com.tangosol.util
Class SafeHashMap

java.lang.Object
  java.util.AbstractMap
      com.tangosol.util.SafeHashMap

All Implemented Interfaces:

Serializable, Cloneable, Map

Direct Known Subclasses:

ObservableHashMap, OldCache

public class SafeHashMap
extends AbstractMap
implements Cloneable, Serializable

An implementation of java.util.Map that is synchronized, but minimally so. This class is for use in situation where high concurrency is required, but so is data integrity.

All additions and removals are synchronized on the map, so to temporarily prevent changes to the map contents, synchronize on the map object.

Author:

cp 1999.04.27

Nested Class Summary

protected static class	SafeHashMap.Entry A map entry (key-value pair).
protected class	SafeHashMap.EntrySet A set of entries backed by this map.
protected class	SafeHashMap.KeySet A set of entries backed by this map.
protected class	SafeHashMap.ValuesCollection A collection of values backed by this map.

Field Summary

protected static int	BIGGEST_MODULO Biggest possible modulo.
static float	DEFAULT_GROWTHRATE Using the default growth rate, the bucket array will grow by a factor of four.
static int	DEFAULT_INITIALSIZE Default initial size provides a prime modulo and is large enough that resize is not immediate.
static float	DEFAULT_LOADFACTOR The default load factor is 100%, which means that the hash map will not resize until there is (on average) one entry in every bucket.
protected SafeHashMap.Entry[]	m_aeBucket The array of hash buckets.
protected int	m_cCapacity The capacity of the hash map (the point at which it must resize), 1 <= n.
protected int	m_cEntries The number of entries stored in the hash map, 0 <= n.
protected SafeHashMap.ValuesCollection	m_colValues The collection of values backed by this map.
protected float	m_flGrowthRate The rate of growth as a fraction of the current number of buckets, 0 < n, such that the hash map grows to bucketcount * (1 + growthrate)
protected float	m_flLoadFactor The determining factor for the hash map capacity given a certain number of buckets, such that capactiy = bucketcount * loadfactor.
protected Object	m_oIterActive A holder for active Iterator(s): either WeakReference() or WeakHashMap(, null)
protected SafeHashMap.EntrySet	m_setEntries The set of entries backed by this map.
protected SafeHashMap.KeySet	m_setKeys The set of keys backed by this map.
protected static int[]	PRIME_MODULO A list of possible modulos to use.
protected Object	RESIZING When resizing completes, a notification is issued against this object.

Constructor Summary

SafeHashMap()
Construct a thread-safe hash map using the default settings.

SafeHashMap(int cInitialBuckets, float flLoadFactor, float flGrowthRate)
Construct a thread-safe hash map using the specified settings.

Method Summary

void	clear() Removes all mappings from this map.
Object	clone() Create a clone of the SafeHashMap.
protected SafeHashMap.Entry	cloneEntryList(SafeHashMap.Entry entryThat) Clone an entire linked list of entries.
boolean	containsKey(Object key) Returns true if this map contains a mapping for the specified key.
Set	entrySet() Returns a set view of the mappings contained in this map.
Object	get(Object key) Returns the value to which this map maps the specified key.
protected int	getBucketIndex(int nHash, int cBuckets) Calculate the bucket number for a particular hash code.
SafeHashMap.Entry	getEntry(Object key) Locate an Entry in the hash map based on its key.
protected SafeHashMap.Entry	getEntryInternal(Object oKey) Locate an Entry in the hash map based on its key.
protected SafeHashMap.Entry[]	getStableBucketArray() Get the bucket array, or if a resize is occurring, wait for the resize to complete and return the new bucket array.
protected void	grow() Resize the bucket array, rehashing all Entries.
protected SafeHashMap.Entry	instantiateEntry() Factory pattern.
protected SafeHashMap.EntrySet	instantiateEntrySet() Factory pattern.
protected SafeHashMap.KeySet	instantiateKeySet() Factory pattern.
protected SafeHashMap.ValuesCollection	instantiateValuesCollection() Factory pattern.
protected boolean	isActiveIterator() Determine if there are any active Iterators, which may mean that they are in the middle of iterating over the Map.
boolean	isEmpty() Returns true if this map contains no key-value mappings.
protected void	iteratorActivated(Iterator iter) Register the activation of an Iterator.
protected void	iteratorDeactivated(Iterator iter) Unregister the (formerly active) Iterator.
Set	keySet() Returns a Set view of the keys contained in this map.
Object	put(Object key, Object value) Associates the specified value with the specified key in this map.
Object	remove(Object oKey) Removes the mapping for this key from this map if present.
protected void	removeEntryInternal(SafeHashMap.Entry entry) Removes the passed Entry from the map.
int	size() Returns the number of key-value mappings in this map.
Collection	values() Returns a collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

containsValue, equals, hashCode, putAll, toString

Field Detail

PRIME_MODULO

protected static final int[] PRIME_MODULO

A list of possible modulos to use.

