Package org.glassfish.pfl.basic.concurrent

Class WeakHashMapSafeReadLock<K,​V>

  • Type Parameters:
    K - the type of keys maintained by this map
    V - the type of mapped values
    All Implemented Interfaces:
    java.util.Map<K,​V>
    public class WeakHashMapSafeReadLock<K,​V>
extends java.util.AbstractMap<K,​V>
implements java.util.Map<K,​V>
    A hashtable-based Map implementation with weak keys. An entry in a WeakHashMap will automatically be removed when its key is no longer in ordinary use. More precisely, the presence of a mapping for a given key will not prevent the key from being discarded by the garbage collector, that is, made finalizable, finalized, and then reclaimed. When a key has been discarded its entry is effectively removed from the map, so this class behaves somewhat differently from other Map implementations.

    Both null values and the null key are supported. This class has performance characteristics similar to those of the HashMap class, and has the same efficiency parameters of initial capacity and load factor.

    Like most collection classes, this class is not synchronized. A synchronized WeakHashMap may be constructed using the Collections.synchronizedMap method.

    This class is intended primarily for use with key objects whose equals methods test for object identity using the == operator. Once such a key is discarded it can never be recreated, so it is impossible to do a lookup of that key in a WeakHashMap at some later time and be surprised that its entry has been removed. This class will work perfectly well with key objects whose equals methods are not based upon object identity, such as String instances. With such recreatable key objects, however, the automatic removal of WeakHashMap entries whose keys have been discarded may prove to be confusing.

    The behavior of the WeakHashMap class depends in part upon the actions of the garbage collector, so several familiar (though not required) Map invariants do not hold for this class. Because the garbage collector may discard keys at any time, a WeakHashMap may behave as though an unknown thread is silently removing entries. In particular, even if you synchronize on a WeakHashMap instance and invoke none of its mutator methods, it is possible for the size method to return smaller values over time, for the isEmpty method to return false and then true, for the containsKey method to return true and later false for a given key, for the get method to return a value for a given key but later return null, for the put method to return null and the remove method to return false for a key that previously appeared to be in the map, and for successive examinations of the key set, the value collection, and the entry set to yield successively smaller numbers of elements.

    Each key object in a WeakHashMap is stored indirectly as the referent of a weak reference. Therefore a key will automatically be removed only after the weak references to it, both inside and outside of the map, have been cleared by the garbage collector.

    Implementation note: The value objects in a WeakHashMap are held by ordinary strong references. Thus care should be taken to ensure that value objects do not strongly refer to their own keys, either directly or indirectly, since that will prevent the keys from being discarded. Note that a value object may refer indirectly to its key via the WeakHashMap itself; that is, a value object may strongly refer to some other key object whose associated value object, in turn, strongly refers to the key of the first value object. One way to deal with this is to wrap values themselves within WeakReferences before inserting, as in: m.put(key, new WeakReference(value)), and then unwrapping upon each get.

    The iterators returned by the iterator method of the collections returned by all of this class's "collection view methods" are fail-fast: if the map is structurally modified at any time after the iterator is created, in any way except through the iterator's own remove method, the iterator will throw a ConcurrentModificationException. Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.

    Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.

    This class is a member of the Java Collections Framework.

    This is copied directly from JDK 6 to get a version with the fix for but 6425537.

    Since:
    1.2
    Version:
    %I%, %G%
    See Also:
    HashMap, WeakReference

    • Field Summary

      Fields 
      Modifier and Type Field Description
      private static int DEFAULT_INITIAL_CAPACITY
      The default initial capacity -- MUST be a power of two.
      private static float DEFAULT_LOAD_FACTOR
      The load fast used when none specified in constructor.
      private java.util.Set<java.util.Map.Entry<K,​V>> entrySet  
      private float loadFactor
      The load factor for the hash table.
      private static int MAXIMUM_CAPACITY
      The maximum capacity, used if a higher value is implicitly specified by either of the constructors with arguments.
      private int modCount
      The number of times this WeakHashMap has been structurally modified.
      private java.util.Set<K> myKeySet  
      private java.util.Collection<V> myValues  
      private static java.lang.Object NULL_KEY
      Value representing null keys inside tables.
      private java.lang.ref.ReferenceQueue<K> queue
      Reference queue for cleared WeakEntries
      private int size
      The number of key-value mappings contained in this weak hash map.
      private WeakHashMapSafeReadLock.Entry[] table
      The table, resized as necessary.
      private int threshold
      The next size value at which to resize (capacity * load factor).

