Package org.apache.shiro.util

Class SoftHashMap<K,​V>

  • All Implemented Interfaces:
    Map<K,​V>
    public class SoftHashMap<K,​V>
extends Object
implements Map<K,​V>
    A SoftHashMap is a memory-constrained map that stores its values in SoftReferences. (Contrast this with the JDK's WeakHashMap, which uses weak references for its keys, which is of little value if you want the cache to auto-resize itself based on memory constraints).

    Having the values wrapped by soft references allows the cache to automatically reduce its size based on memory limitations and garbage collection. This ensures that the cache will not cause memory leaks by holding strong references to all of its values.

    This class is a generics-enabled Map based on initial ideas from Heinz Kabutz's and Sydney Redelinghuys's publicly posted version (with their approval), with continued modifications.

    This implementation is thread-safe and usable in concurrent environments.

    Since:
    1.0

    • Constructor Detail

      • SoftHashMap

        public SoftHashMap​(int retentionSize)
        Creates a new SoftHashMap with the specified retention size.

        The retention size (n) is the total number of most recent entries in the map that will be strongly referenced (ie 'retained') to prevent them from being eagerly garbage collected. That is, the point of a SoftHashMap is to allow the garbage collector to remove as many entries from this map as it desires, but there will always be (n) elements retained after a GC due to the strong references.

        Note that in a highly concurrent environments the exact total number of strong references may differ slightly than the actual retentionSize value. This number is intended to be a best-effort retention low water mark.

        Parameters:
        retentionSize - the total number of most recent entries in the map that will be strongly referenced (retained), preventing them from being eagerly garbage collected by the JVM.

      • SoftHashMap

        public SoftHashMap​(Map<K,​V> source,
                   int retentionSize)
        Creates a SoftHashMap backed by the specified source, with the specified retention size.

        The retention size (n) is the total number of most recent entries in the map that will be strongly referenced (ie 'retained') to prevent them from being eagerly garbage collected. That is, the point of a SoftHashMap is to allow the garbage collector to remove as many entries from this map as it desires, but there will always be (n) elements retained after a GC due to the strong references.

        Note that in a highly concurrent environments the exact total number of strong references may differ slightly than the actual retentionSize value. This number is intended to be a best-effort retention low water mark.

        Parameters:
        source - the backing map to populate this SoftHashMap
        retentionSize - the total number of most recent entries in the map that will be strongly referenced (retained), preventing them from being eagerly garbage collected by the JVM.