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com.opensymphony.oscache.base.algorithm
Class AbstractConcurrentReadCache

java.lang.Object
  extended by java.util.AbstractMap
      extended by com.opensymphony.oscache.base.algorithm.AbstractConcurrentReadCache
All Implemented Interfaces:
Serializable, Cloneable, Map
Direct Known Subclasses:
FIFOCache, LRUCache, UnlimitedCache
public abstract class AbstractConcurrentReadCache
extends AbstractMap
implements Map, Cloneable, Serializable

A version of Hashtable that supports mostly-concurrent reading, but exclusive writing. Because reads are not limited to periods without writes, a concurrent reader policy is weaker than a classic reader/writer policy, but is generally faster and allows more concurrency. This class is a good choice especially for tables that are mainly created by one thread during the start-up phase of a program, and from then on, are mainly read (with perhaps occasional additions or removals) in many threads. If you also need concurrency among writes, consider instead using ConcurrentHashMap.

Successful retrievals using get(key) and containsKey(key) usually run without locking. Unsuccessful ones (i.e., when the key is not present) do involve brief synchronization (locking). Also, the size and isEmpty methods are always synchronized.

Because retrieval operations can ordinarily overlap with writing operations (i.e., put, remove, and their derivatives), retrievals can only be guaranteed to return the results of the most recently completed operations holding upon their onset. Retrieval operations may or may not return results reflecting in-progress writing operations. However, the retrieval operations do always return consistent results -- either those holding before any single modification or after it, but never a nonsense result. For aggregate operations such as putAll and clear, concurrent reads may reflect insertion or removal of only some entries. In those rare contexts in which you use a hash table to synchronize operations across threads (for example, to prevent reads until after clears), you should either encase operations in synchronized blocks, or instead use java.util.Hashtable.

This class also supports optional guaranteed exclusive reads, simply by surrounding a call within a synchronized block, as in
AbstractConcurrentReadCache t; ... Object v;
synchronized(t) { v = t.get(k); }
But this is not usually necessary in practice. For example, it is generally inefficient to write: 

   AbstractConcurrentReadCache t; ...            // Inefficient version
   Object key; ...
   Object value; ...
   synchronized(t) {
     if (!t.containsKey(key))
       t.put(key, value);
       // other code if not previously present
     }
     else {
       // other code if it was previously present
     }
   }
Instead, just take advantage of the fact that put returns null if the key was not previously present: 
   AbstractConcurrentReadCache t; ...                // Use this instead
   Object key; ...
   Object value; ...
   Object oldValue = t.put(key, value);
   if (oldValue == null) {
     // other code if not previously present
   }
   else {
     // other code if it was previously present
   }

Iterators and Enumerations (i.e., those returned by keySet().iterator(), entrySet().iterator(), values().iterator(), keys(), and elements()) return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They will return at most one instance of each element (via next()/nextElement()), but might or might not reflect puts and removes that have been processed since they were created. They do not throw ConcurrentModificationException. However, these iterators are designed to be used by only one thread at a time. Sharing an iterator across multiple threads may lead to unpredictable results if the table is being concurrently modified. Again, you can ensure interference-free iteration by enclosing the iteration in a synchronized block.

This class may be used as a direct replacement for any use of java.util.Hashtable that does not depend on readers being blocked during updates. Like Hashtable but unlike java.util.HashMap, this class does NOT allow null to be used as a key or value. This class is also typically faster than ConcurrentHashMap when there is usually only one thread updating the table, but possibly many retrieving values from it.

Implementation note: A slightly faster implementation of this class will be possible once planned Java Memory Model revisions are in place.

[ Introduction to this package. ]

See Also:
Serialized Form

Nested Class Summary
protected static class AbstractConcurrentReadCache.Entry
          AbstractConcurrentReadCache collision list entry.
protected  class AbstractConcurrentReadCache.HashIterator
           
protected  class AbstractConcurrentReadCache.KeyIterator
           
protected  class AbstractConcurrentReadCache.ValueIterator
           
 
Field Summary
protected  Boolean barrierLock
          Lock used only for its memory effects.
protected  int count
          The total number of mappings in the hash table.
static int DEFAULT_INITIAL_CAPACITY
          The default initial number of table slots for this table (32).
static float DEFAULT_LOAD_FACTOR
          The default load factor for this table.
protected  int DEFAULT_MAX_ENTRIES
          Default cache capacity (number of entries).
protected  Set entrySet
           
