Hibernate学习29 -- Hibernate查询语言（HQL）3 -- 实体对象查询

实体对象查询：

²        N + 1问题，在默认情况下，使用query.iterate查询，有可以能出现N+1问题，所谓的N+1是在查询的时候发出了N+1条sql语句：

1：首先发出一条查询对象id列表的sql

N：根据id列表到缓存中查询，如果缓存中不存在与之匹配的数据，那么会根据id发出相应的sql语句

²        list和iterate的区别？

list每次都会发出sql语句，list会向缓存中放入数据，而不利用缓存中的数据。

iterate：在默认情况下iterate利用缓存数据，但如果缓存中不存在数据有可以能出现N+1问题。

例子1代码如下：

/**  * 实体对象查询  */ public class SimpleObjectQueryTest1 extends TestCase {       public void testQuery1() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   //返回Student对象的集合            //可以忽略select            List students = session.createQuery ("from Student").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }       public void testQuery2() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   //返回Student对象的集合            //可以忽略select,表可以使用别名            List students = session.createQuery ("from Student s").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }       public void testQuery3() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                       //返回Student对象的集合            //可以忽略select,表可以使用as命名别名            List students = session.createQuery ("from Student as s").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }       public void testQuery4() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   //返回Student对象的集合            //使用select查询实体对象，必须采用别名            List students = session.createQuery ("select s from Student as s").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }              public void testQuery5() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   //不支持select * from .....这样的查询语句            List students = session.createQuery ("select * from Student").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }   }

例子2代码如下：

/**  * 实体对象查询  */ public class SimpleObjectQueryTest2 extends TestCase {     public void testQuery1() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   /**             * 采用list查询发出一条查询语句，取得Student对象数据、             *             * Hibernate: select student0_.id as id1_, student0_.name as name1_,             * student0_.createTime as createTime1_, student0_.classesid as classesid1_             * from t_student student0_             *             */            List students = session.createQuery ("from Student").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }       public void testQuery2() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   /**             * 出现N+1问题             * 1：发出查询id列表的sql             *   Hibernate: select student0_.id as col_0_0_ from t_student student0_             *             * N：在依次发出根据id查询Student对象的sql             * Hibernate: select student0_.id as id1_0_, student0_.name as name1_0_,             * student0_.createTime as createTime1_0_, student0_.classesid as classesid1_0_             * from t_student student0_ where student0_.id=?             */            Iterator iter = session.createQuery ("from Student").iterate();            while(iter.hasNext()) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }          public void testQuery3() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   List students = session.createQuery ("from Student").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }             System.out.println("----------------------");            /**             * 不会出现N+1问题             * 因为list操作已经将Student对象放到了一级缓存中，所以再次使用 iterate操作的时候             * 它首先发出一条查询id列表的sql，在根据id到缓存中去数据，只有 在缓存中找不到相应的             * 数据时，才会发出sql到数据库中查询             */            Iterator iter = session.createQuery ("from Student").iterate();            while(iter.hasNext()) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }                     session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }     public void testQuery4() {        Session session = null;        try {            session = HibernateUtils.getSession();            session.beginTransaction();                   List students = session.createQuery ("from Student").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            System.out.println("-------------------");                   /**             * 再次发出查询sql             * 在默认情况下list每次都会向数据库发出查询对象的sql，除非配置 查询缓存，所以下面的list操作             * 虽然在一级缓存中已经有了对象数据，但list默认情况下不会利用缓 存，而再次发出sql             * 默认情况下，list会向缓存中放入数据，但不会利用数据             */            students = session.createQuery("from Student").list();            for (Iterator iter=students.iterator(); iter.hasNext();) {               Student student = (Student)iter.next();               System.out.println(student.getName());            }            session.getTransaction().commit();        }catch(Exception e) {            e.printStackTrace();            session.getTransaction().rollback();        }finally {            HibernateUtils.closeSession(session);        }     }   }