Hibernate的延遲加載的項目實踐

更新時間：2026年01月11日 09:30:07 作者：辭暮爾爾-煙火年年

Hibernate延遲加載(Lazy Loading)是一種性能優(yōu)化技術(shù),僅在需要時加載關(guān)聯(lián)數(shù)據(jù),減少初始查詢和內(nèi)存消耗,本文就來介紹一下Hibernate 延遲加載的實現(xiàn),感興趣的可以了解一下

Hibernate的延遲加載（Lazy Loading）是一種性能優(yōu)化技術(shù)，通過在需要訪問數(shù)據(jù)時才實際加載它們，而不是在初始加載時將所有相關(guān)數(shù)據(jù)全部加載。這種方式可以減少不必要的數(shù)據(jù)查詢和內(nèi)存開銷，特別是在處理大型數(shù)據(jù)集和復(fù)雜的對象關(guān)系時。

延遲加載的實現(xiàn)

在Hibernate中，延遲加載通常應(yīng)用于集合及關(guān)聯(lián)關(guān)系中，比如@OneToMany、@ManyToOne、@ManyToMany等。通過指定加載策略，可以控制關(guān)聯(lián)實體的加載時機。

示例代碼

下面是一個完整的示例，展示如何在Hibernate中使用延遲加載。

配置文件hibernate.cfg.xml

<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- 數(shù)據(jù)庫連接配置 -->
        <property name="hibernate.connection.driver_class">com.mysql.cj.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://localhost:3306/your_database</property>
        <property name="hibernate.connection.username">your_username</property>
        <property name="hibernate.connection.password">your_password</property>

        <!-- Hibernate 屬性配置 -->
        <property name="hibernate.dialect">org.hibernate.dialect.MySQLDialect</property>
        <property name="hibernate.show_sql">true</property>
        <property name="hibernate.format_sql">true</property>
        <property name="hibernate.hbm2ddl.auto">update</property>

        <!-- 映射類配置 -->
        <mapping class="com.example.domain.Student"/>
        <mapping class="com.example.domain.Course"/>
    </session-factory>
</hibernate-configuration>

HibernateUtil 類

import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {
    private static final SessionFactory sessionFactory;

    static {
        try {
            // 從配置文件創(chuàng)建SessionFactory
            sessionFactory = new Configuration().configure("hibernate.cfg.xml").buildSessionFactory();
        } catch (Throwable ex) {
            // 記錄啟動失敗的錯誤
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

    public static SessionFactory getSessionFactory() {
        return sessionFactory;
    }
}

實體類 Student 和 Course

Student 類

package com.example.domain;

import javax.persistence.*;
import java.util.HashSet;
import java.util.Set;

@Entity
public class Student {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;
    private int age;

    @OneToMany(mappedBy = "student", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
    private Set<Course> courses = new HashSet<>();

    public Student() {}

    public Student(String name, int age) {
        this.name = name;
        this.age = age;
    }

    // getters 和 setters
    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public Set<Course> getCourses() {
        return courses;
    }

    public void setCourses(Set<Course> courses) {
        this.courses = courses;
    }
}

Course 類

package com.example.domain;

import javax.persistence.*;

@Entity
public class Course {
    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;
    
    private String name;

    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "student_id")
    private Student student;

    public Course() {}

    public Course(String name, Student student) {
        this.name = name;
        this.student = student;
    }

    // getters 和 setters
    public Long getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public Student getStudent() {
        return student;
    }

    public void setStudent(Student student) {
        this.student = student;
    }
}

延遲加載示例代碼

下面的示例展示了如何在并發(fā)環(huán)境中使用延遲加載。

import org.hibernate.Hibernate;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

public class HibernateLazyLoadingExample {
    public static void main(String[] args) {
        // 獲取SessionFactory
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

        // 插入示例數(shù)據(jù)
        insertSampleData(sessionFactory);

        // 示例: 在打開和關(guān)閉會話時訪問延遲加載的集合
        Session session = null;
        Transaction transaction = null;

        try {
            session = sessionFactory.openSession();
            transaction = session.beginTransaction();

            // 獲取Student對象
            Student student = session.get(Student.class, 1L);
            System.out.println("Student: " + student.getName() + ", Age: " + student.getAge());

            // 在會話關(guān)閉前訪問延遲加載的集合
            System.out.println("Courses (before session close): " + student.getCourses().size());

            // 顯式初始化延遲加載的集合
            Hibernate.initialize(student.getCourses());

            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }

        // 在會話關(guān)閉后訪問延遲加載的集合
        session = sessionFactory.openSession();
        transaction = session.beginTransaction();

        try {
            Student student = session.get(Student.class, 1L);
            System.out.println("Student: " + student.getName() + ", Age: " + student.getAge());

            // 會話關(guān)閉后訪問延遲加載的集合會引發(fā) LazyInitializationException
            System.out.println("Courses (after session close): " + student.getCourses().size());

            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }

        // 關(guān)閉SessionFactory
        sessionFactory.close();
    }

    private static void insertSampleData(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        Transaction transaction = session.beginTransaction();
        try {
            Student student = new Student("John Doe", 20);
            Course course1 = new Course("Mathematics", student);
            Course course2 = new Course("Physics", student);
            student.getCourses().add(course1);
            student.getCourses().add(course2);
            session.save(student);
            transaction.commit();
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}

詳細(xì)解釋

配置延遲加載：在實體類的關(guān)聯(lián)字段上添加fetch = FetchType.LAZY注解。

@OneToMany(mappedBy = "student", fetch = FetchType.LAZY, cascade = CascadeType.ALL)
private Set<Course> courses = new HashSet<>();

示例場景：
- 在一個會話中獲取Student對象及其關(guān)聯(lián)的Course集合。
- 在會話關(guān)閉前訪問延遲加載的集合會觸發(fā)集合的初始化。
- 使用Hibernate.initialize方法顯式地初始化延遲加載的集合。
- 會話關(guān)閉后訪問延遲加載的集合會引發(fā)LazyInitializationException異常。
事務(wù)管理：在事務(wù)中進行數(shù)據(jù)操作，并在操作完成后提交事務(wù)。如果操作失敗，則回滾事務(wù)以確保數(shù)據(jù)一致性。

總結(jié)

延遲加載是一種有效的性能優(yōu)化技術(shù)，通過在需要訪問數(shù)據(jù)時才實際加載它們，可以減少不必要的數(shù)據(jù)查詢和內(nèi)存開銷。在Hibernate中，通過fetch = FetchType.LAZY注解可以方便地實現(xiàn)延遲加載。理解并正確應(yīng)用延遲加載，可以有效地提高應(yīng)用的性能和資源利用率。

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