Class MapMaker
- java.lang.Object
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- com.google.common.collect.MapMaker
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@GwtCompatible(emulated=true) public final class MapMaker extends java.lang.Object
A builder of
ConcurrentMap
instances having any combination of the following features:- keys or values automatically wrapped in weak or soft references
- notification of evicted (or otherwise removed) entries
Usage example:
ConcurrentMap<Request, Stopwatch> timers = new MapMaker() .concurrencyLevel(4) .weakKeys() .makeMap();
These features are all optional;
new MapMaker().makeMap()
returns a valid concurrent map that behaves similarly to aConcurrentHashMap
.The returned map is implemented as a hash table with similar performance characteristics to
ConcurrentHashMap
. It supports all optional operations of theConcurrentMap
interface. It does not permit null keys or values.Note: by default, the returned map uses equality comparisons (the
equals
method) to determine equality for keys or values. However, ifweakKeys()
was specified, the map uses identity (==
) comparisons instead for keys. Likewise, ifweakValues()
orsoftValues()
was specified, the map uses identity comparisons for values.The view collections of the returned map have weakly consistent iterators. This means that they are safe for concurrent use, but if other threads modify the map after the iterator is created, it is undefined which of these changes, if any, are reflected in that iterator. These iterators never throw
ConcurrentModificationException
.If
weakKeys()
,weakValues()
, orsoftValues()
are requested, it is possible for a key or value present in the map to be reclaimed by the garbage collector. Entries with reclaimed keys or values may be removed from the map on each map modification or on occasional map accesses; such entries may be counted byMap.size()
, but will never be visible to read or write operations. A partially-reclaimed entry is never exposed to the user. AnyMap.Entry
instance retrieved from the map's entry set is a snapshot of that entry's state at the time of retrieval; such entries do, however, supportMap.Entry.setValue(V)
, which simply callsMap.put(K, V)
on the entry's key.The maps produced by
MapMaker
are serializable, and the deserialized maps retain all the configuration properties of the original map. During deserialization, if the original map had used soft or weak references, the entries are reconstructed as they were, but it's not unlikely they'll be quickly garbage-collected before they are ever accessed.new MapMaker().weakKeys().makeMap()
is a recommended replacement forWeakHashMap
, but note that it compares keys using object identity whereasWeakHashMap
usesObject.equals(java.lang.Object)
.- Since:
- 2.0
- Author:
- Bob Lee, Charles Fry, Kevin Bourrillion
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Constructor Summary
Constructors Constructor Description MapMaker()
Constructs a newMapMaker
instance with default settings, including strong keys, strong values, and no automatic eviction of any kind.
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Method Summary
All Methods Instance Methods Concrete Methods Deprecated Methods Modifier and Type Method Description MapMaker
concurrencyLevel(int concurrencyLevel)
Guides the allowed concurrency among update operations.MapMaker
initialCapacity(int initialCapacity)
Sets the minimum total size for the internal hash tables.<K,V>
java.util.concurrent.ConcurrentMap<K,V>makeComputingMap(Function<? super K,? extends V> computingFunction)
Deprecated.Caching functionality inMapMaker
has been moved toCacheBuilder
, withmakeComputingMap(com.google.common.base.Function<? super K, ? extends V>)
being replaced byCacheBuilder.build(com.google.common.cache.CacheLoader<? super K1, V1>)
.<K,V>
java.util.concurrent.ConcurrentMap<K,V>makeMap()
Builds a thread-safe map.java.lang.String
toString()
Returns a string representation for this MapMaker instance.MapMaker
weakKeys()
Specifies that each key (not value) stored in the map should be wrapped in aWeakReference
(by default, strong references are used).MapMaker
weakValues()
Specifies that each value (not key) stored in the map should be wrapped in aWeakReference
(by default, strong references are used).
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Method Detail
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initialCapacity
public MapMaker initialCapacity(int initialCapacity)
Sets the minimum total size for the internal hash tables. For example, if the initial capacity is60
, and the concurrency level is8
, then eight segments are created, each having a hash table of size eight. Providing a large enough estimate at construction time avoids the need for expensive resizing operations later, but setting this value unnecessarily high wastes memory.- Throws:
java.lang.IllegalArgumentException
- ifinitialCapacity
is negativejava.lang.IllegalStateException
- if an initial capacity was already set
-
concurrencyLevel
public MapMaker concurrencyLevel(int concurrencyLevel)
Guides the allowed concurrency among update operations. Used as a hint for internal sizing. The table is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because assignment of entries to these partitions is not necessarily uniform, the actual concurrency observed may vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the table. Using a significantly higher value than you need can waste space and time, and a significantly lower value can lead to thread contention. But overestimates and underestimates within an order of magnitude do not usually have much noticeable impact. A value of one permits only one thread to modify the map at a time, but since read operations can proceed concurrently, this still yields higher concurrency than full synchronization. Defaults to 4.Note: Prior to Guava release 9.0, the default was 16. It is possible the default will change again in the future. If you care about this value, you should always choose it explicitly.
