准备

模型及映射文件

package com.zze.bean;

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

public class Class {
    public Class() {
    }

    public Class(Integer id, String name) {
        this.id = id;
        this.name = name;
    }

    private Integer id;
    private String name;

    private Set<Student> students = new HashSet<>();

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public Set<Student> getStudents() {
        return students;
    }

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

    @Override
    public String toString() {
        return "Class{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

班级模型：com.zze.bean.Class

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.zze.bean.Class" table="class">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name" length="32"/>
        <set name="students" cascade="save-update">
            <key column="cid"/>
            <one-to-many class="com.zze.bean.Student"/>
        </set>
    </class>
</hibernate-mapping>

班级映射：com/zze/bean/Class.hbm.xml

package com.zze.bean;

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

public class Student {
    public Student() {
    }

    public Student(Integer id, String name, Integer age,String gender) {
        this.id = id;
        this.name = name;
        this.age = age;
        this.gender = gender;
    }

    private Integer id;
    private String name;
    private Integer age;
    private String gender;

    private Class clazz;

    public Integer getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

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

    public Integer getAge() {
        return age;
    }

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

    public String getGender() {
        return gender;
    }

    public void setGender(String gender) {
        this.gender = gender;
    }

    public Class getClazz() {
        return clazz;
    }

    public void setClazz(Class clazz) {
        this.clazz = clazz;
    }

    @Override
    public String toString() {
        return "Student{" +
                "id=" + id +
                ", name='" + name + '\'' +
                '}';
    }
}

学生模型：com.zze.bean.Student

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-mapping-3.0.dtd">

<hibernate-mapping>
    <class name="com.zze.bean.Student" table="student">
        <id name="id">
            <generator class="native"/>
        </id>
        <property name="name" length="32"/>
        <property name="age"/>
        <property name="gender"/>
        <many-to-one name="clazz" column="cid" class="com.zze.bean.Class"/>
    </class>
</hibernate-mapping>

学生映射：com/zze/bean/Student.hbm.xml

全局配置

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE hibernate-configuration PUBLIC
        "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
        "http://www.hibernate.org/dtd/hibernate-configuration-3.0.dtd">
<hibernate-configuration>
    <session-factory>
        <property name="hibernate.connection.driver_class">com.mysql.jdbc.Driver</property>
        <property name="hibernate.connection.url">jdbc:mysql://192.168.208.153:3306/1221</property>
        <property name="hibernate.connection.username">root</property>
        <property name="hibernate.connection.password">root</property>
        <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="current_session_context_class">thread</property>
        <property name="hibernate.hbm2ddl.auto">create</property>
        <mapping resource="com/zze/bean/Class.hbm.xml"></mapping>
        <mapping resource="com/zze/bean/Student.hbm.xml"></mapping>
    </session-factory>
</hibernate-configuration>

hibernate.cfg.xml

工具类

package com.zze.util;

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

public class HibernateUtil {
    public static final Configuration cfg;
    public static final SessionFactory sf;

    static {
        cfg = new Configuration().configure();
        sf = cfg.buildSessionFactory();
    }

    public static Session openSession() {
        return sf.openSession();
    }

    public static Session getCurrentSession() {
        return sf.getCurrentSession();
    }
}

com.zze.util.HibernateUtil

初始化数据

@Test
public void initData(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz1 = new Class(null,"软件一班");
    Class clazz2 = new Class(null,"软件二班");
    Class clazz3 = new Class(null,"汽修一班");
    Class clazz4 = new Class(null,"汽修二班");

    Student student1 = new Student(null, "张三", 12,"男");
    Student student2 = new Student(null, "李四", 13,"女");
    Student student3 = new Student(null, "王五", 14,"男");
    Student student4 = new Student(null, "赵六", 15,"男");
    Student student5 = new Student(null, "吴刚", 39,"男");
    Student student6 = new Student(null, "王如花", 30,"女");
    Student student7 = new Student(null, "李志静", 49,"女");
    Student student8 = new Student(null, "何王天", 30,"男");

    clazz1.getStudents().add(student1);
    clazz1.getStudents().add(student2);
    clazz2.getStudents().add(student3);
    clazz2.getStudents().add(student4);
    clazz3.getStudents().add(student5);
    clazz3.getStudents().add(student6);
    clazz4.getStudents().add(student7);
    clazz4.getStudents().add(student8);

    session.save(clazz1);
    session.save(clazz2);
    session.save(clazz3);
    session.save(clazz4);
    transaction.commit();
}

init data

几种检索方式

OID检索

  • 简介

    Hibernate 根据对象的 OID（主键）进行检索。

  • 示例

    @Test
public void test1(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.get(Class.class, 1);
    System.out.println(clazz);
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=1, name='软件一班'}
     */
}

    例 1：使用 get 方法

    @Test
public void test2() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.load(Class.class, 1);
    System.out.println(clazz);
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=1, name='软件一班'}
     */
}

    例 2：使用 load 方法

对象导航检索

  • 简介

    Hibernate 根据一个已经查询到的对象，获得其关联的对象的一种查询方式。

  • 示例

    @Test
public void test3(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.load(Class.class, 1);
    System.out.println(clazz);
    System.out.println(Arrays.toString(clazz.getStudents().toArray()));
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=1, name='软件一班'}
    Hibernate:
        select
            students0_.cid as cid5_1_0_,
            students0_.id as id1_1_0_,
            students0_.id as id1_1_1_,
            students0_.name as name2_1_1_,
            students0_.age as age3_1_1_,
            students0_.gender as gender4_1_1_,
            students0_.cid as cid5_1_1_
        from
            student students0_
        where
            students0_.cid=?
    [Student{id=2, name='张三'}, Student{id=1, name='李四'}]
     */
}

