Package org.codehaus.groovy.runtime.metaclass

Class ConcurrentReaderHashMap

  • All Implemented Interfaces:
    Serializable, Cloneable, Map
    public class ConcurrentReaderHashMap
extends AbstractMap
implements Cloneable, Serializable
    A hash table that supports mostly-concurrent reading, but exclusive writing. Because reads are not limited to periods without writes, a concurrent reader policy is weaker than a classic reader/writer policy, but is generally faster and allows more concurrency. This class is a good choice especially for tables that are mainly created by one thread during the start-up phase of a program, and from then on, are mainly read (with perhaps occasional additions or removals) in many threads. If you also need concurrency among writes, consider instead using ConcurrentHashMap.

    Successful retrievals using get(key) and containsKey(key) usually run without locking. Unsuccessful ones (i.e., when the key is not present) do involve brief synchronization (locking). Also, the size and isEmpty methods are always synchronized.

    Because retrieval operations can ordinarily overlap with writing operations (i.e., put, remove, and their derivatives), retrievals can only be guaranteed to return the results of the most recently completed operations holding upon their onset. Retrieval operations may or may not return results reflecting in-progress writing operations. However, the retrieval operations do always return consistent results -- either those holding before any single modification or after it, but never a nonsense result. For aggregate operations such as putAll and clear, concurrent reads may reflect insertion or removal of only some entries. In those rare contexts in which you use a hash table to synchronize operations across threads (for example, to prevent reads until after clears), you should either encase operations in synchronized blocks, or instead use java.util.Hashtable.

    This class also supports optional guaranteed exclusive reads, simply by surrounding a call within a synchronized block, as in
    ConcurrentReaderHashMap t; ... Object v;
    synchronized(t) { v = t.get(k); }
    But this is not usually necessary in practice. For example, it is generally inefficient to write: 

       ConcurrentReaderHashMap t; ...            // Inefficient version
   Object key; ...
   Object value; ...
   synchronized(t) { 
     if (!t.containsKey(key))
       t.put(key, value);
       // other code if not previously present
     }
     else {
       // other code if it was previously present
     }
   }
    Instead, if the values are intended to be the same in each case, just take advantage of the fact that put returns null if the key was not previously present: 
       ConcurrentReaderHashMap t; ...                // Use this instead
   Object key; ...
   Object value; ...
   Object oldValue = t.put(key, value);
   if (oldValue == null) {
     // other code if not previously present
   }
   else {
     // other code if it was previously present
   }

    Iterators and Enumerations (i.e., those returned by keySet().iterator(), entrySet().iterator(), values().iterator(), keys(), and elements()) return elements reflecting the state of the hash table at some point at or since the creation of the iterator/enumeration. They will return at most one instance of each element (via next()/nextElement()), but might or might not reflect puts and removes that have been processed since they were created. They do not throw ConcurrentModificationException. However, these iterators are designed to be used by only one thread at a time. Sharing an iterator across multiple threads may lead to unpredictable results if the table is being concurrently modified. Again, you can ensure interference-free iteration by enclosing the iteration in a synchronized block.

    This class may be used as a direct replacement for any use of java.util.Hashtable that does not depend on readers being blocked during updates. Like Hashtable but unlike java.util.HashMap, this class does NOT allow null to be used as a key or value. This class is also typically faster than ConcurrentHashMap when there is usually only one thread updating the table, but possibly many retrieving values from it.

    Implementation note: A slightly faster implementation of this class will be possible once planned Java Memory Model revisions are in place.

    [ Introduction to this package. ]

    See Also:
    Serialized Form

    • Field Detail

      • lastWrite

        protected transient Object lastWrite
        field written to only to guarantee lock ordering.

      • DEFAULT_INITIAL_CAPACITY

        public static final int DEFAULT_INITIAL_CAPACITY
        The default initial number of table slots for this table (32). Used when not otherwise specified in constructor.
        See Also:
        Constant Field Values

      • DEFAULT_LOAD_FACTOR

        public static final float DEFAULT_LOAD_FACTOR
        The default load factor for this table (1.0). Used when not otherwise specified in constructor.
        See Also:
        Constant Field Values

      • count

        protected transient int count
        The total number of mappings in the hash table.

      • threshold

        protected int threshold
        The table is rehashed when its size exceeds this threshold. (The value of this field is always (int)(capacity * loadFactor).)

      • loadFactor

        protected float loadFactor
        The load factor for the hash table.

      • keySet

        protected transient Set keySet

      • entrySet

        protected transient Set entrySet

    • Constructor Detail

      • ConcurrentReaderHashMap

        public ConcurrentReaderHashMap​(int initialCapacity,
                               float loadFactor)
        Constructs a new, empty map with the specified initial capacity and the specified load factor.
        Parameters:
        initialCapacity - the initial capacity The actual initial capacity is rounded to the nearest power of two.
        loadFactor - the load factor of the ConcurrentReaderHashMap
        Throws:
        IllegalArgumentException - if the initial maximum number of elements is less than zero, or if the load factor is non-positive.

      • ConcurrentReaderHashMap

        public ConcurrentReaderHashMap​(int initialCapacity)
        Constructs a new, empty map with the specified initial capacity and default load factor.
        Parameters:
        initialCapacity - the initial capacity of the ConcurrentReaderHashMap.
        Throws:
        IllegalArgumentException - if the initial maximum number of elements is less than zero.

