hibernate 所提供的一級快取(Session Level)，在同一個 session 下，如果對主鍵 select 兩次 相同的資料，其只會對 db 進行一次的 select，這個即為一級快取，很好了解。
Session level 快取會隨著 Session 生命週期起始與消滅。
來看良葛格的解釋 :
Session會維護一個Map容器，並保留與目前Session發生關係的資料，當您透過主鍵來載入資料時，Session會先依據所要載入的類別與所給定的主鍵，看看Map中是否已有資料，如果有的話就返回，若沒有就對資料庫進行查詢，並在載入資料後在Map中維護。
來看一下程式內容
public String firstCache() {
Users userA = usersService.getById(1L);
Users userB = usersService.getById(1L);
System.out.println("我們相同嗎 ? " + (userA == userB));
System.out.println("我是一級快取");
return SUCCESS;
}
result as below我們相同嗎 ? true 我是一級快取
結果顯示 : from 良格葛 -- 第二次查詢資料時，在快取中找到資料物件，於是直接返回，這與第一次查詢到的資料物件是同一個實例，所以會顯示 true 的結果。
那要怎麼將快取清掉呢?
可以透過evict()將某個物件從快取中移去，或使用clear()清除快取中的所有物件，
例如 :
public String firstCache() {
Users userA = usersService.getById(1L);
// session.clear() || session.evict(userA);
Users userB = usersService.getById(1L);
System.out.println("我們相同嗎 ? " + (userA == userB));
System.out.println("我是一級快取");
return SUCCESS;
}
則答案就會變成 false
我們相同嗎 ? false 我是一級快取
小結：當讀取的資料很大時，每一次 request 進來就去讀取一次，會造成相當的負擔，所以有了二級快取這個東西。
簡單講就是當資料沒有修改時，不管讀幾次，只要有設定二級快取，session 就算關閉了以後，二級快取裡如果有的話，也不會對 DB select。
以下開始介紹二級快取：
首先，先來看一下程式片段：
public String noCache() {
Users uA = usersService.getByAccount("markyeh");
System.out.println("我沒有 快取 : " + uA.getUpdateTime());
return SUCCESS;
}
然後我對點了這個連結兩次 http://localhost:8080/Cache/noCache.doresult as below：
Hibernate:
/*
FROM
Users
WHERE
account = ? */ select
users0_.id as id0_,
users0_.account as account0_,
users0_.address as address0_,
users0_.area1 as area4_0_,
users0_.area2 as area5_0_,
users0_.create_time as create6_0_,
users0_.email as email0_,
users0_.icon as icon0_,
users0_.mobile as mobile0_,
users0_.name as name0_,
users0_.password as password0_,
users0_.reg_ip as reg12_0_,
users0_.reg_random_code as reg13_0_,
users0_.roc_id as roc14_0_,
users0_.score as score0_,
users0_.state as state0_,
users0_.update_time as update17_0_
from
users users0_
where
users0_.account=?
我沒有 快取 : 2012-03-15 10:37:41.395
Hibernate:
/*
FROM
Users
WHERE
account = ? */ select
users0_.id as id0_,
users0_.account as account0_,
users0_.address as address0_,
users0_.area1 as area4_0_,
users0_.area2 as area5_0_,
users0_.create_time as create6_0_,
users0_.email as email0_,
users0_.icon as icon0_,
users0_.mobile as mobile0_,
users0_.name as name0_,
users0_.password as password0_,
users0_.reg_ip as reg12_0_,
users0_.reg_random_code as reg13_0_,
users0_.roc_id as roc14_0_,
users0_.score as score0_,
users0_.state as state0_,
users0_.update_time as update17_0_
from
users users0_
where
users0_.account=?
