Package org.codehaus.groovy.runtime.metaclass

Class ConcurrentReaderHashMap

java.lang.Object
java.util.AbstractMap
org.codehaus.groovy.runtime.metaclass.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:

  • Field Details

    • barrierLock

      protected final ConcurrentReaderHashMap.BarrierLock barrierLock
      Lock used only for its memory effects.

    • lastWrite

      protected transient Object lastWrite
      field written 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:

    • 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:

    • table

      protected transient ConcurrentReaderHashMap.Entry[] table
      The hash table data.

    • 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

    • values

      protected transient Collection values

  • Constructor Details

    • 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 Details

    • 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:

    • 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:

    • 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:

    • 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:

    • putAll

      public void putAll(Map t)
      Copies all 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

    • keys

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

    • 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:

    • capacity

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

    • loadFactor

      public float loadFactor()
      Returns:
      the load factor