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package org.openslx.util;
import java.util.concurrent.atomic.AtomicReference;
import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
public abstract class GenericDataCache<T>
{
private static final Logger LOGGER = LogManager.getLogger( GenericDataCache.class );
/**
* How long the cached data is valid after fetching
*/
private final int validMs;
/**
* Deadline when the cache goes invalid
*/
private long validUntil = 0;
/**
* The data being held
*/
private final AtomicReference<T> item = new AtomicReference<>();
public GenericDataCache( int validMs )
{
this.validMs = validMs;
}
/**
* Get the cached object, but refresh the cache first if
* the cached instance is too old.
*
* @return
*/
public T get()
{
return get( CacheMode.DEFAULT );
}
/**
* Get the cached object, using the given cache access strategy.
* ALWAYS_CACHED: Never refresh the cache, except if it has never been fetched before
* DEFAULT: Only fetch from remote if the cached value is too old
* NEVER_CACHED: Always fetch from remote. If it fails, return null
*
* @param mode Cache access strategy as described above
* @return T
*/
public T get( CacheMode mode )
{
switch ( mode ) {
case FORCE_CACHED:
break;
case PREFER_CACHED:
if ( validUntil == 0 )
ensureUpToDate( true );
break;
case DEFAULT:
ensureUpToDate( false );
break;
case NEVER_CACHED:
if ( !ensureUpToDate( true ) )
return null;
break;
}
return item.get();
}
private synchronized boolean ensureUpToDate( boolean force )
{
final long now = System.currentTimeMillis();
if ( !force && now < validUntil )
return true;
T fetched;
try {
fetched = update();
if ( fetched == null )
return false;
} catch ( Exception e ) {
LOGGER.warn( "Could not fetch fresh data", e );
return false;
}
item.set( fetched );
validUntil = now + validMs;
return true;
}
protected abstract T update() throws Exception;
//
public static enum CacheMode
{
/**
* Use cache, even if the item has never been fetched before.
*/
FORCE_CACHED,
/**
* Use cache if it's not empty, no matter how old it is.
*/
PREFER_CACHED,
/**
* Obey the cache timeout value of this cache.
*/
DEFAULT,
/**
* Always fetch a fresh instance of the item.
*/
NEVER_CACHED
}
}
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