ehcache 缓存技术

一 ehcache API:

1: Using the CacheManager

1.1所有ehcache的使用, 都是从 CacheManager. 开始的.
有多种方法创建CacheManager实例:

//Create a singleton CacheManager using defaults, then list caches.
CacheManager.getInstance()

 

或者:

//Create a CacheManager instance using defaults, then list caches.
CacheManager manager = new CacheManager();
String[] cacheNames = manager.getCacheNames();

 

如果需要从指定配置文件创建 CacheManager:

Create two CacheManagers, each with a different configuration, and list the caches in each.
CacheManager manager1 = new CacheManager("src/config/ehcache1.xml");
CacheManager manager2 = new CacheManager("src/config/ehcache2.xml");
String[] cacheNamesForManager1 = manager1.getCacheNames();
String[] cacheNamesForManager2 = manager2.getCacheNames();

 

1.2 Adding and Removing Caches Programmatically
手动创建一个cache, 而不是通过配置文件:

//creates a cache called testCache, which
//will be configured using defaultCache from the configuration
CacheManager singletonManager = CacheManager.create();
singletonManager.addCache("testCache");
Cache test = singletonManager.getCache("testCache");

 

或者:

//Create a Cache and add it to the CacheManager, then use it. Note that Caches are not usable until they have
//been added to a CacheManager.
    public void testCreatCacheByProgram()
    {
        CacheManager singletonManager = CacheManager.create();
        Cache memoryOnlyCache = new Cache("testCache", 5000, false, false, 5, 2);
        singletonManager.addCache(memoryOnlyCache);
        Cache testCache = singletonManager.getCache("testCache");
        assertNotNull(testCache);
    }

 

    
手动移除一个cache:

//Remove cache called sampleCache1
CacheManager singletonManager = CacheManager.create();
singletonManager.removeCache("sampleCache1");

 

1.3 Shutdown the CacheManager
ehcache应该在使用后关闭, 最佳实践是在code中显式调用:

//Shutdown the singleton CacheManager
CacheManager.getInstance().shutdown();

 2 Using Caches
比如我有这样一个cache:

<cache name="sampleCache1" maxElementsInMemory="10000"
    maxElementsOnDisk="1000" eternal="false" overflowToDisk="true"
    diskSpoolBufferSizeMB="20" timeToIdleSeconds="300"
    timeToLiveSeconds="600" memoryStoreEvictionPolicy="LFU" />

 

        
2.1 Obtaining a reference to a Cache
获得该cache的引用:

String cacheName = "sampleCache1";
CacheManager manager = new CacheManager("src/ehcache1.xml");
Cache cache = manager.getCache(cacheName);

 
2.2 Performing CRUD operations
下面的代码演示了ehcache的增删改查:

public void testCRUD()
{
    String cacheName = "sampleCache1";
    CacheManager manager = new CacheManager("src/ehcache1.xml");
    Cache cache = manager.getCache(cacheName);
    //Put an element into a cache
    Element element = new Element("key1", "value1");
    cache.put(element);
    //This updates the entry for "key1"
    cache.put(new Element("key1", "value2"));
    //Get a Serializable value from an element in a cache with a key of "key1".
    element = cache.get("key1");
    Serializable value = element.getValue();
    //Get a NonSerializable value from an element in a cache with a key of "key1".
    element = cache.get("key1");
    assertNotNull(element);
    Object valueObj = element.getObjectValue();
    assertNotNull(valueObj);
    //Remove an element from a cache with a key of "key1".
    assertNotNull(cache.get("key1"));
    cache.remove("key1");
    assertNull(cache.get("key1"));

}

     
2.3    Disk Persistence on demand

//sampleCache1 has a persistent diskStore. We wish to ensure that the data //and index are written immediately.
        public void testDiskPersistence()
    {
        String cacheName = "sampleCache1";
        CacheManager manager = new CacheManager("src/ehcache1.xml");
        Cache cache = manager.getCache(cacheName);
        for (int i = 0; i < 50000; i++)
        {
            Element element = new Element("key" + i, "myvalue" + i);
            cache.put(element);
        }
        cache.flush();
        Log.debug("java.io.tmpdir = " + System.getProperty("java.io.tmpdir"));
    }

 

备注: 持久化到硬盘的路径由虚拟机参数"java.io.tmpdir"决定.
例如, 在windows中, 会在此路径下
C:\Documents and Settings\li\Local Settings\Temp
在linux中, 通常会在: /tmp 下

2.4  Obtaining Cache Sizes
以下代码演示如何获得cache个数:

public void testCachesizes()
{
    long count = 5;
    String cacheName = "sampleCache1";
    CacheManager manager = new CacheManager("src/ehcache1.xml");
    Cache cache = manager.getCache(cacheName);
    for (int i = 0; i < count; i++)
    {
        Element element = new Element("key" + i, "myvalue" + i);
        cache.put(element);
    }
    //Get the number of elements currently in the Cache.
    int elementsInCache = cache.getSize();
    assertTrue(elementsInCache == 5);
    //Cache cache = manager.getCache("sampleCache1");
    long elementsInMemory = cache.getMemoryStoreSize();
    //Get the number of elements currently in the DiskStore.
    long elementsInDiskStore = cache.getDiskStoreSize();
    assertTrue(elementsInMemory + elementsInDiskStore == count);
}

 

3: Registering CacheStatistics in an MBeanServer
ehCache 提供jmx支持:

CacheManager manager = new CacheManager();
MBeanServer mBeanServer = ManagementFactory.getPlatformMBeanServer();
ManagementService.registerMBeans(manager, mBeanServer, false, false, false, true);

 

把该程序打包, 然后:

java -Dcom.sun.management.jmxremote -jar 程序名.jar

 
再到javahome/bin中运行jconsole.exe, 便可监控cache.

