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測(cè)試代碼如下：

package com.hb3.pack_22;

import java.io.IOException;
import java.sql.SQLException;
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

import com.hb3.pack_22.model.User;

public class BusinessService {

    public static void main(String[] args) throws IOException, SQLException {
        
        Configuration config = new Configuration().configure();
        SessionFactory sessionFactory = config.buildSessionFactory();      
        Session session = sessionFactory.openSession();

        String hql = "from com.hb3.pack_22.model.User";
        
        Query query = session.createQuery(hql);
        List<?> users = query.list();
                
        for(int i = 0; i < users.size(); i++) {
            User user = (User) users.get(i);
            System.out.println(user.getName());
        }
                
        query = session.createQuery(hql);
        users = query.list();
                
        for(int i = 0; i < users.size(); i++) {
            User user = (User) users.get(i);
            System.out.println(user.getName());
        }

        session.close();
        sessionFactory.close();
    }
}

執(zhí)行的結(jié)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(jiàn)，沒(méi)有啟用Query的快取功能。

如果打算打開(kāi)Query的快取功能，首先在hibernate.cfg.xml中設(shè)定hibernate.cache.use_query_cache屬性：

<?xml version="1.0" encoding="utf-8"?> 
<!DOCTYPE hibernate-configuration PUBLIC
    "-//Hibernate/Hibernate Configuration DTD 3.0//EN"
    "http://hibernate.sourceforge.net/hibernate-configuration-3.0.dtd">
    
<hibernate-configuration> 

    <session-factory> 
        .... 

        <property name="hibernate.cache.use_query_cache">true</property> 
        ....
        
    </session-factory> 

</hibernate-configuration>

然后在每次簡(jiǎn)歷Query時(shí)，執(zhí)行setCacheable(true)：

package com.hb3.pack_22;

import java.io.IOException;
import java.sql.SQLException;
import java.util.List;

import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;

import com.hb3.pack_22.model.User;

public class BusinessService {

    public static void main(String[] args) throws IOException, SQLException {
        
        Configuration config = new Configuration().configure();
        SessionFactory sessionFactory = config.buildSessionFactory();      
        Session session = sessionFactory.openSession();

        String hql = "from com.hb3.pack_22.model.User";
        
        Query query = session.createQuery(hql);
        //使用Query快取 
        query.setCacheable(true);
        List<?> users = query.list();
                
        for(int i = 0; i < users.size(); i++) {
            User user = (User) users.get(i);
            System.out.println(user.getName());
        }
                
        query = session.createQuery(hql);
        //使用Query快取 
        query.setCacheable(true);
        users = query.list();
                
        for(int i = 0; i < users.size(); i++) {
            User user = (User) users.get(i);
            System.out.println(user.getName());
        }

        session.close();
        sessionFactory.close();
    }
}

執(zhí)行結(jié)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(shí)，Hibernate在啟用Query的緩存機(jī)制后，會(huì)保留執(zhí)行過(guò)的查詢SQL和查詢結(jié)果，在下次查詢后會(huì)看看SQL是否相同，并看看對(duì)應(yīng)的資料庫(kù)表格是否有變動(dòng)(Update/Delete/Insert),如果SQL相同且且資料庫(kù)也沒(méi)有變動(dòng)則將緩存中的結(jié)果直接返回。

值得一提的是，Query上有l(wèi)ist()與iterator()2個(gè)方法用于結(jié)果集返回。他們的區(qū)別是，list()方法在讀取數(shù)據(jù)庫(kù)時(shí)不會(huì)使用緩存機(jī)制而直接向數(shù)據(jù)庫(kù)發(fā)起查詢，而iterator()則會(huì)將讀取到的結(jié)果寫(xiě)到緩存中，以便于讀取時(shí)再次利用。

Session session = sessionFactory.openSession();
        
Query query = session.createQuery("from User");
List users = query.list();
users = query.list();

session.close();

執(zhí)行結(jié)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_

Session session = sessionFactory.openSession();

Query query = session.createQuery("from User");
Iterator users = query.iterate();
users = query.iterate();

session.close();

執(zhí)行結(jié)果：

Hibernate: select user0_.id as col_0_0_ from user user0_

由于使用iterator()方法時(shí)會(huì)使用到Session level緩存，所以在查詢大量記錄時(shí)會(huì)好用大量的記憶體。必要時(shí)可以使用Session的evict()或clear()方法來(lái)清除緩存。