- Throws:
java.lang.IllegalArgumentException
- ifconcurrencyLevel
is nonpositivejava.lang.IllegalStateException
- if a concurrency level was already set
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weakKeys
@GwtIncompatible("java.lang.ref.WeakReference") public MapMaker weakKeys()
Specifies that each key (not value) stored in the map should be wrapped in aWeakReference
(by default, strong references are used).Warning: when this method is used, the resulting map will use identity (
==
) comparison to determine equality of keys, which is a technical violation of theMap
specification, and may not be what you expect.- Throws:
java.lang.IllegalStateException
- if the key strength was already set- See Also:
WeakReference
-
weakValues
@GwtIncompatible("java.lang.ref.WeakReference") public MapMaker weakValues()
Specifies that each value (not key) stored in the map should be wrapped in aWeakReference
(by default, strong references are used).Weak values will be garbage collected once they are weakly reachable. This makes them a poor candidate for caching; consider
softValues()
instead.Warning: when this method is used, the resulting map will use identity (
==
) comparison to determine equality of values. This technically violates the specifications of the methodscontainsValue
,remove(Object, Object)
andreplace(K, V, V)
, and may not be what you expect.- Throws:
java.lang.IllegalStateException
- if the value strength was already set- See Also:
WeakReference
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makeMap
public <K,V> java.util.concurrent.ConcurrentMap<K,V> makeMap()
Builds a thread-safe map. This method does not alter the state of thisMapMaker
instance, so it can be invoked again to create multiple independent maps.The bulk operations
putAll
,equals
, andclear
are not guaranteed to be performed atomically on the returned map. Additionally,size
andcontainsValue
are implemented as bulk read operations, and thus may fail to observe concurrent writes.- Returns:
- a serializable concurrent map having the requested features
-
makeComputingMap
@Deprecated public <K,V> java.util.concurrent.ConcurrentMap<K,V> makeComputingMap(Function<? super K,? extends V> computingFunction)
Deprecated.Caching functionality inMapMaker
has been moved toCacheBuilder
, withmakeComputingMap(com.google.common.base.Function<? super K, ? extends V>)
being replaced byCacheBuilder.build(com.google.common.cache.CacheLoader<? super K1, V1>)
. See the MapMaker Migration Guide for more details.Builds a map that supports atomic, on-demand computation of values.Map.get(java.lang.Object)
either returns an already-computed value for the given key, atomically computes it using the supplied function, or, if another thread is currently computing the value for this key, simply waits for that thread to finish and returns its computed value. Note that the function may be executed concurrently by multiple threads, but only for distinct keys.New code should use
CacheBuilder
, which supports statistics collection, introduces theCacheLoader
interface for loading entries into the cache (allowing checked exceptions to be thrown in the process), and more cleanly separates computation from the cache'sMap
view.If an entry's value has not finished computing yet, query methods besides
get
return immediately as if an entry doesn't exist. In other words, an entry isn't externally visible until the value's computation completes.Map.get(java.lang.Object)
on the returned map will never returnnull
. It may throw:NullPointerException
if the key is null or the computing function returns a null resultComputationException
if an exception was thrown by the computing function. If that exception is already of typeComputationException
it is propagated directly; otherwise it is wrapped.
Note: Callers of
get
must ensure that the key argument is of typeK
. Theget
method acceptsObject
, so the key type is not checked at compile time. Passing an object of a type other thanK
can result in that object being unsafely passed to the computing function as typeK
, and unsafely stored in the map.If
Map.put(K, V)
is called before a computation completes, other threads waiting on the computation will wake up and return the stored value.This method does not alter the state of this
MapMaker
instance, so it can be invoked again to create multiple independent maps.Insertion, removal, update, and access operations on the returned map safely execute concurrently by multiple threads. Iterators on the returned map are weakly consistent, returning elements reflecting the state of the map at some point at or since the creation of the iterator. They do not throw
ConcurrentModificationException
, and may proceed concurrently with other operations.The bulk operations
putAll
,equals
, andclear
are not guaranteed to be performed atomically on the returned map. Additionally,size
andcontainsValue
are implemented as bulk read operations, and thus may fail to observe concurrent writes.- Parameters:
computingFunction
- the function used to compute new values- Returns:
- a serializable concurrent map having the requested features
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toString
public java.lang.String toString()
Returns a string representation for this MapMaker instance. The exact form of the returned string is not specificed.- Overrides:
toString
in classjava.lang.Object
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