Hibernate（33） Hibernate的投影（Projections）是什么？

Victor3562026-01-10 8:04

Hibernate的投影（Projections）

在Hibernate中，投影（Projection）是指查询结果中只包含实体的某些属性，而不是整个实体对象。这对于优化查询性能和减少数据传输量非常有用。

Hibernate的投影可以通过Criteria API中的Projections类来实现。Projections类提供了多种方法，可以对查询结果进行投影，包括单属性、多属性、聚合（如计数、求和、平均值等）和分组等功能。

使用Projections的示例代码

实体类定义

我们继续使用之前定义的Person和Address实体类：

Person类
java 复制代码 
package com.example.domain;

import javax.persistence.*;

@Entity
@Table(name = "person")
public class Person {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "name")
    private String name;

    @Column(name = "age")
    private int age;

    public Person() {}

    public Person(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;
    }
}
Address类
java 复制代码 
package com.example.domain;

import javax.persistence.*;

@Entity
@Table(name = "address")
public class Address {

    @Id
    @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Long id;

    @Column(name = "street")
    private String street;

    @ManyToOne
    @JoinColumn(name = "person_id", nullable = false)
    private Person person;

    public Address() {}

    public Address(String street, Person person) {
        this.street = street;
        this.person = person;
    }

    // Getters 和 Setters

    public Long getId() {
        return id;
    }

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

    public String getStreet() {
        return street;
    }

    public void setStreet(String street) {
        this.street = street;
    }

    public Person getPerson() {
        return person;
    }

    public void setPerson(Person person) {
        this.person = person;
    }
}

Hibernate配置文件 hibernate.cfg.xml

xml 复制代码 
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <!-- 数据库连接配置 -->
        <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 属性配置 -->
        <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>

        <!-- 映射类 -->
        <mapping class="com.example.domain.Person"/>
        <mapping class="com.example.domain.Address"/>
    </session-factory>
</hibernate-configuration>

HibernateUtil类

java 复制代码 
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

public class HibernateUtil {
    private static final SessionFactory sessionFactory;

    static {
        try {
            // 从配置文件创建SessionFactory
            sessionFactory = new Configuration().configure("hibernate.cfg.xml").buildSessionFactory();
        } catch (Throwable ex) {
            // 记录启动失败的错误
            System.err.println("Initial SessionFactory creation failed." + ex);
            throw new ExceptionInInitializerError(ex);
        }
    }

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

使用Projections示例

插入示例数据

java 复制代码 
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;

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

        // 插入示例数据
        insertSampleData(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void insertSampleData(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        Transaction transaction = session.beginTransaction();
        try {
            Person person1 = new Person("John Doe", 30);
            Person person2 = new Person("Jane Doe", 28);

            session.save(person1);
            session.save(person2);

            Address address1 = new Address("123 Main St", person1);
            Address address2 = new Address("456 Oak St", person2);

            session.save(address1);
            session.save(address2);

            transaction.commit();
            System.out.println("Inserted sample data");
        } catch (Exception e) {
            if (transaction != null) {
                transaction.rollback();
            }
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}

使用Projections查询

查询Person的姓名和年龄
java 复制代码 
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.criterion.Projections;
import org.hibernate.criterion.CriteriaQuery;
import org.hibernate.criterion.Restrictions;
import org.hibernate.query.criteria.HibernateCriteriaBuilder;
import org.hibernate.query.criteria.JpaCriteriaQuery;

import javax.persistence.criteria.Root;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;
import java.util.List;

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

        // 使用Projections查询Person的姓名和年龄
        queryPersonNameAndAge(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void queryPersonNameAndAge(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        try {
            CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();
            CriteriaQuery<Object[]> criteriaQuery = criteriaBuilder.createQuery(Object[].class);
            Root<Person> root = criteriaQuery.from(Person.class);

            criteriaQuery.multiselect(root.get("name"), root.get("age"));

            List<Object[]> results = session.createQuery(criteriaQuery).getResultList();
            System.out.println("Query Person name and age:");
            for (Object[] row : results) {
                String name = (String) row[0];
                Integer age = (Integer) row[1];
                System.out.println(name + " - " + age);
            }
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}
使用聚合函数查询

假设我们想计算所有Person的平均年龄。

java 复制代码 
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.Transaction;
import org.hibernate.criterion.Projections;
import org.hibernate.query.criteria.HibernateCriteriaBuilder;
import org.hibernate.query.criteria.JpaCriteriaQuery;

import javax.persistence.criteria.Root;
import javax.persistence.criteria.CriteriaBuilder;
import javax.persistence.criteria.CriteriaQuery;
import javax.persistence.criteria.Root;

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

        // 使用Projections和聚合函数查询平均年龄
        queryAverageAge(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void queryAverageAge(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        try {
            CriteriaBuilder criteriaBuilder = session.getCriteriaBuilder();
            CriteriaQuery<Double> criteriaQuery = criteriaBuilder.createQuery(Double.class);
            Root<Person> root = criteriaQuery.from(Person.class);

            criteriaQuery.select(criteriaBuilder.avg(root.get("age")));

            Double averageAge = session.createQuery(criteriaQuery).getSingleResult();
            System.out.println("Average age of persons: " + averageAge);
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            if (session != null) {
                session.close();
            }
        }
    }
}

Projections详细解释

  1. 插入示例数据 ：向数据库中插入两个Person对象和两个Address对象供查询使用。

    java 复制代码 
    public class HibernateInsertData {
    public static void main(String[] args) {
        // 获取SessionFactory
        SessionFactory sessionFactory = HibernateUtil.getSessionFactory();

        // 插入示例数据
        insertSampleData(sessionFactory);

        // 关闭SessionFactory
        sessionFactory.close();
    }

    private static void insertSampleData(SessionFactory sessionFactory) {
        Session session = sessionFactory.openSession();
        Transaction transaction = session.beginTransaction();
        try {
            Person person1 = new Person("John Doe", 30);
            Person person2 = new Person("Jane Doe",28);
相关推荐
a程序小傲10 小时前
【Node】单线程的Node.js为什么可以实现多线程？
java·数据库·后端·面试·node.js
奋进的芋圆19 小时前
DataSyncManager 详解与 Spring Boot 迁移指南
java·spring boot·后端
计算机程序设计小李同学19 小时前
个人数据管理系统
java·vue.js·spring boot·后端·web安全
Echo娴19 小时前
Spring的开发步骤
java·后端·spring
追逐时光者20 小时前
TIOBE 公布 C# 是 2025 年度编程语言
后端·.net
Victor35620 小时前
Hibernate（32）什么是Hibernate的Criteria查询？
后端
Victor35620 小时前
Hibernate（31）Hibernate的原生SQL查询是什么？
后端
热门推荐
01GitHub 镜像站点02Labelme从安装到标注：零基础完整指南03安娜的档案(Anna’s Archive) 镜像网站/国内最新可访问入口（持续更新）04Linux下V2Ray安装配置指南05jdk21下载、安装（Windows、Linux、macOS）06【踩坑笔记】50系显卡适配的 PyTorch 安装07Opencode CLI 安装成功,但是启动失败08手把手教你通过Gemini3 pro 学生认证，白用一年，手慢无！09全球最强模型Grok4，国内已可免费使用！（附教程）10Claude Code Plan 模式完全指南：从入门到精通