org.apache.commons.collections
public final class StaticBucketMap extends Object implements Map
Deprecated: Moved to map subpackage. Due to be removed in v4.0.
A StaticBucketMap is an efficient, thread-safe implementation ofjava.util.Map
that performs well in in a highly
thread-contentious environment. The map supports very efficient
{@link #get(Object) get}, {@link #put(Object,Object) put},
{@link #remove(Object) remove} and {@link #containsKey(Object) 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 {@link 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 {@link #putAll(Map) putAll} or the {@link Collection#retainAll(Collection) 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.Also, much like an encyclopedia, the results of {@link #size()} and {@link #isEmpty()} are out-of-date as soon as they are produced.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 {@link 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 {@link 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 {@link #atomic(Runnable)} method is provided to allow atomic iterations and bulk operations; however, overuse of {@link #atomic(Runnable) atomic} will basically result in a map that's slower than an ordinary synchronized {@link 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.
Since: Commons Collections 2.1
Version: $Revision: 1.18 $ $Date: 2004/02/18 01:15:42 $
Constructor Summary | |
---|---|
StaticBucketMap()
Initializes the map with the default number of buckets (255). | |
StaticBucketMap(int numBuckets)
Initializes the map with a specified number of buckets. |
Method Summary | |
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void | atomic(Runnable r)
Prevents any operations from occurring on this map while the
given {@link Runnable} executes. |
void | clear()
Implements {@link Map#clear()}. |
boolean | containsKey(Object key)
Implements {@link Map#containsKey(Object)}. |
boolean | containsValue(Object value)
Implements {@link Map#containsValue(Object)}. |
Set | entrySet()
Implements {@link Map#entrySet()}. |
boolean | equals(Object obj)
Implements {@link Map#equals(Object)}. |
Object | get(Object key)
Implements {@link Map#get(Object)}. |
int | hashCode()
Implements {@link Map#hashCode()}. |
boolean | isEmpty()
Implements {@link Map#isEmpty()}. |
Set | keySet()
Implements {@link Map#keySet()}. |
Object | put(Object key, Object value)
Implements {@link Map#put(Object, Object)}. |
void | putAll(Map other)
Implements {@link Map#putAll(Map)}. |
Object | remove(Object key)
Implements {@link Map#remove(Object)}. |
int | size()
Implements {@link Map#size()}. |
Collection | values()
Implements {@link Map#values()}. |
Parameters: numBuckets the number of buckets for this map
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.
Parameters: r the code to execute atomically