    例 3：

HQL检索

  • 简介

    Hibernate Language Query，Hibernate 本身提供语法支持的一种检索方式。

  • 示例

    /**
 * 查询所有班级
 */
@Test
public void test4(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql  ="from Class";
    Query query = session.createQuery(hql);
    List<Class> list = query.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_,
            class0_.name as name2_0_
        from
            class class0_
    [Class{id=1, name='软件一班'}, Class{id=2, name='软件二班'}, Class{id=3, name='汽修一班'}, Class{id=4, name='汽修二班'}]
     */
}

    例 4：简单查询

    @Test
public void test5(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql  ="select c from Class c";
    Query query = session.createQuery(hql);
    List<Class> list = query.list();
    System.out.println(list);
    transaction.commit();
    /*
        Hibernate:
            select
                class0_.id as id1_0_,
                class0_.name as name2_0_
            from
                class class0_
        [Class{id=1, name='软件一班'}, Class{id=2, name='软件二班'}, Class{id=3, name='汽修一班'}, Class{id=4, name='汽修二班'}]
     */
}

    例 5：别名查询

    /**
 * 根据年龄排序
 */
@Test
public void test6(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    // 升序
    String hql = "from Student order by age";
    // 降序
    // String hql = "from Student order by age desc";
    Query query = session.createQuery(hql);
    List<Class> list = query.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_,
            student0_.name as name2_1_,
            student0_.age as age3_1_,
            student0_.gender as gender4_1_,
            student0_.cid as cid5_1_
        from
            student student0_
        order by
            student0_.age
    [Student{id=2, name='张三'}, Student{id=1, name='李四'}, Student{id=3, name='王五'}, Student{id=4, name='赵六'}, Student{id=6, name='王如花'}, Student{id=8, name='何王天'}, Student{id=5, name='吴刚'}, Student{id=7, name='李志静'}]
     */
}

    例 6：排序查询

    /**
 * 一个条件，查询 name='张三' 的 Student
 * 参数按位置绑定
 */
@Test
public void test7() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "from Student where name = ?";
    Query query = session.createQuery(hql);
    query.setParameter(0, "张三");
    List<Student> list = query.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_,
            student0_.name as name2_1_,
            student0_.age as age3_1_,
            student0_.gender as gender4_1_,
            student0_.cid as cid5_1_
        from
            student student0_
        where
            student0_.name=?
    [Student{id=2, name='张三'}]
     */
}

/**
 * 多个条件 查询 name 中包含 '王' 且 age 大于 18 的Student
 * 参数按名称绑定
 */
@Test
public void test8() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "from Student where name like :name and age>:age";
    Query query = session.createQuery(hql);
    query.setParameter("name", "%王%");
    query.setParameter("age", 18);
    List<Student> list = query.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_,
            student0_.name as name2_1_,
            student0_.age as age3_1_,
            student0_.gender as gender4_1_,
            student0_.cid as cid5_1_
        from
            student student0_
        where
            (
                student0_.name like ?
            )
            and student0_.age>?
    [Student{id=6, name='王如花'}, Student{id=8, name='何王天'}]
     */
}

    例 7：条件查询

    /**
 * 查询单个属性 返回 Object List
 */
@Test
public void test9() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "select s.name from Student s";
    Query query = session.createQuery(hql);
    List<Object> list = query.list();
    System.out.println(list);
    transaction.commit();
/*
Hibernate:
    select
        student0_.name as col_0_0_
    from
        student student0_
[李四, 张三, 王五, 赵六, 吴刚, 王如花, 李志静, 何王天]
 */
}

/**
 * 查询多个属性 返回 Object[] List
 */
@Test
public void test10() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "select s.name,s.age from Student s";
    Query query = session.createQuery(hql);
    List<Object[]> list = query.list();
    for (Object[] objects : list) {
        System.out.println(Arrays.toString(objects));
    }
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.name as col_0_0_,
            student0_.age as col_1_0_
        from
            student student0_
    [李四, 13]
    [张三, 12]
    [王五, 14]
    [赵六, 15]
    [吴刚, 39]
    [王如花, 30]
    [李志静, 49]
    [何王天, 30]
     */
}

/**
 * 查询多个属性 返回对应对象类型 List
 */
@Test
public void test11() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    /*
    需要让模型提供对应构造：
        public Student(String name, Integer age) {
            this.name = name;
            this.age = age;
        }
     */
    String hql = "select new Student(name,age) from Student";
    Query query = session.createQuery(hql);
    List<Student> list = query.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.name as col_0_0_,
            student0_.age as col_1_0_
        from
            student student0_
    [Student{id=null, name='李四'}, Student{id=null, name='张三'}, Student{id=null, name='王五'}, Student{id=null, name='赵六'}, Student{id=null, name='吴刚'}, Student{id=null, name='王如花'}, Student{id=null, name='李志静'}, Student{id=null, name='何王天'}]
     */
}

