A StaticBucketMap is an efficient, thread-safe implementation of
java.util.Map
that performs well in in a highly
thread-contentious environment. The map supports very efficient
get
,
put
,
remove
and
containsKey
operations, assuming (approximate) uniform hashing and
that the number of entries does not exceed the number of buckets. If the
number of entries exceeds the number of buckets or if the hash codes of the
objects are not uniformly distributed, these operations have a worst case
scenario that is proportional to the number of elements in the map
(
O(n)).
Each bucket in the hash table has its own monitor, so two threads can
safely operate on the map at the same time, often without incurring any
monitor contention. This means that you don't have to wrap instances
of this class with
java.util.Collections.synchronizedMap(Map)
;
instances are already thread-safe. Unfortunately, however, this means
that this map implementation behaves in ways you may find disconcerting.
Bulk operations, such as
putAll
or the
retainAll
operation in collection
views, are
not atomic. If two threads are simultaneously
executing
staticBucketMapInstance.putAll(map);
and
staticBucketMapInstance.entrySet().removeAll(map.entrySet());
then the results are generally random. Those two statement could cancel
each other out, leaving
staticBucketMapInstance
essentially
unchanged, or they could leave some random subset of
map
in
staticBucketMapInstance
.
Also, much like an encyclopedia, the results of
size()
and
isEmpty()
are out-of-date as soon as they are produced.
The iterators returned by the collection views of this class are
not
fail-fast. They will
never raise a
java.util.ConcurrentModificationException
. Keys and values
added to the map after the iterator is created do not necessarily appear
during iteration. Similarly, the iterator does not necessarily fail to
return keys and values that were removed after the iterator was created.
Finally, unlike
java.util.HashMap
-style implementations, this
class
never rehashes the map. The number of buckets is fixed
at construction time and never altered. Performance may degrade if
you do not allocate enough buckets upfront.
The
atomic(Runnable)
method is provided to allow atomic iterations
and bulk operations; however, overuse of
atomic
will basically result in a map that's slower than an ordinary synchronized
java.util.HashMap
.
Use this class if you do not require reliable bulk operations and
iterations, or if you can make your own guarantees about how bulk
operations will affect the map.
atomic
public void atomic(Runnable r)
Prevents any operations from occurring on this map while the
given
Runnable
executes. This method can be used, for
instance, to execute a bulk operation atomically:
staticBucketMapInstance.atomic(new Runnable() {
public void run() {
staticBucketMapInstance.putAll(map);
}
});
It can also be used if you need a reliable iterator:
staticBucketMapInstance.atomic(new Runnable() {
public void run() {
Iterator iterator = staticBucketMapInstance.iterator();
while (iterator.hasNext()) {
foo(iterator.next();
}
}
});
Implementation note: This method requires a lot of time
and a ton of stack space. Essentially a recursive algorithm is used
to enter each bucket's monitor. If you have twenty thousand buckets
in your map, then the recursive method will be invoked twenty thousand
times. You have been warned.
r
- the code to execute atomically
clear
public final void clear()
Implements Map.clear()
.
containsKey
public boolean containsKey(Object key)
Implements Map.containsKey(Object)
.
containsValue
public boolean containsValue(Object value)
Implements Map.containsValue(Object)
.
entrySet
public Set entrySet()
Implements Map.entrySet()
.
equals
public final boolean equals(Object obj)
Implements Map.equals(Object)
.
get
public Object get(Object key)
Implements Map.get(Object)
.
hashCode
public final int hashCode()
Implements Map.hashCode()
.
isEmpty
public final boolean isEmpty()
Implements Map.isEmpty()
.
keySet
public Set keySet()
Implements Map.keySet()
.
put
public Object put(Object key,
Object value)
Implements Map.put(Object, Object)
.
putAll
public void putAll(Map other)
Implements Map.putAll(Map)
.
remove
public Object remove(Object key)
Implements Map.remove(Object)
.
size
public int size()
Implements Map.size()
.
values
public Collection values()
Implements Map.values()
.