Package org.apache.commons.collections4.map

Class ConcurrentReferenceHashMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
org.apache.commons.collections4.map.ConcurrentReferenceHashMap<K,V>
Type Parameters:
K - the type of keys maintained by this map.
V - the type of mapped values.
All Implemented Interfaces:
ConcurrentMap<K,V>, Map<K,V>
public class ConcurrentReferenceHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>
An advanced hash map supporting configurable garbage collection semantics of keys and values, optional referential-equality, full concurrency of retrievals, and adjustable expected concurrency for updates.

This map is designed around specific advanced use-cases. If there is any doubt whether this map is for you, you most likely should be using ConcurrentHashMap instead.

This map supports strong, weak, and soft keys and values. By default, keys are weak, and values are strong. Such a configuration offers similar behavior to WeakHashMap, entries of this map are periodically removed once their corresponding keys are no longer referenced outside of this map. In other words, this map will not prevent a key from being discarded by the garbage collector. Once a key has been discarded by the collector, the corresponding entry is no longer visible to this map; however, the entry may occupy space until a future map operation decides to reclaim it. For this reason, summary functions such as size and isEmpty might return a value greater than the observed number of entries. In order to support a high level of concurrency, stale entries are only reclaimed during blocking (usually mutating) operations.

Enabling soft keys allows entries in this map to remain until their space is absolutely needed by the garbage collector. This is unlike weak keys which can be reclaimed as soon as they are no longer referenced by a normal strong reference. The primary use case for soft keys is a cache, which ideally occupies memory that is not in use for as long as possible.

By default, values are held using a normal strong reference. This provides the commonly desired guarantee that a value will always have at least the same life-span as its key. For this reason, care should be taken to ensure that a value never refers, either directly or indirectly, to its key, thereby preventing reclamation. If this is unavoidable, then it is recommended to use the same reference type in use for the key. However, it should be noted that non-strong values may disappear before their corresponding key.

While this map does allow the use of both strong keys and values, it is recommended you use ConcurrentHashMap for such a configuration, since it is optimized for that case.

Just like ConcurrentHashMap, 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, retrieval operations do not entail locking, and there is not any support for locking the entire map 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.

Retrieval operations (including get) generally do not block, so they may overlap with update operations (including put and remove). Retrievals reflect the results of the most recently completed update operations holding upon their onset. For aggregate operations such as putAll and clear, concurrent retrievals may reflect insertion or removal of only some entries. Similarly, Iterators and Enumerations return elements reflecting the state of the hash map 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.

The allowed concurrency among update operations is guided by the optional concurrencyLevel constructor argument (default 16), which is used as a hint for internal sizing. The map is internally partitioned to try to permit the indicated number of concurrent updates without contention. Because placement in hash tables is essentially random, the actual concurrency will vary. Ideally, you should choose a value to accommodate as many threads as will ever concurrently modify the map. 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 is appropriate when it is known that only one thread will modify and all others will only read. Also, resizing this or any other kind of hash map is a relatively slow operation, so, when possible, it is a good idea that you provide estimates of expected map sizes in constructors.

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.

Provenance: Copied and edited from Apache Groovy git master at commit 77dc80a7512ceb2168b1bc866c3d0c69b002fe11; via Doug Lea, Jason T. Greene, with assistance from members of JCP JSR-166, and Hazelcast.

  • Method Details

    • builder

      public static <K, V> ConcurrentReferenceHashMap.Builder<K,V> builder()
      Creates a new Builder.

      By default, keys are weak, and values are strong.

      The default values are:

      Type Parameters:
      K - the type of keys.
      V - the type of values.
      Returns:
      a new Builder.

    • clear

      public void clear()
      Removes all of the mappings from this map.
      Specified by:
      clear in interface Map<K,V>
      Overrides:
      clear in class AbstractMap<K,V>

    • compute

      public V compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
      Specified by:
      compute in interface ConcurrentMap<K,V>
      Specified by:
      compute in interface Map<K,V>

    • computeIfAbsent

      public V computeIfAbsent(K key, Function<? super K,? extends V> mappingFunction)
      The default implementation is equivalent to the following steps for this map, then returning the current value or null if now absent: 
      
 if (map.get(key) == null) {
     V newValue = mappingFunction.apply(key);
     if (newValue != null)
         return map.putIfAbsent(key, newValue);
 }

      The default implementation may retry these steps when multiple threads attempt updates including potentially calling the mapping function multiple times.

