hibernate3学习笔记(二十二)｜Query缓存机制

接前面的例子：http://blog.csdn.net/kunshan_shenbin/archive/2008/09/03/2874375.aspx

测试代码如下：

1. package com.hb3.pack_22;
3. import java.io.IOException;
4. import java.sql.SQLException;
5. import java.util.List;
7. import org.hibernate.Query;
8. import org.hibernate.Session;
9. import org.hibernate.SessionFactory;
10. import org.hibernate.cfg.Configuration;
12. import com.hb3.pack_22.model.User;
14. public class BusinessService {
16.     public static void main(String[] args) throws IOException, SQLException {
17.         
18.         Configuration config = new Configuration().configure();
19.         SessionFactory sessionFactory = config.buildSessionFactory();      
20.         Session session = sessionFactory.openSession();
22.         
23.         String hql = "from com.hb3.pack_22.model.User";
24.         
25.         Query query = session.createQuery(hql);
26.         List users = query.list();
27.                 
28.         for(int i = 0; i < users.size(); i++) {
29.             User user = (User) users.get(i);
30.             System.out.println(user.getName());
31.         }
32.                 
33.         query = session.createQuery(hql);
34.         users = query.list();
35.                 
36.         for(int i = 0; i < users.size(); i++) {
37.             User user = (User) users.get(i);
38.             System.out.println(user.getName());
39.         }
42.         session.close();
43.         sessionFactory.close();
44.     }
45. }

执行的结果为：

Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_
chenyan
shenbin
Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_
chenyan
shenbin

 

可见，没有启用Query的快取功能。

如果打算打开Query的快取功能，首先在hibernate.cfg.xml中设定hibernate.cache.use_query_cache属性：

1. xml version="1.0" encoding="utf-8"?> 
3.     "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
4.     "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
5.     
6. <hibernate-configuration> 
8.     <session-factory> 
9.         .... 
11.         <property name="hibernate.cache.use_query_cache">trueproperty> 
12.         ....
13.         
14.     session-factory> 
16. hibernate-configuration>

然后在每次简历Query时，执行setCacheable(true)：

1. package com.hb3.pack_22;
3. import java.io.IOException;
4. import java.sql.SQLException;
5. import java.util.List;
7. import org.hibernate.Query;
8. import org.hibernate.Session;
9. import org.hibernate.SessionFactory;
10. import org.hibernate.cfg.Configuration;
12. import com.hb3.pack_22.model.User;
14. public class BusinessService {
16.     public static void main(String[] args) throws IOException, SQLException {
17.         
18.         Configuration config = new Configuration().configure();
19.         SessionFactory sessionFactory = config.buildSessionFactory();      
20.         Session session = sessionFactory.openSession();
22.         
23.         String hql = "from com.hb3.pack_22.model.User";
24.         
25.         Query query = session.createQuery(hql);
26.         //使用Query快取
27.         query.setCacheable(true);
28.         List users = query.list();
29.                 
30.         for(int i = 0; i < users.size(); i++) {
31.             User user = (User) users.get(i);
32.             System.out.println(user.getName());
33.         }
34.                 
35.         query = session.createQuery(hql);
36.         //使用Query快取
37.         query.setCacheable(true);
38.         users = query.list();
39.                 
40.         for(int i = 0; i < users.size(); i++) {
41.             User user = (User) users.get(i);
42.             System.out.println(user.getName());
43.         }
46.         session.close();
47.         sessionFactory.close();
48.     }
49. }

执行结果为：

Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_
chenyan
shenbin
chenyan
shenbin

 

其实，Hibernate在启用Query的缓存机制后，会保留执行过的查询SQL和查询结果，在下次查询后会看看SQL是否相同，并看看对应的资料库表格是否有变动(Update/Delete/Insert),如果SQL相同且且资料库也没有变动则将缓存中的结果直接返回。

 

值得一提的是，Query上有list()与iterator()2个方法用于结果集返回。他们的区别是，list()方法在读取数据库时不会使用缓存机制而直接向数据库发起查询，而iterator()则会将读取到的结果写到缓存中，以便于读取时再次利用。

1. Session session = sessionFactory.openSession();
2.         
3. Query query = session.createQuery("from User");
4. List users = query.list();
5. users = query.list();
7. session.close();

执行结果：

Hibernate: select user0_.id as id, user0_.name as name0_, user0_.age as age0_ from user user0_
Hibernate: select user0_.id as id, user0_.name as name0_, user0_.age as age0_ from user user0_

1. Session session = sessionFactory.openSession();
3. Query query = session.createQuery("from User");
4. Iterator users = query.iterate();
5. users = query.iterate();
7. session.close();

执行结果：

Hibernate: select user0_.id as col_0_0_ from user user0_

Hibernate: select user0_.id as col_0_0_ from user user0_

 

由于使用iterator()方法时会使用到Session level缓存，所以在查询大量记录时会好用大量的记忆体。必要时可以使用Session的evict()或clear()方法来清除缓存。