    例 8：投影查询

    @Test
public void test12() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "from Student";
    Query query = session.createQuery(hql);
    query.setFirstResult(0); // 起始行索引
    query.setMaxResults(3); // 每页条数
    List<Student> list = query.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_,
            student0_.name as name2_1_,
            student0_.age as age3_1_,
            student0_.gender as gender4_1_,
            student0_.cid as cid5_1_
        from
            student student0_ limit ?
    [Student{id=1, name='李四'}, Student{id=2, name='张三'}, Student{id=3, name='王五'}]

    Process finished with exit code 0
     */
}

    例 9：分页查询

    /**
 * 聚合查询
 */
@Test
public void test13() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    // 聚合函数使用 count max min avg sum
    String hql = "select count(1) from Student";
    Query query = session.createQuery(hql);
    Object o = query.uniqueResult();
    System.out.println(o.getClass());
    System.out.println(o);
    transaction.commit();
    /*
    Hibernate:
        select
            count(1) as col_0_0_
        from
            student student0_
    class java.lang.Long
    8
     */
}

/**
 * 分组查询 返回 Object[] List
 * 根据 gender 分组，查询每个 gender 的对应数据条数
 */
@Test
public void test14() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "select gender,count(1) from Student group by gender";
    Query query = session.createQuery(hql);
    List<Object[]> list = query.list();
    for (Object[] objects : list) {
        System.out.println(Arrays.toString(objects));
    }
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.gender as col_0_0_,
            count(1) as col_1_0_
        from
            student student0_
        group by
            student0_.gender
    [女, 3]
    [男, 5]
     */
}

    例 10：聚合及分组统计查询

    @Test
public void test23() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "from Student c join c.clazz";
    Query query = session.createQuery(hql);
    List<Object[]> list = query.list();
    for (Object[] objects : list) {
        System.out.println(Arrays.toString(objects));
    }
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_0_,
            class1_.id as id1_0_1_,
            student0_.name as name2_1_0_,
            student0_.age as age3_1_0_,
            student0_.gender as gender4_1_0_,
            student0_.cid as cid5_1_0_,
            class1_.name as name2_0_1_
        from
            student student0_
        inner join
            class class1_
                on student0_.cid=class1_.id
    [Student{id=1, name='李四'}, Class{id=1, name='软件一班'}]
    [Student{id=2, name='张三'}, Class{id=1, name='软件一班'}]
    [Student{id=3, name='王五'}, Class{id=2, name='软件二班'}]
    [Student{id=4, name='赵六'}, Class{id=2, name='软件二班'}]
    [Student{id=5, name='吴刚'}, Class{id=3, name='汽修一班'}]
    [Student{id=6, name='王如花'}, Class{id=3, name='汽修一班'}]
    [Student{id=7, name='李志静'}, Class{id=4, name='汽修二班'}]
    [Student{id=8, name='何王天'}, Class{id=4, name='汽修二班'}]
     */
}

    例 11：普通内连接

    /**
 * 查询所有学生及其班级
 */
@Test
public void test24() {
    /*
    迫切内连接
    其实就是在普通的内连接 inner join 后添加关键字 fetch
    将关联表数据封装到查询对象的属性中
     */
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "from Student c join fetch c.clazz";
    Query query = session.createQuery(hql);
    List<Student> list = query.list();
    for (Student student : list) {
        System.out.println(String.format("%s %s", student, student.getClazz()));
    }
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_0_,
            class1_.id as id1_0_1_,
            student0_.name as name2_1_0_,
            student0_.age as age3_1_0_,
            student0_.gender as gender4_1_0_,
            student0_.cid as cid5_1_0_,
            class1_.name as name2_0_1_
        from
            student student0_
        inner join
            class class1_
                on student0_.cid=class1_.id
    Student{id=1, name='李四'} Class{id=1, name='软件一班'}
    Student{id=2, name='张三'} Class{id=1, name='软件一班'}
    Student{id=3, name='王五'} Class{id=2, name='软件二班'}
    Student{id=4, name='赵六'} Class{id=2, name='软件二班'}
    Student{id=5, name='吴刚'} Class{id=3, name='汽修一班'}
    Student{id=6, name='王如花'} Class{id=3, name='汽修一班'}
    Student{id=7, name='李志静'} Class{id=4, name='汽修二班'}
    Student{id=8, name='何王天'} Class{id=4, name='汽修二班'}
     */
}

/**
 * 查询所有班级及属于该班级的学生
 */
@Test
public void test25() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String hql = "select distinct c from Class c join fetch c.students";
    Query query = session.createQuery(hql);
    List<Class> list = query.list();
    for (Class clazz : list) {
        System.out.println(String.format("%s %s", clazz, clazz.getStudents()));
    }
    transaction.commit();
    /*
    Class{id=1, name='软件一班'} [Student{id=2, name='张三'}, Student{id=1, name='李四'}]
    Class{id=2, name='软件二班'} [Student{id=4, name='赵六'}, Student{id=3, name='王五'}]
    Class{id=3, name='汽修一班'} [Student{id=5, name='吴刚'}, Student{id=6, name='王如花'}]
    Class{id=4, name='汽修二班'} [Student{id=7, name='李志静'}, Student{id=8, name='何王天'}]
     */
}

    例 12：迫切内连接

QBC检索

  • 简介

    Query By Criteria，条件查询，是一种更加面向对象化的查询方式。

  • 示例

    @Test
public void test15(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = session.createCriteria(Class.class);
    List<Class> list = criteria.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            this_.id as id1_0_0_,
            this_.name as name2_0_0_
        from
            class this_
    [Class{id=1, name='软件一班'}, Class{id=2, name='软件二班'}, Class{id=3, name='汽修一班'}, Class{id=4, name='汽修二班'}]
     */
}

