public class ConcurrentReaderHashMap extends AbstractMap implements Cloneable, Serializable
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. ]
| Modifiers | Name | Description |
|---|---|---|
protected static class |
ConcurrentReaderHashMap.BarrierLock |
A Serializable class for barrier lock * |
protected static class |
ConcurrentReaderHashMap.Entry |
ConcurrentReaderHashMap collision list entry. |
protected class |
ConcurrentReaderHashMap.HashIterator |
|
protected class |
ConcurrentReaderHashMap.KeyIterator |
|
protected class |
ConcurrentReaderHashMap.ValueIterator |
| Modifiers | Name | Description |
|---|---|---|
static int |
DEFAULT_INITIAL_CAPACITY |
The default initial number of table slots for this table (32). |
static float |
DEFAULT_LOAD_FACTOR |
|
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 |
|
protected Object |
lastWrite |
field written to only to guarantee lock ordering. |
protected float |
loadFactor |
The load factor for the hash table. |
protected Map.Entry[] |
table |
The hash table data. |
protected int |
threshold |
The table is rehashed when its size exceeds this threshold. |
protected Collection |
values |
| Constructor and description |
|---|
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. |
| Type Params | Return Type | Name and description |
|---|---|---|
|
public int |
capacity()
|
|
public void |
clear()Removes all mappings from this map. |
|
public Object |
clone()Returns a shallow copy of this ConcurrentReaderHashMap instance: the keys and values themselves are not cloned. |
|
public boolean |
contains(Object value)Tests if some key maps into the specified value in this table. |
|
public boolean |
containsKey(Object key)Tests if the specified object is a key in this table. |
|
public boolean |
containsValue(Object value)Returns true if this map maps one or more keys to the specified value. |
|
public Enumeration |
elements()Returns an enumeration of the values in this table. |
|
public Set |
entrySet()Returns a collection view of the mappings contained in this map. |
|
protected boolean |
eq(Object x, Object y)Check for equality of non-null references x and y. |
|
protected boolean |
findAndRemoveEntry(Map.Entry entry)Helper method for entrySet.remove |
|
public Object |
get(Object key)Returns the value to which the specified key is mapped in this table. |
|
protected final Map.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 |
|
public boolean |
isEmpty()Returns true if this map contains no key-value mappings. |
|
public Set |
keySet()Returns a set view of the keys contained in this map. |
|
public Enumeration |
keys()Returns an enumeration of the keys in this table. |
|
public float |
loadFactor()
|
|
public Object |
put(Object key, Object value)Maps the specified key to the specified
value in this table. |
|
public void |
putAll(Map t)Copies all of the mappings from the specified map to this one. |
|
protected final 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. |
|
public Object |
remove(Object key)Removes the key (and its corresponding value) from this table. |
|
public 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 sync lock is held and interference has been detected. |
|
protected Object |
sremove(Object key, int hash)Continuation of remove(), called only when sync lock is held and interference has been detected. |
|
public Collection |
values()Returns a collection view of the values contained in this map. |
| Methods inherited from class | Name |
|---|---|
class AbstractMap |
remove, get, put, equals, toString, values, hashCode, clear, isEmpty, size, entrySet, putAll, containsKey, containsValue, keySet, wait, wait, wait, getClass, notify, notifyAll, remove, replace, replace, replaceAll, merge, putIfAbsent, compute, forEach, computeIfAbsent, getOrDefault, computeIfPresent |
The default initial number of table slots for this table (32). Used when not otherwise specified in constructor.
Lock used only for its memory effects.
The total number of mappings in the hash table.
field written to only to guarantee lock ordering.
The load factor for the hash table.
The hash table data.
The table is rehashed when its size exceeds this threshold. (The value of this field is always (int)(capacity * loadFactor).)
Constructs a new, empty map with the specified initial capacity and the specified load factor.
initialCapacity - the initial capacity
The actual initial capacity is rounded to the nearest power of two.loadFactor - the load factor of the ConcurrentReaderHashMapConstructs a new, empty map with the specified initial capacity and default load factor.
initialCapacity - the initial capacity of the
ConcurrentReaderHashMap.Constructs a new, empty map with a default initial capacity and load factor.
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.
Removes all mappings from this map.
Returns a shallow copy of this ConcurrentReaderHashMap instance: the keys and values themselves are not cloned.
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).
value - a value to search for.true if and only if some key maps to the
value argument in this table as
determined by the equals method;
false otherwise.null.Tests if the specified object is a key in this table.
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.null.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.
value - value whose presence in this map is to be tested.null.Returns an enumeration of the values in this table. Use the Enumeration methods on the returned object to fetch the elements sequentially.
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.
Helper method for entrySet.remove
Returns the value to which the specified key is mapped in this table.
key - a key in the table.null if the key is not mapped to any value in
this table.null.Get ref to table; the reference and the cells it accesses will be at least as fresh as from last use of barrierLock
Returns true if this map contains no key-value mappings.
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 an enumeration of the keys in this table.
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.
key - the table key.value - the value.null if it did not have one.null.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.
t - Mappings to be stored in this map.Force a memory synchronization that will cause all readers to see table. Call only when already holding main sync lock.
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.
Removes the key (and its corresponding value) from this table. This method does nothing if the key is not in the table.
key - the key that needs to be removed.null if the key did not have a mapping.null.Returns the number of key-value mappings in this map.
Continuation of put(), called only when sync lock is held and interference has been detected.
Continuation of remove(), called only when sync lock is held and interference has been detected.
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.
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