System.out.println

本文深入探讨了使用Hibernate进行数据库查询时,缓存与迭代器的使用方式,包括list查询与iterate查询的区别,以及如何利用缓存提高查询效率。

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System.out.println("--------------------------------------");

  Iterator iter = session.createQuery("from User s where s.id<3")。iterate();

  while (iter.hasNext()) {

  User age = (User) iter.next();

  System.out.println(age);

  }

  session.getTransaction()。commit();

  session.close();

  结果如下:

  Hibernate: select user0_.id as id0_, user0_.username as username0_, user0_.age as age0_, user0_.password as password0_ from User user0_ where user0_.id<3

  User [id=1, username=JAY, age=8, password=JAY]

  User [id=2, username=aa, age=3, password=aa]

  --------------------------------------

  Hibernate: select user0_.id as col_0_0_ from User user0_ where user0_.id<3

  User [id=1, username=JAY, age=8, password=JAY]

  User [id=2, username=aa, age=3, password=aa]

  从结果可知,第一次采用list查询,将对象全部取出,然后当第二次采用iterate查询时,只执行了对id(主键)的查询,然后通过主键从缓存中取出结果;可见采用list查询时在也会将结果放置缓存,只是它每次依旧从数据库中重新查询取得。

  Iterate实例(查询对象属性):

  SessionFactory factory = new Configuration()。configure()。buildSessionFactory();

  Session session=null;

  session=factory.openSession();

  session.beginTransaction();

  Iterator iter= session.createQuery

  ("select s.username from User s where s.id<3")。iterate();

  while (iter.hasNext()) {

  String age = (String) iter.next();

  System.out.println(age);

  }

  System.out.println("--------------------------------------");

  iter= session.createQuery

  ("select s.username from User s where s.id<3")。iterate();

  while (iter.hasNext()) {

  String age = (String) iter.next();

  System.out.println(age);

  }

  session.getTransaction()。commit();

  session.close();

  结果如下:

  Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3

  JAY

  aa

  --------------------------------------

  Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3

  JAY

  aa

  从结果可知,当查询对象属性的时候,则是直接查询对象相应的属性,而不再查询对象id(主键);并且第二查询的时候是从数据库中重新查询取出结果,而不是从缓存中直接取出。

  List实例(查询对象属性):

  SessionFactory factory = new Configuration()。configure()。buildSessionFactory();

  Session session=null;

  session=factory.openSession();

  session.beginTransaction();

  List<String> list = session.createQuery

  ("select s.username from User s where s.id<3")。list();

  for(String s:list){

  System.out.println(s.toString());

  }

  System.out.println("--------------------------------------");

  list =(List) session.createQuery

  ("select s.username from User s where s.id<3")。list();

  for(String s:list){

  System.out.println(s.toString());

  }

  session.getTransaction()。commit();

  session.close();

  结果如下:

  Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3

  JAY

  aa

  --------------------------------------

  Hibernate: select user0_.username as col_0_0_ from User user0_ where user0_.id<3

  JAY

  aa

  结论和Iterate实例(查询对象属性)的结论一样

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