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/* |
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* Written by Doug Lea with assistance from members of JCP JSR-166 |
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* Expert Group and released to the public domain. Use, modify, and |
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* redistribute this code in any way without acknowledgement. |
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*/ |
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|
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package java.util.concurrent; |
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|
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import java.util.*; |
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import java.io.Serializable; |
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import java.io.IOException; |
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import java.io.ObjectInputStream; |
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import java.io.ObjectOutputStream; |
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|
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/** |
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* A version of Hashtable supporting |
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* concurrency for both retrievals and updates. |
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* |
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* <dl> |
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* <dt> Retrievals |
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* |
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* <dd> Retrievals may overlap updates. Successful retrievals using |
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* get(key) and containsKey(key) usually run without |
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* locking. Unsuccessful retrievals (i.e., when the key is not |
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* present) do involve brief locking. Because |
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* retrieval operations can ordinarily overlap with update operations |
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* (i.e., put, remove, and their derivatives), retrievals can only be |
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* guaranteed to return the results of the most recently |
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* <em>completed</em> operations holding upon their onset. Retrieval |
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* operations may or may not return results reflecting in-progress |
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* writing operations. However, the retrieval operations do always |
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* return consistent results -- either those holding before any single |
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* modification or after it, but never a nonsense result. For |
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* aggregate operations such as putAll and clear, concurrent reads may |
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* reflect insertion or removal of only some entries. <p> |
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* |
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* Iterators and Enumerations (i.e., those returned by |
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* keySet().iterator(), entrySet().iterator(), values().iterator(), |
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* keys(), and elements()) return elements reflecting the state of the |
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* hash table at some point at or since the creation of the |
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* iterator/enumeration. They will return at most one instance of |
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* each element (via next()/nextElement()), but might or might not |
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* reflect puts and removes that have been processed since |
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* construction if the Iterator. They do <em>not</em> throw |
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* ConcurrentModificationException. However, these iterators are |
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* designed to be used by only one thread at a time. Passing an |
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* iterator across multiple threads may lead to unpredictable traversal |
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* if the table is being concurrently modified. <p> |
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* |
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* |
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* <dt> Updates |
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* |
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* <dd> This class supports a hard-wired preset <em>concurrency |
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* level</em> of 32. This allows a maximum of 32 put and/or remove |
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* operations to proceed concurrently. This level is an upper bound on |
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* concurrency, not a guarantee, since it interacts with how |
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* well-strewn elements are across bins of the table. (The preset |
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* value in part reflects the fact that even on large multiprocessors, |
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* factors other than synchronization tend to be bottlenecks when more |
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* than 32 threads concurrently attempt updates.) |
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* Additionally, operations triggering internal resizing and clearing |
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* do not execute concurrently with any operation. |
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* <p> |
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* |
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* There is <em>NOT</em> any support for locking the entire table to |
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* prevent updates. |
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* |
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* </dl> |
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* |
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* |
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* This class may be used as a direct replacement for |
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* java.util.Hashtable in any application that does not rely |
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* on the ability to lock the entire table to prevent updates. |
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* Like Hashtable but unlike java.util.HashMap, |
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* this class does NOT allow <tt>null</tt> to be used as a key or |
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* value. |
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* <p> |
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* |
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**/ |
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public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> |
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implements ConcurrentMap<K, V>, Cloneable, Serializable { |
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|
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/* |
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The basic strategy is an optimistic-style scheme based on |
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the guarantee that the hash table and its lists are always |
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kept in a consistent enough state to be read without locking: |
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|
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* Read operations first proceed without locking, by traversing the |
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apparently correct list of the apparently correct bin. If an |
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entry is found, but not invalidated (value field null), it is |
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returned. If not found, operations must recheck (after a memory |
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barrier) to make sure they are using both the right list and |
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the right table (which can change under resizes). If |
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invalidated, reads must acquire main update lock to wait out |
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the update, and then re-traverse. |
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|
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* All list additions are at the front of each bin, making it easy |
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to check changes, and also fast to traverse. Entry next |
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pointers are never assigned. Remove() builds new nodes when |
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necessary to preserve this. |
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|
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* Remove() (also clear()) invalidates removed nodes to alert read |
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operations that they must wait out the full modifications. |
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|
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* Locking for puts, removes (and, when necessary gets, etc) |
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is controlled by Segments, each covering a portion of the |
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table. During operations requiring global exclusivity (mainly |
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resize and clear), ALL of these locks are acquired at once. |
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Note that these segments are NOT contiguous -- they are based |
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on the least 5 bits of hashcodes. This ensures that the same |
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segment controls the same slots before and after resizing, which |
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is necessary for supporting concurrent retrievals. This |
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comes at the price of a mismatch of logical vs physical locality, |
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but this seems not to be a performance problem in practice. |
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|
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*/ |
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|
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/** |
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* The hash table data. |
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*/ |
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private transient Entry<K,V>[] table; |
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|
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|
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/** |
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* The number of concurrency control segments. |