    例 13：简单查询

    @Test
public void test16(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = session.createCriteria(Class.class);
    // 升序
    criteria.addOrder(Order.asc("id"));
    // 降序
    // criteria.addOrder(Order.desc("id"));
    List<Class> list = criteria.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            this_.id as id1_0_0_,
            this_.name as name2_0_0_
        from
            class this_
        order by
            this_.id asc
    [Class{id=1, name='软件一班'}, Class{id=2, name='软件二班'}, Class{id=3, name='汽修一班'}, Class{id=4, name='汽修二班'}]
     */
}

    例 14：排序查询

    @Test
public void test17() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = session.createCriteria(Student.class);
    criteria.setFirstResult(0); // 数据起始索引
    criteria.setMaxResults(2); // 每页数据条数
    List<Student> list = criteria.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            this_.id as id1_1_0_,
            this_.name as name2_1_0_,
            this_.age as age3_1_0_,
            this_.gender as gender4_1_0_,
            this_.cid as cid5_1_0_
        from
            student this_ limit ?
    [Student{id=1, name='李四'}, Student{id=2, name='张三'}]
     */
}

    例 15：分页查询

    /**
 * 简单 and 条件查询
 * 查询 name 包含 '王' 且 age>10
 */
@Test
public void test18() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = session.createCriteria(Student.class);
    // 默认多个 criteria.add 的条件是用 and 连接
    criteria.add(Restrictions.like("name", "%王%"));
    criteria.add(Restrictions.gt("age", 10));
    List<Student> list = criteria.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            this_.id as id1_1_0_,
            this_.name as name2_1_0_,
            this_.age as age3_1_0_,
            this_.gender as gender4_1_0_,
            this_.cid as cid5_1_0_
        from
            student this_
        where
            this_.name like ?
            and this_.age>?
    [Student{id=3, name='王五'}, Student{id=6, name='王如花'}, Student{id=8, name='何王天'}]
     */
}

/**
 * 简单 or 条件查询
 * 查询 name 包含 '王' 或 name 包含 '张'
 */
@Test
public void test19() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = session.createCriteria(Student.class);
    // or 条件可以通过 Restrictions.or 方法实现
    criteria.add(Restrictions.or(Restrictions.like("name", "%王%"), Restrictions.like("name", "%张%")));
    List<Student> list = criteria.list();
    System.out.println(list);
    transaction.commit();

    /*
    Hibernate:
        select
            this_.id as id1_1_0_,
            this_.name as name2_1_0_,
            this_.age as age3_1_0_,
            this_.gender as gender4_1_0_,
            this_.cid as cid5_1_0_
        from
            student this_
        where
            (
                this_.name like ?
                or this_.name like ?
            )
    [Student{id=2, name='张三'}, Student{id=3, name='王五'}, Student{id=6, name='王如花'}, Student{id=8, name='何王天'}]
     */
}

/**
 * 复杂 and 和 or 查询
 * 查询 (name 包含 '王' 且 age>10) 或 name 包含 '张'
 */
@Test
public void test20() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = session.createCriteria(Student.class);
    // 通过 conjunction.add 添加的条件是 and 连接
    Conjunction conjunction = Restrictions.conjunction();
    conjunction.add(Restrictions.like("name", "%王%"));
    conjunction.add(Restrictions.gt("age", 10));
    // 通过 disjunction.add 添加的条件是 or 连接
    Disjunction disjunction = Restrictions.disjunction();
    disjunction.add(conjunction);
    disjunction.add(Restrictions.like("name", "%张%"));
    criteria.add(disjunction);
    List<Student> list = criteria.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            this_.id as id1_1_0_,
            this_.name as name2_1_0_,
            this_.age as age3_1_0_,
            this_.gender as gender4_1_0_,
            this_.cid as cid5_1_0_
        from
            student this_
        where
            (
                (
                    this_.name like ?
                    and this_.age>?
                )
                or this_.name like ?
            )
    [Student{id=2, name='张三'}, Student{id=3, name='王五'}, Student{id=6, name='王如花'}, Student{id=8, name='何王天'}]
     */
}

    例 16：条件查询

    /**
 * 聚合统计查询
 * 统计男女人数及最大年龄
 */
@Test
public void test21() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = session.createCriteria(Student.class);
    /*
    criteria :
        add : 普通条件，where
        addOrder : 排序
        setProjection : 聚合函数和 group by having
     */
    ProjectionList projectionList = Projections.projectionList();
    projectionList.add(Projections.groupProperty("gender"));
    projectionList.add(Projections.rowCount());
    projectionList.add(Projections.max("age"));
    criteria.setProjection(projectionList);

    List<Object[]> list = criteria.list();
    for (Object[] objects : list) {
        System.out.println(Arrays.toString(objects));
    }
    transaction.commit();
    /*
    Hibernate:
        select
            this_.gender as y0_,
            count(*) as y1_,
            max(this_.age) as y2_
        from
            student this_
        group by
            this_.gender
    [女, 3, 49]
    [男, 5, 39]
     */
}