我沒有 快取 : 2012-03-15 10:37:41.395
執行結果：hibernate 對 db 進行了兩次的 select，但明明相同的東西，怎麼做才能只對 db select 一次就好呢？這裡就開始說明如何設定 hibernate 的二級快取：
我的環境如下
struts2 + hibernate + spring + ehcache(第三方實作 cache 的套件)
1. 在 spring 設定檔中，在 hibernate 注入的 bean 裡，增加二級快取的設定
<bean id="sessionFactory"
class="org.springframework.orm.hibernate3.annotation.AnnotationSessionFactoryBean">
<property name="dataSource" ref="ihergoSqlDataSource" />
<property name="hibernateProperties">
<props>
<!--
<prop key="hibernate.dialect">org.hibernate.dialect.MySQLInnoDBDialect</prop>
-->
<prop key="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</prop>
<prop key="hibernate.connection.characterEncoding">utf-8</prop>
<prop key="hibernate.show_sql">true</prop>
<prop key="hibernate.format_sql">true</prop>
<prop key="hibernate.use_sql_comments">true</prop>
<prop key="hibernate.cache.use_second_level_cache">true</prop> <!-- 要不要使用二級快取 -->
<prop key="hibernate.cache.provider_class">org.hibernate.cache.EhCacheProvider</prop> <!-- 指定為 EHCache -->
<prop key="hibernate.cache.region.factory_class">net.sf.ehcache.hibernate.SingletonEhCacheRegionFactory</prop>
<prop key="hibernate.cache.use_query_cache">true</prop> <!-- query 也要快取-->
<prop key="hibernate.hbm2ddl.auto">update</prop>
</props>
</property>
<property name="packagesToScan">
<list>
<value>mt.buyer.db.entity</value>
</list>
</property>
</bean>
2. 設定 entity (以 annotation 為例)@Entity
@Cache(usage = CacheConcurrencyStrategy.READ_WRITE) // <---這裡, 只定為 read_write
@Table(name = "users")
public class Users implements Serializable {
@Id
@GeneratedValue(strategy = GenerationType.AUTO)
private long id;
/**
* 帳號
*/
@Column(name = "account", length = 20, nullable = false)
private String account;
...
}
3. 編寫 dao // 快取
public Users getByAccountUserCache(final String account) {
final String hql = "FROM Users WHERE account = :account ";
List memberList = getHibernateTemplate().executeFind(
new HibernateCallback>() {
public List doInHibernate(Session hbmSs)
throws HibernateException, SQLException {
// TODO Auto-generated method stub
Query query = hbmSs.createQuery(hql);
query.setString("account", account);
query.setCacheable(true); // <---- 這裡
return query.list();
}
});
if (memberList != null && memberList.size() > 0) {
return (Users) memberList.get(0);
}
return null;
}
// 沒有快取
public Users getByAccount(String account) {
final String hql = "FROM Users WHERE account = ?";
List memberList = getHibernateTemplate().find(hql, account);
if (memberList != null && memberList.size() > 0) {
return (Users) memberList.get(0);
}
return null;
}
4. 增加設定檔 ehcache.xml 並放在 src 下(預設讀取入徑, 也可用 spring 注入) <?xml version="1.0" encoding="UTF-8"?> <ehcache> <diskStore path="java.io.tmpdir" /> <defaultCache maxElementsInMemory="10000" eternal="false" overflowToDisk="true" timeToIdleSeconds="300" timeToLiveSeconds="180" diskPersistent="false" diskExpiryThreadIntervalSeconds="120" /> <!-- Hibernate --> <cache name="org.hibernate.cache.StandardQueryCache" maxElementsInMemory="5" eternal="false" timeToLiveSeconds="120" overflowToDisk="true"> </cache> <cache name="org.hibernate.cache.UpdateTimestampsCache" maxElementsInMemory="5000" eternal="true" overflowToDisk="true"> </cache> </ehcache>5. 測試程式 (action)
public String cache() {
Users uA = usersService.getByAccountUserCache("markyeh");
Users uB = usersService.getByAccountUserCache("markyeh");
System.out.println("快取 : " + uA.getUpdateTime());
System.out.println("快取 : " + uB.getUpdateTime());
return SUCCESS;
}
6. 執行連結 : http://localhost:8080/Cache/cache.do Hibernate:
/*
FROM
Users
WHERE
account = :account */ select
users0_.id as id0_,
users0_.account as account0_,
users0_.address as address0_,
users0_.area1 as area4_0_,
users0_.area2 as area5_0_,
users0_.create_time as create6_0_,
users0_.email as email0_,
users0_.icon as icon0_,
users0_.mobile as mobile0_,
users0_.name as name0_,
users0_.password as password0_,
users0_.reg_ip as reg12_0_,
users0_.reg_random_code as reg13_0_,
users0_.roc_id as roc14_0_,
users0_.score as score0_,
users0_.state as state0_,
users0_.update_time as update17_0_
from
users users0_
where
users0_.account=?
