类型安全的准服务注册类，这是当前应用的单例对象，不作修改，所以声明为final
        // 在configure("cfg/hibernate.cfg.xml")方法中，如果不指定资源路径，默认在类路径下寻找名为hibernate.cfg.xml的文件
        final StandardServiceRegistry registry = new StandardServiceRegistryBuilder()
                .configure("hibernate.cfg.xml").build();
        // 2. 根据服务注册类创建一个元数据资源集，同时构建元数据并生成应用一般唯一的的session工厂
        SessionFactory sessionFactory = new MetadataSources(registry)
                .buildMetadata().buildSessionFactory();

        /**** 上面是配置准备，下面开始我们的数据库操作 ******/
        Session session = sessionFactory.openSession();// 从会话工厂获取一个session

        // creating transaction object
        Transaction t = session.beginTransaction();

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

        Iterator<Question> iterator = list.iterator();
        while (iterator.hasNext()) {
            Question question = iterator.next();
            System.out.println("question id:" + question.getId());
            System.out.println("question name:" + question.getName());
            System.out.println("answers.....");
            Map<String, User> map = question.getAnswers();
            Set<Map.Entry<String, User>> set = map.entrySet();

            Iterator<Map.Entry<String, User>> iteratoranswer = set.iterator();
            while (iteratoranswer.hasNext()) {
                Map.Entry<String, User> entry = (Map.Entry<String, User>) iteratoranswer
                        .next();
                System.out.println("answer name:" + entry.getKey());
                System.out.println("answer posted by.........");
                User user = entry.getValue();
                System.out