sql - Hibernate session.load 的工作原理很奇怪

Question

我正在使用 Hibernate 3.2，而且负载很正常。这是我的问题。

    package entity;

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.JoinColumn;
import javax.persistence.OneToMany;

@Entity
public class Teacher {

    private int id;
    private String name;
    private Set<Student> students = new HashSet<Student>();
    private Date time;

    public Teacher() {
        super();
    }

    public Teacher(int id, String name, Set<Student> students, Date time) {
        super();
        this.id = id;
        this.name = name;
        this.students = students;
        this.time = time;
    }

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }

    public String getName() {
        return name;
    }

    @OneToMany(mappedBy="teacher",cascade={CascadeType.ALL},fetch=FetchType.EAGER)
    @JoinColumn(name="teacher_id")
    public Set<Student> getStudents() {
        return students;
    }

    public Date getTime() {
        return time;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setName(String name) {
        this.name = name;
    }

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

    public void setTime(Date time) {
        this.time = time;
    }

    @Override
    public String toString() {
        return "Teacher+"+this.name+",student number:"+this.students.size();
    }
}


Teacher.java

另一个班级

package entity;

import java.util.Date;

import javax.persistence.CascadeType;
import javax.persistence.Entity;
import javax.persistence.FetchType;
import javax.persistence.GeneratedValue;
import javax.persistence.Id;
import javax.persistence.ManyToOne;

@Entity
public class Student implements Cloneable{
    private Date brith;
    private int id;
    private Teacher teacher;

    public Student() {
        super();
    }

    public Student(Date brith, int id, Teacher teacher) {
        super();
        this.brith = brith;
        this.id = id;
        this.teacher = teacher;
    }

    public Date getBrith() {
        return brith;
    }

    @Id
    @GeneratedValue
    public int getId() {
        return id;
    }

    @ManyToOne(cascade={CascadeType.ALL},fetch=FetchType.LAZY)
    public Teacher getTeacher() {
        return teacher;
    }


    public void setBrith(Date brith) {
        this.brith = brith;
    }

    public void setId(int id) {
        this.id = id;
    }

    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }

    @Override
    public String toString() {
        return "student brith="+this.brith+"teacher="+teacher.getName();
    }

}


Student.java

这是我的测试代码，

package entity;

import java.util.Date;
import java.util.HashSet;
import java.util.Set;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.AnnotationConfiguration;
import org.junit.AfterClass;
import org.junit.BeforeClass;
import org.junit.Test;

public class Hibernate {

    static SessionFactory sessionFactory;

    @BeforeClass
    public static void setUpBeforeClass() throws Exception {
        sessionFactory = new AnnotationConfiguration().configure().buildSessionFactory();
    }

    @AfterClass
    public static void tearDownAfterClass() throws Exception {
        sessionFactory.close();
    }

    @Test
    public void testSaveStudent(){
        Student s = new Student();
        s.setBrith(new Date());
        Student s1 = new Student();
        s1.setBrith(new Date());
        Teacher t = new Teacher();
        t.setName("teacher");
        t.setTime(new Date());
        Set<Student> set = new HashSet<Student>();
        set.add(s1);
        set.add(s);
        t.setStudents(set);
        s1.setTeacher(t);
        s.setTeacher(t);
        Session session = sessionFactory.getCurrentSession();
        session.beginTransaction();
        session.save(s1);
        session.getTransaction().commit();
    }

    @Test
    public void testFetchStudent(){
        testSaveStudent();
        Session session = sessionFactory.getCurrentSession();
        session.beginTransaction();
        Student s = (Student)session.load(Student.class, 1);
        s.getBrith();
        session.getTransaction().commit();
        System.out.println(s);
    }

    @Test
    public void testFetchStudent2(){
        testSaveStudent();
        Session session = sessionFactory.getCurrentSession();
        session.beginTransaction();
        Student s = (Student)session.load(Student.class, 1);
        System.out.println(s);
        session.getTransaction().commit();
        System.out.println(s);
    }

    @Test
    public void testFetchTeacher(){
        testSaveStudent();
        Session session = sessionFactory.getCurrentSession();
        session.beginTransaction();
        Teacher t = (Teacher)session.load(Teacher.class, 1);
        /*System.out.println(t);
        t.setName("what are you doing");
        session.update(t);*/
        session.getTransaction().commit();
        System.out.println(t);
    }

    public static void main(String[] args) throws Exception {
        setUpBeforeClass();
    }
}

让它更容易，方法 testStudent 失败，但 testStudent2 成功，session.load 方法有什么魔力？为什么他们不能得到相同的结果？