public class

SoftLimitMRUCache

extends Object
implements Serializable
java.lang.Object
   ↳ org.hibernate.util.SoftLimitMRUCache

Class Overview

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 java.lang.ref.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.

See Also

Summary

Constants
int DEFAULT_SOFT_REF_COUNT The default soft reference count.
int DEFAULT_STRONG_REF_COUNT The default strong reference count.
Public Constructors
SoftLimitMRUCache()
Constructs a cache with the default settings.
SoftLimitMRUCache(int strongRefCount, int softRefCount)
Constructs a cache with the specified settings.
Public Methods
synchronized void clear()
Clears the cache.
synchronized Object get(Object key)
Gets an object from the cache.
synchronized Object put(Object key, Object value)
Puts a value in the cache.
synchronized int size()
Gets the strong reference cache size.
synchronized int softSize()
Gets the soft reference cache size.
[Expand]
Inherited Methods
From class java.lang.Object

Constants

public static final int DEFAULT_SOFT_REF_COUNT

The default soft reference count.

Constant Value: 2048 (0x00000800)

public static final int DEFAULT_STRONG_REF_COUNT

The default strong reference count.

Constant Value: 128 (0x00000080)

Public Constructors

public SoftLimitMRUCache ()

Constructs a cache with the default settings.

See Also

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.

Public Methods

public synchronized void clear ()

Clears the cache.

public synchronized 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.

public synchronized 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.

public synchronized int size ()

Gets the strong reference cache size.

Returns
  • the strong reference cache size.

public synchronized int softSize ()

Gets the soft reference cache size.

Returns
  • the soft reference cache size.