de.unihalle.informatik.Alida.helpers
Class ALDConcReadWeakHashMap

java.lang.Object
  extended by de.unihalle.informatik.Alida.helpers.ALDConcReadWeakHashMap

public class ALDConcReadWeakHashMap
extends java.lang.Object

A version of Hashtable that supports concurrent reading/exclusive writing.

Note:
This file in its original version was taken from: http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html

According to the webpage there are no license restrictions on using the file:

"Do I need a license to use it? Can I get one? No!"

You can find the original header of the file, which originally was named 'ConcurrentReaderHashMap.java', below. For using it with Alida the class was significantly modified and also simplified as Alida requires only all very small amount of the original functionality.

Details (non-modified comment of original version):
This version of Hashtable 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 class ALDConcReadWeakHashMap.BarrierLock
          A Serializable class for barrier lock.
 
Field Summary
protected  ALDConcReadWeakHashMap.BarrierLock 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 (1.0).
protected  java.lang.Object lastWrite
          field written to only to guarantee lock ordering.
protected  float loadFactor
          The load factor for the hash table.
protected static java.lang.ref.ReferenceQueue<java.lang.Object> refQueue
          Queue for managing references to deleted objects.
protected  de.unihalle.informatik.Alida.helpers.ALDWeakHashMapEntry[] table
          The hash table data.
protected  int threshold
          The table is rehashed when its size exceeds this threshold.
 
Constructor Summary
ALDConcReadWeakHashMap()
          Constructs a new, empty map with a default initial capacity and load factor.
ALDConcReadWeakHashMap(int initialCapacity)
          Constructs a new, empty map with the specified initial capacity and default load factor.
 
Method Summary
 int capacity()
          Returns the number of slots in this table.
 void clear()
          Removes all mappings from the map.
 java.lang.Object clone()
          Returns a shallow copy of this ConcurrentReaderHashMap instance.
 boolean containsKey(java.lang.Object key)
          Tests if the specified object is a key in this table.
protected  boolean eq(java.lang.Object x, java.lang.Object y)
          Check for equality of non-null references x and y.
 java.lang.Object get(java.lang.Object key)
          Returns the value to which the specified key is mapped in this table.
protected  de.unihalle.informatik.Alida.helpers.ALDWeakHashMapEntry[] 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.
 float loadFactor()
          Returns the load factor.
 java.lang.Object put(java.lang.Object key, java.lang.Object value)
          Maps the specified key to the specified value in this table.
protected  void recordModification(java.lang.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.
 java.lang.Object remove(java.lang.Object key)
          Removes the key (and its corresponding value) from the table.
 int size()
          Returns the number of key-value mappings in this map.
protected  java.lang.Object sput(java.lang.Object key, java.lang.Object value, int hash)
          Continuation of put(), called only when synch lock is held and interference has been detected.
protected  java.lang.Object sremove(java.lang.Object key, int hash)
          Continuation of remove(), called only when synch lock is held and interference has been detected.
 
Methods inherited from class java.lang.Object
equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
 

Field Detail

barrierLock

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


lastWrite

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


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.

See Also:
Constant Field Values

table

protected transient de.unihalle.informatik.Alida.helpers.ALDWeakHashMapEntry[] table
The hash table data.


count

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


threshold

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


loadFactor

protected float loadFactor
The load factor for the hash table.


refQueue

protected static java.lang.ref.ReferenceQueue<java.lang.Object> refQueue
Queue for managing references to deleted objects.

Constructor Detail

ALDConcReadWeakHashMap

public ALDConcReadWeakHashMap(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:
java.lang.IllegalArgumentException - if the initial maximum number of elements is less than zero.

ALDConcReadWeakHashMap

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

Method Detail

recordModification

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


getTableForReading

protected final de.unihalle.informatik.Alida.helpers.ALDWeakHashMapEntry[] getTableForReading()
Get ref to table; the reference and the cells it accesses will be at least as fresh as from last use of barrierLock


eq

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

We are going to check for object references, not equality!


size

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


isEmpty

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


get

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

Parameters:
key - Key in the table.
Returns:
Value to which the key is mapped in this table; null if the key is not mapped to any value.
Throws:
java.lang.NullPointerException - if the key is null.
See Also:
put(Object, Object)

containsKey

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

the method returns true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise.

Parameters:
key - Questionable key.
Returns:
true if and only if object is a key.
Throws:
java.lang.NullPointerException - if the key is null.

put

public java.lang.Object put(java.lang.Object key,
                            java.lang.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:
java.lang.NullPointerException - if the key or value is null.
See Also:
Object.equals(Object), get(Object)

sput

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


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 java.lang.Object remove(java.lang.Object key)
Removes the key (and its corresponding value) from the table.

This method does nothing if the key is not in the table.

Parameters:
key - Key that needs to be removed.
Returns:
Value to which the key had been mapped in this table, or null if the key did not have a mapping.
Throws:
java.lang.NullPointerException - if the key is null.

sremove

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


clear

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


clone

public java.lang.Object clone()
Returns a shallow copy of this ConcurrentReaderHashMap instance.

Note that the keys and values themselves are not cloned.

Overrides:
clone in class java.lang.Object
Returns:
A shallow copy of this map.

capacity

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


loadFactor

public float loadFactor()
Returns the load factor.



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