Package gw.util.concurrent

Class ConcurrentWeakHashMap<K,​V>

  • Type Parameters:
    K - the type of keys maintained by this map
    V - the type of mapped values
    All Implemented Interfaces:
    java.io.Serializable, java.util.concurrent.ConcurrentMap<K,​V>, java.util.Map<K,​V>
    public class ConcurrentWeakHashMap<K,​V>
extends java.util.AbstractMap<K,​V>
implements java.util.concurrent.ConcurrentMap<K,​V>, java.io.Serializable
    A hash table with weak keys, full concurrency of retrievals, and adjustable expected concurrency for updates. Similar to WeakHashMap, entries of this table are periodically removed once their corresponding keys are no longer referenced outside of this table. In other words, this table 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 table; however, the entry may occupy space until a future table 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.

    While keys in this table are only held using a weak reference, values are held using a normal strong reference. This provides the guarantee that a value will always have at least the same life-span as it's 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 weak values are desired, one can simply use a WeakReference for the value type.

    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 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.

    Retrieval operations (including get) generally do not block, so 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 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.

    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 table 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 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 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 table is a relatively slow operation, so, when possible, it is a good idea to provide estimates of expected table 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.

    This class is a member of the Java Collections Framework.

    See Also:
    Serialized Form

    • Field Summary

      Fields 
      Modifier and Type Field Description
      (package private) static int DEFAULT_CONCURRENCY_LEVEL
      The default concurrency level for this table, used when not otherwise specified in a constructor.
      (package private) static int DEFAULT_INITIAL_CAPACITY
      The default initial capacity for this table, used when not otherwise specified in a constructor.
      (package private) static float DEFAULT_LOAD_FACTOR
      The default load factor for this table, used when not otherwise specified in a constructor.
      (package private) java.util.Set<java.util.Map.Entry<K,​V>> entrySet  
      (package private) java.util.Set<K> keySet  
      (package private) static int MAX_SEGMENTS
      The maximum number of segments to allow; used to bound constructor arguments.
      (package private) static int MAXIMUM_CAPACITY
      The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.
      (package private) static int RETRIES_BEFORE_LOCK
      Number of unsynchronized retries in size and containsValue methods before resorting to locking.
      (package private) int segmentMask
      Mask value for indexing into segments.
      (package private) ConcurrentWeakHashMap.Segment<K,​V>[] segments
      The segments, each of which is a specialized hash table
      (package private) int segmentShift
      Shift value for indexing within segments.
      private static long serialVersionUID  
      (package private) java.util.Collection<V> values  

    • Constructor Summary

      Constructors 
      Constructor Description
      ConcurrentWeakHashMap()
      Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).
      ConcurrentWeakHashMap​(int initialCapacity)
      Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).
      ConcurrentWeakHashMap​(int initialCapacity, float loadFactor)
      Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).
      ConcurrentWeakHashMap​(int initialCapacity, float loadFactor, int concurrencyLevel)
      Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
      ConcurrentWeakHashMap​(java.util.Map<? extends K,​? extends V> m)
      Creates a new map with the same mappings as the given map.

    • Method Summary

      All Methods Static Methods Instance Methods Concrete Methods 
      Modifier and Type Method Description
      void clear()
      Removes all of the mappings from this map.
      boolean contains​(java.lang.Object value)
      Legacy method testing if some key maps into the specified value in this table.
      boolean containsKey​(java.lang.Object key)
      Tests if the specified object is a key in this table.
      boolean containsValue​(java.lang.Object value)
      Returns true if this map maps one or more keys to the specified value.
      java.util.Enumeration<V> elements()
      Returns an enumeration of the values in this table.
      java.util.Set<java.util.Map.Entry<K,​V>> entrySet()
      Returns a Set view of the mappings contained in this map.
      V get​(java.lang.Object key)
      Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
      private static int hash​(int h)
      Applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions.
      boolean isEmpty()
      Returns true if this map contains no key-value mappings.
      java.util.Enumeration<K> keys()
      Returns an enumeration of the keys in this table.
      java.util.Set<K> keySet()
      Returns a Set view of the keys contained in this map.
      V put​(K key, V value)
      Maps the specified key to the specified value in this table.
      void putAll​(java.util.Map<? extends K,​? extends V> m)
      Copies all of the mappings from the specified map to this one.
      V putIfAbsent​(K key, V value)
      private void readObject​(java.io.ObjectInputStream s)
      Reconstitute the ConcurrentWeakHashMap instance from a stream (i.e., deserialize it).
      V remove​(java.lang.Object key)
      Removes the key (and its corresponding value) from this map.
      boolean remove​(java.lang.Object key, java.lang.Object value)
      V replace​(K key, V value)
      boolean replace​(K key, V oldValue, V newValue)
      (package private) ConcurrentWeakHashMap.Segment<K,​V> segmentFor​(int hash)
      Returns the segment that should be used for key with given hash
      int size()
      Returns the number of key-value mappings in this map.
      java.util.Collection<V> values()
      Returns a Collection view of the values contained in this map.
      private void writeObject​(java.io.ObjectOutputStream s)
      Save the state of the ConcurrentWeakHashMap instance to a stream (i.e., serialize it).