    • Constructor Summary

      Constructors 
      Constructor Description
      WeakHashMapSafeReadLock()
      Constructs a new, empty WeakHashMap with the default initial capacity (16) and load factor (0.75).
      WeakHashMapSafeReadLock​(int initialCapacity)
      Constructs a new, empty WeakHashMap with the given initial capacity and the default load factor (0.75).
      WeakHashMapSafeReadLock​(int initialCapacity, float loadFactor)
      Constructs a new, empty WeakHashMap with the given initial capacity and the given load factor.
      WeakHashMapSafeReadLock​(java.util.Map<? extends K,​? extends V> m)
      Constructs a new WeakHashMap with the same mappings as the specified 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.
      (package private) static int computeHash​(int h)  
      boolean containsKey​(java.lang.Object key)
      Returns true if this map contains a mapping for the specified key.
      private boolean containsNullValue()
      Special-case code for containsValue with null argument
      boolean containsValue​(java.lang.Object value)
      Returns true if this map maps one or more keys to the specified value.
      java.util.Set<java.util.Map.Entry<K,​V>> entrySet()
      Returns a Set view of the mappings contained in this map.
      (package private) static boolean eq​(java.lang.Object x, java.lang.Object y)
      Checks for equality of non-null reference x and possibly-null y.
      private void expungeStaleEntries()
      Expunges stale entries from the table.
      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.
      (package private) WeakHashMapSafeReadLock.Entry<K,​V> getEntry​(java.lang.Object key)
      Returns the entry associated with the specified key in this map.
      private WeakHashMapSafeReadLock.Entry[] getTable()
      Returns the table after first expunging stale entries.
      (package private) static int indexFor​(int h, int length)
      Returns index for hash code h.
      boolean isEmpty()
      Returns true if this map contains no key-value mappings.
      java.util.Set<K> keySet()
      Returns a Set view of the keys contained in this map.
      private static java.lang.Object maskNull​(java.lang.Object key)
      Use NULL_KEY for key if it is null.
      V put​(K key, V value)
      Associates the specified value with the specified key in this map.
      void putAll​(java.util.Map<? extends K,​? extends V> m)
      Copies all of the mappings from the specified map to this map.
      V remove​(java.lang.Object key)
      Removes the mapping for a key from this weak hash map if it is present.
      (package private) WeakHashMapSafeReadLock.Entry<K,​V> removeMapping​(java.lang.Object o)
      Special version of remove needed by Entry set
      (package private) void resize​(int newCapacity)
      Rehashes the contents of this map into a new array with a larger capacity.
      int size()
      Returns the number of key-value mappings in this map.
      private void transfer​(WeakHashMapSafeReadLock.Entry[] src, WeakHashMapSafeReadLock.Entry[] dest)
      Transfers all entries from src to dest tables
      private static <K> K unmaskNull​(java.lang.Object key)
      Returns internal representation of null key back to caller as null.
      java.util.Collection<V> values()
      Returns a Collection view of the values contained in this map.

      • 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.Map

        compute, computeIfAbsent, computeIfPresent, equals, forEach, getOrDefault, hashCode, merge, putIfAbsent, remove, replace, replace, replaceAll

    • Field Detail

      • DEFAULT_INITIAL_CAPACITY

        private static final int DEFAULT_INITIAL_CAPACITY
        The default initial capacity -- MUST be a power of two.
        See Also:
        Constant Field Values

      • MAXIMUM_CAPACITY

        private 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.
        See Also:
        Constant Field Values

      • DEFAULT_LOAD_FACTOR

        private static final float DEFAULT_LOAD_FACTOR
        The load fast used when none specified in constructor.
        See Also:
        Constant Field Values

      • size

        private int size
        The number of key-value mappings contained in this weak hash map.

      • threshold

        private int threshold
        The next size value at which to resize (capacity * load factor).

      • loadFactor

        private final float loadFactor
        The load factor for the hash table.

      • queue

        private final java.lang.ref.ReferenceQueue<K> queue
        Reference queue for cleared WeakEntries

      • modCount

        private volatile int modCount
        The number of times this WeakHashMap has been structurally modified. Structural modifications are those that change the number of mappings in the map or otherwise modify its internal structure (e.g., rehash). This field is used to make iterators on Collection-views of the map fail-fast.
        See Also:
        ConcurrentModificationException

      • NULL_KEY

        private static final java.lang.Object NULL_KEY
        Value representing null keys inside tables.

      • entrySet

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

      • myKeySet

        private transient volatile java.util.Set<K> myKeySet

      • myValues

        private transient volatile java.util.Collection<V> myValues

    • Constructor Detail

      • WeakHashMapSafeReadLock

        public WeakHashMapSafeReadLock​(int initialCapacity,
                               float loadFactor)
        Constructs a new, empty WeakHashMap with the given initial capacity and the given load factor.
        Parameters:
        initialCapacity - The initial capacity of the WeakHashMap
        loadFactor - The load factor of the WeakHashMap
        Throws:
        java.lang.IllegalArgumentException - if the initial capacity is negative, or if the load factor is nonpositive.

      • WeakHashMapSafeReadLock

        public WeakHashMapSafeReadLock​(int initialCapacity)
        Constructs a new, empty WeakHashMap with the given initial capacity and the default load factor (0.75).
        Parameters:
        initialCapacity - The initial capacity of the WeakHashMap
        Throws:
        java.lang.IllegalArgumentException - if the initial capacity is negative

      • WeakHashMapSafeReadLock

        public WeakHashMapSafeReadLock()
        Constructs a new, empty WeakHashMap with the default initial capacity (16) and load factor (0.75).