      • ConcurrentReaderHashMap

        public ConcurrentReaderHashMap()
        Constructs a new, empty map with a default initial capacity and load factor.

      • ConcurrentReaderHashMap

        public ConcurrentReaderHashMap​(Map t)
        Constructs a new map with the same mappings as the given map. The map is created with a capacity of twice the number of mappings in the given map or 16 (whichever is greater), and a default load factor.

    • Method Detail

      • recordModification

        protected final void recordModification​(Object x)
        Force a memory synchronization that will cause all readers to see table. Call only when already holding main sync lock.

      • getTableForReading

        protected final ConcurrentReaderHashMap.Entry[] getTableForReading()
        Get ref to table; the reference and the cells it accesses will be at least as fresh as from last use of barrierLock

      • eq

        protected boolean eq​(Object x,
                     Object y)
        Check for equality of non-null references x and y.

      • size

        public int size()
        Returns the number of key-value mappings in this map.
        Specified by:
        size in interface Map
        Overrides:
        size in class AbstractMap
        Returns:
        the number of key-value mappings in this map.

      • isEmpty

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

      • get

        public Object get​(Object key)
        Returns the value to which the specified key is mapped in this table.
        Specified by:
        get in interface Map
        Overrides:
        get in class AbstractMap
        Parameters:
        key - a key in the table.
        Returns:
        the value to which the key is mapped in this table; null if the key is not mapped to any value in this table.
        Throws:
        NullPointerException - if the key is null.
        See Also:
        put(Object, Object)

      • containsKey

        public boolean containsKey​(Object key)
        Tests if the specified object is a key in this table.
        Specified by:
        containsKey in interface Map
        Overrides:
        containsKey in class AbstractMap
        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 key is null.
        See Also:
        contains(Object)

      • put

        public Object put​(Object key,
                  Object 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
        Overrides:
        put in class AbstractMap
        Parameters:
        key - the table key.
        value - the value.
        Returns:
        the previous value of the specified key in this table, or null if it did not have one.
        Throws:
        NullPointerException - if the key or value is null.
        See Also:
        Object.equals(Object), get(Object)

      • sput

        protected Object sput​(Object key,
                      Object value,
                      int hash)
        Continuation of put(), called only when sync lock is held and interference has been detected.

      • rehash

        protected void rehash()
        Rehashes the contents of this map into a new table with a larger capacity. This method is called automatically when the number of keys in this map exceeds its capacity and load factor.

      • remove

        public Object remove​(Object key)
        Removes the key (and its corresponding value) from this table. This method does nothing if the key is not in the table.
        Specified by:
        remove in interface Map
        Overrides:
        remove in class AbstractMap
        Parameters:
        key - the key that needs to be removed.
        Returns:
        the value to which the key had been mapped in this table, or null if the key did not have a mapping.
        Throws:
        NullPointerException - if the key is null.

      • sremove

        protected Object sremove​(Object key,
                         int hash)
        Continuation of remove(), called only when sync lock is held and interference has been detected.

      • 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, and so is much slower than method containsKey.
        Specified by:
        containsValue in interface Map
        Overrides:
        containsValue in class AbstractMap
        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 value is null.

      • contains

        public boolean contains​(Object value)
        Tests if some key maps into the specified value in this table. This operation is more expensive than the containsKey method.

        Note that this method is identical in functionality to containsValue, (which is part of the Map interface in the 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:
        NullPointerException - if the value is null.
        See Also:
        containsKey(Object), containsValue(Object), Map

      • putAll

        public void putAll​(Map t)
        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
        Overrides:
        putAll in class AbstractMap
        Parameters:
        t - Mappings to be stored in this map.

      • clear

        public void clear()
        Removes all mappings from this map.
        Specified by:
        clear in interface Map
        Overrides:
        clear in class AbstractMap

      • clone

        public Object clone()
        Returns a shallow copy of this ConcurrentReaderHashMap instance: the keys and values themselves are not cloned.
        Overrides:
        clone in class AbstractMap
        Returns:
        a shallow copy of this map.

      • keySet

        public Set 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.
        Specified by:
        keySet in interface Map
        Overrides:
        keySet in class AbstractMap
        Returns:
        a set view of the keys contained in this map.

      • values

        public Collection 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.
        Specified by:
        values in interface Map
        Overrides:
        values in class AbstractMap
        Returns:
        a collection view of the values contained in this map.

      • entrySet

        public Set entrySet()
        Returns a collection view of the mappings contained in this map. Each element in the returned collection is a Map.Entry. 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 the map, via the Iterator.remove, Collection.remove, removeAll, retainAll, and clear operations. It does not support the add or addAll operations.
        Specified by:
        entrySet in interface Map
        Specified by:
        entrySet in class AbstractMap
        Returns:
        a collection view of the mappings contained in this map.

      • findAndRemoveEntry

        protected boolean findAndRemoveEntry​(Map.Entry entry)
        Helper method for entrySet.remove

      • elements

        public Enumeration elements()
        Returns an enumeration of the values in this table. Use the Enumeration methods on the returned object to fetch the elements sequentially.
        Returns:
        an enumeration of the values in this table.
        See Also:
        Enumeration, keys(), values(), Map

      • capacity

        public int capacity()
        Returns:
        the number of slots in this table

      • loadFactor

        public float loadFactor()
        Returns:
        the load factor