protected  HashMap groups
          A HashMap containing the group information.
protected  Set keySet
           
protected  Object lastWrite
          field written to only to guarantee lock ordering.
protected  float loadFactor
          The load factor for the hash table.
protected  int maxEntries
          Cache capacity (number of entries).
protected  boolean memoryCaching
          Use memory cache or not.
protected static String NULL
           
protected  PersistenceListener persistenceListener
          Persistence listener.
protected  AbstractConcurrentReadCache.Entry[] table
          The hash table data.
protected  int threshold
          The table is rehashed when its size exceeds this threshold.
protected  int UNLIMITED
          Max number of element in cache when considered unlimited.
protected  boolean unlimitedDiskCache
          Use unlimited disk caching.
protected  Collection values
           
 
Constructor Summary
AbstractConcurrentReadCache()
          Constructs a new, empty map with a default initial capacity and load factor.
AbstractConcurrentReadCache(int initialCapacity)
          Constructs a new, empty map with the specified initial capacity and default load factor.
AbstractConcurrentReadCache(int initialCapacity, float loadFactor)
          Constructs a new, empty map with the specified initial capacity and the specified load factor.
AbstractConcurrentReadCache(Map t)
          Constructs a new map with the same mappings as the given map.
 
Method Summary
 int capacity()
          Return the number of slots in this table.
 void clear()
          Removes all mappings from this map.
 Object clone()
          Returns a shallow copy of this.
 boolean contains(Object value)
          Tests if some key maps into the specified value in this table.
 boolean containsKey(Object key)
          Tests if the specified object is a key in this table.
 boolean containsValue(Object value)
          Returns true if this map maps one or more keys to the specified value.
 Enumeration elements()
          Returns an enumeration of the values in this table.
 Set entrySet()
          Returns a collection view of the mappings contained in this map.
protected  boolean findAndRemoveEntry(Map.Entry entry)
          Helper method for entrySet remove.
 Object get(Object key)
          Returns the value to which the specified key is mapped in this table.
 Set getGroup(String groupName)
          Returns a set of the cache keys that reside in a particular group.
protected  Set getGroupForReading(String groupName)
          Get ref to group.
protected  Map getGroupsForReading()
          Get ref to groups.
 int getMaxEntries()
          Retrieve the cache capacity (number of entries).
 PersistenceListener getPersistenceListener()
          Get the persistence listener.
protected  AbstractConcurrentReadCache.Entry[] getTableForReading()
          Get ref to table; the reference and the cells it accesses will be at least as fresh as from last use of barrierLock
 boolean isEmpty()
          Returns true if this map contains no key-value mappings.
 boolean isMemoryCaching()
          Check if memory caching is used.
 boolean isOverflowPersistence()
          Check if we use overflowPersistence
 boolean isUnlimitedDiskCache()
          Check if we use unlimited disk cache.
protected abstract  void itemPut(Object key)
          Notify the underlying implementation that an item was put in the cache.
protected abstract  void itemRemoved(Object key)
          Notify the underlying implementation that an item was removed from the cache.
protected abstract  void itemRetrieved(Object key)
          Notify any underlying algorithm that an item has been retrieved from the cache.
 Enumeration keys()
          Returns an enumeration of the keys in this table.
 Set keySet()
          Returns a set view of the keys contained in this map.
 float loadFactor()
          Return the load factor
protected  void persistClear()
          Removes the entire cache from persistent storage.
protected  void persistRemove(Object key)
          Remove an object from the persistence.
protected  void persistRemoveGroup(String groupName)
          Removes a cache group using the persistence listener.
protected  Object persistRetrieve(Object key)
          Retrieve an object from the persistence listener.
protected  Set persistRetrieveGroup(String groupName)
          Retrieves a cache group using the persistence listener.
protected  void persistStore(Object key, Object obj)
          Store an object in the cache using the persistence listener.
protected  void persistStoreGroup(String groupName, Set group)
          Creates or Updates a cache group using the persistence listener.
 Object put(Object key, Object value)
          OpenSymphony BEGIN
 void putAll(Map t)
          Copies all of the mappings from the specified map to this one.
protected  void recordModification(Object x)
          Force a memory synchronization that will cause all readers to see table.
protected  void rehash()
          Rehashes the contents of this map into a new table with a larger capacity.
 Object remove(Object key)
          OpenSymphony BEGIN
 Object removeForce(Object key)
          Like remove(Object), but ensures that the entry will be removed from the persistent store, too, even if overflowPersistence or unlimitedDiskcache are true.
protected abstract  Object removeItem()
          The cache has reached its cacpacity and an item needs to be removed.
 void setMaxEntries(int newLimit)
          Set the cache capacity
 void setMemoryCaching(boolean memoryCaching)
          Sets the memory caching flag.
 void setOverflowPersistence(boolean overflowPersistence)
          Sets the overflowPersistence flag
 void setPersistenceListener(PersistenceListener listener)
          Set the persistence listener to use.
 void setUnlimitedDiskCache(boolean unlimitedDiskCache)
          Sets the unlimited disk caching flag.
 int size()
          Returns the total number of cache entries held in this map.
protected  Object sput(Object key, Object value, int hash, boolean persist)
          OpenSymphony BEGIN
protected  Object sremove(Object key, int hash, boolean invokeAlgorithm)
          OpenSymphony BEGIN
 Collection values()
          Returns a collection view of the values contained in this map.
 
