EhCache default cache in java

问题:

I have this configuration for ehCache:

<ehcache>
    <defaultCache
            name="defaut"
            maxElementsInMemory="5"
            eternal="false"
            timeToIdleSeconds="20"
            timeToLiveSeconds="20"
            overflowToDisk="false"
            diskPersistent="false"
            memoryStoreEvictionPolicy="LRU"
            />           
</ehcache>

How can I get access to default cache of EhCache?

CacheManager.getInstance().getCache("default"); // returns null

回答1:

My understanding is that the "default cache" is actually a template for new caches that get created, rather than being a specific named cache.

CacheManager.getCache will only return a cache instance if it has already been created, so you'll need to tell it to create a new one, using something like addCacheIfAbsent(). The name doesn't matter, it will be created on demand using the default cache settings.

回答2:

I came across the same problem trying to create a new cache.

Using EhCache 2.7.3 I couldn't use the addCacheIfAbsent(..) method since it returns the deprecated EhCache class, and not the Cache class.

My initial attempt was as follows:

private Cache addCache(Class<?> cacheClazz) {
    CacheConfiguration cacheConfiguration = null;
    {
        Cache defaultCache = getCacheManager().getCache("default");
        cacheConfiguration = defaultCache.getCacheConfiguration();
    }
    Cache newCache = new Cache(cacheConfiguration);
    getCacheManager().addCache(newCache);
    return newCache;
}

But using CacheManager.getCache("default") returns null- so yeah, it doesn't seem like one can get a reference to the default (template) cache.

I got the code working as follows:

private Cache addCache(Class<?> cacheClazz) {
    // get the default (template) cache configuration
    CacheConfiguration cacheConfiguration = getCacheManager().getConfiguration().getDefaultCacheConfiguration();
    // give it a unique name or the process will fail
    cacheConfiguration.setName(cacheClazz.getName());
    Cache newCache = new Cache(cacheConfiguration);
    getCacheManager().addCache(newCache);
    return newCache;
}

It wasn't thread safe (tested using TestNg concurrent tests). The final implementation looks as follows:

private final static Map<String, String> firstRunMap = new HashMap<>(); // Not using ConcurrentHashMap so be careful if you wanted to iterate over the Map (https://stackoverflow.com/questions/27753184/java-hashmap-add-new-entry-while-iterating)
private Cache addCache(Class<?> cacheClazz) {
    final String mapKey = getMapKey(cacheClazz);

    if (firstRunMap.get(mapKey) == null) {
        synchronized(mapKey) {
            if (firstRunMap.get(mapKey) == null) {

                // -----------------------------------------------------
                // First run for this cache!!!
                // -----------------------------------------------------

                // get the default (template) cache configuration
                CacheConfiguration cacheConfiguration = getCacheManager().getConfiguration().getDefaultCacheConfiguration();
                // give it a unique name or the process will fail
                cacheConfiguration.setName(cacheClazz.getName());
                Cache newCache = new Cache(cacheConfiguration);
                getCacheManager().addCache(newCache);

                // -----------------------------------------------------
                // First run complete!!!
                // -----------------------------------------------------

                firstRunMap.put(mapKey, "");

                return newCache;
            }
        }
    }

    // Not the first thread
    return getCache(cacheClazz);
}

    // This class is AbstractEhCache - change it to your class
private String getMapKey(Class<?> cacheClazz) {
    String mapKey = AbstractEhCache.class.getName() // to differentiate from similar keys in other classes
            + "-" + cacheClazz.getName();
    // Using intern() on the key as I want to synchronize on it.
    // (Strings with different hashCodes represent different locks)
    return mapKey.intern();
}

private Cache getCache(Class<?> cacheClazz) {
    return getCacheManager().getCache(cacheClazz.getName());
}

回答3:

please refer to the api of ehcache

addCache

public void addCache(String cacheName)

          throws IllegalStateException,
                 ObjectExistsException,
                 CacheException  

Adds a Ehcache based on the defaultCache with the given name.

from: http://ehcache.org/apidocs/2.7.6/

hope to help you!