org.cliffc.high_scale_lib
Class NonBlockingHashMap<TypeK,TypeV>

java.lang.Object
  java.util.AbstractMap<TypeK,TypeV>
      org.cliffc.high_scale_lib.NonBlockingHashMap<TypeK,TypeV>

Type Parameters:

TypeK - the type of keys maintained by this map

TypeV - the type of mapped values

All Implemented Interfaces:

Serializable, Cloneable, ConcurrentMap<TypeK,TypeV>, Map<TypeK,TypeV>

public class NonBlockingHashMap<TypeK,TypeV>
extends AbstractMap<TypeK,TypeV>
implements ConcurrentMap<TypeK,TypeV>, Cloneable, Serializable

A lock-free alternate implementation of ConcurrentHashMap with better scaling properties and generally lower costs to mutate the Map. It provides identical correctness properties as ConcurrentHashMap. All operations are non-blocking and multi-thread safe, including all update operations. NonBlockingHashMap scales substatially better than ConcurrentHashMap for high update rates, even with a large concurrency factor. Scaling is linear up to 768 CPUs on a 768-CPU Azul box, even with 100% updates or 100% reads or any fraction in-between. Linear scaling up to all cpus has been observed on a 32-way Sun US2 box, 32-way Sun Niagra box, 8-way Intel box and a 4-way Power box. This class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable. However, even though all operations are thread-safe, operations do not entail locking and there is not any support for locking the entire table in a way that prevents all access. This class is fully interoperable with Hashtable in programs that rely on its thread safety but not on its synchronization details.

Operations (including put) generally do not block, so may overlap with other update operations (including other puts and removes). Retrievals reflect the results of the most recently completed update operations holding upon their onset. For aggregate operations such as putAll, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They do not throw ConcurrentModificationException. However, iterators are designed to be used by only one thread at a time.

Very full tables, or tables with high reprobe rates may trigger an internal resize operation to move into a larger table. Resizing is not terribly expensive, but it is not free either; during resize operations table throughput may drop somewhat. All threads that visit the table during a resize will 'help' the resizing but will still be allowed to complete their operation before the resize is finished (i.e., a simple 'get' operation on a million-entry table undergoing resizing will not need to block until the entire million entries are copied).

This class and its views and iterators implement all of the optional methods of the Map and Iterator interfaces.

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

Since:

1.5

Author:

Cliff Click

See Also:

Serialized Form

Nested Class Summary

Nested classes/interfaces inherited from interface java.util.Map

Map.Entry<K,V>

Constructor Summary

NonBlockingHashMap()
Create a new NonBlockingHashMap with default minimum size (currently set to 8 K/V pairs or roughly 84 bytes on a standard 32-bit JVM).

NonBlockingHashMap(int initial_sz)
Create a new NonBlockingHashMap with initial room for the given number of elements, thus avoiding internal resizing operations to reach an appropriate size.

Method Summary

void	clear() Removes all of the mappings from this map.
Object	clone() Creates a shallow copy of this hashtable.
boolean	contains(Object val) Legacy method testing if some key maps into the specified value in this table.
boolean	containsKey(Object key) Tests if the key in the table using the equals method.
boolean	containsValue(Object val) Returns true if this Map maps one or more keys to the specified value.
Enumeration<TypeV>	elements() Returns an enumeration of the values in this table.
Set<Map.Entry<TypeK,TypeV>>	entrySet() Returns a Set view of the mappings contained in this map.
TypeV	get(Object key) Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
boolean	isEmpty() Returns size() == 0.
Enumeration<TypeK>	keys() Returns an enumeration of the keys in this table.
Set<TypeK>	keySet() Returns a Set view of the keys contained in this map.
void	print() Verbose printout of table internals, useful for debugging.
TypeV	put(TypeK key, TypeV val) Maps the specified key to the specified value in the table.
void	putAll(Map<? extends TypeK,? extends TypeV> m) Copies all of the mappings from the specified map to this one, replacing any existing mappings.
TypeV	putIfAbsent(TypeK key, TypeV val) Atomically, do a put(TypeK, TypeV) if-and-only-if the key is not mapped.
TypeV	remove(Object key) Removes the key (and its corresponding value) from this map.
boolean	remove(Object key, Object val) Atomically do a remove(Object) if-and-only-if the key is mapped to a value which is equals to the given value.
TypeV	replace(TypeK key, TypeV val) Atomically do a put(key,val) if-and-only-if the key is mapped to some value already.
boolean	replace(TypeK key, TypeV oldValue, TypeV newValue) Atomically do a put(key,newValue) if-and-only-if the key is mapped a value which is equals to oldValue.
long	reprobes() Get and clear the current count of reprobes.
int	size() Returns the number of key-value mappings in this map.
String	toString() Returns a string representation of this map.
Collection<TypeV>	values() Returns a Collection view of the values contained in this map.

