hibernate 数据库查询

hibernate提供了多种方式进行数据库数据的查询

HQL查询

代码如下

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        Query query = session.createQuery("from Student student where student.name like :name order by student.id asc");
        query.setParameter("name", "student%");
        //实现分页
        query.setFirstResult(0);
        query.setMaxResults(2);
        List<Student> students = query.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

如果结果是单一的对象而不是集合，可以使用query. uniqueResult()

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        Query query = session.createQuery("select count(*) from Student");
        Long count = (Long)query.uniqueResult();

        tx.commit();
        HibernateFactory.closeSession();
    }
}

也可以直接访问对象中的对象属性（String hql = "from Student where grade.name= 'grade1'"；） 这样相当于两张表的联合查询

如果一次查询多个对象，可以使用以下方式

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Query query = session.createQuery("from Student s,Grade g where s.id=g.id and s.name like :name");
        query.setParameter("name", "student%");
        List list = query.list();
        for(int i = 0;i < list.size();i++) {
            Object[] obj = (Object[]) list.get(i);
            Student student = (Student) obj[0];
            Grade grade = (Grade)obj[1];
            System.out.println(student.getName() + "|" + grade.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

Criteria方式查询

代码如下

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        Criteria criteria = session.createCriteria(Student.class);
        criteria.add(Restrictions.like("name", "student%"));
        
        criteria.setFirstResult(0);
        criteria.setMaxResults(2);
        criteria.addOrder(Order.desc("name"));
        
        List<Student> students = criteria.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

使用criteria方式查询有以下三个步骤

1.使用Session实例 的createCriteria()方法创建Criteria对象

2.使用工具类Restrictions的方法为Criteria对象设置查询条件，Order工具类的方法设置排序方式，Projections工具类的方法进行统计和分组

3.使用Criteria对象的list()方法进行查询并返回结果

例子查询

例子查询将一个对象的非空属性作为查询条件进行查询，代码如下

public class HibernateTest {
    public static void main(String[] args) {
                //建立一个例子对象，它的非空属性作为删选条件
        Student studentExample = new Student();
        studentExample.setName("student1");
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Criteria criteria = session.createCriteria(Student.class);
        criteria.add(Example.create(studentExample));
        
        List<Student> students = criteria.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }

        tx.commit();
        HibernateFactory.closeSession();
    }
}

离线查询

离线查询建立一个DetachedCriteria对象，将查询的条件等指定好，然后在session.beginTransaction()后将这个对象传入，代码如下

public class HibernateTest {
    public static void main(String[] args) {
        DetachedCriteria dc = DetachedCriteria.forClass(Student.class);
        dc.add(Restrictions.like("name", "student%"));
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Criteria criteria = dc.getExecutableCriteria(session);
        List<Student> students = criteria.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }

        tx.commit();
        HibernateFactory.closeSession();
    }
}

load/get方式

Student student = (Student) session.get(Student.class, 1);

Student student = (Student) session.load(Student.class, 1);

两者区别

1.get()采用立即加载方式,而load()采用延迟加载;get()方法执行的时候,会立即向数据库发出查询语句, 而load()方法返回的是一个代理(此代理中只有一个id属性),只有等真正使用该对象属性的时候,才会发出sql语句

2.如果数据库中没有对应的记录,get()方法返回的是null.而load()方法出现异常ObjectNotFoundException

命名查询

1.配置hbm文件，将sql或者hql语句写入hbm，并给与一个唯一标示这个查询语句的name

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE hibernate-mapping PUBLIC
        "-//Hibernate/Hibernate Mapping DTD 3.0//EN"
        "http://hibernate.sourceforge.net/hibernate-mapping-3.0.dtd">
<hibernate-mapping >
    <class>...</class>
    
    <query name="hqlquery">
        <![CDATA[
            from Student where name like :name
        ]]>
    </query>
    
    <query name="sqlquery">
        <![CDATA[
            select s from Student s where name like ?
        ]]>
    </query>
</hibernate-mapping>

2.通过name在代码中调用事先写好的查询语句

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Query query = session.getNamedQuery("hqlquery");
        query.setParameter("name", "student%");
        List<Student> students = query.list();
        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        Query sqlquery = session.getNamedQuery("sqlquery");
        sqlquery.setParameter(0, "student%");
        List<Student> students2 = query.list();
        for(Student student : students2) {
            System.out.println(student.getName());
        }
        
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

SQL查询

hibernate支持原生的sql查询语句，具体的代码如下

返回单表的查询

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();

