Hibernate 单向一对一关联

纸上得来终觉浅

开始学习Hibernate的关联关系。一共有七种关联关系，可以分为单向和双向，单向只是A能够加载B，但B不能够加载A，双向可互相加载，详细如下：

1)单向 一对一

2)单向多对一

3)单向一对多

4)单向多对多

5)双向一对一

6)双向一对多

7)双向多对多

这里讲的是单向一对一关联：单向一对一关联有两种情况，一个是一对一主键关联，一个是一对一外键关联（使用的较多）；

1. 一对一主键关联就是A的主键是由B的主键生成，与B的主键值相同，通过A能够加载到B，示例如下：

<pre name="code" class="java">package roadArchitectWeb;

public class Student {
	private Integer Id;
	private String sName;
	private String Age;
	private Teacher teacher;
	
	public Integer getId() {
		return Id;
	}
	public void setId(Integer id) {
		Id = id;
	}
	public Teacher getTeacher() {
		return teacher;
	}
	public void setTeacher(Teacher teacher) {
		this.teacher = teacher;
	}
	public String getsName() {
		return sName;
	}
	public void setsName(String sName) {
		this.sName = sName;
	}
	public String getAge() {
		return Age;
	}
	public void setAge(String age) {
		Age = age;
	}
	@Override
	public String toString() {
		return "Student [Id=" + Id + ", sName=" + sName + ", Age=" + Age + ", teacher=" + teacher + "]";
	}
	public Student(){
	}
}

package roadArchitectWeb;

public class Teacher {
	private Integer Id;
	private String Name;
	private String PhoneNum;
	public Integer getId() {
		return Id;
	}
	public void setId(Integer id) {
		Id = id;
	}
	public String getName() {
		return Name;
	}
	public void setName(String name) {
		Name = name;
	}
	public String getPhoneNum() {
		return PhoneNum;
	}
	public void setPhoneNum(String phoneNum) {
		PhoneNum = phoneNum;
	}
	public  Teacher() {
	}
	@Override
	public String toString() {
		return "Teacher [Id=" + Id + ", Name=" + Name + ", PhoneNum=" + PhoneNum + "]";
	}
}

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated 2016-4-21 19:12:13 by Hibernate Tools 3.5.0.Final -->
<hibernate-mapping package="roadArchitectWeb">
    <class name="Student" table="STUDENT">
        <id name="Id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="foreign">
            <param name="property">teacher</param>
            </generator>
        </id>
        <property name="sName" type="java.lang.String" access="field">
            <column name="SNAME" />
        </property>
        <property name="Age" type="java.lang.String">
            <column name="AGE" />
        </property>
        <one-to-one name="teacher" class="Teacher" constrained="true"></one-to-one>
    </class>
</hibernate-mapping>

<?xml version="1.0"?>
<!DOCTYPE hibernate-mapping PUBLIC "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
"http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<!-- Generated 2016-4-21 19:24:31 by Hibernate Tools 3.5.0.Final -->
<hibernate-mapping>
    <class name="roadArchitectWeb.Teacher" table="TEACHER">
        <id name="Id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native" />
        </id>
        <property name="Name" type="java.lang.String">
            <column name="NAME" />
        </property>
        <property name="PhoneNum" type="java.lang.String">
            <column name="PHONENUM" />
        </property>
    </class>
</hibernate-mapping>

<?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">123456</property>
    <property name="connection.driver_class">com.mysql.jdbc.Driver</property>
    <property name="connection.url">jdbc:mysql://localhost/roadarchitectweb</property>
    
    <property name="dialect">org.hibernate.dialect.MySQL5InnoDBDialect</property>
    <property name="show_sql">true</property>
    <property name="format_sql">true</property>
    <property name="hbm2ddl.auto">update</property>
    
    <property name="hibernate.c3p0.max_size">10</property>
    <property name="hibernate.c3p0.min_size">5</property>
    <property name="c3p0.acquire_increment">2</property>
    
    <property name="c3p0.idle_test_period">2000</property>
    <property name="c3p0.timeout">2000</property>
    
    <property name="c3p0.max_statements">10</property>
    
    <property name="hibernate.temp.use_jdbc_metadata_defaults">false</property>
    <mapping resource="roadArchitectWeb/Student.hbm.xml"/>
    <mapping resource="roadArchitectWeb/Teacher.hbm.xml"/>
    </session-factory>
</hibernate-configuration>

package roadArchitectWeb;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.cfg.Configuration;
import org.junit.Test;

import com.mysql.jdbc.interceptors.ServerStatusDiffInterceptor;
public class HibernateTest {
	@Test
	public void test() {
		SessionFactory sessionFactory = null;
		Configuration configuration = new Configuration().configure();
		sessionFactory = configuration.buildSessionFactory();
		Session session = sessionFactory.openSession();
		Transaction transaction = session.beginTransaction();
		
		/*begin*/
		Teacher teacher = new Teacher();
		Student student = new Student();
		teacher.setName("chenT");
		teacher.setPhoneNum("18788837117");
		student.setsName("ma");
		student.setAge("11");
		student.setTeacher(teacher);
		session.save(student);
		session.save(teacher);
//		student = (Student)session.get(Student.class, 1);
		/*通过A加载到B*/
//		System.out.println("HibernateTest.test():"+student.getTeacher().getName());
		/*end*/
		
		transaction.commit();
		session.close();
		sessionFactory.close();
	}
}

要在A（Student）类中设置B属性，指定ID的生成方式根据B生成，设置<one-to-one>标签，之后A就可以加载到B。

注：one-to-one标签中的constrained属性，默认为false；设置为true有两个特点：

1) 影响save和delete的先后顺序，先增加B表，然后增加A表，删除的时候反之。

2)如果constrained=false，则会在查询时就全部取出来，用left outer join的方式。如果constrained=true，hibernate即会延迟加载sql，只把A的查出来，等有用到B的时候再发sql取

2.一对一外键关联，这种方式，就是在一般的多对一中增加了uniqe为true；  A表的外键要对应B表的主键

接着上面的代码，只需要把A的主键生成方式改为正常即可：

 <id name="Id" type="java.lang.Integer">
            <column name="ID" />
            <generator class="native">
            </generator>
        </id>

同时设置<many-to-one>属性：column指定A表新增的列名

<many-to-one name="teacher" column="teacherId" unique="true"></many-to-one>