Methods inherited from class java.util.AbstractMap

equals, hashCode

Methods inherited from class java.lang.Object

getClass, notify, notifyAll, wait, wait, wait

Methods inherited from interface java.util.Map

equals, hashCode

Constructor Detail

NonBlockingHashMap

public NonBlockingHashMap()

Create a new NonBlockingHashMap with default minimum size (currently set to 8 K/V pairs or roughly 84 bytes on a standard 32-bit JVM).

NonBlockingHashMap

public NonBlockingHashMap(int initial_sz)

Create a new NonBlockingHashMap with initial room for the given number of elements, thus avoiding internal resizing operations to reach an appropriate size. Large numbers here when used with a small count of elements will sacrifice space for a small amount of time gained. The initial size will be rounded up internally to the next larger power of 2.

Method Detail

print

public final void print()

Verbose printout of table internals, useful for debugging.

reprobes

public long reprobes()

Get and clear the current count of reprobes. Reprobes happen on key collisions, and a high reprobe rate may indicate a poor hash function or weaknesses in the table resizing function.

Returns:

the count of reprobes since the last call to reprobes() or since the table was created.

size

public int size()

Returns the number of key-value mappings in this map.

Specified by:

size in interface Map<TypeK,TypeV>

Overrides:

size in class AbstractMap<TypeK,TypeV>

Returns:

the number of key-value mappings in this map

isEmpty

public boolean isEmpty()

Returns size() == 0.

Specified by:

isEmpty in interface Map<TypeK,TypeV>

Overrides:

isEmpty in class AbstractMap<TypeK,TypeV>

Returns:

size() == 0

containsKey

public boolean containsKey(Object key)

Tests if the key in the table using the equals method.

Specified by:

containsKey in interface Map<TypeK,TypeV>

Overrides:

containsKey in class AbstractMap<TypeK,TypeV>

Returns:

true if the key is in the table using the equals method

Throws:

NullPointerException - if the specified key is null

contains

public boolean contains(Object val)

Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to containsValue(java.lang.Object), and exists solely to ensure full compatibility with class Hashtable, which supported this method prior to introduction of the Java Collections framework.

Parameters:

val - a value to search for

Returns:

true if this map maps one or more keys to the specified value

Throws:

NullPointerException - if the specified value is null

put

public TypeV put(TypeK key,
                 TypeV val)

Maps the specified key to the specified value in the table. Neither key nor value can be null.

The value can be retrieved by calling get(java.lang.Object) with a key that is equal to the original key.

Specified by:

put in interface Map<TypeK,TypeV>

Overrides:

put in class AbstractMap<TypeK,TypeV>

Parameters:

key - key with which the specified value is to be associated

val - value to be associated with the specified key

Returns:

the previous value associated with key, or null if there was no mapping for key

Throws:

NullPointerException - if the specified key or value is null

putIfAbsent

public TypeV putIfAbsent(TypeK key,
                         TypeV val)

Atomically, do a put(TypeK, TypeV) if-and-only-if the key is not mapped. Useful to ensure that only a single mapping for the key exists, even if many threads are trying to create the mapping in parallel.

Specified by:

putIfAbsent in interface ConcurrentMap<TypeK,TypeV>

Returns:

the previous value associated with the specified key, or null if there was no mapping for the key

Throws:

NullPointerException - if the specified key or value is null

remove

public TypeV remove(Object key)

Removes the key (and its corresponding value) from this map. This method does nothing if the key is not in the map.

