一对多与多对一区别:

映射原理是一致的,都是在多的一端加入一个外键,指向一的一端

区别在于维护的关系不同:

多对一维护的关系是:多指向一的,有了此关系,在加载多的时候可以将一加载上来

一对多维护的关系是:一指向多的关系,有了此关系,加载一

的时候可以将多加载上来

单向

适用情况:一个班级有多个学生(班级依赖学生,必须先有学生,班级维护关系)

关系图:

注:都是在多的一端加上外键,指向一的一端

<span style="font-size:18px;">实体:
	Student:
		public class Student {

			private int id;

			private String name;

			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;
			}
		}

	Class:
		public class Classes {

			private int id;

			private String name;

			private Set students;

			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 Set getStudents() {
				return students;
			}

			public void setStudents(Set students) {
				this.students = students;
			}

		}
映射文件:
   Student:
	<hibernate-mapping>
	    <class name="com.bjpowernode.hibernate.Student" table="t_student">
		<id name="id">
		     <generator class="native"/>
		</id>
		<property name="name"/>
	    </class>
	</hibernate-mapping>
    Class:
	<hibernate-mapping>
	     <class name="com.bjpowernode.hibernate.Classes" table="t_classes">
		<id name="id">
			<generator class="native"/>
		</id>
		<property name="name"/>
		<set name="students">
	        <key column="classesid"/>
		    <one-to-many class="com.bjpowernode.hibernate.Student"/>
		</set>
	     </class>
          </hibernate-mapping>
客户端:
	public void testSave2() {
		Session session = null;
		try {
			session = HibernateUtils.getSession();
			session.beginTransaction();
			Student student1 = new Student();
			student1.setName("张三");
			session.save(student1);

			Student student2 = new Student();
			student2.setName("李四");
			session.save(student2);

			Classes classes = new Classes();
			classes.setName("动力节点");

			Set students = new HashSet();
			students.add(student1);
			students.add(student2);
			classes.setStudents(students);

			// 可以成功保存数据
			// 但是会发出多余的update语句来维持关系
			session.save(classes);
			session.getTransaction().commit();
		} catch (Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		} finally {
			HibernateUtils.closeSession(session);
		}
	}</span>

结果:

双向

适用情况:

两端都知道关系，解决一对多单向关联的缺陷

关系图:

<span style="font-size:18px;">实体:
student：
	public class Student {

		private int id;

		private String name;

		private Classes classes;

		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 Classes getClasses() {
			return classes;
		}

		public void setClasses(Classes classes) {
			this.classes = classes;
		}
	}
class：
	public class Classes {

		private int id;

		private String name;

		private Set students;

		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 Set getStudents() {
			return students;
		}

		public void setStudents(Set students) {
			this.students = students;
		}

	}

映射文件:
student：
   <hibernate-mapping>
      <class name="com.bjpowernode.hibernate.Student" table="t_student">
	<id name="id">
		<generator class="native"/>
	</id>
	<property name="name"/>
	<many-to-one name="classes" column="classesid"/>
       </class>
    </hibernate-mapping></span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-size:18px;">class：</span></span>

<span style="font-family: Arial, Helvetica, sans-serif;"><span style="font-size:18px;">        <hibernate-mapping></span></span>

<span style="font-size:18px;">       <class name="com.bjpowernode.hibernate.Classes" table="t_classes">
	   <id name="id">
		<generator class="native"/>
	   </id>
	   <property name="name"/>
	   <set name="students" inverse="true">
		<key column="classesid"/>
	        <one-to-many class="com.bjpowernode.hibernate.Student"/>
	   </set>
	</class>
</hibernate-mapping></span>

客户端:

  <span style="font-size:18px;"> 情况1：
	public void testSave2() {
	    Session session = null;
	    try {
		session = HibernateUtils.getSession();
		session.beginTransaction();
		Student student1 = new Student();
		student1.setName("张三");
		session.save(student1);

		Student student2 = new Student();
		student2.setName("李四");
		session.save(student2);

		Classes classes = new Classes();
		classes.setName("动力节点");

		Set students = new HashSet();
		students.add(student1);
		students.add(student2);
		classes.setStudents(students);

	       //可以成功保存数据
		//但是会发出多余的update语句来维持关系
		session.save(classes);
		session.getTransaction().commit();
		}catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
	        finally {
			HibernateUtils.closeSession(session);
		}
	}</span>

注：可以保存数据，但会发出多余的update语句来维护关系

情况2：

<span style="font-size:18px;">public void testSave3() {
	Session session = null;
	try {
	<span style="white-space:pre">	</span>session = HibernateUtils.getSession();
		session.beginTransaction();

		Classes classes = new Classes();
		classes.setName("动力节点");
		session.save(classes);

		Student student1 = new Student();
		student1.setName("张三");
		student1.setClasses(classes);
		session.save(student1);

		Student student2 = new Student();
		student2.setName("李四");
		student2.setClasses(classes);
		session.save(student2);

		session.getTransaction().commit();
	       catch(Exception e) {
			e.printStackTrace();
			session.getTransaction().rollback();
		}finally {
			HibernateUtils.closeSession(session);
		}

}</span>

结果：