DEFAULT_INITIALSIZE

public static final int DEFAULT_INITIALSIZE

Default initial size provides a prime modulo and is large enough that resize is not immediate. (A hash map probably uses less than 128 bytes initially.)

BIGGEST_MODULO

protected static final int BIGGEST_MODULO

Biggest possible modulo.

DEFAULT_LOADFACTOR

public static final float DEFAULT_LOADFACTOR

The default load factor is 100%, which means that the hash map will not resize until there is (on average) one entry in every bucket. The cost of scanning a linked list in a particular bucket is very low, so there is little reason for having this value below 1.0, and the goal is constant order access, so assuming a perfect hash this will provide the optimal access speed relative to size.

See Also:

Constant Field Values

DEFAULT_GROWTHRATE

public static final float DEFAULT_GROWTHRATE

Using the default growth rate, the bucket array will grow by a factor of four. The relatively high growth rate helps to ensure less resize operations, an important consideration in a high-concurrency map.

See Also:

Constant Field Values

RESIZING

protected Object RESIZING

When resizing completes, a notification is issued against this object.

Due to custom serialization this field cannot be marked as "final", but must be treated as such.

m_cEntries

protected volatile int m_cEntries

The number of entries stored in the hash map, 0 <= n. This field is declared volatile to avoid synchronization for the size() operation.

m_aeBucket

protected volatile SafeHashMap.Entry[] m_aeBucket

The array of hash buckets. This field is declared volatile in order to reduce synchronization.

m_cCapacity

protected int m_cCapacity

The capacity of the hash map (the point at which it must resize), 1 <= n.

m_flLoadFactor

protected float m_flLoadFactor

The determining factor for the hash map capacity given a certain number of buckets, such that capactiy = bucketcount * loadfactor.

m_flGrowthRate

protected float m_flGrowthRate

The rate of growth as a fraction of the current number of buckets, 0 < n, such that the hash map grows to bucketcount * (1 + growthrate)

m_setEntries

protected transient SafeHashMap.EntrySet m_setEntries

The set of entries backed by this map.

m_setKeys

protected transient SafeHashMap.KeySet m_setKeys

The set of keys backed by this map.

m_colValues

protected transient SafeHashMap.ValuesCollection m_colValues

The collection of values backed by this map.

m_oIterActive

protected transient Object m_oIterActive

A holder for active Iterator(s): either WeakReference() or WeakHashMap(, null)

Constructor Detail

SafeHashMap

public SafeHashMap()

Construct a thread-safe hash map using the default settings.

SafeHashMap

public SafeHashMap(int cInitialBuckets,
                   float flLoadFactor,
                   float flGrowthRate)

Construct a thread-safe hash map using the specified settings.

Parameters:

cInitialBuckets - the initial number of hash buckets, 0 < n

flLoadFactor - the acceptable load factor before resizing occurs, 0 < n, such that a load factor of 1.0 causes resizing when the number of entries exceeds the number of buckets

flGrowthRate - the rate of bucket growth when a resize occurs, 0 < n, such that a growth rate of 1.0 will double the number of buckets: bucketcount = bucketcount * (1 + growthrate)

Method Detail

size

public int size()

Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.

This method is not synchronized; it returns the size at the moment that the method is invoked. To ensure that the size does not change from the returned value, the caller must synchronize on the map before calling the size method.

Specified by:

size in interface Map

Overrides:

size in class AbstractMap

Returns:

the number of key-value mappings in this map

isEmpty

public boolean isEmpty()

Returns true if this map contains no key-value mappings.

This method is not synchronized; it returns the state of the map at the moment that the method is invoked. To ensure that the size does not change, the caller must synchronize on the map before calling the method.

Specified by:

isEmpty in interface Map

Overrides:

isEmpty in class AbstractMap

Returns:

true if this map contains no key-value mappings

containsKey

public boolean containsKey(Object key)

Returns true if this map contains a mapping for the specified key.

This method is not synchronized; it returns true if the map contains the key at the moment that the method is invoked. To ensure that the key is still in (or is still not in) the table when the method returns, the caller must synchronize on the map before calling the method.

Specified by:

containsKey in interface Map

Overrides:

containsKey in class AbstractMap

Parameters:

key - key whose presence in this map is to be tested

Returns:

true if this map contains a mapping for the specified key

get

public Object get(Object key)

Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key. A return value of null does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key to null. The containsKey operation may be used to distinguish these two cases.

