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java.lang.Object java.util.AbstractMap org.codehaus.groovy.runtime.metaclass.ConcurrentReaderHashMap
public class ConcurrentReaderHashMap extends AbstractMap
A hash table 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 | |
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protected static class |
ConcurrentReaderHashMap.BarrierLock
A Serializable class for barrier lock * |
protected static class |
ConcurrentReaderHashMap.Entry
|
protected class |
ConcurrentReaderHashMap.HashIterator
|
protected class |
ConcurrentReaderHashMap.KeyIterator
|
protected class |
ConcurrentReaderHashMap.ValueIterator
|
Field Summary | |
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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 ConcurrentReaderHashMap.BarrierLock |
barrierLock
Lock used only for its memory effects. |
protected int |
count
The total number of mappings in the hash table. |
protected Set |
entrySet
|
protected Set |
keySet
Returns a set view of the keys contained in this map. |
protected Object |
lastWrite
field written to only to guarantee lock ordering. |
protected float |
loadFactor
The load factor for the hash table. |
protected ConcurrentReaderHashMap.Entry[] |
table
The hash table data. |
protected int |
threshold
The table is rehashed when its size exceeds this threshold. |
protected Collection |
values
|
Method Summary | |
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def
|
ConcurrentReaderHashMap(int initialCapacity, float loadFactor)
Constructs a new, empty map with the specified initial capacity and the specified load factor. |
def
|
ConcurrentReaderHashMap(int initialCapacity)
Constructs a new, empty map with the specified initial capacity and default load factor. |
def
|
ConcurrentReaderHashMap()
Constructs a new, empty map with a default initial capacity and load factor. |
def
|
ConcurrentReaderHashMap(Map t)
Constructs a new map with the same mappings as the given map. |
int
|
capacity()
|
void
|
clear()
Returns a shallow copy of this ConcurrentReaderHashMap instance: the keys and values themselves are not cloned. |
Object
|
clone()
|
boolean
|
contains(Object value)
Copies all of the mappings from the specified map to this one. |
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()
|
Set
|
entrySet()
|
protected boolean
|
eq(Object x, Object y)
Check for equality of non-null references x and y. |
protected boolean
|
findAndRemoveEntry(Map.Entry entry)
|
Object
|
get(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 |
boolean
|
isEmpty()
Returns true if this map contains no key-value mappings. |
Set
|
keySet()
|
Enumeration
|
keys()
ConcurrentReaderHashMap collision list entry. |
float
|
loadFactor()
|
Object
|
put(Object key, Object value)
|
void
|
putAll(Map t)
|
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)
Removes the key (and its corresponding value) from this table. |
int
|
size()
Returns the number of key-value mappings in this map. |
protected Object
|
sput(Object key, Object value, int hash)
Continuation of put(), called only when synch lock is held and interference has been detected. |
protected Object
|
sremove(Object key, int hash)
Continuation of remove(), called only when synch lock is held and interference has been detected. |
Collection
|
values()
|
Methods inherited from class AbstractMap | |
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remove, get, put, equals, toString, values, hashCode, clear, isEmpty, size, entrySet, putAll, keySet, containsKey, containsValue, wait, wait, wait, getClass, notify, notifyAll |
Methods inherited from class Object | |
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wait, wait, wait, equals, toString, hashCode, getClass, notify, notifyAll |
Field Detail |
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public static final int DEFAULT_INITIAL_CAPACITY
public static final float DEFAULT_LOAD_FACTOR
protected final ConcurrentReaderHashMap.BarrierLock barrierLock
protected int count
protected Set entrySet
protected Set keySet
protected Object lastWrite
protected float loadFactor
protected ConcurrentReaderHashMap.Entry[] table
protected int threshold
protected Collection values
Method Detail |
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public def ConcurrentReaderHashMap(int initialCapacity, float loadFactor)
initialCapacity
- the initial capacity
The actual initial capacity is rounded to the nearest power of two.loadFactor
- the load factor of the ConcurrentReaderHashMap
public def ConcurrentReaderHashMap(int initialCapacity)
initialCapacity
- the initial capacity of the
ConcurrentReaderHashMap.
public def ConcurrentReaderHashMap()
public def ConcurrentReaderHashMap(Map t)
public int capacity()
public void clear()
public Object clone()
public boolean contains(Object value)
t
- Mappings to be stored in this map.
public boolean containsKey(Object key)
null
.key
- possible key.true
if and only if the specified object
is a key in this table, as determined by the
equals method; false
otherwise.
public boolean containsValue(Object value)
null
.value
- value whose presence in this map is to be tested.
public Enumeration elements()
public Set entrySet()
protected boolean eq(Object x, Object y)
protected boolean findAndRemoveEntry(Map.Entry entry)
public Object get(Object key)
null
.key
- a key in the table.null
if the key is not mapped to any value in
this table.
protected final ConcurrentReaderHashMap.Entry[] getTableForReading()
public boolean isEmpty()
public Set keySet()
public Enumeration keys()
public float loadFactor()
public Object put(Object key, Object value)
public void putAll(Map t)
protected final void recordModification(Object x)
protected void rehash()
public Object remove(Object key)
null
.key
- the key that needs to be removed.null
if the key did not have a mapping.
public int size()
protected Object sput(Object key, Object value, int hash)
protected Object sremove(Object key, int hash)
public Collection values()
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