Package org.zkoss.util

Class CacheMap<K,V>

java.lang.Object
org.zkoss.util.CacheMap<K,V>
All Implemented Interfaces:
Serializable, Cloneable, Map<K,V>, Cache<K,V>
Direct Known Subclasses:
ResourceCache
public class CacheMap<K,V> extends Object implements Map<K,V>, Cache<K,V>, Serializable, Cloneable
The cache map. The key-to-value mappings hold in this map is temporary. They are removed when GC demanding memory and a criteria is met. The criteria is whether the mapping is old enough (called lifetime), or the upper bound is hit (called max-size).

The criteria can be changed by overriding canExpunge(int, org.zkoss.util.CacheMap.Value<V>). When to check the criteria can be changed by overriding shallExpunge().

If the criteria is totally independent of GC, you could override shallExpunge() to always return true (rather than when GC is activated).

It is different from WeakHashMap:

  • The mapping might be removed even if the key is hold somewhere (i.e., strong reachable).
  • The mapping might not be removed when GC demanding memory if the criteria doesn't meet.
  • It is not serializable.

Like other maps, it is not thread-safe. To get one, use java.util.Collections.synchronizedMap.

Implementation Note: there is another version of CacheMap that uses WeakReference for each value (refer to obsolete). The drawback is that all mapping will be queued and need to be examined, because GC tends to release all reference at once.

We don't use PhantomReference because it is still required to re-create the reference after enqueued.

Author:
tomyeh
See Also:

  • Field Details

  • Constructor Details

    • CacheMap

      public CacheMap(int maxSize, int lifetime)
      Constructs a cache map with the specified max size and lifetime. Unlike LinkedHashMap, the default order is the access order, i.e., the order is changed once accessed, including get().
      Since:
      3.0.0

    • CacheMap

      public CacheMap()
      Constructs a cache map. Unlike LinkedHashMap, the default order is the access order, i.e., the order is changed once accessed, including get().

    • CacheMap

      public CacheMap(int cap)
      Constructs a cache map. Unlike LinkedHashMap, the default order is the access order, i.e., the order is changed once accessed, including get().

    • CacheMap

      public CacheMap(int cap, float load)
      Constructs a cache map. Unlike LinkedHashMap, the default order is the access order, i.e., the order is changed once accessed, including get().

    • CacheMap

      public CacheMap(boolean accessOrder)
      Constructs a cache map.
      Parameters:
      accessOrder - whether to use the access order. Specify false for the insertion order.
      Since:
      6.0.0

    • CacheMap

      public CacheMap(int maxSize, int lifetime, boolean accessOrder)
      Constructs a cache map with the specified max size and lifetime.
      Parameters:
      accessOrder - whether to use the access order. Specify false for the insertion order.
      Since:
      6.0.0

    • CacheMap

      public CacheMap(int cap, float load, boolean accessOrder)
      Constructs a cache map.
      Parameters:
      accessOrder - whether to use the access order. Specify false for the insertion order.
      Since:
      6.0.0

  • Method Details

    • onExpunge

      protected void onExpunge(CacheMap.Value<V> v)
      Called when a pair of key and value having been expunged. This method is called after it is removed, so you could add it back.

      Default: does nothing

    • shallExpunge

      protected boolean shallExpunge()
      Returns whether it is time to expunge. Once shallExpunge returns true, values are examined one-by-one thru canExpunge(int, org.zkoss.util.CacheMap.Value<V>), and expunged if EXPUNGE_YES.

      This implementation returns true only if GC was activated. You might override it to return true, such that expunge is enforced no matter GC was activated.

      See Also:

    • canExpunge

      protected int canExpunge(int size, CacheMap.Value<V> v)
      Tests whether certain value is OK to expunge.

      Note: values are tested thru canExpunge(int, org.zkoss.util.CacheMap.Value<V>) only if shallExpunge() returns true.

      Deriving classes might override this method to return different value for different criteria.

      The return value could be a combination of EXPUNGE_xxx. One of EXPUNGE_YES and EXPUNGE_NO is returned to denote whether to expunge the mapping. One of EXPUNGE_CONTINUE and EXPUNGE_STOP is returned to denote whether to continue the searching of the next mapping for expunging.

      Normally, you return either (EXPUNGE_YES|EXPUNGE_CONTINUE) or (EXPUNG_NO|EXPUNGE_STOP). Notice that the mapping is queried in the last-access order. Thus, you rarely needs to return (EXPUNGE_NO|EXPUNGE_CONTINUE) unless the appropriate one might be out of this order.

      This implementation compares the access time and size. It returns (EXPUNGE_YES|EXPUNGE_CONTINUE) if OK, and (EXPUNGE_NO|EXPUNGE_STOP) if not.

