hibernate3學習筆記(二十二)｜Query緩存機制

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

測試代碼如下：

1.package com.hb3.pack_22;
2.
3.import java.io.IOException;
4.import java.sql.SQLException;
5.import java.util.List;
6.
7.import org.hibernate.Query;
8.import org.hibernate.Session;
9.import org.hibernate.SessionFactory;
10.import org.hibernate.cfg.Configuration;
11.
12.import com.hb3.pack_22.model.User;
13.
14.public class BusinessService {
15.
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();
21.
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.        }
40.
41.
42.        session.close();
43.        sessionFactory.close();
44.    }
45.}

執行的結果為：

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

可見，沒有啟用Query的快取功能。

如果打算打開Query的快取功能，首先在hibernate.cfg.xml中設定hibernate.cache.use_query_cache屬性：

1.<?xml version="1.0" encoding="utf-8"?> 
2.<!DOCTYPE hibernate-configuration PUBLIC
3.    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
4.    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
5.
6.<hibernate-configuration> 
7.
8.    <session-factory> 
9.        ....
10.
11.        <property name="hibernate.cache.use_query_cache">true</property> 
12.        ....
13.
14.    </session-factory> 
15.
16.</hibernate-configuration>

然後在每次簡歷Query時，執行setCacheable(true)：

1.package com.hb3.pack_22;
2.
3.import java.io.IOException;
4.import java.sql.SQLException;
5.import java.util.List;
6.
7.import org.hibernate.Query;
8.import org.hibernate.Session;
9.import org.hibernate.SessionFactory;
10.import org.hibernate.cfg.Configuration;
11.
12.import com.hb3.pack_22.model.User;
13.
14.public class BusinessService {
15.
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();
21.
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.        }
44.
45.
46.        session.close();
47.        sessionFactory.close();
48.    }
49.}

執行結果為：

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

其實，Hibernate在啟用Query的緩存機制後，會保留執行過的查詢SQL和查詢結果，在下次查詢後會看看SQL是否相同，並看看對應的資料庫表格是否有變動(Update/Delete/Insert),如果SQL相同且且資料庫也沒有變動則將緩存中的結果直接返回。

值得一提的是，Query上有list()與iterator()2個方法用於結果集返回。他們的區別是，list()方法在讀取數據庫時不會使用緩存機制而直接向數據庫發起查詢，而iterator()則會將讀取到的結果寫到緩存中，以便於讀取時再次利用。

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

執行結果：

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();
2.
3.Query query = session.createQuery("from User");
4.Iterator users = query.iterate();
5.users = query.iterate();
6.
7.session.close();

執行結果：

Hibernate: select user0_.id as col_0_0_ from user user0_
Hibernate: select user0_.id as col_0_0_ from user user0_

由於使用iterator()方法時會使用到Session level緩存，所以在查詢大量記錄時會好用大量的記憶體。必要時可以使用Session的evict()或clear()方法來清除緩存。

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

請注意，盡管iterator()方法時會使用到Session level緩存,但這不能就說在任何情況下都是最有效率的做法。

請看下面的測試：

1.package com.hb3.pack_22;
2.
3.import java.io.IOException;
4.import java.sql.SQLException;
5.import java.util.Iterator;
6.import java.util.List;
7.
8.import org.hibernate.Query;
9.import org.hibernate.Session;
10.import org.hibernate.SessionFactory;
11.import org.hibernate.cfg.Configuration;
12.
13.import com.hb3.pack_22.model.User;
14.
15.public class BusinessService {
16.
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();
22.
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.}

執行結果：

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

看見，Iterator中的元素並不是一次性返回的，而是一條一條返回的。當數據量龐大時，特性就會下降。

再看下面的測試代碼：

1.package com.hb3.pack_22;
2.
3.import java.io.IOException;
4.import java.sql.SQLException;
5.import java.util.Iterator;
6.import java.util.List;
7.
8.import org.hibernate.Query;
9.import org.hibernate.Session;
10.import org.hibernate.SessionFactory;
11.import org.hibernate.cfg.Configuration;
12.
13.import com.hb3.pack_22.model.User;
14.
15.public class BusinessService {
16.
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();
22.
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.}

執行的結果為：

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

這明顯在性能上要優於剛才的做法。可見，首次查詢使用query.list性能要優於query.iterate，之後再做相同的查詢請求時，則借助緩存機制，使用query.iterate會大大提高性能。