hibernate初探之一对多映射

客官°小女子只卖身不卖艺 2022-07-11 13:59 214阅读 0赞

# 1、映射类型 #

一对多(one-to-many)

多对一(many-to-one)

一对一(one-to-one)

多对多（many-to-many）

# 2、一对多关联 #

如下图的班级与学生的一对多的关系

![Center][]

在数据库中，可以通过添加主外键的关联，表现一对多的关系。

在hibernate中通过在一方持有多方的集合实现，即在“一”的一端中使用<set>元素表示持有“多”的一端的对象

测试用例如下所示：

hibernate.cfg.xml文件

<?xml version="1.0" encoding="UTF-8"?>
    <!DOCTYPE hibernate-configuration PUBLIC
    		"-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    		"http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
    <hibernate-configuration>
        <session-factory>
        	<property name="connection.username">root</property>
        	<property name="connection.password">123</property>
        	<property name="connection.driver_class">com.mysql.jdbc.Driver</property>
        	<property name="connection.url">
        		jdbc:mysql:///hibernate?userUnicode=true&characterEncoding=UTF-8
        	</property>
        	<property name="dialect">org.hibernate.dialect.MySQL5Dialect</property>
        	
        	<property name="show_sql">true</property>
        	<property name="format_sql">true</property>
        	<property name="hbm2ddl.auto">create</property>
        	
        	<mapping resource="hibernate/Grade.hbm.xml"/>
        	<mapping resource="hibernate/Student.hbm.xml"/>
        </session-factory>
    </hibernate-configuration>

student类

package hibernate;
    
    import java.io.Serializable;
    
    public class Student implements Serializable{
    	private int sid;
    	private String sname;
    	private String sex;
    	public int getSid() {
    		return sid;
    	}
    	public void setSid(int sid) {
    		this.sid = sid;
    	}
    	public String getSname() {
    		return sname;
    	}
    	public void setSname(String sname) {
    		this.sname = sname;
    	}
    	public String getSex() {
    		return sex;
    	}
    	public void setSex(String sex) {
    		this.sex = sex;
    	}
    	@Override
    	public String toString() {
    		return "Student [sid=" + sid + ", sname=" + sname + ", sex=" + sex + "]";
    	}
    	public Student(String sname, String sex) {
    		super();
    		this.sname = sname;
    		this.sex = sex;
    	}
    	public Student() {
    		super();
    	}
    	
    
    }

Grade班级类

package hibernate;
    
    import java.io.Serializable;
    import java.util.HashSet;
    import java.util.Set;
    
    public class Grade implements Serializable{
    	private int gid;
    	private String gname;
    	private String gdesc;//班级对应的详细信息
    	//在以防定义一个多方的集合
    	private Set<Student> students = new HashSet<Student>();//一定要注意初始化不然会报空指针异常错误
    	public int getGid() {
    		return gid;
    	}
    	public void setGid(int gid) {
    		this.gid = gid;
    	}
    	public String getGname() {
    		return gname;
    	}
    	public void setGname(String gname) {
    		this.gname = gname;
    	}
    	public String getGdesc() {
    		return gdesc;
    	}
    	public void setGdesc(String gdesc) {
    		this.gdesc = gdesc;
    	}
    	public Set<Student> getStudents() {
    		return students;
    	}
    	public void setStudents(Set<Student> students) {
    		this.students = students;
    	}
    	public Grade(int gid, String gname, String gdesc, Set<Student> students) {
    		super();
    		this.gid = gid;
    		this.gname = gname;
    		this.gdesc = gdesc;
    		this.students = students;
    	}
    	public Grade() {
    		super();
    	}
    	public Grade(String gname, String gdesc) {
    		super();
    		this.gname = gname;
    		this.gdesc = gdesc;
    	}
    	
    	
    }

student持久化映射文件

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
    
    <hibernate-mapping>
        <class name="hibernate.Student" table="STUDENT">
            <id name="sid" type="int">
                <column name="SID" />
                <generator class="native" />
            </id>
            <property name="sname" type="java.lang.String">
                <column name="SNAME" length="20" not-null="true"/>
            </property>
            <property name="sex" type="java.lang.String">
                <column name="SEX" />
            </property>
        </class>
    </hibernate-mapping>

grade持久化映射文件

<?xml version="1.0"?>
    <!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
    
    <hibernate-mapping>
        <class name="hibernate.Grade" table="GRADE">
            <id name="gid" column="gid" type="java.lang.Integer">
            	<generator class="native"/>
            </id>
            <property name="gname" type="java.lang.String">
            	<column name="gname" length="20" not-null="true"></column>
            </property>
            <property name="gdesc" type="java.lang.String">
            	<column name="gdesc"></column>
            </property>
            
            <set name="students" table="STUDENT">
            	
            	<key column="gid"></key>
            	<one-to-many class="hibernate.Student"/>
            </set>
        </class>
    </hibernate-mapping>

测试文件

package test;
    
    import org.hibernate.Session;
    import org.hibernate.SessionFactory;
    import org.hibernate.Transaction;
    import org.hibernate.cfg.Configuration;
    import org.hibernate.service.ServiceRegistry;
    import org.hibernate.service.ServiceRegistryBuilder;
    import org.junit.Test;
    
    
    import hibernate.Grade;
    import hibernate.Student;
    
    /**
     * 单向一对多关系（班级--->学生）
     * @author Administrator
     *
     */
    public class TestManyToOne {
    	//将学生添加到班级
    	@Test
    	public void TestAddStudentToClass()
    	{
    		//创建Configuration对象，读取hibernate.cfg.xml文件，完成初始化
    		Configuration configuration=new Configuration().configure();
    		//创建服务注册对象
    		ServiceRegistry serviceRegistry= new ServiceRegistryBuilder().applySettings(configuration.getProperties()).buildServiceRegistry();
    		//创建会话工厂对象
    		SessionFactory sessionFactory = configuration.buildSessionFactory(serviceRegistry);
    		//会话对象
    		Session session = sessionFactory.openSession();
    		//开启事务
    		Transaction tx= session.beginTransaction();
    		Grade g =new Grade("Java2班", "Java分类的班级");
    		Student s= new Student("xqh", "女");
    		Student s1= new Student("txr", "女");
    		Student s2=new Student("sn","女");
    		Student s3 =new Student("pha","男");
    		Student s4 =new Student("lyh","男");
    		Student s5=new Student("zzc","男");
    		g.getStudents().add(s);
    		g.getStudents().add(s1);
    		g.getStudents().add(s2);
    		g.getStudents().add(s3);
    		g.getStudents().add(s4);
    		g.getStudents().add(s5);
    		
    		
    		session.save(g);
    		session.save(s);
    		session.save(s1);
    		session.save(s2);
    		session.save(s3);
    		session.save(s4);
    		session.save(s5);
    		tx.commit();
    		session.close();
    		
    	}
    	
    	
    
    }

测试运行最后通过，途中所报错误用红色标记了。

注意：1、查询的时候要将hibernate.cfg.xml中的create改为update

2、注意关联的方向

[Center]: /images/20220711/300e5945f9154797a4dc5c535b4a1756.png