Specified by:

remove in interface Map<TypeK,TypeV>

Overrides:

remove in class AbstractMap<TypeK,TypeV>

Returns:

the previous value associated with key, or null if there was no mapping for key

Throws:

NullPointerException - if the specified key is null

remove

public boolean remove(Object key,
                      Object val)

Atomically do a remove(Object) if-and-only-if the key is mapped to a value which is equals to the given value.

Specified by:

remove in interface ConcurrentMap<TypeK,TypeV>

Throws:

NullPointerException - if the specified key or value is null

replace

public TypeV replace(TypeK key,
                     TypeV val)

Atomically do a put(key,val) if-and-only-if the key is mapped to some value already.

Specified by:

replace in interface ConcurrentMap<TypeK,TypeV>

Throws:

NullPointerException - if the specified key or value is null

replace

public boolean replace(TypeK key,
                       TypeV oldValue,
                       TypeV newValue)

Atomically do a put(key,newValue) if-and-only-if the key is mapped a value which is equals to oldValue.

Specified by:

replace in interface ConcurrentMap<TypeK,TypeV>

Throws:

NullPointerException - if the specified key or value is null

putAll

public void putAll(Map<? extends TypeK,? extends TypeV> m)

Copies all of the mappings from the specified map to this one, replacing any existing mappings.

Specified by:

putAll in interface Map<TypeK,TypeV>

Overrides:

putAll in class AbstractMap<TypeK,TypeV>

Parameters:

m - mappings to be stored in this map

clear

public void clear()

Removes all of the mappings from this map.

Specified by:

clear in interface Map<TypeK,TypeV>

Overrides:

clear in class AbstractMap<TypeK,TypeV>

containsValue

public boolean containsValue(Object val)

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 is much slower than containsKey(java.lang.Object).

Specified by:

containsValue in interface Map<TypeK,TypeV>

Overrides:

containsValue in class AbstractMap<TypeK,TypeV>

Parameters:

val - value whose presence in this map is to be tested

Returns:

true if this map maps one or more keys to the specified value

Throws:

NullPointerException - if the specified value is null

clone

public Object clone()

Creates a shallow copy of this hashtable. All the structure of the hashtable itself is copied, but the keys and values are not cloned. This is a relatively expensive operation.

Overrides:

clone in class AbstractMap<TypeK,TypeV>

Returns:

a clone of the hashtable.

toString

public String toString()

Returns a string representation of this map. The string representation consists of a list of key-value mappings in the order returned by the map's entrySet view's iterator, enclosed in braces ("{}"). Adjacent mappings are separated by the characters ", " (comma and space). Each key-value mapping is rendered as the key followed by an equals sign ("=") followed by the associated value. Keys and values are converted to strings as by String.valueOf(Object).

Overrides:

toString in class AbstractMap<TypeK,TypeV>

Returns:

a string representation of this map

get

public TypeV get(Object key)

Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

More formally, if this map contains a mapping from a key k to a value v such that key.equals(k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

Specified by:

get in interface Map<TypeK,TypeV>

Overrides:

get in class AbstractMap<TypeK,TypeV>

Throws:

NullPointerException - if the specified key is null

elements

public Enumeration<TypeV> elements()

Returns an enumeration of the values in this table.

Returns:

an enumeration of the values in this table

See Also:

values()

values

public Collection<TypeV> 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.

The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

Specified by:

values in interface Map<TypeK,TypeV>

Overrides:

values in class AbstractMap<TypeK,TypeV>

keys

public Enumeration<TypeK> keys()

Returns an enumeration of the keys in this table.

Returns:

an enumeration of the keys in this table

See Also:

keySet()

keySet

public Set<TypeK> 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.

The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

Specified by:

keySet in interface Map<TypeK,TypeV>

Overrides:

keySet in class AbstractMap<TypeK,TypeV>

entrySet

public Set<Map.Entry<TypeK,TypeV>> entrySet()

Returns a Set view of the mappings 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 the map, via the Iterator.remove, Set.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.

The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction.

Warning: the iterator associated with this Set requires the creation of Map.Entry objects with each iteration. The NonBlockingHashMap does not normally create or using Map.Entry objects so they will be created soley to support this iteration. Iterating using keySet() or values() will be more efficient.

Specified by:

entrySet in interface Map<TypeK,TypeV>

Specified by:

entrySet in class AbstractMap<TypeK,TypeV>