Class WeakHashMapSafeReadLock<K,V>
- Type Parameters:
K
- the type of keys maintained by this mapV
- the type of mapped values
- All Implemented Interfaces:
Map<K,
V>
Map
implementation with weak keys.
An entry in a WeakHashMap
will automatically be removed when
its key is no longer in ordinary use. More precisely, the presence of a
mapping for a given key will not prevent the key from being discarded by the
garbage collector, that is, made finalizable, finalized, and then reclaimed.
When a key has been discarded its entry is effectively removed from the map,
so this class behaves somewhat differently from other Map
implementations.
Both null values and the null key are supported. This class has
performance characteristics similar to those of the HashMap
class, and has the same efficiency parameters of initial capacity
and load factor.
Like most collection classes, this class is not synchronized.
A synchronized WeakHashMap
may be constructed using the
Collections.synchronizedMap method.
This class is intended primarily for use with key objects whose
equals
methods test for object identity using the
==
operator. Once such a key is discarded it can never be
recreated, so it is impossible to do a lookup of that key in a
WeakHashMap
at some later time and be surprised that its entry
has been removed. This class will work perfectly well with key objects
whose equals
methods are not based upon object identity, such
as String
instances. With such recreatable key objects,
however, the automatic removal of WeakHashMap
entries whose
keys have been discarded may prove to be confusing.
The behavior of the WeakHashMap
class depends in part upon
the actions of the garbage collector, so several familiar (though not
required) Map
invariants do not hold for this class. Because
the garbage collector may discard keys at any time, a
WeakHashMap
may behave as though an unknown thread is silently
removing entries. In particular, even if you synchronize on a
WeakHashMap
instance and invoke none of its mutator methods, it
is possible for the size
method to return smaller values over
time, for the isEmpty
method to return false
and
then true
, for the containsKey
method to return
true
and later false
for a given key, for the
get
method to return a value for a given key but later return
null
, for the put
method to return
null
and the remove
method to return
false
for a key that previously appeared to be in the map, and
for successive examinations of the key set, the value collection, and
the entry set to yield successively smaller numbers of elements.
Each key object in a WeakHashMap
is stored indirectly as
the referent of a weak reference. Therefore a key will automatically be
removed only after the weak references to it, both inside and outside of the
map, have been cleared by the garbage collector.
Implementation note: The value objects in a
WeakHashMap
are held by ordinary strong references. Thus care
should be taken to ensure that value objects do not strongly refer to their
own keys, either directly or indirectly, since that will prevent the keys
from being discarded. Note that a value object may refer indirectly to its
key via the WeakHashMap
itself; that is, a value object may
strongly refer to some other key object whose associated value object, in
turn, strongly refers to the key of the first value object. One way
to deal with this is to wrap values themselves within
WeakReferences
before
inserting, as in: m.put(key, new WeakReference(value))
,
and then unwrapping upon each get
.
The iterators returned by the iterator
method of the collections
returned by all of this class's "collection view methods" are
fail-fast: if the map is structurally modified at any time after the
iterator is created, in any way except through the iterator's own
remove
method, the iterator will throw a ConcurrentModificationException
. Thus, in the face of concurrent
modification, the iterator fails quickly and cleanly, rather than risking
arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed
as it is, generally speaking, impossible to make any hard guarantees in the
presence of unsynchronized concurrent modification. Fail-fast iterators
throw ConcurrentModificationException
on a best-effort basis.
Therefore, it would be wrong to write a program that depended on this
exception for its correctness: the fail-fast behavior of iterators
should be used only to detect bugs.
This class is a member of the Java Collections Framework.
This is copied directly from JDK 6 to get a version with the fix for but 6425537.