快取 : 2012-03-15 10:37:41.395
快取 : 2012-03-15 10:37:41.395
7. 可以發現, 第二次的查詢就已經沒有再進 db search 了, 如果再執行相同連結一次(也就是再發一個 request 到後台) 快取 : 2012-03-15 10:37:41.395 快取 : 2012-03-15 10:37:41.395可以看到，都是從二級快取的地方取得資料，這時，大家會問，那修改時怎麼判斷要不要取得新資料 8. 我們加入下面的程式碼：
public String update() {
// Users u = usersService.getByAccountUserCache("markyeh");
Users u = usersService.getByAccount("markyeh");
u.setArea1(3);
u.setUpdateTime(new Date());
usersService.update(u);
System.out.println("我修改了 : " + u.getUpdateTime());
return SUCCESS;
}
9. 分別依順序執行 3 個連結 a) http://localhost:8080/Cache/cache.do b) http://localhost:8080/Cache/update.do c) http://localhost:8080/Cache/cache.do // a) select
Hibernate:
/*
FROM
Users
WHERE
account = :account */ select
users0_.id as id0_,
users0_.account as account0_,
users0_.address as address0_,
users0_.area1 as area4_0_,
users0_.area2 as area5_0_,
users0_.create_time as create6_0_,
users0_.email as email0_,
users0_.icon as icon0_,
users0_.mobile as mobile0_,
users0_.name as name0_,
users0_.password as password0_,
users0_.reg_ip as reg12_0_,
users0_.reg_random_code as reg13_0_,
users0_.roc_id as roc14_0_,
users0_.score as score0_,
users0_.state as state0_,
users0_.update_time as update17_0_
from
users users0_
where
users0_.account=?
快取 : 2012-03-15 16:39:16.944
快取 : 2012-03-15 16:39:16.944
第一次執行, 結果一樣, 不會撈兩次
// b) update
Hibernate:
/*
FROM
Users
WHERE
account = ? */ select
users0_.id as id0_,
users0_.account as account0_,
users0_.address as address0_,
users0_.area1 as area4_0_,
users0_.area2 as area5_0_,
users0_.create_time as create6_0_,
users0_.email as email0_,
users0_.icon as icon0_,
users0_.mobile as mobile0_,
users0_.name as name0_,
users0_.password as password0_,
users0_.reg_ip as reg12_0_,
users0_.reg_random_code as reg13_0_,
users0_.roc_id as roc14_0_,
users0_.score as score0_,
users0_.state as state0_,
users0_.update_time as update17_0_
from
users users0_
where
users0_.account=?
Hibernate:
/* update
mt.buyer.db.entity.Users */ update
users
set
account=?,
address=?,
area1=?,
area2=?,
create_time=?,
email=?,
icon=?,
mobile=?,
name=?,
password=?,
reg_ip=?,
reg_random_code=?,
roc_id=?,
score=?,
state=?,
update_time=?
where
id=?
