Package com.tangosol.util

Class SafeHashMap<K,​V>

  • All Implemented Interfaces:
    Serializable, Cloneable, Map<K,​V>
    Direct Known Subclasses:
    ObservableHashMap, OldCache
    public class SafeHashMap<K,​V>
extends AbstractMap<K,​V>
implements Cloneable, Serializable
    An implementation of java.util.Map that is synchronized, but minimally so. This class is for use in situation where high concurrency is required, but so is data integrity.

    All additions and removals are synchronized on the map, so to temporarily prevent changes to the map contents, synchronize on the map object.

    Author:
    cp 1999.04.27
    See Also:
    Serialized Form

    • Field Detail

      • DEFAULT_INITIALSIZE

        public static final int DEFAULT_INITIALSIZE
        Default initial size provides a prime modulo and is large enough that resize is not immediate. (A hash map probably uses less than 128 bytes initially.)
        See Also:
        Constant Field Values

      • BIGGEST_MODULO

        protected static final int BIGGEST_MODULO
        Biggest possible modulo.
        See Also:
        Constant Field Values

      • DEFAULT_LOADFACTOR

        public static final float DEFAULT_LOADFACTOR
        The default load factor is 100%, which means that the hash map will not resize until there is (on average) one entry in every bucket. The cost of scanning a linked list in a particular bucket is very low, so there is little reason for having this value below 1.0, and the goal is constant order access, so assuming a perfect hash this will provide the optimal access speed relative to size.
        See Also:
        Constant Field Values

      • DEFAULT_GROWTHRATE

        public static final float DEFAULT_GROWTHRATE
        Using the default growth rate, the bucket array will grow by a factor of four. The relatively high growth rate helps to ensure less resize operations, an important consideration in a high-concurrency map.
        See Also:
        Constant Field Values

      • RESIZING

        protected Object RESIZING
        When resizing completes, a notification is issued against this object.

        Due to custom serialization this field cannot be marked as "final", but must be treated as such.

      • m_cEntries

        protected volatile int m_cEntries
        The number of entries stored in the hash map, 0 <= n. This field is declared volatile to avoid synchronization for the size() operation.

      • m_aeBucket

        protected volatile SafeHashMap.Entry[] m_aeBucket
        The array of hash buckets. This field is declared volatile in order to reduce synchronization.

      • m_cCapacity

        protected int m_cCapacity
        The capacity of the hash map (the point at which it must resize), 1 <= n.

      • m_flLoadFactor

        protected float m_flLoadFactor
        The determining factor for the hash map capacity given a certain number of buckets, such that capacity = bucketCount * loadFactor.

      • m_flGrowthRate

        protected float m_flGrowthRate
        The rate of growth as a fraction of the current number of buckets, 0 < n, such that the hash map grows to bucketCount * (1 + growthRate)

      • m_setEntries

        protected transient SafeHashMap.EntrySet m_setEntries
        The set of entries backed by this map.

      • m_setKeys

        protected transient SafeHashMap.KeySet m_setKeys
        The set of keys backed by this map.

      • m_oIterActive

        protected transient Object m_oIterActive
        A holder for active Iterator(s): either WeakReference(<Iterator>) or WeakHashMap(<Iterator>, null)

    • Constructor Detail

      • SafeHashMap

        public SafeHashMap()
        Construct a thread-safe hash map using the default settings.

      • SafeHashMap

        public SafeHashMap​(int cInitialBuckets,
                   float flLoadFactor,
                   float flGrowthRate)
        Construct a thread-safe hash map using the specified settings.
        Parameters:
        cInitialBuckets - the initial number of hash buckets, 0 < n
        flLoadFactor - the acceptable load factor before resizing occurs, 0 < n, such that a load factor of 1.0 causes resizing when the number of entries exceeds the number of buckets
        flGrowthRate - the rate of bucket growth when a resize occurs, 0 < n, such that a growth rate of 1.0 will double the number of buckets: bucketCount = bucketCount * (1 + growthRate)

    • Method Detail

      • 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.

        This method is not synchronized; it returns the size at the moment that the method is invoked. To ensure that the size does not change from the returned value, the caller must synchronize on the map before calling the size method.

        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

      • isEmpty

        public boolean isEmpty()
        Returns true if this map contains no key-value mappings.

        This method is not synchronized; it returns the state of the map at the moment that the method is invoked. To ensure that the size does not change, the caller must synchronize on the map before calling the method.

        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

      • containsKey

        public boolean containsKey​(Object key)
        Returns true if this map contains a mapping for the specified key.

        This method is not synchronized; it returns true if the map contains the key at the moment that the method is invoked. To ensure that the key is still in (or is still not in) the table when the method returns, the caller must synchronize on the map before calling the method.

        Specified by:
        containsKey in interface Map<K,​V>
        Overrides:
        containsKey in class AbstractMap<K,​V>
        Parameters:
        key - key whose presence in this map is to be tested
        Returns:
        true if this map contains a mapping for the specified key

      • get

        public V get​(Object key)
        Returns the value to which this map maps the specified key. Returns null if the map contains no mapping for this key. 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 to null. The containsKey operation may be used to distinguish these two cases.
        Specified by:
        get in interface Map<K,​V>
        Overrides:
        get in class AbstractMap<K,​V>
        Parameters:
        key - key whose associated value is to be returned
        Returns:
        the value to which this map maps the specified key, or null if the map contains no mapping for this key

      • getEntry

        public SafeHashMap.Entry<K,​V> getEntry​(Object key)
        Locate an Entry in the hash map based on its key.
        Parameters:
        key - the key object to search for
        Returns:
        the Entry or null

      • put

        public V put​(K key,
             V value)
        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.