Methods inherited from class java.util.AbstractMap
equals, hashCode, toString
 
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
 
Methods inherited from interface java.util.Map
equals, hashCode
 

Field Detail

DEFAULT_INITIAL_CAPACITY

public static final int DEFAULT_INITIAL_CAPACITY
The default initial number of table slots for this table (32). Used when not otherwise specified in constructor.

See Also:
Constant Field Values

DEFAULT_LOAD_FACTOR

public static final float DEFAULT_LOAD_FACTOR
The default load factor for this table. Used when not otherwise specified in constructor, the default is 0.75f.

See Also:
Constant Field Values

NULL

protected static final String NULL
See Also:
Constant Field Values

barrierLock

protected final Boolean barrierLock
Lock used only for its memory effects. We use a Boolean because it is serializable, and we create a new one because we need a unique object for each cache instance.

lastWrite

protected transient Object lastWrite
field written to only to guarantee lock ordering.

table

protected transient AbstractConcurrentReadCache.Entry[] table
The hash table data.

count

protected transient int count
The total number of mappings in the hash table.

persistenceListener

protected transient PersistenceListener persistenceListener
Persistence listener.

memoryCaching

protected boolean memoryCaching
Use memory cache or not.

unlimitedDiskCache

protected boolean unlimitedDiskCache
Use unlimited disk caching.

loadFactor

protected float loadFactor
The load factor for the hash table.

DEFAULT_MAX_ENTRIES

protected final int DEFAULT_MAX_ENTRIES
Default cache capacity (number of entries).

See Also:
Constant Field Values

UNLIMITED

protected final int UNLIMITED
Max number of element in cache when considered unlimited.

See Also:
Constant Field Values

values

protected transient Collection values

groups

protected HashMap groups
A HashMap containing the group information. Each entry uses the group name as the key, and holds a Set of containing keys of all the cache entries that belong to that particular group.

entrySet

protected transient Set entrySet

keySet

protected transient Set keySet

maxEntries

protected int maxEntries
Cache capacity (number of entries).

threshold

protected int threshold
The table is rehashed when its size exceeds this threshold. (The value of this field is always (int)(capacity * loadFactor).)

Constructor Detail

AbstractConcurrentReadCache

public AbstractConcurrentReadCache(int initialCapacity,
                                   float loadFactor)
Constructs a new, empty map with the specified initial capacity and the specified load factor.

Parameters:
initialCapacity - the initial capacity The actual initial capacity is rounded to the nearest power of two.
loadFactor - the load factor of the AbstractConcurrentReadCache
Throws:
IllegalArgumentException - if the initial maximum number of elements is less than zero, or if the load factor is nonpositive.

AbstractConcurrentReadCache

public AbstractConcurrentReadCache(int initialCapacity)
Constructs a new, empty map with the specified initial capacity and default load factor.

Parameters:
initialCapacity - the initial capacity of the AbstractConcurrentReadCache.
Throws:
IllegalArgumentException - if the initial maximum number of elements is less than zero.

AbstractConcurrentReadCache

public AbstractConcurrentReadCache()
Constructs a new, empty map with a default initial capacity and load factor.

AbstractConcurrentReadCache

public AbstractConcurrentReadCache(Map t)
Constructs a new map with the same mappings as the given map. The map is created with a capacity of twice the number of mappings in the given map or 11 (whichever is greater), and a default load factor.