- Since:
- 1.2
- Version:
- %I%, %G%
- See Also:
-
Nested Class Summary
Nested ClassesModifier and TypeClassDescriptionprivate static class
The entries in this hash table extend WeakReference, using its main ref field as the key.private class
private class
private class
private class
private class
private class
private class
Nested classes/interfaces inherited from class java.util.AbstractMap
AbstractMap.SimpleEntry<K,
V>, AbstractMap.SimpleImmutableEntry<K, V> -
Field Summary
FieldsModifier and TypeFieldDescriptionprivate static final int
The default initial capacity -- MUST be a power of two.private static final float
The load fast used when none specified in constructor.private final float
The load factor for the hash table.private static final int
The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.private int
The number of times this WeakHashMap has been structurally modified.private Collection
<V> private static final Object
Value representing null keys inside tables.private final ReferenceQueue
<K> Reference queue for cleared WeakEntriesprivate int
The number of key-value mappings contained in this weak hash map.private WeakHashMapSafeReadLock.Entry[]
The table, resized as necessary.private int
The next size value at which to resize (capacity * load factor). -
Constructor Summary
ConstructorsConstructorDescriptionConstructs a new, emptyWeakHashMap
with the default initial capacity (16) and load factor (0.75).WeakHashMapSafeReadLock
(int initialCapacity) Constructs a new, emptyWeakHashMap
with the given initial capacity and the default load factor (0.75).WeakHashMapSafeReadLock
(int initialCapacity, float loadFactor) Constructs a new, emptyWeakHashMap
with the given initial capacity and the given load factor.WeakHashMapSafeReadLock
(Map<? extends K, ? extends V> m) Constructs a newWeakHashMap
with the same mappings as the specified map. -
Method Summary
Modifier and TypeMethodDescriptionvoid
clear()
Removes all of the mappings from this map.(package private) static int
computeHash
(int h) boolean
containsKey
(Object key) Returnstrue
if this map contains a mapping for the specified key.private boolean
Special-case code for containsValue with null argumentboolean
containsValue
(Object value) Returnstrue
if this map maps one or more keys to the specified value.entrySet()
Returns aSet
view of the mappings contained in this map.(package private) static boolean
Checks for equality of non-null reference x and possibly-null y.private void
Expunges stale entries from the table.Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.(package private) WeakHashMapSafeReadLock.Entry
<K, V> Returns the entry associated with the specified key in this map.private WeakHashMapSafeReadLock.Entry[]
getTable()
Returns the table after first expunging stale entries.(package private) static int
indexFor
(int h, int length) Returns index for hash code h.boolean
isEmpty()
Returnstrue
if this map contains no key-value mappings.keySet()
Returns aSet
view of the keys contained in this map.private static Object
Use NULL_KEY for key if it is null.Associates the specified value with the specified key in this map.void
Copies all of the mappings from the specified map to this map.Removes the mapping for a key from this weak hash map if it is present.(package private) WeakHashMapSafeReadLock.Entry
<K, V> Special version of remove needed by Entry set(package private) void
resize
(int newCapacity) Rehashes the contents of this map into a new array with a larger capacity.int
size()
Returns the number of key-value mappings in this map.private void
transfer
(WeakHashMapSafeReadLock.Entry[] src, WeakHashMapSafeReadLock.Entry[] dest) Transfers all entries from src to dest tablesprivate static <K> K
unmaskNull
(Object key) Returns internal representation of null key back to caller as null.values()
Returns aCollection
view of the values contained in this map.Methods inherited from class java.util.AbstractMap
clone, equals, hashCode, toString
Methods inherited from class java.lang.Object
finalize, getClass, notify, notifyAll, wait, wait, wait
Methods inherited from interface java.util.Map
compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll
-
Field Details
-
DEFAULT_INITIAL_CAPACITY
private static final int DEFAULT_INITIAL_CAPACITYThe default initial capacity -- MUST be a power of two.- See Also:
-
MAXIMUM_CAPACITY
private static final int MAXIMUM_CAPACITYThe maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments. MUST be a power of two invalid input: '<'= 1invalid input: '<'invalid input: '<'30.- See Also:
-
DEFAULT_LOAD_FACTOR
private static final float DEFAULT_LOAD_FACTORThe load fast used when none specified in constructor.- See Also:
-
table
The table, resized as necessary. Length MUST Always be a power of two. -
size
private int sizeThe number of key-value mappings contained in this weak hash map. -
threshold
private int thresholdThe next size value at which to resize (capacity * load factor). -
loadFactor
private final float loadFactorThe load factor for the hash table. -
queue
Reference queue for cleared WeakEntries -
modCount
private volatile int modCountThe number of times this WeakHashMap has been structurally modified. Structural modifications are those that change the number of mappings in the map or otherwise modify its internal structure (e.g., rehash). This field is used to make iterators on Collection-views of the map fail-fast.- See Also:
-
NULL_KEY
Value representing null keys inside tables. -
entrySet
-
myKeySet
-
myValues
-
-
Constructor Details
-
WeakHashMapSafeReadLock
public WeakHashMapSafeReadLock(int initialCapacity, float loadFactor) Constructs a new, emptyWeakHashMap
with the given initial capacity and the given load factor.- Parameters:
initialCapacity
- The initial capacity of theWeakHashMap
loadFactor
- The load factor of theWeakHashMap
- Throws:
IllegalArgumentException
- if the initial capacity is negative, or if the load factor is nonpositive.