    例 17：聚合及分组统计查询

    /**
 * 离线条件查询
 * 查询 name 包含 '一'
 */
@Test
public void test22(){
    DetachedCriteria detachedCriteria = DetachedCriteria.forClass(Class.class);
    detachedCriteria.add(Restrictions.like("name", "%一%"));
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Criteria criteria = detachedCriteria.getExecutableCriteria(session);
    List<Class> list = criteria.list();
    System.out.println(list);
    transaction.commit();
    /*
    Hibernate:
        select
            this_.id as id1_0_0_,
            this_.name as name2_0_0_
        from
            class this_
        where
            this_.name like ?
    [Class{id=1, name='软件一班'}, Class{id=3, name='汽修一班'}]
     */
}

    例 18：离线条件查询

SQL检索

  • 简介

    Hibernate 支持的原生 SQL 查询。

  • 示例

    /**
 * 查询所有 Class，返回 Object[] List
 */
@Test
public void test26() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String sql = "select * from class";
    SQLQuery sqlQuery = session.createSQLQuery(sql);
    List<Object[]> list = sqlQuery.list();
    for (Object[] objects : list) {
        System.out.println(Arrays.toString(objects));
    }
    transaction.commit();
    /*
    Hibernate:
        select
            *
        from
            class
    [1, 软件一班]
    [2, 软件二班]
    [3, 汽修一班]
    [4, 汽修二班]
     */
}

/**
 * 查询所有 Class，返回 Class List
 */
@Test
public void test27() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    String sql = "select * from class";
    SQLQuery sqlQuery = session.createSQLQuery(sql);
    sqlQuery.addEntity(Class.class);
    List<Class> list = sqlQuery.list();
    for (Class clazz : list) {
        System.out.println(clazz);
    }
    transaction.commit();
    /*
    Hibernate:
        select
            *
        from
            class
    Class{id=1, name='软件一班'}
    Class{id=2, name='软件二班'}
    Class{id=3, name='汽修一班'}
    Class{id=4, name='汽修二班'}
     */
}

    例 19：原生 SQL 查询

延迟加载

概述

只有真正使用对象的属性时才会发送 SQL 进行查询。如： session.load(Student.class,1); 。

分类

  • 类级别的延迟加载

    指的是查询某个类的对象时，针对这个类的对象是否使用延迟加载。

    除了修改 lazy="false" 这种方式，还可通过如下两种方式让 lazy 失效：

    1、将持久化类以 final 修饰。

    2、通过调用 Hibernate.initialize(proxy); 。

  • 关联级别的延迟加载

    指的是查询到某个对象后，再查询其关联的对象是否使用延迟加载。

示例

/**
 * 懒加载
 * 修改映射文件 ：
 * com/zze/bean/Class.hbm.xml :
 * <class name="com.zze.bean.Class" table="class" lazy="true">
 * lazy 默认为 true
 */
@Test
public void test27() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.load(Class.class, 1);
    System.out.println(clazz.getClass()); // class com.zze.bean.Class_$$_jvst718_0
    System.out.println(clazz); // 发出 SQL
    transaction.commit();
}

/**
 * 立即加载
 * 修改映射文件 :
 * com/zze/bean/Class.hbm.xml :
 * <class name="com.zze.bean.Class" table="class" lazy="false">
 */
@Test
public void test28() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.load(Class.class, 1); // 发出 SQL
    System.out.println(clazz.getClass()); // class com.zze.bean.Class
    System.out.println(clazz);
    transaction.commit();
}

例 20：类级别的延迟加载

/**
 * 懒加载
 * 修改映射文件 ：
 * com/zze/bean/Class.hbm.xml :
 * <set name="students" cascade="save-update" lazy="true">
 * lazy 默认为 true
 */
@Test
public void test29() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.get(Class.class, 1);
    Set<Student> students = clazz.getStudents();
    System.out.println(students); // 发出 SQL
    transaction.commit();
}

/**
 * 即时加载
 * 修改映射文件 ：
 * com/zze/bean/Class.hbm.xml :
 * <set name="students" cascade="save-update" lazy="false">
 */
@Test
public void test30() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.get(Class.class, 1);
    Set<Student> students = clazz.getStudents(); // 发出 SQL
    System.out.println(students);
    transaction.commit();
}

/**
 * 懒加载
 * 修改映射文件 :
 * com/zze/bean/Student.hbm.xml :
 * <many-to-one name="clazz" lazy="proxy" column="cid" class="com.zze.bean.Class"/>
 * com/zze/bean/Class.hbm.xml :
 * <class name="com.zze.bean.Class" table="class" lazy="true">
 * <p>
 * proxy : 默认值，表示引用关联类的加载策略。
 */
@Test
public void test31() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Student student = session.get(Student.class, 1);
    Class clazz = student.getClazz();
    System.out.println(clazz); // 发出 SQL
    transaction.commit();
}

/**
 * 即时加载
 * 当关联类的 lazy="false" 时，无论当前类 many-to-one 标签上的 lazy 如何配置，都是即时加载。
 * 修改映射文件 :
 * com/zze/bean/Student.hbm.xml :
 * * <many-to-one name="clazz" lazy="proxy" column="cid" class="com.zze.
 * com/zze/bean/Class.hbm.xml :
 * * <class name="com.zze.bean.Class" table="class" lazy="false">
 * 或 :
 * com/zze/bean/Student.hbm.xml :
 * * <many-to-one name="clazz" lazy="false" column="cid" class="com.zze.bean.Class"/>
 * com/zze/bean/Class.hbm.xml :
 * * <class name="com.zze.bean.Class" table="class" lazy="true">
 */
@Test
public void test32() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Student student = session.get(Student.class, 1);
    Class clazz = student.getClazz(); // 发出 SQL
    System.out.println(clazz);
    transaction.commit();
}