Method Detail

isEmpty

public boolean isEmpty()
Returns true if this map contains no key-value mappings.

Specified by:
isEmpty in interface Map
Overrides:
isEmpty in class AbstractMap
Returns:
true if this map contains no key-value mappings.

getGroup

public Set getGroup(String groupName)
Returns a set of the cache keys that reside in a particular group.

Parameters:
groupName - The name of the group to retrieve.
Returns:
a set containing all of the keys of cache entries that belong to this group, or null if the group was not found.
Throws:
NullPointerException - if the groupName is null.

setMaxEntries

public void setMaxEntries(int newLimit)
Set the cache capacity

getMaxEntries

public int getMaxEntries()
Retrieve the cache capacity (number of entries).

setMemoryCaching

public void setMemoryCaching(boolean memoryCaching)
Sets the memory caching flag.

isMemoryCaching

public boolean isMemoryCaching()
Check if memory caching is used.

setPersistenceListener

public void setPersistenceListener(PersistenceListener listener)
Set the persistence listener to use.

getPersistenceListener

public PersistenceListener getPersistenceListener()
Get the persistence listener.

setUnlimitedDiskCache

public void setUnlimitedDiskCache(boolean unlimitedDiskCache)
Sets the unlimited disk caching flag.

isUnlimitedDiskCache

public boolean isUnlimitedDiskCache()
Check if we use unlimited disk cache.

isOverflowPersistence

public boolean isOverflowPersistence()
Check if we use overflowPersistence

Returns:
Returns the overflowPersistence.

setOverflowPersistence

public void setOverflowPersistence(boolean overflowPersistence)
Sets the overflowPersistence flag

Parameters:
overflowPersistence - The overflowPersistence to set.

capacity

public int capacity()
Return the number of slots in this table.

clear

public void clear()
Removes all mappings from this map.

Specified by:
clear in interface Map
Overrides:
clear in class AbstractMap

clone

public Object clone()
Returns a shallow copy of this. AbstractConcurrentReadCache instance: the keys and values themselves are not cloned.

Overrides:
clone in class AbstractMap
Returns:
a shallow copy of this map.

contains

public boolean contains(Object value)
Tests if some key maps into the specified value in this table. This operation is more expensive than the containsKey method.

Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the collections framework).

Parameters:
value - a value to search for.
Returns:
true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise.
Throws:
NullPointerException - if the value is null.
See Also:
containsKey(Object), containsValue(Object), Map

containsKey

public boolean containsKey(Object key)
Tests if the specified object is a key in this table.

Specified by:
containsKey in interface Map
Overrides:
containsKey in class AbstractMap
Parameters:
key - possible key.
Returns:
true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise.
Throws:
NullPointerException - if the key is null.
See Also:
contains(Object)

containsValue

public boolean containsValue(Object value)
Returns true if this map maps one or more keys to the specified value. Note: This method requires a full internal traversal of the hash table, and so is much slower than method containsKey.

Specified by:
containsValue in interface Map
Overrides:
containsValue in class AbstractMap
Parameters:
value - value whose presence in this map is to be tested.
Returns:
true if this map maps one or more keys to the specified value.
Throws:
NullPointerException - if the value is null.

elements

public Enumeration elements()
Returns an enumeration of the values in this table. Use the Enumeration methods on the returned object to fetch the elements sequentially.

Returns:
an enumeration of the values in this table.
See Also:
Enumeration, keys(), values(), Map

entrySet

public Set entrySet()
Returns a collection view of the mappings contained in this map. Each element in the returned collection is a Map.Entry. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:
entrySet in interface Map
Specified by:
entrySet in class AbstractMap
Returns:
a collection view of the mappings contained in this map.

get

public Object get(Object key)
Returns the value to which the specified key is mapped in this table.

Specified by:
get in interface Map
Overrides:
get in class AbstractMap
Parameters:
key - a key in the table.
Returns:
the value to which the key is mapped in this table; null if the key is not mapped to any value in this table.
Throws:
NullPointerException - if the key is null.
See Also:
put(Object, Object)

keySet

public Set keySet()
Returns a set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:
keySet in interface Map
Overrides:
keySet in class AbstractMap
Returns:
a set view of the keys contained in this map.

keys

public Enumeration keys()
Returns an enumeration of the keys in this table.