        List<Student> students = session.createSQLQuery("select * from student").addEntity(Student.class).list();
        //或者采用以下语句
        // List<Student> students = session.createSQLQuery("select s.* from student as s").addEntity("s",Student.class).list();

        for(Student student : students) {
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

返回多个表的查询

public class HibernateTest {
    public static void main(String[] args) {
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        SQLQuery query = session.createSQLQuery("select s.*,g.* from student as s,grade as g where s.gradeid=g.id");
        query.addEntity("s",Student.class).addEntity("g",Grade.class);
        
        List list = query.list();
        //list的每一条数据都由Object数组组成，数组的每个约束对应一个对象
        for (int i = 0; i < list.size(); i++) {
            Object[] obj = (Object[]) list.get(i);
            Student student = (Student) obj[0];
            Grade grade = (Grade) obj[1];
        }

        tx.commit();
        HibernateFactory.closeSession();
    }
}

使用jdbc

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        session.doWork(new Work() {
            public void execute(Connection connection) throws SQLException {  
                // 这里已经得到connection了，可以继续你的JDBC代码。
                PreparedStatement ps = connection.prepareStatement("insert into student(id,name) values(?,?)");
                for(int i = 0;i < 500;i++) {
                    ps.setInt(1,i);
                    ps.setString(2,"student" + i);
                    ps.addBatch();    //添加到批处理命令中
                }
                ps.executeBatch();     //执行批处理命令
                // 注意不要close了这个connection。  
            }
        });
        
        //事务提交后才会执行插入操作
        tx.commit();
        HibernateFactory.closeSession();
    }
}

使用原生的jdbc操作，hibernate4之前可以用session.connection()获取Connection对象，Hibernate4之后这个方法已经废弃

Query.iterator与Query.list的比较

iterator的使用方法如下

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        
        Query query = session.createQuery("from Student");
        Iterator iterator = query.iterate();
        while(iterator.hasNext()) {
            Student student = (Student)iterator.next();
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

iterator方式查询产生的sql语句如下

Hibernate: select student0_.id as col_0_0_ from student student0_

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

Hibernate: select student0_.id as id1_1_0_, student0_.name as name2_1_0_, student0_.gradeid as gradeid3_1_0_ from student student0_ where student0_.id=?

通过list方式查询产生的sql语句如下

Hibernate: select student0_.id as id1_1_, student0_.name as name2_1_, student0_.gradeid as gradeid3_1_ from student student0_

两者相比较，list()只发一条语句将符合条件的数据全部查出，而iterator()却现将id查出来，然后根据id再将符合条件的数据查出，这就构成了N＋1的问题；既然list更高效，为什么hibernate还将iterator存在呢？

执行以下程序

public class HibernateTest {
    public static void main(String[] args) {
        
        Session session = HibernateFactory.currentSession();
        Transaction tx = session.beginTransaction();
        String hql = "from Student";
        
        Query listQuery = session.createQuery(hql);
        List<Student> list = listQuery.list();
        for(Student student : list) {
            System.out.println(student.getName());
        }
        
        Query iteratorQuery = session.createQuery(hql);
        Iterator iterator = iteratorQuery.iterate();
        while(iterator.hasNext()) {
            Student student = (Student)iterator.next();
            System.out.println(student.getName());
        }
        
        tx.commit();
        HibernateFactory.closeSession();
    }
}

最终产生的sql语句如下

Hibernate: select student0_.id as id1_1_, student0_.name as name2_1_, student0_.gradeid as gradeid3_1_ from student student0_
Hibernate: select student0_.id as col_0_0_ from student student0_

将list()和iterator()放在一起使用，这时的iterator只执行了一条SQL，原因在于hibernate的缓存机制

list()方法将执行Select SQL从数据库中获取所有符合满足条件的记录并构造相应的实体对象，实体对象构建完毕后，就将其纳入缓存；

这样等到iterator()执行时，首先会执行一条SQL来查询符合条件数据的id，随即，iterator方法首先在本地缓存内根据id查找对应的实体对象是否存在，如果缓存中已经存在对应的数据，则直接以此数据对象作为查询结果；如果没有找到，则再次执行Select语句获得对应数据库中的表记录（如果iterator在数据库中查到并构建了完整的数据对象，也会将其纳入缓存中）;

list()方法无法读取缓存，但它可以写入缓存，在上面这个实例中，list()将读取的数据放入缓存中，iterator()直接可以用于是出现了以上的结果；

如果目标数据只读或者读取相当频繁，可以使用iterator()来减少性能上的消耗；