例 21：关联级别的延迟加载

抓取策略

概述

通过一个对象抓取关联对象需要发送 SQL ，SQL 如何发送及 SQL 的发送规则就可以通过抓取策略进行配置。

通过 <set> 或 <many-to-one> 上的 fetch 属性进行配置。

要考虑fetch 和这些标签上的 lazy 如何配置才能最大化优化发送的 SQL 语句。

set标签上的fetch和lazy

  • fetch

    抓取策略，控制 SQL 语句格式。

    在这里 fetch 有三个可选值：

    select : 默认值，发送普通 select 语句查询关联对象。

    join: 发送一条迫切左外连接查询关联对象。

    subselect : 发送子查询查询关联对象。

  • lazy

    延迟加载，控制查询关联对象时是否使用懒加载。

    在这里 lazy 有三个可选值：

    true : 默认值，查询关联对象时，使用延迟加载。

    false : 查询关联对象时，不使用延迟加载。

    extra : 及其懒惰。

  • 示例

    /**
 * com/zze/bean/Class.hbm.xml
 * 默认值：
 * <set name="students" cascade="save-update" fetch="select" lazy="true">
 */
@Test
public void test33() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.get(Class.class, 1);
    for (Student student : clazz.getStudents()) {
        System.out.println(student);// 发送 SQL
    }
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Hibernate:
        select
            students0_.cid as cid5_1_0_,
            students0_.id as id1_1_0_,
            students0_.id as id1_1_1_,
            students0_.name as name2_1_1_,
            students0_.age as age3_1_1_,
            students0_.gender as gender4_1_1_,
            students0_.cid as cid5_1_1_
        from
            student students0_
        where
            students0_.cid=?
    Student{id=2, name='张三'}
    Student{id=1, name='李四'}
     */
}

/**
 * com/zze/bean/Class.hbm.xml
 *
 * <set name="students" cascade="save-update" fetch="select" lazy="false">
 */
@Test
public void test34() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.get(Class.class, 1); //发送 2 条 SQL
    for (Student student : clazz.getStudents()) {
        System.out.println(student);
    }
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Hibernate:
        select
            students0_.cid as cid5_1_0_,
            students0_.id as id1_1_0_,
            students0_.id as id1_1_1_,
            students0_.name as name2_1_1_,
            students0_.age as age3_1_1_,
            students0_.gender as gender4_1_1_,
            students0_.cid as cid5_1_1_
        from
            student students0_
        where
            students0_.cid=?
    Student{id=2, name='张三'}
    Student{id=1, name='李四'}
     */
}

/**
 * com/zze/bean/Class.hbm.xml
 *
 * <set name="students" cascade="save-update" fetch="select" lazy="extra">
 */
@Test
public void test35() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.get(Class.class, 1);
    System.out.println(clazz.getStudents().size()); // 针对数量发送一条sql
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Hibernate:
        select
            count(id)
        from
            student
        where
            cid =?
     */
}

/**
 * com/zze/bean/Class.hbm.xml
 * fetch="join" 时，lazy 失效
 * <set name="students" cascade="save-update" fetch="join" lazy="true">
 */
public void test36() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Class clazz = session.get(Class.class, 1);// 发送 SQL
    for (Student student : clazz.getStudents()) {
        System.out.println(student);
    }
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_,
            students1_.cid as cid5_1_1_,
            students1_.id as id1_1_1_,
            students1_.id as id1_1_2_,
            students1_.name as name2_1_2_,
            students1_.age as age3_1_2_,
            students1_.gender as gender4_1_2_,
            students1_.cid as cid5_1_2_
        from
            class class0_
        left outer join
            student students1_
                on class0_.id=students1_.cid
        where
            class0_.id=?
    Student{id=2, name='张三'}
    Student{id=1, name='李四'}
     */
}

/**
 * com/zze/bean/Class.hbm.xml
 *
 * <set name="students" cascade="save-update" fetch="subselect" lazy="true">
 */
@Test
public void test37() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Query query = session.createQuery("from Class");
    List<Class> list = query.list();
    for (Class clazz : list) {
        Set<Student> students = clazz.getStudents(); // 发送 SQL
        System.out.println(students);
    }
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_,
            class0_.name as name2_0_
        from
            class class0_
    Hibernate:
        select
            students0_.cid as cid5_1_1_,
            students0_.id as id1_1_1_,
            students0_.id as id1_1_0_,
            students0_.name as name2_1_0_,
            students0_.age as age3_1_0_,
            students0_.gender as gender4_1_0_,
            students0_.cid as cid5_1_0_
        from
            student students0_
        where
            students0_.cid in (
                select
                    class0_.id
                from
                    class class0_
            )
    [Student{id=1, name='李四'}, Student{id=2, name='张三'}]
    [Student{id=4, name='赵六'}, Student{id=3, name='王五'}]
    [Student{id=5, name='吴刚'}, Student{id=6, name='王如花'}]
    [Student{id=7, name='李志静'}, Student{id=8, name='何王天'}]
     */
}

