EDU.oswego.cs.dl.util.concurrent

Class ConcurrentReaderHashMap

public class ConcurrentReaderHashMap 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
ConcurrentReaderHashMap 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:

   ConcurrentReaderHashMap 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, if the values are intended to be the same in each case, just take advantage of the fact that put returns null if the key was not previously present:
   ConcurrentReaderHashMap 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. ]

Nested Class Summary
protected static classConcurrentReaderHashMap.BarrierLock
A Serializable class for barrier lock *
protected static classConcurrentReaderHashMap.Entry
ConcurrentReaderHashMap collision list entry.
protected classConcurrentReaderHashMap.HashIterator
protected classConcurrentReaderHashMap.KeyIterator
protected classConcurrentReaderHashMap.ValueIterator
Field Summary
protected ConcurrentReaderHashMap.BarrierLockbarrierLock
Lock used only for its memory effects.
protected intcount
The total number of mappings in the hash table.
static intDEFAULT_INITIAL_CAPACITY
The default initial number of table slots for this table (32).
static floatDEFAULT_LOAD_FACTOR
The default load factor for this table (1.0).
protected SetentrySet
protected SetkeySet
protected ObjectlastWrite
field written to only to guarantee lock ordering.
protected floatloadFactor
The load factor for the hash table.
protected ConcurrentReaderHashMap.Entry[]table
The hash table data.
protected intthreshold
The table is rehashed when its size exceeds this threshold.
protected Collectionvalues
Constructor Summary
ConcurrentReaderHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty map with the specified initial capacity and the specified load factor.
ConcurrentReaderHashMap(int initialCapacity)
Constructs a new, empty map with the specified initial capacity and default load factor.
ConcurrentReaderHashMap()
Constructs a new, empty map with a default initial capacity and load factor.
ConcurrentReaderHashMap(Map t)
Constructs a new map with the same mappings as the given map.
Method Summary
intcapacity()
Return the number of slots in this table
voidclear()
Removes all mappings from this map.
Objectclone()
Returns a shallow copy of this ConcurrentReaderHashMap instance: the keys and values themselves are not cloned.
booleancontains(Object value)
Tests if some key maps into the specified value in this table.
booleancontainsKey(Object key)
Tests if the specified object is a key in this table.
booleancontainsValue(Object value)
Returns true if this map maps one or more keys to the specified value.
Enumerationelements()
Returns an enumeration of the values in this table.
SetentrySet()
Returns a collection view of the mappings contained in this map.
protected booleaneq(Object x, Object y)
Check for equality of non-null references x and y.
protected booleanfindAndRemoveEntry(Entry entry)
Helper method for entrySet.remove
Objectget(Object key)
Returns the value to which the specified key is mapped in this table.
protected ConcurrentReaderHashMap.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
booleanisEmpty()
Returns true if this map contains no key-value mappings.
Enumerationkeys()
Returns an enumeration of the keys in this table.
SetkeySet()
Returns a set view of the keys contained in this map.
floatloadFactor()
Return the load factor
Objectput(Object key, Object value)
Maps the specified key to the specified value in this table.
voidputAll(Map t)
Copies all of the mappings from the specified map to this one.
protected voidrecordModification(Object x)
Force a memory synchronization that will cause all readers to see table.
protected voidrehash()
Rehashes the contents of this map into a new table with a larger capacity.
Objectremove(Object key)
Removes the key (and its corresponding value) from this table.
intsize()
Returns the number of key-value mappings in this map.
protected Objectsput(Object key, Object value, int hash)
Continuation of put(), called only when synch lock is held and interference has been detected.
protected Objectsremove(Object key, int hash)
Continuation of remove(), called only when synch lock is held and interference has been detected.
Collectionvalues()
Returns a collection view of the values contained in this map.

Field Detail

barrierLock

protected final ConcurrentReaderHashMap.BarrierLock barrierLock
Lock used only for its memory effects.

count

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

DEFAULT_INITIAL_CAPACITY

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

DEFAULT_LOAD_FACTOR

public static final float DEFAULT_LOAD_FACTOR
The default load factor for this table (1.0). Used when not otherwise specified in constructor.

entrySet

protected transient Set entrySet

keySet

protected transient Set keySet

lastWrite

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

loadFactor

protected float loadFactor
The load factor for the hash table.

Serial:

table

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

threshold

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

Serial:

values

protected transient Collection values

Constructor Detail

ConcurrentReaderHashMap

public ConcurrentReaderHashMap(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 ConcurrentReaderHashMap

Throws: IllegalArgumentException if the initial maximum number of elements is less than zero, or if the load factor is nonpositive.

ConcurrentReaderHashMap

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

Parameters: initialCapacity the initial capacity of the ConcurrentReaderHashMap.

Throws: IllegalArgumentException if the initial maximum number of elements is less than zero.

ConcurrentReaderHashMap

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

ConcurrentReaderHashMap

public ConcurrentReaderHashMap(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 16 (whichever is greater), and a default load factor.

Method Detail

capacity

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

clear

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

clone

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

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 containsValue Map

containsKey

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

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

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.

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: java.util.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.

Returns: a collection view of the mappings contained in this map.

eq

protected boolean eq(Object x, Object y)
Check for equality of non-null references x and y.

findAndRemoveEntry

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

get

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

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: ConcurrentReaderHashMap

getTableForReading

protected final ConcurrentReaderHashMap.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

isEmpty

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

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

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

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.

Returns: a set view of the keys contained in this map.

loadFactor

public float loadFactor()
Return the load factor

put

public Object put(Object key, Object value)
Maps the specified key to the specified value in this table. Neither the key nor the value can be null.

The value can be retrieved by calling the get method with a key that is equal to the original key.

Parameters: key the table key. value the value.

Returns: the previous value of the specified key in this table, or null if it did not have one.

Throws: NullPointerException if the key or value is null.

See Also: Object#equals(Object) get

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.

Parameters: t Mappings to be stored in this map.

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.

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.

remove

public Object remove(Object key)
Removes the key (and its corresponding value) from this table. This method does nothing if the key is not in the table.

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.

Throws: NullPointerException if the key is null.

size

public int size()
Returns the number of key-value mappings in this map.

Returns: the number of key-value mappings in this map.

sput

protected Object sput(Object key, Object value, int hash)
Continuation of put(), called only when synch lock is held and interference has been detected.

sremove

protected Object sremove(Object key, int hash)
Continuation of remove(), called only when synch lock is held and interference has been detected.

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.

Returns: a collection view of the values contained in this map.