缓存：
是计算机领域的概念，它介于应用程序和永久性数据存储源之间。
缓存：
一般人的理解是在内存中的一块空间，可以将二级缓存配置到硬盘。用白话来说，就是一个存储数据的容器。我们关注的是，哪些数据需要被放入二级缓存。
缓存作用：
降低应用程序直接读写数据库的频率，从而提高程序的运行性能。缓存中的数据是数据存储源中数据的拷贝。缓存的物理介质通常是【内存】。
缓存分类：
1.Session缓存（又称作事务缓存）：Hibernate内置的，不能卸除。
2.SessionFactory缓存（又称作应用缓存）：使用第三方插件，可插拔。
缓存范围：
缓存只能被当前Session对象访问。
缓存的生命周期依赖于Session的生命周期，当Session被关闭后，缓存也就结束生命周期。
缓存被应用范围内的所有session共享,不同的Session可以共享。
这些session有可能是并发访问缓存，因此必须对缓存进行更新。
缓存的生命周期依赖于应用的生命周期，应用结束时，缓存也就结束了生命周期，二级缓存存在于应用程序范围。
一级缓存：
1.当我们通过session的save、update、saveOrUpdate方法进行操作时，如果一级缓存中没有对象，那么会从数据库中查询到这些对象，并存储到一级缓存中。
2.当我们通过session的load、get、Query的list等方法进行操作时，会先判断一级缓存中是否存在数据，如果没有才会从数据库获取，并且将查询的数据存储到一级缓存中。
3.当调用session的close方法时，session缓存将清空。
public class SessionFactoryUtil {
// private static Configuration configuration;
//
// private static SessionFactory factory;
//
// static{
// //默认加载项目根目录下的hibernate.cfg.xml
// configuration=new Configuration().configure();
// factory=configuration.buildSessionFactory();
// }
// public static synchronized Session getCureentSession()
// {
// return factory.getCurrentSession();
// }
//线程变量 get set
static ThreadLocal<Session> tlSession=new ThreadLocal<Session>();
//得有SessionFactory
public static SessionFactory factory;
static Configuration cfg=null;
static {
cfg=new Configuration().configure("hibernate.cfg.xml");
factory=cfg.buildSessionFactory();
}
//01.获取连接
public static Session getSession(){
//01.从线程变量中尝试获取
Session session = tlSession.get();
if (session==null){
//用户第一次获取连接：发现线程变量中没有session，创建一个，并且放入线程变量
session = factory.openSession();
tlSession.set(session);
}
return session;
}
//02.释放连接
public static void closeSession(){
Session session = tlSession.get();
if (session!=null){
//线程变量set成null
tlSession.set(null);
session.close();
}
}
public class AppTest
{
private static Transaction transaction;
private static Session session; @Before
public void one(){ //创建一个会话
session= SessionFactoryUtil.getSession();
//开启事务
transaction=session.beginTransaction(); } @After
public void close(){
transaction.commit();
} /**
* 是否存在一级缓存
*/ @Test
public void test13(){
Criteria criteria=session.createCriteria(District.class);
District load = (District) session.load(District.class, 1);
System.out.println(load.getName());
System.out.println("==================");
District load2 = (District) session.get(District.class, 1);
System.out.println(load2.getName());
} 在这里控制台输出一条SQL时证明一级缓存存在
二级缓存：
SessionFactory级别的缓存，可以跨越Session存在，可以被多个Session所共享。
在cfg.xml文件中配置相关的节点
<!--开启hibernate需要的2级缓存 -->
<property name="cache.use_second_level_cache">true</property>
<property name="cache.provider_class">org.hibernate.cache.EhCacheProvider</property> <!--指定那个类需要开启二级缓存 -->
<class-cache class="com.msss.pojo.District" usage="read-write"></class-cache>
<ehcache xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../config/ehcache.xsd">
<!--
The ehcache-failsafe.xml is a default configuration for ehcache, if an ehcache.xml is not configured.
The diskStore element is optional. It must be configured if you have overflowToDisk or diskPersistent enabled
for any cache. If it is not configured, a warning will be issues and java.io.tmpdir will be used.
diskStore has only one attribute - "path". It is the path to the directory where .data and .index files will be created.
If the path is a Java System Property it is replaced by its value in the
running VM.
The following properties are translated:
* user.home - User's home directory
* user.dir - User's current working directory
* java.io.tmpdir - Default temp file path
* ehcache.disk.store.dir - A system property you would normally specify on the command line
e.g. java -Dehcache.disk.store.dir=/u01/myapp/diskdir ...
Subdirectories can be specified below the property e.g. java.io.tmpdir/one
-->
<diskStore path="java.io.tmpdir"/>
<!--
Specifies a CacheManagerEventListenerFactory, be used to create a CacheManagerPeerProvider,
which is notified when Caches are added or removed from the CacheManager.
