public class

SoftCache

extends AbstractMap<K, V>
implements Map<K, V>
java.lang.Object
   ↳ java.util.AbstractMap<K, V>
     ↳ sun.misc.SoftCache

Class Overview

A memory-sensitive implementation of the Map interface.

A SoftCache object uses soft references to implement a memory-sensitive hash map. If the garbage collector determines at a certain point in time that a value object in a SoftCache entry is no longer strongly reachable, then it may remove that entry in order to release the memory occupied by the value object. All SoftCache objects are guaranteed to be completely cleared before the virtual machine will throw an OutOfMemoryError. Because of this automatic clearing feature, the behavior of this class is somewhat different from that of other Map implementations.

Both null values and the null key are supported. This class has the same performance characteristics as the HashMap class, and has the same efficiency parameters of initial capacity and load factor.

Like most collection classes, this class is not synchronized. A synchronized SoftCache may be constructed using the Collections.synchronizedMap method.

In typical usage this class will be subclassed and the fill method will be overridden. When the get method is invoked on a key for which there is no mapping in the cache, it will in turn invoke the fill method on that key in an attempt to construct a corresponding value. If the fill method returns such a value then the cache will be updated and the new value will be returned. Thus, for example, a simple URL-content cache can be constructed as follows: 

     public class URLCache extends SoftCache {
         protected Object fill(Object key) {
             return ((URL)key).getContent();
         }
     }

The behavior of the SoftCache class depends in part upon the actions of the garbage collector, so several familiar (though not required) Map invariants do not hold for this class.

Because entries are removed from a SoftCache in response to dynamic advice from the garbage collector, a SoftCache may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a SoftCache instance and invoke none of its mutator methods, it is possible for the size method to return smaller values over time, for the isEmpty method to return false and then true, for the containsKey method to return true and later false for a given key, for the get method to return a value for a given key but later return null, for the put method to return null and the remove method to return false for a key that previously appeared to be in the map, and for successive examinations of the key set, the value set, and the entry set to yield successively smaller numbers of elements.

See Also

Summary

Public Constructors
SoftCache(int initialCapacity, float loadFactor)
Construct a new, empty SoftCache with the given initial capacity and the given load factor.
SoftCache(int initialCapacity)
Construct a new, empty SoftCache with the given initial capacity and the default load factor.
SoftCache()
Construct a new, empty SoftCache with the default capacity and the default load factor.
Public Methods
void clear()
Remove all mappings from this cache.
boolean containsKey(Object key)
Return true if this cache contains a mapping for the specified key.
Set entrySet()
Return a Set view of the mappings in this cache.
Object get(Object key)
Return the value to which this cache maps the specified key.
boolean isEmpty()
Return true if this cache contains no key-value mappings.
Object put(Object key, Object value)
Update this cache so that the given key maps to the given value.
Object remove(Object key)
Remove the mapping for the given key from this cache, if present.
int size()
Return the number of key-value mappings in this cache.
Protected Methods
Object fill(Object key)
Create a value object for the given key.
[Expand]
Inherited Methods
From class java.util.AbstractMap
From class java.lang.Object
From interface java.util.Map

Public Constructors

public SoftCache (int initialCapacity, float loadFactor)

Construct a new, empty SoftCache with the given initial capacity and the given load factor.

Parameters
initialCapacity The initial capacity of the cache
loadFactor A number between 0.0 and 1.0
Throws
IllegalArgumentException If the initial capacity is less than or equal to zero, or if the load factor is less than zero

public SoftCache (int initialCapacity)

Construct a new, empty SoftCache with the given initial capacity and the default load factor.

Parameters
initialCapacity The initial capacity of the cache
Throws
IllegalArgumentException If the initial capacity is less than or equal to zero

public SoftCache ()

Construct a new, empty SoftCache with the default capacity and the default load factor.

Public Methods

public void clear ()

Remove all mappings from this cache.

public boolean containsKey (Object key)

Return true if this cache contains a mapping for the specified key. If there is no mapping for the key, this method will not attempt to construct one by invoking the fill method.

Parameters
key The key whose presence in the cache is to be tested
Returns
  • true if this map contains a mapping for the specified key

public Set entrySet ()

Return a Set view of the mappings in this cache.

Returns
  • a set view of the mappings contained in this map

public Object get (Object key)

Return the value to which this cache maps the specified key. If the cache does not presently contain a value for this key, then invoke the fill method in an attempt to compute such a value. If that method returns a non-null value, then update the cache and return the new value. Otherwise, return null.

Note that because this method may update the cache, it is considered a mutator and may cause ConcurrentModificationExceptions to be thrown if invoked while an iterator is in use.

Parameters
key The key whose associated value, if any, is to be returned
Returns
  • the value to which the specified key is mapped, or null if this map contains no mapping for the key
See Also

public boolean isEmpty ()

Return true if this cache contains no key-value mappings.

Returns
  • true if this map contains no key-value mappings

public Object put (Object key, Object value)

Update this cache so that the given key maps to the given value. If the cache previously contained a mapping for key then that mapping is replaced and the old value is returned.

Parameters
key The key that is to be mapped to the given value
value The value to which the given key is to be mapped
Returns
  • The previous value to which this key was mapped, or null if if there was no mapping for the key

public Object remove (Object key)

Remove the mapping for the given key from this cache, if present.

Parameters
key The key whose mapping is to be removed
Returns
  • The value to which this key was mapped, or null if there was no mapping for the key

public int size ()

Return the number of key-value mappings in this cache. The time required by this operation is linear in the size of the map.

Returns
  • the number of key-value mappings in this map

Protected Methods

protected Object fill (Object key)

Create a value object for the given key. This method is invoked by the get method when there is no entry for key. If this method returns a non-null value, then the cache will be updated to map key to that value, and that value will be returned by the get method.

The default implementation of this method simply returns null for every key value. A subclass may override this method to provide more useful behavior.

Parameters
key The key for which a value is to be computed
Returns
  • A value for key, or null if one could not be computed
See Also