Specified by:

get in interface Map

Overrides:

get in class AbstractMap

Parameters:

key - key whose associated value is to be returned

Returns:

the value to which this map maps the specified key, or null if the map contains no mapping for this key

getEntry

public SafeHashMap.Entry getEntry(Object key)

Locate an Entry in the hash map based on its key.

Parameters:

key - the key object to search for

Returns:

the Entry or null

put

public Object put(Object key,
                  Object value)

Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.

This method is not synchronized; it only synchronizes internally if it has to add a new Entry. To ensure that the value does not change (or the Entry is not removed) before this method returns, the caller must synchronize on the map before calling this method.

Specified by:

put in interface Map

Overrides:

put in class AbstractMap

Parameters:

key - key with which the specified value is to be associated

value - value to be associated with the specified key

Returns:

previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values

grow

protected void grow()

Resize the bucket array, rehashing all Entries.

remove

public Object remove(Object oKey)

Removes the mapping for this key from this map if present.

Specified by:

remove in interface Map

Overrides:

remove in class AbstractMap

Parameters:

oKey - key whose mapping is to be removed from the map

Returns:

previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values

clear

public void clear()

Removes all mappings from this map.

Specified by:

clear in interface Map

Overrides:

clear in class AbstractMap

entrySet

public Set entrySet()

Returns a set view of the mappings contained in this map. Each element in the returned set is a Map.Entry. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. If the map is modified while an iteration over the set is in progress, the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.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 set view of the mappings contained in this map.

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. (If the map is modified while an iteration over the Set is in progress, the results of the iteration are undefined.) The Set supports element removal, which removes the corresponding entry from the 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

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. If the map is modified while an iteration over the collection is in progress, the results of the iteration are undefined. 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:

values in interface Map

Overrides:

values in class AbstractMap

Returns:

a collection view of the values contained in this map

clone

public Object clone()

Create a clone of the SafeHashMap. Synchronization is necessary to prevent resizing (or to wait for resizing to complete), and also prevents other changes to the SafeHashMap while the deep clone is occurring.

Overrides:

clone in class AbstractMap

Returns:

a clone of the SafeHashMap

cloneEntryList

protected SafeHashMap.Entry cloneEntryList(SafeHashMap.Entry entryThat)

Clone an entire linked list of entries.

This method must be called on the map that will contain the clones, to allow the map to be the parent of the entries if the entries are not static inner classes.

Parameters:

entryThat - the entry that is the head of the list to clone

Returns:

null if the entry is null, otherwise an entry that is a clone of the passed entry, and a linked list of clones for each entry in the linked list that was passed

getEntryInternal

protected SafeHashMap.Entry getEntryInternal(Object oKey)

Locate an Entry in the hash map based on its key.

Unlike the getEntry(java.lang.Object) method, there must be no side-effects of calling this method.

Parameters:

oKey - the key object to search for

Returns:

the Entry or null

removeEntryInternal

protected void removeEntryInternal(SafeHashMap.Entry entry)

Removes the passed Entry from the map.

Note: Unlike calling the "remove" method, which is overriden at subclasses, calling this method directly does not generate any events.

Parameters:

entry - the entry to remove from this map

getBucketIndex

protected int getBucketIndex(int nHash,
                             int cBuckets)

Calculate the bucket number for a particular hash code.

Parameters:

nHash - the hash code

cBuckets - the number of buckets

Returns:

the bucket index for the specified hash code

getStableBucketArray

protected SafeHashMap.Entry[] getStableBucketArray()

Get the bucket array, or if a resize is occurring, wait for the resize to complete and return the new bucket array.

Returns:

the latest bucket array

iteratorActivated

protected void iteratorActivated(Iterator iter)

Register the activation of an Iterator.

Parameters:

iter - the activated iterator

iteratorDeactivated

protected void iteratorDeactivated(Iterator iter)

Unregister the (formerly active) Iterator.

Parameters:

iter - the deactivated iterator

isActiveIterator

protected boolean isActiveIterator()

Determine if there are any active Iterators, which may mean that they are in the middle of iterating over the Map.

Returns:

true iff there is at least one active Iterator

instantiateEntry

protected SafeHashMap.Entry instantiateEntry()

Factory pattern.

Returns:

a new instance of the Entry class (or a subclass thereof)

instantiateEntrySet

protected SafeHashMap.EntrySet instantiateEntrySet()

Factory pattern.

Returns:

a new instance of the EntrySet class (or a subclass thereof)

instantiateKeySet

protected SafeHashMap.KeySet instantiateKeySet()

Factory pattern.

Returns:

a new instance of the KeySet class (or subclass thereof)

instantiateValuesCollection

protected SafeHashMap.ValuesCollection instantiateValuesCollection()

Factory pattern.

Returns:

a new instance of the ValuesCollection class (or subclass thereof)