A memory-sensitive implementation of the Map interface.
A SoftCache object uses
java.lang.ref.SoftReferencesoft 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.
version:
1.4, 00/02/02
author:
Mark Reinhold
since:
JDK1.2
See Also: java.util.HashMap
See Also: java.lang.ref.SoftReference | 