Hibernate-為什么延遲加載在這里不起作用？

我正在通過在線課程學習 Hibernate，而現在，我正在學習 Eager vs Lazy loading。

例如，我有三個物體和一個測驗程式，如下所示：

@Entity
@Table(name = "instructor")
public class Instructor {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private int id;
    
    @Column(name = "first_name")
    private String firstName;
    
    @Column(name = "last_name")
    private String lastName;
    
    @Column(name = "email")
    private String email;
    
    @OneToOne(cascade = CascadeType.ALL)
    @JoinColumn(name = "instructor_detail_id")
    private InstructorDetail instructorDetail;
    
    @OneToMany(fetch = FetchType.LAZY,
            mappedBy = "instructor",
            cascade = {
                    CascadeType.DETACH,
                    CascadeType.MERGE,
                    CascadeType.PERSIST,
                    CascadeType.REFRESH
            })
    private List<Course> courses;
    
    // constructors
    
    public void add(Course tempCourse) {
        if (courses == null) {
            courses = new ArrayList<>();
        }
        
        if (tempCourse != null) {
            courses.add(tempCourse);
            tempCourse.setInstructor(this);
        }
    }
    
    // getters and setters
    
    @Override
    public String toString() {
        return "Instructor{"  
                "id="   id  
                ", firstName='"   firstName   '\''  
                ", lastName='"   lastName   '\''  
                ", email='"   email   '\''  
                ", instructorDetail="   instructorDetail  
                '}';
    }
    
}

@Entity
@Table(name = "instructor_detail")

public class InstructorDetail {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private int id;
    
    @Column(name = "youtube_channel")
    private String youtubeChannel;
    
    @Column(name = "hobby")
    private String hobby;
    
    @OneToOne(mappedBy = "instructorDetail",
            cascade = {
                    CascadeType.DETACH,
                    CascadeType.MERGE,
                    CascadeType.PERSIST,
                    CascadeType.REFRESH
            })
    private Instructor instructor;
    
    // constructors
    
    // getters and setters
    
    @Override
    public String toString() {
        return "InstructorDetail{"  
                "id="   id  
                ", youtubeChannel='"   youtubeChannel   '\''  
                ", hobby='"   hobby   '\''  
                '}';
    }
    
}

@Entity
@Table(name = "course")
public class Course {
    
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    @Column(name = "id")
    private int id;
    
    @Column(name = "title")
    private String title;
    
    @ManyToOne(fetch = FetchType.LAZY,
            cascade = {
                    CascadeType.DETACH,
                    CascadeType.MERGE,
                    CascadeType.PERSIST,
                    CascadeType.REFRESH})
    @JoinColumn(name = "instructor_id")
    private Instructor instructor;
    
    // constructors
    
    // getters and setters
    
    @Override
    public String toString() {
        return "Course{"  
                "id="   id  
                ", title='"   title   '\''  
                '}';
    }
    
}

主程式：

package pt.hmsk.hibernate.demo;

import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;
import pt.hmsk.hibernate.demo.entity.Course;
import pt.hmsk.hibernate.demo.entity.Instructor;
import pt.hmsk.hibernate.demo.entity.InstructorDetail;

public class EagerLazyDemo {
    
    public static void main(String[] args) {
        try (SessionFactory factory = new Configuration()
                .configure("hibernate.cfg.xml")
                .addAnnotatedClass(Instructor.class)
                .addAnnotatedClass(InstructorDetail.class)
                .addAnnotatedClass(Course.class)
                .buildSessionFactory();
             Session session = factory.getCurrentSession()) {
            
            // start a transaction
            session.beginTransaction();
    
            // get instructor from db
            int theId = 1;
            Instructor tempInstructor = session.get(Instructor.class, theId);
    
            System.out.println("luv2code: Instructor: "   tempInstructor);
            
            // get courses for the instructor
            System.out.println("luv2code: Courses: "   tempInstructor.getCourses());
            
            // commit transaction
            session.getTransaction().commit();
            
            System.out.println("luv2code: Done!");
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    
}

然后，我在主程式的行中放置了一個斷點：

System.out.println("luv2code: Instructor: "   tempInstructor);

假設在這里，還沒有呼叫 course 欄位，所以它不應該在 tempInstructor 物件中，但除錯程式顯示課程已加載到 Instructor 物件中：

為什么會這樣？？

uj5u.com熱心網友回復：

您懶惰地加載課程，但因為您通過線路參考它們

System.out.println("luv2code: Courses: "   tempInstructor.getCourses());

他們被加載了。

通過添加到您的hibernate.cfg.xml

<property name="hibernate.show_sql">true</property>

并且不要從講師那里獲得課程，然后您會發現它們實際上是延遲加載的。

uj5u.com熱心網友回復：

哦，另一種驗證方式是在提交事務后呼叫課程。事務是可能發生延遲加載的范圍。

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