使用注解的Hibernate many-to-many映射

客官°小女子只卖身不卖艺 2022-08-18 14:29 125阅读 0赞

many to many映射是在两个实体间，一个实体可以和其他的多个实体建立关系。例如一个捐赠服务的实体Subscription实体和读取实体都是两个实体，一个捐赠可以有多个读者，一个读者可以有多个捐赠，在这篇文中我们将会学习在Hibernate3中怎么建立这种关系。

### 提出解决方案 ###

为了演示多对多的关系，我们连接两个实体Reader实体和Subscription实体。他们的实体关系如下：

![Center][]

### 拥有者实体 ###

拥有者实体负责建立和维护关系，在我们的例子中，我让Reader实体成为拥有者，@JoinTable注解使用来建立这种关系

package hibernate.test.manyToMany.joinTable;
     
    import java.io.Serializable;
    import java.util.Set;
     
    import javax.persistence.CascadeType;
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.GenerationType;
    import javax.persistence.Id;
    import javax.persistence.JoinColumn;
    import javax.persistence.JoinTable;
    import javax.persistence.ManyToMany;
    import javax.persistence.Table;
    import javax.persistence.UniqueConstraint;
     
    @Entity(name = "ReaderEntity")
    @Table(name = "READER", uniqueConstraints = {
            @UniqueConstraint(columnNames = "ID"),
            @UniqueConstraint(columnNames = "EMAIL") })
    public class ReaderEntity implements Serializable {
     
        private static final long serialVersionUID = -1798070786993154676L;
     
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        @Column(name = "ID", unique = true, nullable = false)
        private Integer readerId;
     
        @Column(name = "EMAIL", unique = true, nullable = false, length = 100)
        private String email;
     
        @Column(name = "FIRST_NAME", unique = false, nullable = false, length = 100)
        private String firstName;
     
        @Column(name = "LAST_NAME", unique = false, nullable = false, length = 100)
        private String lastName;
     
        @ManyToMany(cascade=CascadeType.ALL)
        @JoinTable(name="READER_SUBSCRIPTIONS", joinColumns={@JoinColumn(referencedColumnName="ID")}
                                            , inverseJoinColumns={@JoinColumn(referencedColumnName="ID")}) 
        private Set<SubscriptionEntity> subscriptions;
     
        public Integer getReaderId() {
            return readerId;
        }
     
        public void setReaderId(Integer readerId) {
            this.readerId = readerId;
        }
     
        public String getEmail() {
            return email;
        }
     
        public void setEmail(String email) {
            this.email = email;
        }
     
        public String getFirstName() {
            return firstName;
        }
     
        public void setFirstName(String firstName) {
            this.firstName = firstName;
        }
     
        public String getLastName() {
            return lastName;
        }
     
        public void setLastName(String lastName) {
            this.lastName = lastName;
        }
     
        public Set<SubscriptionEntity> getSubscriptions() {
            return subscriptions;
        }
     
        public void setSubscriptions(Set<SubscriptionEntity> subscriptions) {
            this.subscriptions = subscriptions;
        }
    }

### 关系实体 ###

我们的映射实体是Subscription实体，使用mappedBy建立和Reader实体的关系

package hibernate.test.manyToMany.joinTable;
     
    import java.io.Serializable;
    import java.util.Set;
     
    import javax.persistence.Column;
    import javax.persistence.Entity;
    import javax.persistence.GeneratedValue;
    import javax.persistence.GenerationType;
    import javax.persistence.Id;
    import javax.persistence.ManyToMany;
    import javax.persistence.Table;
    import javax.persistence.UniqueConstraint;
     
    @Entity(name = "SubscriptionEntity")
    @Table(name = "SUBSCRIPTION", uniqueConstraints = {
            @UniqueConstraint(columnNames = "ID")})
    public class SubscriptionEntity implements Serializable
    {
     
        private static final long serialVersionUID = -6790693372846798580L;
     