      • 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.concurrent.ConcurrentMap

        compute, computeIfAbsent, computeIfPresent, forEach, getOrDefault, merge, replaceAll

      • Methods inherited from interface java.util.Map

        equals, hashCode

    • Field Detail

      • DEFAULT_INITIAL_CAPACITY

        static final int DEFAULT_INITIAL_CAPACITY
        The default initial capacity for this table, used when not otherwise specified in a constructor.
        See Also:
        Constant Field Values

      • DEFAULT_LOAD_FACTOR

        static final float DEFAULT_LOAD_FACTOR
        The default load factor for this table, used when not otherwise specified in a constructor.
        See Also:
        Constant Field Values

      • DEFAULT_CONCURRENCY_LEVEL

        static final int DEFAULT_CONCURRENCY_LEVEL
        The default concurrency level for this table, used when not otherwise specified in a constructor.
        See Also:
        Constant Field Values

      • MAXIMUM_CAPACITY

        static final int MAXIMUM_CAPACITY
        The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments. MUST be a power of two <= 1<<30 to ensure that entries are indexable using ints.
        See Also:
        Constant Field Values

      • MAX_SEGMENTS

        static final int MAX_SEGMENTS
        The maximum number of segments to allow; used to bound constructor arguments.
        See Also:
        Constant Field Values

      • RETRIES_BEFORE_LOCK

        static final int RETRIES_BEFORE_LOCK
        Number of unsynchronized retries in size and containsValue methods before resorting to locking. This is used to avoid unbounded retries if tables undergo continuous modification which would make it impossible to obtain an accurate result.
        See Also:
        Constant Field Values

      • segmentMask

        final int segmentMask
        Mask value for indexing into segments. The upper bits of a key's hash code are used to choose the segment.

      • segmentShift

        final int segmentShift
        Shift value for indexing within segments.

      • keySet

        transient java.util.Set<K> keySet

      • entrySet

        transient java.util.Set<java.util.Map.Entry<K,​V>> entrySet

      • values

        transient java.util.Collection<V> values

    • Constructor Detail

      • ConcurrentWeakHashMap

        public ConcurrentWeakHashMap​(int initialCapacity,
                             float loadFactor,
                             int concurrencyLevel)
        Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
        Parameters:
        initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.
        loadFactor - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.
        concurrencyLevel - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
        Throws:
        java.lang.IllegalArgumentException - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.

      • ConcurrentWeakHashMap

        public ConcurrentWeakHashMap​(int initialCapacity,
                             float loadFactor)
        Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).
        Parameters:
        initialCapacity - The implementation performs internal sizing to accommodate this many elements.
        loadFactor - the load factor threshold, used to control resizing. Resizing may be performed when the average number of elements per bin exceeds this threshold.
        Throws:
        java.lang.IllegalArgumentException - if the initial capacity of elements is negative or the load factor is nonpositive
        Since:
        1.6

      • ConcurrentWeakHashMap

        public ConcurrentWeakHashMap​(int initialCapacity)
        Creates a new, empty map with the specified initial capacity, and with default load factor (0.75) and concurrencyLevel (16).
        Parameters:
        initialCapacity - the initial capacity. The implementation performs internal sizing to accommodate this many elements.
        Throws:
        java.lang.IllegalArgumentException - if the initial capacity of elements is negative.

      • ConcurrentWeakHashMap

        public ConcurrentWeakHashMap()
        Creates a new, empty map with a default initial capacity (16), load factor (0.75) and concurrencyLevel (16).

      • ConcurrentWeakHashMap

        public ConcurrentWeakHashMap​(java.util.Map<? extends K,​? extends V> m)
        Creates a new map with the same mappings as the given map. The map is created with a capacity of 1.5 times the number of mappings in the given map or 16 (whichever is greater), and a default load factor (0.75) and concurrencyLevel (16).
        Parameters:
        m - the map

    • Method Detail

      • hash

        private static int hash​(int h)
        Applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions. This is critical because ConcurrentWeakHashMap uses power-of-two length hash tables, that otherwise encounter collisions for hashCodes that do not differ in lower or upper bits.

