(8)hibernate中OneToMany双向

以你之姓@ 2022-05-08 13:44 198阅读 0赞

**1.在Classroom.java增加Set<Student> stus:**

public class Classroom {
    private int id;
    private String name;
    private int grade;
    private Set<Student> stus;
    
    public Set<Student> getStus() {
    	return stus;
    }
    public void setStus(Set<Student> stus) {
    	this.stus = stus;
    }
    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 int getGrade() {
    	return grade;
    }
    public void setGrade(int grade) {
    	this.grade = grade;
    }
    
    }

**2.Classroom.hbm.xml:**

<hibernate-mapping>
        <class name="model.Classroom" table="CLASSROOM">
            <id name="id" type="int">
                <column name="ID" />
                <generator class="native" />
            </id>
            <property name="name" type="java.lang.String">
                <column name="NAME" />
            </property>
            <property name="grade" type="int">
                <column name="GRADE" />
            </property>
            <set name="stus" lazy="extra" inverse="true">
                <key>
                    <column name="c_id" />
                </key>
                <one-to-many class="model.Student" />
            </set>
        </class>
    </hibernate-mapping>

注意:(1).lazy=“extra”是取size的时候智能化，inverse="true"是不在ClassRoom这一端维护,key是在对方(Student)的外键名称,class是集合中元素的类型

**test01:**

void test3() {
    		Session s=null;
    		try {
    			s=HibernateUtil.getSession();
    			s.beginTransaction();
    			Student stu1=new Student();
    			stu1.setName("a");
    			stu1.setNumber(1);
    			s.save(stu1);
    			
    			Student stu2=new Student();
    			stu2.setName("b");
    			stu2.setNumber(2);
    			s.save(stu2);
    			
    			Classroom cla=new Classroom();
    			cla.setGrade(2017);
    			cla.setName("计算机");
                Set<Student> stus=new HashSet<>();
                stus.add(stu1);
                stus.add(stu2);
                cla.setStus(stus);
                s.save(cla);
                
    			s.getTransaction().commit();
    		} catch (Exception e) {
    			// TODO 自动生成的 catch 块
    			e.printStackTrace();
    			s.getTransaction().rollback();
    		}finally {
    			HibernateUtil.closeSession(s);
    		}
    	}

注意:(1).5条sql,3条save，2条update，cla.setStus(stus);实际上是在stu端设置c\_id，由于2个stu，所以设置了2个stu，就多发2条update stu的语句

**test02:**

void test4() {
    		Session s=null;
    		try {
    			s=HibernateUtil.getSession();
    			s.beginTransaction();
    			
    			Classroom cla=new Classroom();
    			cla.setGrade(2017);
    			cla.setName("计算机");
                s.save(cla);
                
                Student stu1=new Student();
    			stu1.setName("a");
    			stu1.setNumber(1);
    			stu1.setCla(cla);
    			s.save(stu1);
    			
    			Student stu2=new Student();
    			stu2.setName("b");
    			stu2.setNumber(2);
    			stu2.setCla(cla);
    			s.save(stu2);
                
    			s.getTransaction().commit();
    		} catch (Exception e) {
    			// TODO 自动生成的 catch 块
    			e.printStackTrace();
    			s.getTransaction().rollback();
    		}finally {
    			HibernateUtil.closeSession(s);
    		}
    	}

注意:(1).先设置1再设置多，即发3条sql语句

**test03:**

void test5() {
    		Session s=null;
    		try {
    			s=HibernateUtil.getSession();
    			s.beginTransaction();
    			
    			Classroom cla=s.load(Classroom.class, 3);
    			System.out.println(cla.getStus().size());
                
    			Student stu=s.load(Student.class, 6);
    			System.out.println(stu.getCla().getGrade());
    			s.getTransaction().commit();
    		} catch (Exception e) {
    			// TODO 自动生成的 catch 块
    			e.printStackTrace();
    			s.getTransaction().rollback();
    		}finally {
    			HibernateUtil.closeSession(s);
    		}
    	}

注意:(1).load classroom一条语句，把classroom存在缓存中，然后取size用count再发一条语句，然后取load stu再发一条语句，然后取stu.getCla()不用再发取classroom的语句了，因为缓存中有。

**test04:**

void test6() {
    		Session s=null;
    		try {
    			s=HibernateUtil.getSession();
    			s.beginTransaction();
    			
    			Student stu=s.load(Student.class, 6);
    			System.out.println(stu.getCla().getGrade());
    			
    			s.getTransaction().commit();
    		} catch (Exception e) {
    			// TODO 自动生成的 catch 块
    			e.printStackTrace();
    			s.getTransaction().rollback();
    		}finally {
    			HibernateUtil.closeSession(s);
    		}
    	}

注意:(1).load stu一条语句，stu.getCla再发一条取cla的语句，因为缓存没有cla，所以总共2条语句