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. ]
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.
- 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).
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.
containsKey
public boolean containsKey(Object key)
Tests if the specified object is a key in this table.
true
if and only if the specified object
is a key in this table, as determined by the
equals method; false
otherwise.
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.
value
- value whose presence in this map is to be tested.
- true if this map maps one or more keys to the
specified value.
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.
- an enumeration of the values in this table.
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.
- 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(Map.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.
key
- a key in the table.
- the value to which the key is mapped in this table;
null
if the key is not mapped to any value in
this table.
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.
- true if this map contains no key-value mappings.
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.
- a set view of the keys contained in this map.
keys
public Enumeration keys()
Returns an enumeration of the keys in this table.
- an enumeration of the keys in this table.
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.
key
- the table key.value
- the value.
- the previous value of the specified key in this table,
or
null
if it did not have one.
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.
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.
key
- the key that needs to be removed.
- the value to which the key had been mapped in this table,
or
null
if the key did not have a mapping.
size
public int size()
Returns the number of key-value mappings in this map.
- 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.
- a collection view of the values contained in this map.