      • segmentFor

        final ConcurrentWeakHashMap.Segment<K,​V> segmentFor​(int hash)
        Returns the segment that should be used for key with given hash
        Parameters:
        hash - the hash code for the key
        Returns:
        the segment

      • isEmpty

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

      • 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 java.util.Map<K,​V>
        Overrides:
        size in class java.util.AbstractMap<K,​V>
        Returns:
        the number of key-value mappings in this map

      • get

        public V get​(java.lang.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 java.util.Map<K,​V>
        Overrides:
        get in class java.util.AbstractMap<K,​V>
        Throws:
        java.lang.NullPointerException - if the specified key is null

      • containsKey

        public boolean containsKey​(java.lang.Object key)
        Tests if the specified object is a key in this table.
        Specified by:
        containsKey in interface java.util.Map<K,​V>
        Overrides:
        containsKey in class java.util.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:
        java.lang.NullPointerException - if the specified key is null

      • containsValue

        public boolean containsValue​(java.lang.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, and so is much slower than method containsKey.
        Specified by:
        containsValue in interface java.util.Map<K,​V>
        Overrides:
        containsValue in class java.util.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:
        java.lang.NullPointerException - if the specified value is null

      • contains

        public boolean contains​(java.lang.Object value)
        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:
        value - a value to search for
        Returns:
        true if and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
        Throws:
        java.lang.NullPointerException - if the specified value is null

      • 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 java.util.Map<K,​V>
        Overrides:
        put in class java.util.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:
        java.lang.NullPointerException - if the specified key or value is null

      • putIfAbsent

        public V putIfAbsent​(K key,
                     V value)
        Specified by:
        putIfAbsent in interface java.util.concurrent.ConcurrentMap<K,​V>
        Specified by:
        putIfAbsent in interface java.util.Map<K,​V>
        Returns:
        the previous value associated with the specified key, or null if there was no mapping for the key
        Throws:
        java.lang.NullPointerException - if the specified key or value is null

      • putAll

        public void putAll​(java.util.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 java.util.Map<K,​V>
        Overrides:
        putAll in class java.util.AbstractMap<K,​V>
        Parameters:
        m - mappings to be stored in this map

      • remove

        public V remove​(java.lang.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 java.util.Map<K,​V>
        Overrides:
        remove in class java.util.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:
        java.lang.NullPointerException - if the specified key is null

      • remove

        public boolean remove​(java.lang.Object key,
                      java.lang.Object value)
        Specified by:
        remove in interface java.util.concurrent.ConcurrentMap<K,​V>
        Specified by:
        remove in interface java.util.Map<K,​V>
        Throws:
        java.lang.NullPointerException - if the specified key is null

      • replace

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

      • replace

        public V replace​(K key,
                 V value)
        Specified by:
        replace in interface java.util.concurrent.ConcurrentMap<K,​V>
        Specified by:
        replace in interface java.util.Map<K,​V>
        Returns:
        the previous value associated with the specified key, or null if there was no mapping for the key
        Throws:
        java.lang.NullPointerException - if the specified key or value is null

      • clear

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

      • keySet

        public java.util.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 java.util.Map<K,​V>
        Overrides:
        keySet in class java.util.AbstractMap<K,​V>

      • values

        public java.util.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 java.util.Map<K,​V>
        Overrides:
        values in class java.util.AbstractMap<K,​V>

      • entrySet

        public java.util.Set<java.util.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 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:
        entrySet in interface java.util.Map<K,​V>
        Specified by:
        entrySet in class java.util.AbstractMap<K,​V>

      • keys

        public java.util.Enumeration<K> keys()
        Returns an enumeration of the keys in this table.
        Returns:
        an enumeration of the keys in this table
        See Also:
        keySet()

      • elements

        public java.util.Enumeration<V> elements()
        Returns an enumeration of the values in this table.
        Returns:
        an enumeration of the values in this table
        See Also:
        values()

      • writeObject

        private void writeObject​(java.io.ObjectOutputStream s)
                  throws java.io.IOException
        Save the state of the ConcurrentWeakHashMap instance to a stream (i.e., serialize it).
        Parameters:
        s - the stream
        Throws:
        java.io.IOException

      • readObject

        private void readObject​(java.io.ObjectInputStream s)
                 throws java.io.IOException,
                        java.lang.ClassNotFoundException
        Reconstitute the ConcurrentWeakHashMap instance from a stream (i.e., deserialize it).
        Parameters:
        s - the stream
        Throws:
        java.io.IOException
        java.lang.ClassNotFoundException