請(qǐng)參閱：http://blogger.org.cn/blog/more.asp?name=NaddyLee&id=31540

請(qǐng)注意，盡管iterator()方法時(shí)會(huì)使用到Session level緩存,但這不能就說(shuō)在任何情況下都是最有效率的做法。

請(qǐng)看下面的測(cè)試：

package com.hb3.pack_22;


    
import java.io.IOException;
import java.sql.SQLException;
import java.util.Iterator;
import java.util.List;


    
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;


    
import com.hb3.pack_22.model.User;


    
public class BusinessService {


    
    public static void main(String[] args) throws IOException, SQLException {
        
        Configuration config = new Configuration().configure();
        SessionFactory sessionFactory = config.buildSessionFactory();      
        Session session = sessionFactory.openSession();


    
        
        String hql = "from com.hb3.pack_22.model.User";
        
        Query query = session.createQuery(hql);
        
//        List<?> list = query.list(); 
//        Iterator<?> iterator = list.iterator(); 
//        while (iterator.hasNext()) { 
//          User user = (User)iterator.next(); 
//          System.out.println(user.getRoom().getAddress()); 
//      } 
//        System.out.println("====================================="); 
//        list = query.list(); 
//        iterator = list.iterator(); 
//        while (iterator.hasNext()) { 
//          User user = (User)iterator.next(); 
//          System.out.println(user.getRoom().getAddress()); 
//      } 
//         
//        System.out.println("++++++++++++++++++++++++++++++++++++++"); 
        
        query = session.createQuery(hql);
        Iterator<?> iterator = query.iterate();
        while (iterator.hasNext()) {
            User user = (User)iterator.next();
            System.out.println(user.getName());
        }
        System.out.println("=====================================");
        iterator = query.iterate();
        while (iterator.hasNext()) {
            User user = (User)iterator.next();
            System.out.println(user.getName());
        }
        
        session.close();
        sessionFactory.close();
    }
}

執(zhí)行結(jié)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(jiàn)，Iterator中的元素并不是一次性返回的，而是一條一條返回的。當(dāng)數(shù)據(jù)量龐大時(shí)，特性就會(huì)下降。

再看下面的測(cè)試代碼：

package com.hb3.pack_22;


    
import java.io.IOException;
import java.sql.SQLException;
import java.util.Iterator;
import java.util.List;


    
import org.hibernate.Query;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.cfg.Configuration;


    
import com.hb3.pack_22.model.User;


    
public class BusinessService {


    
    public static void main(String[] args) throws IOException, SQLException {
        
        Configuration config = new Configuration().configure();
        SessionFactory sessionFactory = config.buildSessionFactory();      
        Session session = sessionFactory.openSession();


    
        
        String hql = "from com.hb3.pack_22.model.User";
        
        Query query = session.createQuery(hql);
        
        List<?> list = query.list();
        Iterator<?> iterator = list.iterator();
        while (iterator.hasNext()) {
            User user = (User)iterator.next();
            System.out.println(user.getRoom().getAddress());
        }
//        System.out.println("====================================="); 
//        list = query.list(); 
//        iterator = list.iterator(); 
//        while (iterator.hasNext()) { 
//          User user = (User)iterator.next(); 
//          System.out.println(user.getRoom().getAddress()); 
//      } 
//         
//        System.out.println("++++++++++++++++++++++++++++++++++++++"); 
        
        query = session.createQuery(hql);
//        Iterator<?> iterator = query.iterate(); 
//        while (iterator.hasNext()) { 
//          User user = (User)iterator.next(); 
//          System.out.println(user.getName()); 
//      } 
        System.out.println("=====================================");
        iterator = query.iterate();
        while (iterator.hasNext()) {
            User user = (User)iterator.next();
            System.out.println(user.getName());
        }
        
        session.close();
        sessionFactory.close();
    }
}

執(zhí)行的結(jié)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(yōu)于剛才的做法。可見(jiàn)，首次查詢使用query.list性能要優(yōu)于query.iterate，之后再做相同的查詢請(qǐng)求時(shí)，則借助緩存機(jī)制，使用query.iterate會(huì)大大提高性能。

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發(fā)表于 2008-10-29 19:42 淘聲依舊閱讀(396) 評(píng)論(0) 編輯收藏所屬分類(lèi): 103.Hibernate

hibernate3學(xué)習(xí)筆記(二十二)｜Query緩存機(jī)制

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