Class ClockPolicy.Holder

java.lang.Object
org.apache.derby.impl.services.cache.ClockPolicy.Holder
All Implemented Interfaces:
ReplacementPolicy.Callback
Enclosing class:
ClockPolicy
private class ClockPolicy.Holder extends Object implements ReplacementPolicy.Callback
Holder class which represents an entry in the cache. It maintains a recentlyUsed required by the clock algorithm. The class also implements the Callback interface, so that ConcurrentCache can notify the clock policy about events relevant to the clock algorithm.

  • Field Summary

    Fields
    Modifier and Type
    Field
    Description
    private CacheEntry
    entry
    Reference to the CacheEntry object held by this object.
    private boolean
    evicted
    Flag which tells whether this holder has been evicted from the clock.
    private Cacheable
    freedCacheable
    Cacheable object from a removed object.
    (package private) boolean
    recentlyUsed
    Flag indicating whether or not this entry has been accessed recently.

  • Constructor Summary

    Constructors
    Constructor
    Description
    Holder(CacheEntry e)
     

  • Method Summary

    Modifier and Type
    Method
    Description
    void
    access()
    Mark this entry as recently used.
    (package private) boolean
    evictIfFree()
    Evict this holder from the clock if it is not associated with an entry.
    void
    free()
    Mark this object as free and reusable.
    (package private) CacheEntry
    getEntry()
    Returns the entry that is currently associated with this holder.
    (package private) boolean
    isEvicted()
    Check whether this holder has been evicted from the clock.
    (package private) void
    setEvicted()
    Mark this holder as evicted from the clock, effectively preventing reuse of the holder.
    (package private) void
    switchEntry(CacheEntry e)
    Switch which entry the holder is associated with.
    (package private) boolean
    takeIfFree(CacheEntry e)
    Associate this holder with the specified entry if the holder is free (that is, not associated with any other entry).

    Methods inherited from class Object

    clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

  • Field Details

    • recentlyUsed

      boolean recentlyUsed
      Flag indicating whether or not this entry has been accessed recently. Should only be accessed/modified when the current thread has locked the CacheEntry object stored in the entry field.

    • entry

      private CacheEntry entry
      Reference to the CacheEntry object held by this object. The reference should only be accessed when the thread owns the monitor on this holder. A thread is only allowed to change the reference if it also has locked the entry that the reference points to (if the reference is non-null). This ensures that no other thread can disassociate a holder from its entry while the entry is locked, even though the monitor on the holder has been released.

    • freedCacheable

      private Cacheable freedCacheable
      Cacheable object from a removed object. If this object is non-null, entry must be null (which means that the holder is not associated with any object in the cache).

    • evicted

      private boolean evicted
      Flag which tells whether this holder has been evicted from the clock. If it has been evicted, it can't be reused when a new entry is inserted. Only the owner of this holder's monitor is allowed to access this variable.

  • Constructor Details

  • Method Details

    • access

      public void access()
      Mark this entry as recently used. Caller must have locked entry.
      Specified by:
      access in interface ReplacementPolicy.Callback

    • free

      public void free()
      Mark this object as free and reusable. Caller must have locked entry.
      Specified by:
      free in interface ReplacementPolicy.Callback

    • takeIfFree

      boolean takeIfFree(CacheEntry e)
      Associate this holder with the specified entry if the holder is free (that is, not associated with any other entry).
      Parameters:
      e - the entry to associate the holder with (it must be locked by the current thread)
      Returns:
      true if the holder has been associated with the specified entry, false if someone else has taken it or the holder has been evicted from the clock

    • getEntry

      CacheEntry getEntry()
      Returns the entry that is currently associated with this holder.
      Returns:
      the associated entry

    • switchEntry

      void switchEntry(CacheEntry e)
      Switch which entry the holder is associated with. Will be called when we evict an entry to make room for a new one. When this method is called, the current thread must have locked both the entry that is evicted and the entry that is inserted.
      Parameters:
      e - the entry to associate this holder with

    • evictIfFree

      boolean evictIfFree()
      Evict this holder from the clock if it is not associated with an entry.
      Returns:
      true if the holder was successfully evicted, false otherwise

    • setEvicted

      void setEvicted()
      Mark this holder as evicted from the clock, effectively preventing reuse of the holder. Calling thread must have locked the holder's entry.

    • isEvicted

      boolean isEvicted()
      Check whether this holder has been evicted from the clock.
      Returns:
      true if it has been evicted, false otherwise