/**
 * com/zze/bean/Class.hbm.xml
 *
 * <set name="students" cascade="save-update" fetch="subselect" lazy="false">
 */
@Test
public void test38() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Query query = session.createQuery("from Class");
    List<Class> list = query.list(); // 发送 SQL
    for (Class clazz : list) {
        Set<Student> students = clazz.getStudents();
        System.out.println(students);
    }
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_,
            class0_.name as name2_0_
        from
            class class0_
    Hibernate:
        select
            students0_.cid as cid5_1_1_,
            students0_.id as id1_1_1_,
            students0_.id as id1_1_0_,
            students0_.name as name2_1_0_,
            students0_.age as age3_1_0_,
            students0_.gender as gender4_1_0_,
            students0_.cid as cid5_1_0_
        from
            student students0_
        where
            students0_.cid in (
                select
                    class0_.id
                from
                    class class0_
            )
    [Student{id=1, name='李四'}, Student{id=2, name='张三'}]
    [Student{id=4, name='赵六'}, Student{id=3, name='王五'}]
    [Student{id=5, name='吴刚'}, Student{id=6, name='王如花'}]
    [Student{id=7, name='李志静'}, Student{id=8, name='何王天'}]
     */
}

    例 22：

many-to-one上的fetch和lazy

  • fetch

    抓取策略，控制 SQL 语句格式。

    在这里 fetch 有两个可选值：

    select : 默认值，发送普通的 select 语句查询关联对象。

    join : 发送一条迫切左外连接查询关联对象。

  • lazy

    延迟加载，控制查询关联对象时是否使用懒加载。

    在这里 lazy 有三个可选值：

    proxy : 默认值，引用关联对象类的类级别加载策略。

    flase : 查询关联对象时，不使用延迟加载。

    no-proxy：不使用。

  • 示例

    /**
 * com/zze/bean/Student.hbm.xml
 * 默认值
 * <many-to-one name="clazz" column="cid" class="com.zze.bean.Class" fetch="select" lazy="proxy"/>
 *
 * com/zze/bean/Class.hbm.xml
 * <class name="com.zze.bean.Class" table="class" lazy="true">
 */
@Test
public void test39(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Student student = session.get(Student.class, 1);
    System.out.println(student.getClazz()); // 发送 SQL
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_0_,
            student0_.name as name2_1_0_,
            student0_.age as age3_1_0_,
            student0_.gender as gender4_1_0_,
            student0_.cid as cid5_1_0_
        from
            student student0_
        where
            student0_.id=?
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=1, name='软件一班'}
     */
}

/**
 * com/zze/bean/Student.hbm.xml
 * 默认值
 * <many-to-one name="clazz" column="cid" class="com.zze.bean.Class" fetch="select" lazy="proxy"/>
 *
 * com/zze/bean/Class.hbm.xml
 * <class name="com.zze.bean.Class" table="class" lazy="false">
 */
@Test
public void test40(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Student student = session.get(Student.class, 1);// 一次发送 2 条 SQL
    System.out.println(student.getClazz());
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_0_,
            student0_.name as name2_1_0_,
            student0_.age as age3_1_0_,
            student0_.gender as gender4_1_0_,
            student0_.cid as cid5_1_0_
        from
            student student0_
        where
            student0_.id=?
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=1, name='软件一班'}
     */
}

/**
 * com/zze/bean/Student.hbm.xml
 * fetch="join" 时，lazy 失效
 * <many-to-one name="clazz" column="cid" class="com.zze.bean.Class" fetch="join" lazy="proxy"/>
 */
@Test
public void test41(){
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Student student = session.get(Student.class, 1);// 一条关联 SQL
    System.out.println(student.getClazz());
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_0_,
            student0_.name as name2_1_0_,
            student0_.age as age3_1_0_,
            student0_.gender as gender4_1_0_,
            student0_.cid as cid5_1_0_,
            class1_.id as id1_0_1_,
            class1_.name as name2_0_1_
        from
            student student0_
        left outer join
            class class1_
                on student0_.cid=class1_.id
        where
            student0_.id=?
    Class{id=1, name='软件一班'}
     */
}

    例 23：

批量抓取

一批指定数量的关联数据一起查询。

/**
 * 查询所有班级 及 所有班级下的学生信息
 * 默认情况下，查询所有班级会发一条 SQL，然后每次查询班级上的学生也会发送一条 SQL
 * 即如下有 4 个班级，会发 5 条SQL
 */
@Test
public void test42() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Query query = session.createQuery("from Class");
    List<Class> list = query.list();
    for (Class clazz : list) {
        System.out.println(clazz);
        for (Student student : clazz.getStudents()) {
            System.out.println(student);
        }
    }
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_,
            class0_.name as name2_0_
        from
            class class0_
    Class{id=1, name='软件一班'}
    Hibernate:
        select
            students0_.cid as cid5_1_0_,
            students0_.id as id1_1_0_,
            students0_.id as id1_1_1_,
            students0_.name as name2_1_1_,
            students0_.age as age3_1_1_,
            students0_.gender as gender4_1_1_,
            students0_.cid as cid5_1_1_
        from
            student students0_
        where
            students0_.cid=?
    Student{id=1, name='李四'}
    Student{id=2, name='张三'}
    Class{id=2, name='软件二班'}
    Hibernate:
        select
            students0_.cid as cid5_1_0_,
            students0_.id as id1_1_0_,
            students0_.id as id1_1_1_,
            students0_.name as name2_1_1_,
            students0_.age as age3_1_1_,
            students0_.gender as gender4_1_1_,
            students0_.cid as cid5_1_1_
        from
            student students0_
        where
            students0_.cid=?
    Student{id=4, name='王五'}
    Student{id=3, name='赵六'}
    Class{id=3, name='汽修一班'}
    Hibernate:
        select
            students0_.cid as cid5_1_0_,
            students0_.id as id1_1_0_,
            students0_.id as id1_1_1_,
            students0_.name as name2_1_1_,
            students0_.age as age3_1_1_,
            students0_.gender as gender4_1_1_,
            students0_.cid as cid5_1_1_
        from
            student students0_
        where
            students0_.cid=?
    Student{id=5, name='王如花'}
    Student{id=6, name='吴刚'}
    Class{id=4, name='汽修二班'}
    Hibernate:
        select
            students0_.cid as cid5_1_0_,
            students0_.id as id1_1_0_,
            students0_.id as id1_1_1_,
            students0_.name as name2_1_1_,
            students0_.age as age3_1_1_,
            students0_.gender as gender4_1_1_,
            students0_.cid as cid5_1_1_
        from
            student students0_
        where
            students0_.cid=?
    Student{id=8, name='李志静'}
    Student{id=7, name='何王天'}
     */
}