Returns:
an enumeration of the keys in this table.
See Also:
Enumeration, elements(), keySet(), Map

loadFactor

public float loadFactor()
Return the load factor

put

public Object put(Object key,
                  Object value)
OpenSymphony BEGIN

Specified by:
put in interface Map
Overrides:
put in class AbstractMap

putAll

public void putAll(Map t)
Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified Map.

Specified by:
putAll in interface Map
Overrides:
putAll in class AbstractMap
Parameters:
t - Mappings to be stored in this map.

remove

public Object remove(Object key)
OpenSymphony BEGIN

Specified by:
remove in interface Map
Overrides:
remove in class AbstractMap

removeForce

public Object removeForce(Object key)
Like remove(Object), but ensures that the entry will be removed from the persistent store, too, even if overflowPersistence or unlimitedDiskcache are true.

Parameters:
key - the key that needs to be removed.
Returns:
the value to which the key had been mapped in this table, or null if the key did not have a mapping.

size

public int size()
Returns the total number of cache entries held in this map.

Specified by:
size in interface Map
Overrides:
size in class AbstractMap
Returns:
the number of key-value mappings in this map.

values

public Collection values()
Returns a collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. The collection supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

Specified by:
values in interface Map
Overrides:
values in class AbstractMap
Returns:
a collection view of the values contained in this map.

getGroupForReading

protected final Set getGroupForReading(String groupName)
Get ref to group. CACHE-127 Synchronized copying of the group entry set since the new HashSet(Collection c) constructor uses the iterator. This may slow things down but it is better than a ConcurrentModificationException. We might have to revisit the code if performance is too adversely impacted.

getGroupsForReading

protected final Map getGroupsForReading()
Get ref to groups. The reference and the cells it accesses will be at least as fresh as from last use of barrierLock

getTableForReading

protected final AbstractConcurrentReadCache.Entry[] getTableForReading()
Get ref to table; the reference and the cells it accesses will be at least as fresh as from last use of barrierLock

recordModification

protected final void recordModification(Object x)
Force a memory synchronization that will cause all readers to see table. Call only when already holding main synch lock.

findAndRemoveEntry

protected boolean findAndRemoveEntry(Map.Entry entry)
Helper method for entrySet remove.

persistRemove

protected void persistRemove(Object key)
Remove an object from the persistence.

Parameters:
key - The key of the object to remove

persistRemoveGroup

protected void persistRemoveGroup(String groupName)
Removes a cache group using the persistence listener.

Parameters:
groupName - The name of the group to remove

persistRetrieve

protected Object persistRetrieve(Object key)
Retrieve an object from the persistence listener.

Parameters:
key - The key of the object to retrieve

persistRetrieveGroup

protected Set persistRetrieveGroup(String groupName)
Retrieves a cache group using the persistence listener.

Parameters:
groupName - The name of the group to retrieve

persistStore

protected void persistStore(Object key,
                            Object obj)
Store an object in the cache using the persistence listener.

Parameters:
key - The object key
obj - The object to store

persistStoreGroup

protected void persistStoreGroup(String groupName,
                                 Set group)
Creates or Updates a cache group using the persistence listener.

Parameters:
groupName - The name of the group to update
group - The entries for the group

persistClear

protected void persistClear()
Removes the entire cache from persistent storage.

itemPut

protected abstract void itemPut(Object key)
Notify the underlying implementation that an item was put in the cache.

Parameters:
key - The cache key of the item that was put.

itemRetrieved

protected abstract void itemRetrieved(Object key)
Notify any underlying algorithm that an item has been retrieved from the cache.

Parameters:
key - The cache key of the item that was retrieved.

itemRemoved

protected abstract void itemRemoved(Object key)
Notify the underlying implementation that an item was removed from the cache.

Parameters:
key - The cache key of the item that was removed.

removeItem

protected abstract Object removeItem()
The cache has reached its cacpacity and an item needs to be removed. (typically according to an algorithm such as LRU or FIFO).

Returns:
The key of whichever item was removed.

rehash

protected void rehash()
Rehashes the contents of this map into a new table with a larger capacity. This method is called automatically when the number of keys in this map exceeds its capacity and load factor.

sput

protected Object sput(Object key,
                      Object value,
                      int hash,
                      boolean persist)
OpenSymphony BEGIN

sremove

protected Object sremove(Object key,
                         int hash,
                         boolean invokeAlgorithm)
OpenSymphony BEGIN

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