请参阅：http://blogger.org.cn/blog/more.asp?name=NaddyLee&id=31540

请注意，尽管iterator()方法时会使用到Session level缓存,但这不能就说在任何情况下都是最有效率的做法。

请看下面的测试：

1. package com.hb3.pack_22;
3. import java.io.IOException;
4. import java.sql.SQLException;
5. import java.util.Iterator;
6. import java.util.List;
8. import org.hibernate.Query;
9. import org.hibernate.Session;
10. import org.hibernate.SessionFactory;
11. import org.hibernate.cfg.Configuration;
13. import com.hb3.pack_22.model.User;
15. public class BusinessService {
17.     public static void main(String[] args) throws IOException, SQLException {
18.         
19.         Configuration config = new Configuration().configure();
20.         SessionFactory sessionFactory = config.buildSessionFactory();      
21.         Session session = sessionFactory.openSession();
23.         
24.         String hql = "from com.hb3.pack_22.model.User";
25.         
26.         Query query = session.createQuery(hql);
27.         
28. //        List list = query.list();
29. //        Iterator iterator = list.iterator();
30. //        while (iterator.hasNext()) {
31. //          User user = (User)iterator.next();
32. //          System.out.println(user.getRoom().getAddress());
33. //      }
34. //        System.out.println("=====================================");
35. //        list = query.list();
36. //        iterator = list.iterator();
37. //        while (iterator.hasNext()) {
38. //          User user = (User)iterator.next();
39. //          System.out.println(user.getRoom().getAddress());
40. //      }
41. //        
42. //        System.out.println("++++++++++++++++++++++++++++++++++++++");
43.         
44.         query = session.createQuery(hql);
45.         Iterator iterator = query.iterate();
46.         while (iterator.hasNext()) {
47.             User user = (User)iterator.next();
48.             System.out.println(user.getName());
49.         }
50.         System.out.println("=====================================");
51.         iterator = query.iterate();
52.         while (iterator.hasNext()) {
53.             User user = (User)iterator.next();
54.             System.out.println(user.getName());
55.         }
56.         
57.         session.close();
58.         sessionFactory.close();
59.     }
60. }

执行结果：

Hibernate: select user0_.id as col_0_0_ from user user0_
Hibernate: select user0_.id as id0_1_, user0_.name as name0_1_, user0_.room_id as room3_0_1_, room1_.id as id1_0_, room1_.address as address1_0_ from user user0_ left outer join room room1_ on user0_.room_id=room1_.id where user0_.id=?
chenyan
Hibernate: select user0_.id as id0_1_, user0_.name as name0_1_, user0_.room_id as room3_0_1_, room1_.id as id1_0_, room1_.address as address1_0_ from user user0_ left outer join room room1_ on user0_.room_id=room1_.id where user0_.id=?
shenbin
=====================================
Hibernate: select user0_.id as col_0_0_ from user user0_
chenyan
shenbin

看见，Iterator中的元素并不是一次性返回的，而是一条一条返回的。当数据量庞大时，特性就会下降。

再看下面的测试代码：

1. package com.hb3.pack_22;
3. import java.io.IOException;
4. import java.sql.SQLException;
5. import java.util.Iterator;
6. import java.util.List;
8. import org.hibernate.Query;
9. import org.hibernate.Session;
10. import org.hibernate.SessionFactory;
11. import org.hibernate.cfg.Configuration;
13. import com.hb3.pack_22.model.User;
15. public class BusinessService {
17.     public static void main(String[] args) throws IOException, SQLException {
18.         
19.         Configuration config = new Configuration().configure();
20.         SessionFactory sessionFactory = config.buildSessionFactory();      
21.         Session session = sessionFactory.openSession();
23.         
24.         String hql = "from com.hb3.pack_22.model.User";
25.         
26.         Query query = session.createQuery(hql);
27.         
28.         List list = query.list();
29.         Iterator iterator = list.iterator();
30.         while (iterator.hasNext()) {
31.             User user = (User)iterator.next();
32.             System.out.println(user.getRoom().getAddress());
33.         }
34. //        System.out.println("=====================================");
35. //        list = query.list();
36. //        iterator = list.iterator();
37. //        while (iterator.hasNext()) {
38. //          User user = (User)iterator.next();
39. //          System.out.println(user.getRoom().getAddress());
40. //      }
41. //        
42. //        System.out.println("++++++++++++++++++++++++++++++++++++++");
43.         
44.         query = session.createQuery(hql);
45. //        Iterator iterator = query.iterate();
46. //        while (iterator.hasNext()) {
47. //          User user = (User)iterator.next();
48. //          System.out.println(user.getName());
49. //      }
50.         System.out.println("=====================================");
51.         iterator = query.iterate();
52.         while (iterator.hasNext()) {
53.             User user = (User)iterator.next();
54.             System.out.println(user.getName());
55.         }
56.         
57.         session.close();
58.         sessionFactory.close();
59.     }
60. }

执行的结果为：

Hibernate: select user0_.id as id0_, user0_.name as name0_, user0_.room_id as room3_0_ from user user0_
Hibernate: select room0_.id as id1_0_, room0_.address as address1_0_ from room room0_ where room0_.id=?
NTU-M8-419
NTU-M8-419
=====================================
Hibernate: select user0_.id as col_0_0_ from user user0_
chenyan
shenbin

 

这明显在性能上要优于刚才的做法。可见，首次查询使用query.list性能要优于query.iterate，之后再做相同的查询请求时，则借助缓存机制，使用query.iterate会大大提高性能。