      • WeakHashMapSafeReadLock

        public WeakHashMapSafeReadLock​(java.util.Map<? extends K,​? extends V> m)
        Constructs a new WeakHashMap with the same mappings as the specified map. The WeakHashMap is created with the default load factor (0.75) and an initial capacity sufficient to hold the mappings in the specified map.
        Parameters:
        m - the map whose mappings are to be placed in this map
        Throws:
        java.lang.NullPointerException - if the specified map is null
        Since:
        1.3

    • Method Detail

      • maskNull

        private static java.lang.Object maskNull​(java.lang.Object key)
        Use NULL_KEY for key if it is null.

      • unmaskNull

        private static <K> K unmaskNull​(java.lang.Object key)
        Returns internal representation of null key back to caller as null.

      • eq

        static boolean eq​(java.lang.Object x,
                  java.lang.Object y)
        Checks for equality of non-null reference x and possibly-null y. By default uses Object.equals.

      • indexFor

        static int indexFor​(int h,
                    int length)
        Returns index for hash code h.

      • expungeStaleEntries

        private void expungeStaleEntries()
        Expunges stale entries from the table.

      • size

        public int size()
        Returns the number of key-value mappings in this map. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced.
        Specified by:
        size in interface java.util.Map<K,​V>
        Overrides:
        size in class java.util.AbstractMap<K,​V>

      • isEmpty

        public boolean isEmpty()
        Returns true if this map contains no key-value mappings. This result is a snapshot, and may not reflect unprocessed entries that will be removed before next attempted access because they are no longer referenced.
        Specified by:
        isEmpty in interface java.util.Map<K,​V>
        Overrides:
        isEmpty in class java.util.AbstractMap<K,​V>

      • computeHash

        static int computeHash​(int h)

      • 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==null ? k==null : key.equals(k)), then this method returns v; otherwise it returns null. (There can be at most one such mapping.)

        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 java.util.Map<K,​V>
        Overrides:
        get in class java.util.AbstractMap<K,​V>
        See Also:
        put(Object, Object)

      • containsKey

        public boolean containsKey​(java.lang.Object key)
        Returns true if this map contains a mapping for the specified key.
        Specified by:
        containsKey in interface java.util.Map<K,​V>
        Overrides:
        containsKey in class java.util.AbstractMap<K,​V>
        Parameters:
        key - The key whose presence in this map is to be tested
        Returns:
        true if there is a mapping for key; false otherwise

      • getEntry

        WeakHashMapSafeReadLock.Entry<K,​V> getEntry​(java.lang.Object key)
        Returns the entry associated with the specified key in this map. Returns null if the map contains no mapping for this key.

      • 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.
        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. (A null return can also indicate that the map previously associated null with key.)

      • resize

        void resize​(int newCapacity)
        Rehashes the contents of this map into a new array with a larger capacity. This method is called automatically when the number of keys in this map reaches its threshold. If current capacity is MAXIMUM_CAPACITY, this method does not resize the map, but sets threshold to Integer.MAX_VALUE. This has the effect of preventing future calls.
        Parameters:
        newCapacity - the new capacity, MUST be a power of two; must be greater than current capacity unless current capacity is MAXIMUM_CAPACITY (in which case value is irrelevant).

      • putAll

        public void putAll​(java.util.Map<? extends K,​? extends V> m)
        Copies all of the mappings from the specified map to this map. These mappings will 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.
        Throws:
        java.lang.NullPointerException - if the specified map is null.

      • remove

        public V remove​(java.lang.Object key)
        Removes the mapping for a key from this weak hash map if it is present. More formally, if this map contains a mapping from key k to value v such that (key==null ? k==null : key.equals(k)), that mapping is removed. (The map can contain at most one such mapping.)

        Returns the value to which this map previously associated the key, or null if the map contained no mapping for the key. A return value of null does not necessarily indicate that the map contained no mapping for the key; it's also possible that the map explicitly mapped the key to null.

        The map will not contain a mapping for the specified key once the call returns.

        Specified by:
        remove in interface java.util.Map<K,​V>
        Overrides:
        remove in class java.util.AbstractMap<K,​V>
        Parameters:
        key - key whose mapping is to be removed from the map
        Returns:
        the previous value associated with key, or null if there was no mapping for key

      • clear

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

      • containsValue

        public boolean containsValue​(java.lang.Object value)
        Returns true if this map maps one or more keys to the specified value.
        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

      • containsNullValue

        private boolean containsNullValue()
        Special-case code for containsValue with null argument

      • 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. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation), 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:
        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. If the map is modified while an iteration over the collection is in progress (except through the iterator's own remove operation), 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 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. If the map is modified while an iteration over the set is in progress (except through the iterator's own remove operation, or through the setValue operation on a map entry returned by the iterator) 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 java.util.Map<K,​V>
        Specified by:
        entrySet in class java.util.AbstractMap<K,​V>