org.hibernate.util
Class SoftLimitMRUCache

java.lang.Object
  org.hibernate.util.SoftLimitMRUCache

All Implemented Interfaces:

Serializable

public class SoftLimitMRUCache
extends Object
implements Serializable

Cache following a "Most Recently Used" (MRU) algorithm for maintaining a bounded in-memory size; the "Least Recently Used" (LRU) entry is the first available for removal from the cache.

This implementation uses a "soft limit" to the in-memory size of the cache, meaning that all cache entries are kept within a completely SoftReference-based map with the most recently utilized entries additionally kept in a hard-reference manner to prevent those cache entries soft references from becoming enqueued by the garbage collector. Thus the actual size of this cache impl can actually grow beyond the stated max size bound as long as GC is not actively seeking soft references for enqueuement.

The soft-size is bounded and configurable. This allows controlling memory usage which can grow out of control under some circumstances, especially when very large heaps are in use. Although memory usage per se should not be a problem with soft references, which are cleared when necessary, this can trigger extremely slow stop-the-world GC pauses when nearing full heap usage, even with CMS concurrent GC (i.e. concurrent mode failure). This is most evident when ad-hoc HQL queries are produced by the application, leading to poor soft-cache hit ratios. This can also occur with heavy use of SQL IN clauses, which will generate multiples SQL queries (even if parameterized), one for each collection/array size passed to the IN clause. Many slightly different queries will eventually fill the heap and trigger a full GC to reclaim space, leading to unacceptable pauses in some cases.

Note: This class is serializable, however all entries are discarded on serialization.

Author:

Steve Ebersole, Manuel Dominguez Sarmiento

See Also:

Environment.QUERY_PLAN_CACHE_MAX_STRONG_REFERENCES, Environment.QUERY_PLAN_CACHE_MAX_SOFT_REFERENCES, Serialized Form

Field Summary

static int	DEFAULT_SOFT_REF_COUNT The default soft reference count.
static int	DEFAULT_STRONG_REF_COUNT The default strong reference count.

Constructor Summary

SoftLimitMRUCache()
Constructs a cache with the default settings.

SoftLimitMRUCache(int strongRefCount, int softRefCount)
Constructs a cache with the specified settings.

Method Summary

void	clear() Clears the cache.
Object	get(Object key) Gets an object from the cache.
Object	put(Object key, Object value) Puts a value in the cache.
int	size() Gets the strong reference cache size.
int	softSize() Gets the soft reference cache size.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DEFAULT_STRONG_REF_COUNT

public static final int DEFAULT_STRONG_REF_COUNT

The default strong reference count.

See Also:

Constant Field Values

DEFAULT_SOFT_REF_COUNT

public static final int DEFAULT_SOFT_REF_COUNT

The default soft reference count.

See Also:

Constant Field Values

Constructor Detail

SoftLimitMRUCache

public SoftLimitMRUCache()

Constructs a cache with the default settings.

See Also:

DEFAULT_STRONG_REF_COUNT, DEFAULT_SOFT_REF_COUNT

SoftLimitMRUCache

public SoftLimitMRUCache(int strongRefCount,
                         int softRefCount)

Constructs a cache with the specified settings.

Parameters:

strongRefCount - the strong reference count.

softRefCount - the soft reference count.

Throws:

IllegalArgumentException - if either of the arguments is less than one, or if the strong reference count is higher than the soft reference count.

Method Detail

get

public Object get(Object key)

Gets an object from the cache.

Parameters:

key - the cache key.

Returns:

the stored value, or null if no entry exists.

put

public Object put(Object key,
                  Object value)

Puts a value in the cache.

Parameters:

key - the key.

value - the value.

Returns:

the previous value stored in the cache, if any.

size

public int size()

Gets the strong reference cache size.

Returns:

the strong reference cache size.

softSize

public int softSize()

Gets the soft reference cache size.

Returns:

the soft reference cache size.

clear

public void clear()

Clears the cache.