The attributes of CacheManagerEventListenerFactory are:
* class - a fully qualified factory class name
* properties - comma separated properties having meaning only to the factory.
Sets the fully qualified class name to be registered as the CacheManager event listener.
The events include:
* adding a Cache
* removing a Cache
Callbacks to listener methods are synchronous and unsynchronized. It is the responsibility
of the implementer to safely handle the potential performance and thread safety issues
depending on what their listener is doing.
If no class is specified, no listener is created. There is no default.
<cacheManagerEventListenerFactory class="" properties=""/>
-->
<!--
(Enable for distributed operation)
Specifies a CacheManagerPeerProviderFactory which will be used to create a
CacheManagerPeerProvider, which discovers other CacheManagers in the cluster.
The attributes of cacheManagerPeerProviderFactory are:
* class - a fully qualified factory class name
* properties - comma separated properties having meaning only to the factory.
Ehcache comes with a built-in RMI-based distribution system with two means of discovery of
CacheManager peers participating in the cluster:
* automatic, using a multicast group. This one automatically discovers peers and detects
changes such as peers entering and leaving the group
* manual, using manual rmiURL configuration. A hardcoded list of peers is provided at
configuration time.
Configuring Automatic Discovery:
Automatic discovery is configured as per the following example:
<cacheManagerPeerProviderFactory
class="net.sf.ehcache.distribution.RMICacheManagerPeerProviderFactory"
properties="peerDiscovery=automatic, multicastGroupAddress=230.0.0.1,
multicastGroupPort=4446, timeToLive=32"/>
Valid properties are:
* peerDiscovery (mandatory) - specify "automatic"
* multicastGroupAddress (mandatory) - specify a valid multicast group address
* multicastGroupPort (mandatory) - specify a dedicated port for the multicast heartbeat
traffic
* timeToLive - specify a value between 0 and 255 which determines how far the packets will propagate.
By convention, the restrictions are:
0 - the same host
1 - the same subnet
32 - the same site
64 - the same region
128 - the same continent
255 - unrestricted
Configuring Manual Discovery:
Manual discovery is configured as per the following example:
<cacheManagerPeerProviderFactory class=
"net.sf.ehcache.distribution.RMICacheManagerPeerProviderFactory"
properties="peerDiscovery=manual,
rmiUrls=//server1:40000/sampleCache1|//server2:40000/sampleCache1
| //server1:40000/sampleCache2|//server2:40000/sampleCache2"/>
Valid properties are:
* peerDiscovery (mandatory) - specify "manual"
* rmiUrls (mandatory) - specify a pipe separated list of rmiUrls, in the form
//hostname:port
The hostname is the hostname of the remote CacheManager peer. The port is the listening
port of the RMICacheManagerPeerListener of the remote CacheManager peer.
<cacheManagerPeerProviderFactory
class="net.sf.ehcache.distribution.RMICacheManagerPeerProviderFactory"
properties="peerDiscovery=automatic,
multicastGroupAddress=230.0.0.1,
multicastGroupPort=4446, timeToLive=1"/>
-->
<!--
(Enable for distributed operation)
Specifies a CacheManagerPeerListenerFactory which will be used to create a
CacheManagerPeerListener, which
listens for messages from cache replicators participating in the cluster.
The attributes of cacheManagerPeerListenerFactory are:
class - a fully qualified factory class name
properties - comma separated properties having meaning only to the factory.
Ehcache comes with a built-in RMI-based distribution system. The listener component is
RMICacheManagerPeerListener which is configured using
RMICacheManagerPeerListenerFactory. It is configured as per the following example:
<cacheManagerPeerListenerFactory
class="net.sf.ehcache.distribution.RMICacheManagerPeerListenerFactory"
properties="hostName=fully_qualified_hostname_or_ip,
port=40001,
socketTimeoutMillis=120000"/>
All properties are optional. They are:
* hostName - the hostName of the host the listener is running on. Specify
where the host is multihomed and you want to control the interface over which cluster
messages are received. Defaults to the host name of the default interface if not
specified.
* port - the port the listener listens on. This defaults to a free port if not specified.
* socketTimeoutMillis - the number of ms client sockets will stay open when sending
messages to the listener. This should be long enough for the slowest message.
If not specified it defaults 120000ms.
<cacheManagerPeerListenerFactory
class="net.sf.ehcache.distribution.RMICacheManagerPeerListenerFactory"/>
-->
<!-- Cache configuration.