我修改了 : Thu Mar 15 16:40:19 CST 2012
// c) select
Hibernate:
/*
FROM
Users
WHERE
account = :account */ select
users0_.id as id0_,
users0_.account as account0_,
users0_.address as address0_,
users0_.area1 as area4_0_,
users0_.area2 as area5_0_,
users0_.create_time as create6_0_,
users0_.email as email0_,
users0_.icon as icon0_,
users0_.mobile as mobile0_,
users0_.name as name0_,
users0_.password as password0_,
users0_.reg_ip as reg12_0_,
users0_.reg_random_code as reg13_0_,
users0_.roc_id as roc14_0_,
users0_.score as score0_,
users0_.state as state0_,
users0_.update_time as update17_0_
from
users users0_
where
users0_.account=?
快取 : 2012-03-15 16:40:19.524
快取 : 2012-03-15 16:40:19.524
可以發現，ehcache 會自動判斷 cache 裡的東西跟 db 的記錄是不是不一樣了，如果不一樣會再 select 一次放入二級快取裡小結: ehcache.xml 可以設定的方式很多，這裡只是 demo，而我也還在學習，官方表示還可以做到 tx 的管理，所以還是去看一下文件比較好。
參考文件:
http://www.ehcache.org/documentation/user-guide/hibernate
http://renren.it/a/JAVAbiancheng/Spring/20101022/55282.html
http://caterpillar.onlyfun.net/Gossip/HibernateGossip/SecondLevelCache.html
進階，因公司架構 tomcat 為 cluster ，那...二級快取有辦法共用嗎? 答案是可以的
以下介紹 cluster 環境下怎麼共用二級快取的資料：
參考了官方文件後，必須加入第三方的套件 terracotta 先來看看官方的說明:
http://terracotta.org/documentation/enterprise-ehcache/get-started
其這套件就是一個 server ，經由這個 server 幫你控管各 session
而我測試的流程大概如下
1. 先裝 2 個tomcat，分別設定 port 為 8080, 8081
2. 加入相關 jar 檔
3. 修改 ehcache.xml
4. depoly 程式
5. 將 terracotta server run 起來
6. 測試
就從第二點開始：
2. ehcache 官網有寫，要做到For Terracotta clustering, download a full Ehcache distribution. 所以先下載好檔案，並解壓縮，內容會長這樣
<?xml version="1.0" encoding="UTF-8"?> <ehcache xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="ehcache.xsd" updateCheck="true" monitoring="autodetect"> <terracottaConfig url="localhost:9510" /> <!-- 連接到 terracotta server 的 ip 與 port --> <defaultCache maxElementsInMemory="10000" eternal="false" overflowToDisk="false" timeToIdleSeconds="300" timeToLiveSeconds="180" diskPersistent="false" diskExpiryThreadIntervalSeconds="120" /> <!-- diskPersistent 必須設成 false --> <!-- Hibernate --> <cache name="org.hibernate.cache.StandardQueryCache" maxElementsInMemory="5" eternal="false" timeToLiveSeconds="120" overflowToDisk="false"> <!-- overflowToDisk 必須設成 false --> <terracotta /> </cache> <cache name="org.hibernate.cache.UpdateTimestampsCache" maxElementsInMemory="5000" eternal="true" overflowToDisk="false"> <!-- overflowToDisk 必須設成 false --> <terracotta /> </cache> </ehcache>5. 在剛那個資料夾裡, 找到 \ehcache-2.5.1\terracotta\bin\start-tc-server.bat 並執行, 將 server run 起來
...
補充 :
分散式 cache 參考連結
http://www.informit.com/guides/content.aspx?g=java&seqNum=630
http://bbs.51osos.com/thread-4570-1-1.html
http://terracotta.org/documentation/enterprise-ehcache/configuration-guide
然後許多東西都沒有講, 如: terracotta 也可以加入 xml 設定檔, 設定 port, 而且 terracotta 本身也可以做 cluster
反正看一下官網就對了
以上
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