      Parameters:
      size - the current size. It is used instead of size(), since the entry might not be removed yet (such as FastReadCache).
      Returns:
      a combination of EXPUNGE_xxx
      See Also:

    • expunge

      public int expunge()
      Enforces to expunge items that exceeds the maximal allowed number or lifetime.

      By default, this method is called only GC takes places.

      Returns:
      number of items left (size()) after expunged
      Since:
      3.6.1

    • getLifetime

      public int getLifetime()
      Gets the minimal lifetime, unit=milliseconds. An mapping won't be removed by GC unless the minimal lifetime or the maximal allowed size exceeds.
      Specified by:
      getLifetime in interface Cache<K,V>
      See Also:

    • setLifetime

      public void setLifetime(int lifetime)
      Sets the minimal lifetime. Default: Cache.DEFAULT_LIFETIME.
      Specified by:
      setLifetime in interface Cache<K,V>
      Parameters:
      lifetime - the lifetime, unit=milliseconds; if non-positive, they will be removed immediately.
      See Also:

    • getMaxSize

      public int getMaxSize()
      Gets the maximal allowed size. Default: Cache.DEFAULT_MAX_SIZE. An mapping won't be removed by GC unless the minimal lifetime or the maximal allowed size exceeds.

      Notice: getMaxSize() is only a soft limit. It takes effect only if GC takes place.

      Specified by:
      getMaxSize in interface Cache<K,V>
      See Also:

    • setMaxSize

      public void setMaxSize(int maxsize)
      Sets the maximal allowed size.
      Specified by:
      setMaxSize in interface Cache<K,V>
      See Also:

    • isEmpty

      public boolean isEmpty()
      Specified by:
      isEmpty in interface Map<K,V>

    • isEmptyWithoutExpunge

      public boolean isEmptyWithoutExpunge()
      Returns whether it is empty without trying to expunge first.
      Since:
      3.0.1

    • size

      public int size()
      Specified by:
      size in interface Map<K,V>

    • sizeWithoutExpunge

      public int sizeWithoutExpunge()
      Returns the size without trying to expunge first.
      Since:
      3.0.1

    • clear

      public void clear()
      Description copied from interface: Cache
      Clears all objects being cached.
      Specified by:
      clear in interface Cache<K,V>
      Specified by:
      clear in interface Map<K,V>

    • remove

      public V remove(Object key)
      Description copied from interface: Cache
      Removes an object from the cache.
      Specified by:
      remove in interface Cache<K,V>
      Specified by:
      remove in interface Map<K,V>
      Returns:
      the object if found.

    • get

      public V get(Object key)
      Description copied from interface: Cache
      Returns the object of the specified key, or null if not found.
      Specified by:
      get in interface Cache<K,V>
      Specified by:
      get in interface Map<K,V>

    • getWithoutExpunge

      public V getWithoutExpunge(Object key)
      Returns the value without trying to expunge for more memory. It is useful if you want to preserve all entries.

    • containsKey

      public boolean containsKey(Object key)
      Description copied from interface: Cache
      Returns whether the specified key is stored.
      Specified by:
      containsKey in interface Cache<K,V>
      Specified by:
      containsKey in interface Map<K,V>

    • containsKeyWithoutExpunge

      public boolean containsKeyWithoutExpunge(Object key)
      Tests if the given key exists without trying to expunge for more memory.

    • containsValue

      public boolean containsValue(Object value)
      Specified by:
      containsValue in interface Map<K,V>

    • put

      public V put(K key, V value)
      Description copied from interface: Cache
      Stores an object to the cache.
      Specified by:
      put in interface Cache<K,V>
      Specified by:
      put in interface Map<K,V>
      Returns:
      the previous value of the same, or null if no such value

    • putAll

      public void putAll(Map<? extends K,? extends V> map)
      Specified by:
      putAll in interface Map<K,V>

    • entrySet

      public Set<Map.Entry<K,V>> entrySet()
      Specified by:
      entrySet in interface Map<K,V>

    • keySet

      public Set<K> keySet()
      Specified by:
      keySet in interface Map<K,V>

    • values

      public Collection<V> values()
      Specified by:
      values in interface Map<K,V>

    • hashCode

      public int hashCode()
      Specified by:
      hashCode in interface Map<K,V>
      Overrides:
      hashCode in class Object

    • equals

      public boolean equals(Object o)
      Specified by:
      equals in interface Map<K,V>
      Overrides:
      equals in class Object

    • toString

      public String toString()
      Overrides:
      toString in class Object

    • clone

      public Object clone()
      Overrides:
      clone in class Object