Hibernate查询语言（HQL，Hibernate Query Language）是一种面向对象的查询语言，它类似于SQL，但操作的是Hibernate映射的持久化对象及其属性而不是数据库表。HQL的强大之处在于它能够利用Hibernate的映射和关联，简化复杂的查询操作。
主要特点
- 面向对象：HQL操作的是对象及其属性，而不是数据库表和列。
- 自动转换：HQL查询会自动转换成相应的SQL查询。
- 支持投影和聚合：HQL支持投影、聚合、排序、分组等操作。
- 支持关联和连接：可以通过对象关系导航进行关联和连接操作。
HQL查询示例
下面是一个完整的示例，展示如何使用HQL进行各种查询操作。
配置文件 hibernate.cfg.xml
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>
<!-- 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>
<!-- 映射类配置 -->
<mapping class="com.example.domain.Student"/>
</session-factory>
</hibernate-configuration>HibernateUtil 类
java
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;
}
}实体类 Student
java
package com.example.domain;
public class Student {
private Long id;
private String name;
private int age;
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;
}
}使用HQL的查询示例
java
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.query.Query;
import java.util.List;
public class HibernateHQLExample {
public static void main(String[] args) {
// 获取SessionFactory
SessionFactory sessionFactory = HibernateUtil.getSessionFactory();
// 打开一个新的Session
Session session = sessionFactory.openSession();
// 启动事务
session.beginTransaction();
// 创建并保存Student对象
Student student1 = new Student("John Doe", 20);
Student student2 = new Student("Jane Doe", 22);
session.save(student1);
session.save(student2);
// 提交事务
session.getTransaction().commit();
// 查询所有学生
String hql = "FROM Student";
Query<Student> query = session.createQuery(hql, Student.class);
List<Student> students = query.list();
for (Student student : students) {
System.out.println("Student: " + student.getName() + ", Age: " + student.getAge());
}
// 带条件的查询
hql = "FROM Student S WHERE S.age > :student_age";
query = session.createQuery(hql, Student.class);
query.setParameter("student_age", 21);
students = query.list();
for (Student student : students) {
System.out.println("Filtered Student: " + student.getName() + ", Age: " + student.getAge());
}
// 投影查询
hql = "SELECT S.name FROM Student S";
Query<String> nameQuery = session.createQuery(hql, String.class);
List<String> names = nameQuery.list();
for (String name : names) {
System.out.println("Student Name: " + name);
}
// 聚合查询
hql = "SELECT COUNT(S.id) FROM Student S";
Query<Long> countQuery = session.createQuery(hql, Long.class);
Long count = countQuery.getSingleResult();
System.out.println("Total Students: " + count);
// 排序查询
hql = "FROM Student S ORDER BY S.age DESC";
query = session.createQuery(hql, Student.class);
students = query.list();
for (Student student : students) {
System.out.println("Sorted Student: " + student.getName() + ", Age: " + student.getAge());
}
// 分组查询
hql = "SELECT S.age, COUNT(S.id) FROM Student S GROUP BY S.age";
Query<Object[]> groupQuery = session.createQuery(hql, Object[].class);
List<Object[]> groupResults = groupQuery.list();
for (Object[] result : groupResults) {
System.out.println("Age: " + result[0] + ", Count: " + result[1]);
}
// 关闭Session
session.close();
// 关闭SessionFactory
sessionFactory.close();
}
}详细解释
-
查询所有学生 ：使用
FROM Student语句查询所有学生对象。javaString hql = "FROM Student"; Query<Student> query = session.createQuery(hql, Student.class); List<Student> students = query.list(); -
带条件的查询 ：使用
WHERE子句添加条件查询，例如查找年龄大于21岁的学生。javahql = "FROM Student S WHERE S.age > :student_age"; query = session.createQuery(hql, Student.class); query.setParameter("student_age", 21); List<Student> students = query.list(); -
投影查询 ：使用
SELECT子句选择特定字段，例如只选择学生的名字。javahql = "SELECT S.name FROM Student S"; Query<String> nameQuery = session.createQuery(hql, String.class); List<String> names = nameQuery.list(); -
聚合查询 ：使用聚合函数，例如
COUNT函数统计学生人数。javahql = "SELECT COUNT(S.id) FROM Student S"; Query<Long> countQuery = session.createQuery(hql, Long.class); Long count = countQuery.getSingleResult(); -
排序查询 ：使用
ORDER BY子句进行排序，例如按年龄降序排序。javahql = "FROM Student S ORDER BY S.age DESC"; query = session.createQuery(hql, Student.class); List<Student> students = query.list(); -
分组查询 ：使用
GROUP BY子句进行分组查询，例如按年龄分组统计每个年龄段的学生人数。javahql = "SELECT S.age, COUNT(S.id) FROM Student S GROUP BY S.age"; Query<Object[]> groupQuery = session.createQuery(hql, Object[].class); List<Object[]> groupResults = groupQuery.list();
总结
HQL是一种功能强大的查询语言，它简化了Hibernate持久化对象的查询操作。通过HQL可以方便地进行各种复杂的查询操作，如条件查询、投影查询、聚合查询、排序查询和分组查询。此外，HQL与数据库无关，这使得应用程序在不同数据库之间具有良好的可移植性。通过熟练掌握HQL，可以高效地进行数据库操作，提高应用程序的性能和可维护性。