4. 用户可以自定义处理cacheEventHandler, 处理诸如元素放入cache的各种事件(放入,移除,过期等事件)
只需三步:
4.1 在cache配置中, 增加cacheEventListenerFactory节点.

<cache name="Test" maxElementsInMemory="1" eternal="false"
    overflowToDisk="true" timeToIdleSeconds="1" timeToLiveSeconds="2"
    diskPersistent="false" diskExpiryThreadIntervalSeconds="1"
    memoryStoreEvictionPolicy="LFU">
    <cacheEventListenerFactory class="co.ehcache.EventFactory" />
</cache>

 

4.2: 编写EventFactory, 继承CacheEventListenerFactory:

public class EventFactory extends CacheEventListenerFactory
{
    @Override
    public CacheEventListener createCacheEventListener(Properties properties)
    {
        // TODO Auto-generated method stub
        return new CacheEvent();
    }

}

 

4.3  编写 class: CacheEvent, 实现 CacheEventListener 接口:

public class CacheEvent implements CacheEventListener
{

    public void dispose()
    {
        log("in dispose");
    }

    public void notifyElementEvicted(Ehcache cache, Element element)
    {
        // TODO Auto-generated method stub
        log("in notifyElementEvicted" + element);
    }

    public void notifyElementExpired(Ehcache cache, Element element)
    {
        // TODO Auto-generated method stub
        log("in notifyElementExpired" + element);
    }

    public void notifyElementPut(Ehcache cache, Element element) throws CacheException
    {
        // TODO Auto-generated method stub
        log("in notifyElementPut" + element);
    }

    public void notifyElementRemoved(Ehcache cache, Element element) throws CacheException
    {
        // TODO Auto-generated method stub
        log("in notifyElementRemoved" + element);
    }

    public void notifyElementUpdated(Ehcache cache, Element element) throws CacheException
    {
        // TODO Auto-generated method stub
        log("in notifyElementUpdated" + element);
    }

    public void notifyRemoveAll(Ehcache cache)
    {
        // TODO Auto-generated method stub
        log("in notifyRemoveAll");
    }

    public Object clone() throws CloneNotSupportedException
    {
        return super.clone();
    }

    private void log(String s)
    {
        Log.debug(s);
    }
}

 

现在可以编写测试代码:

public void testEventListener()
{
    String key = "person";
    Person person = new Person("lcl", 100);
    MyCacheManager.getInstance().put("Test", key, person);
    Person p = (Person) MyCacheManager.getInstance().get("Test", key);

    try
    {
        Thread.sleep(10000);
    }
    catch (InterruptedException e)
    {
        // TODO Auto-generated catch block
        e.printStackTrace();
    }

    assertNull(MyCacheManager.getInstance().get("Test", key));
}

 

    
    根据配置, 该缓存对象生命期只有2分钟, 在Thread.sleep(10000)期间, 该缓存元素将过期被销毁, 在销毁前, 触发notifyElementExpired事件.
    
二 Ehcache配置文件
 以如下配置为例说明:

<cache name="CACHE_FUNC"
maxElementsInMemory="2"
eternal="false"
timeToIdleSeconds="10"
timeToLiveSeconds="20"
overflowToDisk="true"
diskPersistent="true"
diskExpiryThreadIntervalSeconds="120"  />

 

maxElementsInMemory ：cache 中最多可以存放的元素的数量。如果放入cache中的元素超过这个数值，有两种情况：
1. 若overflowToDisk的属性值为true，会将cache中多出的元素放入磁盘文件中。
2. 若overflowToDisk的属性值为false，会根据memoryStoreEvictionPolicy的策略替换cache中原有的元素。

eternal ：是否永驻内存。如果值是true，cache中的元素将一直保存在内存中，不会因为时间超时而丢失，所以在这个值为true的时候，timeToIdleSeconds和timeToLiveSeconds两个属性的值就不起作用了。

 3. timeToIdleSeconds ：访问这个cache中元素的最大间隔时间。如果超过这个时间没有访问这个cache中的某个元素，那么这个元素将被从cache中清除。

4. timeToLiveSeconds : cache中元素的生存时间。意思是从cache中的某个元素从创建到消亡的时间，从创建开始计时，当超过这个时间，这个元素将被从cache中清除。

5. overflowToDisk ：溢出是否写入磁盘。系统会根据标签<diskStore path="java.io.tmpdir"/> 中path的值查找对应的属性值，如果系统的java.io.tmpdir的值是 D:\temp，写入磁盘的文件就会放在这个文件夹下。文件的名称是cache的名称，后缀名的data。如：CACHE_FUNC.data。

6. diskExpiryThreadIntervalSeconds  ：磁盘缓存的清理线程运行间隔.

7. memoryStoreEvictionPolicy ：内存存储与释放策略。有三个值：
LRU －least recently used
LFU －least frequently used
FIFO－first in first out, the oldest element by creation time

diskPersistent ： 是否持久化磁盘缓存。当这个属性的值为true时，系统在初始化的时候会在磁盘中查找文件名为cache名称，后缀名为index的的文件，如CACHE_FUNC.index 。这个文件中存放了已经持久化在磁盘中的cache的index，找到后把cache加载到内存。要想把cache真正持久化到磁盘，写程序时必须注意，在是用net.sf.ehcache.Cache的void put (Element element)方法后要使用void flush()方法。
 更多说明可看ehcache自带的ehcache.xml的注释说明.