Package io.debezium.util

Class BoundedConcurrentHashMap<K,V>

java.lang.Object
java.util.AbstractMap<K,V>
io.debezium.util.BoundedConcurrentHashMap<K,V>
Type Parameters:
K - the type of keys maintained by this map
V - the type of mapped values
All Implemented Interfaces:
Serializable, ConcurrentMap<K,V>, Map<K,V>
public class BoundedConcurrentHashMap<K,V> extends AbstractMap<K,V> implements ConcurrentMap<K,V>, Serializable
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 class and its views and iterators implement all of the optional methods of the Map and Iterator interfaces.

This class is copied from Hibernate (which took it from Infinispan), and was originally written by Doug Lea with assistance from members of JCP JSR-166 Expert Group and released to the public domain, as explained at http://creativecommons.org/licenses/publicdomain

Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.

Author:
Doug Lea
See Also:

  • Field Details

    • serialVersionUID

      private static final long serialVersionUID
      See Also:

    • DEFAULT_MAXIMUM_CAPACITY

      static final int DEFAULT_MAXIMUM_CAPACITY
      The default initial capacity for this table, used when not otherwise specified in a constructor.
      See Also:

    • DEFAULT_LOAD_FACTOR

      static final float DEFAULT_LOAD_FACTOR
      The default load factor for this table, used when not otherwise specified in a constructor.
      See Also:

    • DEFAULT_CONCURRENCY_LEVEL

      static final int DEFAULT_CONCURRENCY_LEVEL
      The default concurrency level for this table, used when not otherwise specified in a constructor.
      See Also:

    • MAXIMUM_CAPACITY

      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 to ensure that entries are indexable using ints.
      See Also:

    • MAX_SEGMENTS

      static final int MAX_SEGMENTS
      The maximum number of segments to allow; used to bound constructor arguments.
      See Also:

    • RETRIES_BEFORE_LOCK

      static final int RETRIES_BEFORE_LOCK
      Number of unsynchronized retries in size and containsValue 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:

    • segmentMask

      final int segmentMask
      Mask value for indexing into segments. The upper bits of a key's hash code are used to choose the segment.

    • segmentShift

      final int segmentShift
      Shift value for indexing within segments.

    • segments

      final BoundedConcurrentHashMap.Segment<K,V>[] segments
      The segments, each of which is a specialized hash table

    • keySet

      transient Set<K> keySet

    • entrySet

      transient Set<Map.Entry<K,V>> entrySet

    • values

      transient Collection<V> values

  • Constructor Details

    • BoundedConcurrentHashMap

      public BoundedConcurrentHashMap(int capacity, int concurrencyLevel, BoundedConcurrentHashMap.Eviction evictionStrategy, BoundedConcurrentHashMap.EvictionListener<K,V> evictionListener)
      Creates a new, empty map with the specified maximum capacity, load factor and concurrency level.
      Parameters:
      capacity - is the upper bound capacity for the number of elements in this map
      concurrencyLevel - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
      evictionStrategy - the algorithm used to evict elements from this map
      evictionListener - the evicton listener callback to be notified about evicted elements
      Throws:
      IllegalArgumentException - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.

    • BoundedConcurrentHashMap

      public BoundedConcurrentHashMap(int capacity, int concurrencyLevel)
      Creates a new, empty map with the specified maximum capacity, load factor, concurrency level and LRU eviction policy.
      Parameters:
      capacity - is the upper bound capacity for the number of elements in this map
      concurrencyLevel - 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 concurrencyLevel are nonpositive.

    • BoundedConcurrentHashMap

      public BoundedConcurrentHashMap(int capacity, int concurrencyLevel, BoundedConcurrentHashMap.Eviction evictionStrategy)
      Creates a new, empty map with the specified maximum capacity, load factor, concurrency level and eviction strategy.
      Parameters:
      capacity - is the upper bound capacity for the number of elements in this map
      concurrencyLevel - the estimated number of concurrently updating threads. The implementation performs internal sizing to try to accommodate this many threads.
      evictionStrategy - the algorithm used to evict elements from this map
      Throws:
      IllegalArgumentException - if the initial capacity is negative or the load factor or concurrencyLevel are nonpositive.

    • BoundedConcurrentHashMap

      public BoundedConcurrentHashMap(int capacity)
      Creates a new, empty map with the specified maximum capacity, default concurrency level and LRU eviction policy.
      Parameters:
      capacity - is the upper bound capacity for the number of elements in this map
      Throws:
      IllegalArgumentException - if the initial capacity of elements is negative or the load factor is nonpositive
      Since:
      1.6

    • BoundedConcurrentHashMap

      public BoundedConcurrentHashMap()
      Creates a new, empty map with the default maximum capacity

  • Method Details

    • hash

      private static int hash(int h)
      Applies a supplemental hash function to a given hashCode, which defends against poor quality hash functions. This is critical because ConcurrentHashMap uses power-of-two length hash tables, that otherwise encounter collisions for hashCodes that do not differ in lower or upper bits.

    • segmentFor

      final BoundedConcurrentHashMap.Segment<K,V> segmentFor(int hash)
      Returns the segment that should be used for key with given hash
      Parameters:
      hash - the hash code for the key
      Returns:
      the segment

    • 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

    • 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

    • 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 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<K,V>
      Overrides:
      containsValue in class 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
      Throws:
      NullPointerException - if the specified value is null

    • contains

      public boolean contains(Object value)
      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:
      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 specified value is null

    • put

      public V put(K key, V 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<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:
      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 value)
      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

    • remove

      public boolean remove(Object key, Object value)
      Specified by:
      remove in interface ConcurrentMap<K,V>
      Specified by:
      remove in interface Map<K,V>
      Throws:
      NullPointerException - if the specified key is null

    • replace

      public boolean replace(K key, V oldValue, V newValue)
      Specified by:
      replace in interface ConcurrentMap<K,V>
      Specified by:
      replace in interface Map<K,V>
      Throws:
      NullPointerException - if any of the arguments are null

    • replace

      public V replace(K key, V value)
      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>

    • 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>

    • 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>

    • 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:

    • 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:

    • writeObject

      private void writeObject(ObjectOutputStream s) throws IOException
      Save the state of the ConcurrentHashMap instance to a stream (i.e., serialize it).
      Parameters:
      s - the stream
      Throws:
      IOException

    • readObject

      private void readObject(ObjectInputStream s) throws IOException, ClassNotFoundException
      Reconstitute the ConcurrentHashMap instance from a stream (i.e., deserialize it).
      Parameters:
      s - the stream
      Throws:
      IOException
      ClassNotFoundException