/**
 * com/zze/bean/Class.hbm.xml
 *
 * <set name="students" cascade="save-update" batch-size="4">
 * <p>
 * 一次抓取 4 条，就只需要发送 2 次 SQL
 * 一次查询所有班级，一次查询所有班级下的学生
 */
@Test
public void test43() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Query query = session.createQuery("from Class");
    List<Class> list = query.list();
    for (Class clazz : list) {
        System.out.println(clazz);
        for (Student student : clazz.getStudents()) {
            System.out.println(student);
        }
    }
    transaction.commit();
    /*
    Hibernate:
        select
            class0_.id as id1_0_,
            class0_.name as name2_0_
        from
            class class0_
    Class{id=1, name='软件一班'}
    Hibernate:
        select
            students0_.cid as cid5_1_1_,
            students0_.id as id1_1_1_,
            students0_.id as id1_1_0_,
            students0_.name as name2_1_0_,
            students0_.age as age3_1_0_,
            students0_.gender as gender4_1_0_,
            students0_.cid as cid5_1_0_
        from
            student students0_
        where
            students0_.cid in (
                ?, ?, ?, ?
            )
    Student{id=1, name='李四'}
    Student{id=2, name='张三'}
    Class{id=2, name='软件二班'}
    Student{id=4, name='王五'}
    Student{id=3, name='赵六'}
    Class{id=3, name='汽修一班'}
    Student{id=5, name='王如花'}
    Student{id=6, name='吴刚'}
    Class{id=4, name='汽修二班'}
    Student{id=7, name='何王天'}
    Student{id=8, name='李志静'}
     */
}

/**
 * 查询所有学生 及 所有学生所属班级信息
 * 默认情况下，查询所有学生会发送 1 条 SQL，然后每次查询每个学生的所属班级时也会发送一条 SQL，
 * 有 8 个学生，但一共有 4 个班级，由于一级缓存的作用，查询班级需要发送 4 条 SQL
 * 共发 5 条 SQL
 */
@Test
public void test44() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Query query = session.createQuery("from Student");
    List<Student> list = query.list();
    for (Student student : list) {
        System.out.println(student.getClazz());
    }
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_,
            student0_.name as name2_1_,
            student0_.age as age3_1_,
            student0_.gender as gender4_1_,
            student0_.cid as cid5_1_
        from
            student student0_
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=1, name='软件一班'}
    Class{id=1, name='软件一班'}
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=2, name='软件二班'}
    Class{id=2, name='软件二班'}
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=3, name='汽修一班'}
    Class{id=3, name='汽修一班'}
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=4, name='汽修二班'}
    Class{id=4, name='汽修二班'}
     */
}

/**
 * com/zze/bean/Class.hbm.xml
 * <class name="com.zze.bean.Class" table="class" batch-size="3">
 * <p>
 * many-to-one 关系时需在关联属性类映射文件下配置 batch-size
 * <p>
 * 查询所有学生 1 条 SQL，查询所有班级每次抓取 3 条，一共 4 个班级，会发 2 次 SQL
 */
@Test
public void test45() {
    Session session = HibernateUtil.getCurrentSession();
    Transaction transaction = session.beginTransaction();
    Query query = session.createQuery("from Student");
    List<Student> list = query.list();
    for (Student student : list) {
        System.out.println(student.getClazz());
    }
    transaction.commit();
    /*
    Hibernate:
        select
            student0_.id as id1_1_,
            student0_.name as name2_1_,
            student0_.age as age3_1_,
            student0_.gender as gender4_1_,
            student0_.cid as cid5_1_
        from
            student student0_
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id in (
                ?, ?, ?
            )
    Class{id=1, name='软件一班'}
    Class{id=1, name='软件一班'}
    Class{id=2, name='软件二班'}
    Class{id=2, name='软件二班'}
    Class{id=3, name='汽修一班'}
    Class{id=3, name='汽修一班'}
    Hibernate:
        select
            class0_.id as id1_0_0_,
            class0_.name as name2_0_0_
        from
            class class0_
        where
            class0_.id=?
    Class{id=4, name='汽修二班'}
    Class{id=4, name='汽修二班'}
     */
}

例 24：

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