The following attributes are required.
name:
Sets the name of the cache. This is used to identify the cache. It must be unique.
maxElementsInMemory:
Sets the maximum number of objects that will be created in memory (0 == no limit)
maxElementsOnDisk:
Sets the maximum number of objects that will be maintained in the DiskStore
The default value is zero, meaning unlimited.
eternal:
Sets whether elements are eternal. If eternal, timeouts are ignored and the
element is never expired.
overflowToDisk:
Sets whether elements can overflow to disk when the in-memory cache
has reached the maxInMemory limit.
The following attributes are optional.
timeToIdleSeconds:
Sets the time to idle for an element before it expires.
i.e. The maximum amount of time between accesses before an element expires
Is only used if the element is not eternal.
Optional attribute. A value of 0 means that an Element can idle for infinity.
The default value is 0.
timeToLiveSeconds:
Sets the time to live for an element before it expires.
i.e. The maximum time between creation time and when an element expires.
Is only used if the element is not eternal.
Optional attribute. A value of 0 means that and Element can live for infinity.
The default value is 0.
diskPersistent:
Whether the disk store persists between restarts of the Virtual Machine.
The default value is false.
diskExpiryThreadIntervalSeconds:
The number of seconds between runs of the disk expiry thread. The default value
is 120 seconds.
diskSpoolBufferSizeMB:
This is the size to allocate the DiskStore for a spool buffer. Writes are made
to this area and then asynchronously written to disk. The default size is 30MB.
Each spool buffer is used only by its cache. If you get OutOfMemory errors consider
lowering this value. To improve DiskStore performance consider increasing it. Trace level
logging in the DiskStore will show if put back ups are occurring.
memoryStoreEvictionPolicy:
Policy would be enforced upon reaching the maxElementsInMemory limit. Default
policy is Least Recently Used (specified as LRU). Other policies available -
First In First Out (specified as FIFO) and Less Frequently Used
(specified as LFU)
Cache elements can also contain sub elements which take the same format of a factory class
and properties. Defined sub-elements are:
* cacheEventListenerFactory - Enables registration of listeners for cache events, such as
put, remove, update, and expire.
* bootstrapCacheLoaderFactory - Specifies a BootstrapCacheLoader, which is called by a
cache on initialisation to prepopulate itself.
Each cache that will be distributed needs to set a cache event listener which replicates
messages to the other CacheManager peers. For the built-in RMI implementation this is done
by adding a cacheEventListenerFactory element of type RMICacheReplicatorFactory to each
distributed cache's configuration as per the following example:
<cacheEventListenerFactory class="net.sf.ehcache.distribution.RMICacheReplicatorFactory"
properties="replicateAsynchronously=true,
replicatePuts=true,
replicateUpdates=true,
replicateUpdatesViaCopy=true,
replicateRemovals=true "/>
The RMICacheReplicatorFactory recognises the following properties:
* replicatePuts=true|false - whether new elements placed in a cache are
replicated to others. Defaults to true.
* replicateUpdates=true|false - whether new elements which override an
element already existing with the same key are replicated. Defaults to true.
* replicateRemovals=true - whether element removals are replicated. Defaults to true.
* replicateAsynchronously=true | false - whether replications are
asynchronous (true) or synchronous (false). Defaults to true.
* replicateUpdatesViaCopy=true | false - whether the new elements are
copied to other caches (true), or whether a remove message is sent. Defaults to true.
* asynchronousReplicationIntervalMillis=<number of milliseconds> - The asynchronous
replicator runs at a set interval of milliseconds. The default is 1000. The minimum
is 10. This property is only applicable if replicateAsynchronously=true
The RMIBootstrapCacheLoader bootstraps caches in clusters where RMICacheReplicators are
used. It is configured as per the following example:
<bootstrapCacheLoaderFactory
class="net.sf.ehcache.distribution.RMIBootstrapCacheLoaderFactory"
properties="bootstrapAsynchronously=true, maximumChunkSizeBytes=5000000"/>
The RMIBootstrapCacheLoaderFactory recognises the following optional properties:
* bootstrapAsynchronously=true|false - whether the bootstrap happens in the background
after the cache has started. If false, bootstrapping must complete before the cache is
made available. The default value is true.
* maximumChunkSizeBytes=<integer> - Caches can potentially be very large, larger than the
memory limits of the VM. This property allows the bootstraper to fetched elements in
chunks. The default chunk size is 5000000 (5MB).
-->
<!--
Mandatory Default Cache configuration. These settings will be applied to caches
created programmtically using CacheManager.add(String cacheName)
-->
<defaultCache
maxElementsInMemory="10000"
eternal="false"
timeToIdleSeconds="120"
timeToLiveSeconds="120"
overflowToDisk="true"
maxElementsOnDisk="10000000"
diskPersistent="false"
diskExpiryThreadIntervalSeconds="120"
memoryStoreEvictionPolicy="LRU"
/>
</ehcache>