-
WeakHashMapSafeReadLock
public WeakHashMapSafeReadLock(int initialCapacity) Constructs a new, emptyWeakHashMap
with the given initial capacity and the default load factor (0.75).- Parameters:
initialCapacity
- The initial capacity of theWeakHashMap
- Throws:
IllegalArgumentException
- if the initial capacity is negative
-
WeakHashMapSafeReadLock
public WeakHashMapSafeReadLock()Constructs a new, emptyWeakHashMap
with the default initial capacity (16) and load factor (0.75). -
WeakHashMapSafeReadLock
Constructs a newWeakHashMap
with the same mappings as the specified map. TheWeakHashMap
is created with the default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified map.- Parameters:
m
- the map whose mappings are to be placed in this map- Throws:
NullPointerException
- if the specified map is null- Since:
- 1.3
-
-
Method Details
-
maskNull
Use NULL_KEY for key if it is null. -
unmaskNull
Returns internal representation of null key back to caller as null. -
eq
Checks for equality of non-null reference x and possibly-null y. By default uses Object.equals. -
indexFor
static int indexFor(int h, int length) Returns index for hash code h. -
expungeStaleEntries
private void expungeStaleEntries()Expunges stale entries from the table. -
getTable
Returns the table after first expunging stale entries. -
size
public int size()Returns the number of key-value mappings in this map. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced. -
isEmpty
public boolean isEmpty()Returnstrue
if this map contains no key-value mappings. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced. -
computeHash
static int computeHash(int h) -
get
Returns the value to which the specified key is mapped, ornull
if this map contains no mapping for the key.More formally, if this map contains a mapping from a key
k
to a valuev
such that(key==null ? k==null : key.equals(k))
, then this method returnsv
; otherwise it returnsnull
. (There can be at most one such mapping.)A return value of
null
does not necessarily indicate that the map contains no mapping for the key; it's also possible that the map explicitly maps the key tonull
. ThecontainsKey
operation may be used to distinguish these two cases. -
containsKey
Returnstrue
if this map contains a mapping for the specified key.- Specified by:
containsKey
in interfaceMap<K,
V> - Overrides:
containsKey
in classAbstractMap<K,
V> - Parameters:
key
- The key whose presence in this map is to be tested- Returns:
true
if there is a mapping forkey
;false
otherwise
-
getEntry
Returns the entry associated with the specified key in this map. Returns null if the map contains no mapping for this key. -
put
Associates the specified value with the specified key in this map. If the map previously contained a mapping for this key, the old value is replaced.- Specified by:
put
in interfaceMap<K,
V> - Overrides:
put
in classAbstractMap<K,
V> - Parameters:
key
- key with which the specified value is to be associated.value
- value to be associated with the specified key.- Returns:
- the previous value associated with
key
, ornull
if there was no mapping forkey
. (Anull
return can also indicate that the map previously associatednull
withkey
.)
-
resize
void resize(int newCapacity) Rehashes the contents of this map into a new array with a larger capacity. This method is called automatically when the number of keys in this map reaches its threshold. If current capacity is MAXIMUM_CAPACITY, this method does not resize the map, but sets threshold to Integer.MAX_VALUE. This has the effect of preventing future calls.- Parameters:
newCapacity
- the new capacity, MUST be a power of two; must be greater than current capacity unless current capacity is MAXIMUM_CAPACITY (in which case value is irrelevant).
-
transfer
Transfers all entries from src to dest tables -
putAll
Copies all of the mappings from the specified map to this map. These mappings will replace any mappings that this map had for any of the keys currently in the specified map.- Specified by:
putAll
in interfaceMap<K,
V> - Overrides:
putAll
in classAbstractMap<K,
V> - Parameters:
m
- mappings to be stored in this map.- Throws:
NullPointerException
- if the specified map is null.
-
remove
Removes the mapping for a key from this weak hash map if it is present. More formally, if this map contains a mapping from keyk
to valuev
such that(key==null ? k==null : key.equals(k))
, that mapping is removed. (The map can contain at most one such mapping.)Returns the value to which this map previously associated the key, or
null
if the map contained no mapping for the key. A return value ofnull
does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key tonull
.The map will not contain a mapping for the specified key once the call returns.
-
removeMapping
Special version of remove needed by Entry set -
clear
public void clear()Removes all of the mappings from this map. The map will be empty after this call returns. -
containsValue
Returnstrue
if this map maps one or more keys to the specified value.- Specified by:
containsValue
in interfaceMap<K,
V> - Overrides:
containsValue
in classAbstractMap<K,
V> - Parameters:
value
- value whose presence in this map is to be tested- Returns:
true
if this map maps one or more keys to the specified value
-
containsNullValue
private boolean containsNullValue()Special-case code for containsValue with null argument -
keySet
Returns aSet
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. If the map is modified while an iteration over the set is in progress (except through the iterator's ownremove
operation), the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
, andclear
operations. It does not support theadd
oraddAll
operations. -
values
Returns aCollection
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. If the map is modified while an iteration over the collection is in progress (except through the iterator's ownremove
operation), the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Collection.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations. -
entrySet
Returns aSet
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. If the map is modified while an iteration over the set is in progress (except through the iterator's ownremove
operation, or through thesetValue
operation on a map entry returned by the iterator) the results of the iteration are undefined. The set supports element removal, which removes the corresponding mapping from the map, via theIterator.remove
,Set.remove
,removeAll
,retainAll
andclear
operations. It does not support theadd
oraddAll
operations.
-