        @Id
        @GeneratedValue(strategy = GenerationType.IDENTITY)
        @Column(name = "ID", unique = true, nullable = false)
        private Integer subscriptionId;
     
        @Column(name = "SUBS_NAME", unique = true, nullable = false, length = 100)
        private String subscriptionName;
         
        @ManyToMany(mappedBy="subscriptions")
        private Set<ReaderEntity> readers;
     
        public Integer getSubscriptionId() {
            return subscriptionId;
        }
     
        public void setSubscriptionId(Integer subscriptionId) {
            this.subscriptionId = subscriptionId;
        }
     
        public String getSubscriptionName() {
            return subscriptionName;
        }
     
        public void setSubscriptionName(String subscriptionName) {
            this.subscriptionName = subscriptionName;
        }
     
        public Set<ReaderEntity> getReaders() {
            return readers;
        }
     
        public void setReaders(Set<ReaderEntity> readers) {
            this.readers = readers;
        }
    }

### 在配置文件中配置实体 ###

我们已经有了两个可用的实体，我们必须在配置文件中配置他们

<?xml version="1.0" encoding="utf-8"?>
    <!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
    <hibernate-configuration>
        <session-factory>
            <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
            <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/hibernatetest</property>
            <property name="hibernate.connection.password">XXXXXX</property>
            <property name="hibernate.connection.username">root</property>
            <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
            <property name="show_sql">true</property>
            <property name="hbm2ddl.auto">create</property>
            <mapping class="hibernate.test.manyToMany.joinTable.ReaderEntity"/>
            <mapping class="hibernate.test.manyToMany.joinTable.SubscriptionEntity"/>
        </session-factory>
    </hibernate-configuration>

### 测试代码 ###

package hibernate.test.manyToMany;
     
    import hibernate.test.HibernateUtil;
    import hibernate.test.manyToMany.joinTable.*;
     
    import java.util.HashSet;
    import java.util.Set;
     
    import org.hibernate.Session;
     
    public class TestJoinTable
    {
         
        public static void main(String[] args)
        {
            Session session = HibernateUtil.getSessionFactory().openSession();
            session.beginTransaction();
            
            //Add subscription
            SubscriptionEntity subOne = new SubscriptionEntity();
            subOne.setSubscriptionName("Entertainment");
             
            SubscriptionEntity subTwo = new SubscriptionEntity();
            subTwo.setSubscriptionName("Horror");
             
            Set<SubscriptionEntity> subs = new HashSet<SubscriptionEntity>();
            subs.add(subOne);
            subs.add(subTwo);
             
            //Add readers
            ReaderEntity readerOne = new ReaderEntity();
            readerOne.setEmail("demo-user1@mail.com");
            readerOne.setFirstName("demo");
            readerOne.setLastName("user");
             
            ReaderEntity readerTwo = new ReaderEntity();
            readerTwo.setEmail("demo-user2@mail.com");
            readerTwo.setFirstName("demo");
            readerTwo.setLastName("user");
             
            Set<ReaderEntity> readers = new HashSet<ReaderEntity>();
            readers.add(readerOne);
            readers.add(readerTwo);
             
            readerOne.setSubscriptions(subs);
            readerTwo.setSubscriptions(subs);
     
            session.save(readerOne);
            session.save(readerTwo);
             
            session.getTransaction().commit();
            HibernateUtil.shutdown();
        }
    }
     
    Output:
     
    Hibernate: insert into READER (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
    Hibernate: insert into SUBSCRIPTION (SUBS_NAME) values (?)
    Hibernate: insert into SUBSCRIPTION (SUBS_NAME) values (?)
    Hibernate: insert into READER (EMAIL, FIRST_NAME, LAST_NAME) values (?, ?, ?)
    Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)
    Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)
    Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)
    Hibernate: insert into READER_SUBSCRIPTIONS (readers_ID, subscriptions_ID) values (?, ?)

可以在原文中下载源码，原文地址：http://howtodoinjava.com/2012/11/17/hibernate-many-to-many-mapping-using-annotations/

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