hibernate多映射 数据无法查询

Question

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 sname; private String gdesc; private Set<student1> students=new HashSet<student1>(); public Grade(int gid, String sname, String gdesc, Set<student1> students) { super(); this.gid = gid; this.sname = sname; this.gdesc = gdesc; this.students = students; } public Grade() { super(); } public int getGid() { return gid; } public void setGid(int gid) { this.gid = gid; } public String getSname() { return sname; } public void setSname(String sname) { this.sname = sname; } public String getGdesc() { return gdesc; } public void setGdesc(String gdesc) { this.gdesc = gdesc; } public Set<student1> getStudents() { return students; } public void setStudents(Set<student1> students) { this.students = students; } public Grade(String sname, String gdesc) { super(); this.sname = sname; this.gdesc = gdesc; }}学生类package hibernate;import java.io.Serializable;public class student1 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; } public student1(String sname, String sex) { super(); this.sname = sname; this.sex = sex; } }班级hbm类<?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 2017-3-25 22:16:53 by Hibernate Tools 3.5.0.Final --><hibernate-mapping>    <class name="hibernate.Grade" table="grade1">        <id name="gid" type="int">            <column name="GID" />            <generator class="increment" />        </id>        <property name="sname" type="java.lang.String">            <column name="SNAME" />        </property>        <property name="gdesc" type="java.lang.String">            <column name="GDESC" />        </property>        <set name="students" table="student1" inverse="false" lazy="true">            <key>                <column name="gid" />            </key>            <one-to-many class="hibernate.student1" />        </set>    </class></hibernate-mapping>  学生hbm<?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 2017-3-25 22:16:53 by Hibernate Tools 3.5.0.Final --><hibernate-mapping>    <class name="hibernate.student1" table="student1">        <id name="sid" type="int">            <column name="SID" />            <generator class="increment" />        </id>        <property name="sname" type="java.lang.String">            <column name="SNAME" />        </property>        <property name="sex" type="java.lang.String">            <column name="SEX" />        </property>    </class></hibernate-mapping>配置文档<?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_name">com.mysql.jdbc.Driver</property>    <property name="connection.url">jdbc:mysql://localhost:3306/student3?useUnicode=true&amp;characterEncoding=UTF-8</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">create</property>    <mapping resource="student1.hbm.xml"/>     <mapping resource="Grade.hbm.xml"/>    </session-factory></hibernate-configuration>测试类package hibernate;import java.util.Date;import org.hibernate.Session;import org.hibernate.SessionFactory;import org.hibernate.Transaction;import org.hibernate.boot.registry.StandardServiceRegistryBuilder;import org.hibernate.cfg.Configuration;import org.hibernate.service.ServiceRegistry;import org.junit.After;import org.junit.Before;import org.junit.Test;public class StudentsTest {   private SessionFactory sessionFactory;   private Session session;   private Transaction transaction; @Before public void init(){ //创建配置对象 Configuration config=new Configuration().configure(); config.addClass(Grade.class); config.addClass(student1.class); //创建服务注册对象 ServiceRegistry serviceRegistry =new StandardServiceRegistryBuilder().applySettings(config.getProperties()).build(); //创建会话工厂对象 sessionFactory=config.buildSessionFactory(serviceRegistry); //会话对象 session=sessionFactory.openSession(); //开启事物 transaction=session.beginTransaction(); } @After public void destroy(){ transaction.commit(); transaction.commit();//提交事物 session.close();//关闭会话 sessionFactory.close();//关闭会话工厂 } @Test public void testSaveStudent(){ Grade g=new Grade("java二班","java软件开发一班"); student1 stu1=new student1("王五","男"); student1 stu2=new student1("赵六","男"); g.getStudents().add(stu1); g.getStudents().add(stu2); session.save(g); session.save(stu1); session.save(stu2); } @Test public void testsearchGrade(){ Grade grade=(Grade)session.get(Grade.class, 1); System.out.println(grade); }只要测试 testsearchGrade() 方法 数据表就会把以前的数据清空