        This method is not synchronized; it only synchronizes internally if it has to add a new Entry. To ensure that the value does not change (or the Entry is not removed) before this method returns, the caller must synchronize on the map before calling this method.

        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:
        previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values

      • grow

        protected void grow()
        Resize the bucket array, rehashing all Entries.

      • remove

        public V remove​(Object oKey)
        Removes the mapping for this key from this map if present.
        Specified by:
        remove in interface Map<K,​V>
        Overrides:
        remove in class AbstractMap<K,​V>
        Parameters:
        oKey - key whose mapping is to be removed from the map
        Returns:
        previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the map previously associated null with the specified key, if the implementation supports null values

      • clear

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

      • entrySet

        public Set<Map.Entry<K,​V>> entrySet()
        Returns a set view of the mappings contained in this map. Each element in the returned set is a Map.Entry. 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, the results of the iteration are undefined. 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.
        Specified by:
        entrySet in interface Map<K,​V>
        Specified by:
        entrySet in class AbstractMap<K,​V>
        Returns:
        a set view of the mappings contained in this map.

      • 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. (If the map is modified while an iteration over the Set is in progress, the results of the iteration are undefined.) The Set supports element removal, which removes the corresponding entry from the map, via the Iterator.remove, Set.remove, removeAll retainAll, and clear operations. It does not support the add or addAll operations.

        Specified by:
        keySet in interface Map<K,​V>
        Overrides:
        keySet in class AbstractMap<K,​V>
        Returns:
        a Set view of the keys contained 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. If the map is modified while an iteration over the collection is in progress, the results of the iteration are undefined. The collection supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Collection.remove, removeAll, retainAll and clear operations. It does not support the add or addAll operations.
        Specified by:
        values in interface Map<K,​V>
        Overrides:
        values in class AbstractMap<K,​V>
        Returns:
        a collection view of the values contained in this map

      • clone

        public Object clone()
        Create a clone of the SafeHashMap. Synchronization is necessary to prevent resizing (or to wait for resizing to complete), and also prevents other changes to the SafeHashMap while the deep clone is occurring.
        Overrides:
        clone in class AbstractMap<K,​V>
        Returns:
        a clone of the SafeHashMap

      • cloneEntryList

        protected SafeHashMap.Entry cloneEntryList​(SafeHashMap.Entry entryThat)
        Clone an entire linked list of entries.

        This method must be called on the map that will contain the clones, to allow the map to be the parent of the entries if the entries are not static inner classes.

        Parameters:
        entryThat - the entry that is the head of the list to clone
        Returns:
        null if the entry is null, otherwise an entry that is a clone of the passed entry, and a linked list of clones for each entry in the linked list that was passed

      • getEntryInternal

        protected SafeHashMap.Entry<K,​V> getEntryInternal​(Object oKey)
        Locate an Entry in the hash map based on its key.

        Unlike the getEntry(java.lang.Object) method, there must be no side-effects of calling this method.

        Parameters:
        oKey - the key object to search for
        Returns:
        the Entry or null

      • removeEntryInternal

        protected void removeEntryInternal​(SafeHashMap.Entry<K,​V> entry)
        Removes the passed Entry from the map.

        Note: Unlike calling the "remove" method, which is overridden at subclasses, calling this method directly does not generate any events.

        Parameters:
        entry - the entry to remove from this map

      • getBucketIndex

        protected int getBucketIndex​(int nHash,
                             int cBuckets)
        Calculate the bucket number for a particular hash code.
        Parameters:
        nHash - the hash code
        cBuckets - the number of buckets
        Returns:
        the bucket index for the specified hash code

      • getStableBucketArray

        protected SafeHashMap.Entry[] getStableBucketArray()
        Get the bucket array, or if a resize is occurring, wait for the resize to complete and return the new bucket array.
        Returns:
        the latest bucket array

      • iteratorActivated

        protected void iteratorActivated​(Iterator iter)
        Register the activation of an Iterator.
        Parameters:
        iter - the activated iterator

      • iteratorDeactivated

        protected void iteratorDeactivated​(Iterator iter)
        Unregister the (formerly active) Iterator.
        Parameters:
        iter - the deactivated iterator

      • isActiveIterator

        protected boolean isActiveIterator()
        Determine if there are any active Iterators, which may mean that they are in the middle of iterating over the Map.
        Returns:
        true iff there is at least one active Iterator

      • instantiateEntry

        protected SafeHashMap.Entry<K,​V> instantiateEntry​(K oKey,
                                                        V oValue,
                                                        int iHash)
        Factory pattern: instantiate initialized Entry object.
        Parameters:
        oKey - the key
        oValue - the value
        iHash - the hash value
        Returns:
        a new instance of the Entry class (or a subclass thereof)

      • instantiateEntry

        protected SafeHashMap.Entry<K,​V> instantiateEntry()
        Factory pattern: instantiate an un-initialized Entry object.
        Returns:
        a new instance of the Entry class (or a subclass thereof)

      • instantiateEntrySet

        protected SafeHashMap.EntrySet instantiateEntrySet()
        Factory pattern.
        Returns:
        a new instance of the EntrySet class (or a subclass thereof)

      • instantiateKeySet

        protected SafeHashMap.KeySet instantiateKeySet()
        Factory pattern.
        Returns:
        a new instance of the KeySet class (or subclass thereof)

      • instantiateValuesCollection

        protected SafeHashMap.ValuesCollection instantiateValuesCollection()
        Factory pattern.
        Returns:
        a new instance of the ValuesCollection class (or subclass thereof)