      This implementation assumes that the ConcurrentMap cannot contain null values and get() returning null unambiguously means the key is absent. Implementations which support null values must override this default implementation.

      Specified by:
      computeIfAbsent in interface ConcurrentMap<K,V>
      Specified by:
      computeIfAbsent in interface Map<K,V>

    • computeIfPresent

      public V computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction)
      Specified by:
      computeIfPresent in interface ConcurrentMap<K,V>
      Specified by:
      computeIfPresent in interface Map<K,V>

    • containsKey

      public boolean containsKey(Object key)
      Tests if the specified object is a key in this table.
      Specified by:
      containsKey in interface Map<K,V>
      Overrides:
      containsKey in class AbstractMap<K,V>
      Parameters:
      key - possible key
      Returns:
      true if and only if the specified object is a key in this table, as determined by the equals method; false otherwise.
      Throws:
      NullPointerException - if the specified key is null

    • containsValue

      public boolean containsValue(Object value)
      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, therefore it is much slower than the method containsKey.
      Specified by:
      containsValue in interface Map<K,V>
      Overrides:
      containsValue in class AbstractMap<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
      Throws:
      NullPointerException - if the specified value is null

    • entrySet

      public Set<Map.Entry<K,V>> 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 is guaranteed 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:
      entrySet in interface Map<K,V>
      Specified by:
      entrySet in class AbstractMap<K,V>

    • get

      public V get(Object key)
      Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.

      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<K,V>
      Overrides:
      get in class AbstractMap<K,V>
      Throws:
      NullPointerException - if the specified key is null

    • isEmpty

      public boolean isEmpty()
      Returns true if this map contains no key-value mappings.
      Specified by:
      isEmpty in interface Map<K,V>
      Overrides:
      isEmpty in class AbstractMap<K,V>
      Returns:
      true if this map contains no key-value mappings

    • keySet

      public Set<K> 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<K,V>
      Overrides:
      keySet in class AbstractMap<K,V>

    • purgeStaleEntries

      public void purgeStaleEntries()
      Removes any stale entries whose keys have been finalized. Use of this method is normally not necessary since stale entries are automatically removed lazily, when blocking operations are required. However, there are some cases where this operation should be performed eagerly, such as cleaning up old references to a ClassLoader in a multi-classloader environment.

      Note: this method will acquire locks one at a time across all segments of this table, so this method should be used sparingly.

    • put

      public V put(K key, V value)
      Maps the specified key to the specified value in this table. Neither the key nor the value can be null.

      The value can be retrieved by calling the get method with a key that is equal to the original key.

      Specified by:
      put in interface Map<K,V>
      Overrides:
      put in class AbstractMap<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, or null if there was no mapping for key
      Throws:
      NullPointerException - if the specified key or value is null

    • putAll

      public void putAll(Map<? extends K,? extends V> m)
      Copies all of the mappings from the specified map to this one. These mappings replace any mappings that this map had for any of the keys currently in the specified map.
      Specified by:
      putAll in interface Map<K,V>
      Overrides:
      putAll in class AbstractMap<K,V>
      Parameters:
      m - mappings to be stored in this map

    • putIfAbsent

      public V putIfAbsent(K key, V value)
      Specified by:
      putIfAbsent in interface ConcurrentMap<K,V>
      Specified by:
      putIfAbsent in interface Map<K,V>
      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 V 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<K,V>
      Overrides:
      remove in class AbstractMap<K,V>
      Parameters:
      key - the key that needs to be removed
      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 value)
      Specified by:
      remove in interface ConcurrentMap<K,V>
      Specified by:
      remove in interface Map<K,V>
      Throws:
      NullPointerException - if the specified key is null

    • replace

      public V replace(K key, V value)
      Specified by:
      replace in interface ConcurrentMap<K,V>
      Specified by:
      replace in interface Map<K,V>
      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

    • replace

      public boolean replace(K key, V oldValue, V newValue)
      Specified by:
      replace in interface ConcurrentMap<K,V>
      Specified by:
      replace in interface Map<K,V>
      Throws:
      NullPointerException - if any of the arguments are null

    • size

      public int size()
      Returns the number of key-value mappings in this map. If the map contains more than Integer.MAX_VALUE elements, returns Integer.MAX_VALUE.
      Specified by:
      size in interface Map<K,V>
      Overrides:
      size in class AbstractMap<K,V>
      Returns:
      the number of key-value mappings in this map

    • values

      public Collection<V> 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<K,V>
      Overrides:
      values in class AbstractMap<K,V>