Package org.jsr166

Class ConcurrentLinkedHashMap<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>
    Direct Known Subclasses:
    GridBoundedConcurrentLinkedHashMap
    public class ConcurrentLinkedHashMap<K,​V>
extends AbstractMap<K,​V>
implements ConcurrentMap<K,​V>
    A hash table supporting full concurrency of retrievals and adjustable expected concurrency for updates. 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 implementation differs from HashMap in that it maintains a doubly-linked list running through all of its entries. This linked list defines the iteration ordering, which is the order in which keys were inserted into the map (insertion-order).

    NOTE: Access order is not supported by this map. Note that insertion order is not affected if a key is re-inserted into the map. (A key k is reinserted into a map m if m.put(k, v) is invoked when m.containsKey(k) would return true immediately prior to the invocation.)

    An optional maxCap may be passed to the map constructor to create bounded map that will remove stale mappings automatically when new mappings are added to the map.

    When iterating over the key set in insertion order one should note that iterator will see all removes done since the iterator was created, but will see no inserts to map.

    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.

    • Field Detail

      • DFLT_INIT_CAP

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

      • DFLT_LOAD_FACTOR

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

      • DFLT_CONCUR_LVL

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

      • MAX_CAP_LIMIT

        public static final int MAX_CAP_LIMIT
        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_SEGS

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

      • RETRIES_BEFORE_LOCK

        public static final int RETRIES_BEFORE_LOCK
        Number of unsynchronized retries in size and containsValue(java.lang.Object) 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

    • Constructor Detail

      • ConcurrentLinkedHashMap

        public ConcurrentLinkedHashMap​(int initCap,
                               float loadFactor,
                               int concurLvl,
                               int maxCap,
                               ConcurrentLinkedHashMap.QueuePolicy qPlc)
        Creates a new, empty map with the specified initial capacity, load factor, concurrency level and max capacity.
        Parameters:
        initCap - 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.
        concurLvl - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
        maxCap - Max capacity (0 for unbounded).
        qPlc - Queue policy.
        Throws:
        IllegalArgumentException - if the initial capacity is negative or the load factor or concurLvl are non-positive.

      • ConcurrentLinkedHashMap

        public ConcurrentLinkedHashMap​(int initCap,
                               float loadFactor,
                               int concurLvl,
                               int maxCap)
        Creates a new, empty map with the specified initial capacity, load factor, concurrency level and max capacity.
        Parameters:
        initCap - 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.
        concurLvl - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
        maxCap - Max capacity (0 for unbounded).
        Throws:
        IllegalArgumentException - if the initial capacity is negative or the load factor or concurLvl are non-positive.

      • ConcurrentLinkedHashMap

        public ConcurrentLinkedHashMap​(int initCap,
                               float loadFactor,
                               int concurLvl)
        Creates a new, empty map with the specified initial capacity, load factor and concurrency level.
        Parameters:
        initCap - 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.
        concurLvl - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
        Throws:
        IllegalArgumentException - if the initial capacity is negative or the load factor or concurLvl are non-positive.

      • ConcurrentLinkedHashMap

        public ConcurrentLinkedHashMap​(int initCap,
                               float loadFactor)
        Creates a new, empty map with the specified initial capacity and load factor and with the default concurrencyLevel (16).
        Parameters:
        initCap - 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:
        IllegalArgumentException - if the initial capacity of elements is negative or the load factor is non-positive
        Since:
        1.6

      • ConcurrentLinkedHashMap

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

      • ConcurrentLinkedHashMap

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

      • ConcurrentLinkedHashMap

        public ConcurrentLinkedHashMap​(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

      • 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.

      • 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

      • sizex

        public int sizex()
        Returns:
        The number of key-value mappings in this map (constant-time).

      • isEmptyx

        public boolean isEmptyx()
        Returns:
        true if this map contains no key-value mappings

      • 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.

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

      • getSafe

        public V getSafe​(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.) In contrast with get(Object) this method acquires read lock on segment where the key is mapped.

        Throws:
        NullPointerException - if the specified key is null

      • 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 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 so is much slower than method containsKey.
        Specified by:
        containsValue in interface Map<K,​V>
        Overrides:
        containsValue in class AbstractMap<K,​V>
        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

      • 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 and only if some key maps to the value argument in this table as determined by the equals method; false otherwise
        Throws:
        NullPointerException - if the specified value is null

      • put

        public V put​(K key,
             V val)
        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
        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 V putIfAbsent​(K key,
                     V val)
        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

      • 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

      • 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

      • replacex

        public V replacex​(K key,
                  V oldVal,
                  V newVal)
        Replaces the entry for a key only if currently mapped to a given value. This is equivalent to 
           if (map.containsKey(key)) {
       if (map.get(key).equals(oldValue)) {
           map.put(key, newValue);
           return oldValue;
      } else
          return map.get(key);
   } else return null;
        except that the action is performed atomically.
        Parameters:
        key - key with which the specified value is associated
        oldVal - value expected to be associated with the specified key
        newVal - value to be associated with the specified key
        Returns:
        oldVal, if value was replaced, non-null previous value if map contained some other value and null if there were no such key.

      • replace

        public V replace​(K key,
                 V val)
        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

      • 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>

      • 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>

      • descendingKeySet

        public Set<K> descendingKeySet()
        Returns a Set view of the keys contained in this map in descending order. 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.

      • 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>

      • descendingValues

        public Collection<V> descendingValues()
        Returns a Collection view of the values contained in this map in descending order. 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.

      • 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 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 Map<K,​V>
        Specified by:
        entrySet in class AbstractMap<K,​V>

      • descendingEntrySet

        public Set<Map.Entry<K,​V>> descendingEntrySet()
        Returns a Set view of the mappings contained in this map in descending order. 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.

      • keys

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

      • descendingKeys

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

      • elements

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

      • descendingElements

        public Enumeration<V> descendingElements()
        Returns an enumeration of the values in this table in descending order.
        Returns:
        an enumeration of the values in this table in descending order.
        See Also:
        values()