[hibernate-issues] [Hibernate-JIRA] Closed: (HHH-1328) org.hibernate.util.SimpleMRUCache keeps a soft reference to the cache key, so cached values get collected prematurely

[Steve Ebersole (JIRA)]

     [ http://opensource.atlassian.com/projects/hibernate/browse/HHH-1328?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Steve Ebersole closed HHH-1328.
-------------------------------


Closing stale resolved issues

> org.hibernate.util.SimpleMRUCache keeps a soft reference to the cache key, so cached values get collected prematurely
> ---------------------------------------------------------------------------------------------------------------------
>
>                 Key: HHH-1328
>                 URL: http://opensource.atlassian.com/projects/hibernate/browse/HHH-1328
>             Project: Hibernate Core
>          Issue Type: Bug
>          Components: core
>         Environment: Hibernate 3.1. I believe this hasn't changed in CVS.
>            Reporter: Tzvetan Mikov
>            Assignee: Steve Ebersole
>            Priority: Minor
>             Fix For: 3.1.1
>
>
> org.hibernate.util.SimpleMRUCache uses org.apache.commons.collections.ReferenceMap to map HQLQueryPlanKey to HQLQueryPlan. However both the value and the key are stored with soft references. The last 128 HQLQueryPlan-s are kept in a strong reference array, but the cache keys aren't, so often cache entries cannot be found, even though the values still exist in memory.
> There is a comment in the source saying:
> // both keys and values may be soft since value keeps a hard ref to the key (and there is a hard ref to MRU values)
> but it isn't actually true since the value (HQLQueryPlan) doesn't actually keep a reference to the key (HQLQueryPlanKey).
> This is a simplistic patch that should fix the problem:
> --- SimpleMRUCache.java	2005-12-12 07:03:46.000000000 -0800
> +++ SimpleMRUCache-new.java	2005-12-28 18:34:09.920112661 -0800
> @@ -16,20 +16,24 @@
>  
>  	private static final int MAX_STRONG_REF_COUNT = 128; //TODO: configurable?
>  	private final transient Object[] strongRefs = new Object[MAX_STRONG_REF_COUNT]; //strong reference to MRU queries
> +	private final transient Object[] strongKeyRefs = new Object[MAX_STRONG_REF_COUNT];
>  	private transient int strongRefIndex = 0;
>  	private final transient Map softQueryCache = new ReferenceMap(ReferenceMap.SOFT, ReferenceMap.SOFT) ;
> -	// both keys and values may be soft since value keeps a hard ref to the key (and there is a hard ref to MRU values)
>  
>  	public synchronized Object get(Object key) {
>  		Object result = softQueryCache.get( key );
>  		if( result != null ) {
> -			strongRefs[ ++strongRefIndex % MAX_STRONG_REF_COUNT ] = result;
> +			++strongRefIndex;
> +			strongRefs[ strongRefIndex % MAX_STRONG_REF_COUNT ] = result;
> +			strongKeyRefs[ strongRefIndex % MAX_STRONG_REF_COUNT ] = key;
>  		}
>  		return result;
>  	}
>  
>  	public void put(Object key, Object value) {
>  		softQueryCache.put( key, value );
> -		strongRefs[ ++strongRefIndex % MAX_STRONG_REF_COUNT ] = value;
> +		++strongRefIndex;
> +		strongRefs[ strongRefIndex % MAX_STRONG_REF_COUNT ] = value;
> +		strongKeyRefs[ strongRefIndex % MAX_STRONG_REF_COUNT ] = key;
>  	}
>  }

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