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* The value can be at most 32 since ints are used |
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* as bitsets over segments. Emprically, it doesn't |
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* seem to pay to decrease it either, so the value should be at least 32. |
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* In other words, do not redefine this :-) |
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**/ |
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private static final int CONCURRENCY_LEVEL = 32; |
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|
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/** |
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* Mask value for indexing into segments |
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**/ |
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private static final int SEGMENT_MASK = CONCURRENCY_LEVEL - 1; |
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|
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/** |
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* Bookkeeping for each concurrency control segment. |
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* Each segment contains a local count of the number of |
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* elements in its region. |
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* However, the main use of a Segment is for its lock. |
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**/ |
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private final static class Segment extends ReentrantLock { |
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/** |
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* The number of elements in this segment's region. |
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**/ |
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private int count; |
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|
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/** |
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* Get the count under synch. |
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**/ |
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private int getCount() { |
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lock(); |
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try { |
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return count; |
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} |
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finally { |
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unlock(); |
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} |
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} |
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|
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} |
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|
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/** |
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* The array of concurrency control segments. |
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**/ |
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private transient final Segment[] segments = new Segment[CONCURRENCY_LEVEL]; |
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|
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|
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/** |
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* The default initial number of table slots for this table (32). |
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* Used when not otherwise specified in constructor. |
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**/ |
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public static int DEFAULT_INITIAL_CAPACITY = 32; |
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|
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|
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/** |
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* The minimum capacity, used if a lower value is implicitly specified |
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* by either of the constructors with arguments. |
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* MUST be a power of two. |
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*/ |
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private static final int MINIMUM_CAPACITY = 32; |
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|
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/** |
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* The maximum capacity, used if a higher value is implicitly specified |
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* by either of the constructors with arguments. |
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* MUST be a power of two <= 1<<30. |
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*/ |
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private static final int MAXIMUM_CAPACITY = 1 << 30; |
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|
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/** |
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* The default load factor for this table (0.75) |
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* Used when not otherwise specified in constructor. |
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**/ |
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public static final float DEFAULT_LOAD_FACTOR = 0.75f; |
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|
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/** |
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* The load factor for the hash table. |
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* |
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* @serial |
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*/ |
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private final float loadFactor; |
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|
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/** |
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* Per-segment resize threshold. |
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* |
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* @serial |
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*/ |
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private int threshold; |
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|
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|
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/** |
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* Number of segments voting for resize. The table is |
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* doubled when 1/4 of the segments reach threshold. |
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* Volatile but updated without synch since this is just a heuristic. |
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**/ |
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private transient volatile int votesForResize; |
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|
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|
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/** |
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* Return the number of set bits in w. |
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* For a derivation of this algorithm, see |
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* "Algorithms and data structures with applications to |
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* graphics and geometry", by Jurg Nievergelt and Klaus Hinrichs, |
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* Prentice Hall, 1993. |
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* See also notes by Torsten Sillke at |
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* http://www.mathematik.uni-bielefeld.de/~sillke/PROBLEMS/bitcount |
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**/ |
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private static int bitcount(int w) { |
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w -= (0xaaaaaaaa & w) >>> 1; |
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w = (w & 0x33333333) + ((w >>> 2) & 0x33333333); |
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w = (w + (w >>> 4)) & 0x0f0f0f0f; |
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w += w >>> 8; |
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w += w >>> 16; |
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return w & 0xff; |
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} |
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|
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/** |
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* Returns the appropriate capacity (power of two) for the specified |
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* initial capacity argument. |
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*/ |
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private int p2capacity(int initialCapacity) { |
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int cap = initialCapacity; |
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|
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// Compute the appropriate capacity |
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int result; |
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if (cap > MAXIMUM_CAPACITY || cap < 0) { |
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result = MAXIMUM_CAPACITY; |
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} else { |
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result = MINIMUM_CAPACITY; |
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while (result < cap) |
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result <<= 1; |
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} |
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return result; |
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} |
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|
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/** |
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* Return hash code for Object x. Since we are using power-of-two |
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* tables, it is worth the effort to improve hashcode via |
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* the same multiplicative scheme as used in IdentityHashMap. |
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*/ |
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private static int hash(Object x) { |
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int h = x.hashCode(); |
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// Multiply by 127 (quickly, via shifts), and mix in some high |
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// bits to help guard against bunching of codes that are |
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// consecutive or equally spaced. |
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return ((h << 7) - h + (h >>> 9) + (h >>> 17)); |
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} |
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|
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|
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/** |
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* Check for equality of non-null references x and y. |
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**/ |
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private boolean eq(Object x, Object y) { |
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return x == y || x.equals(y); |
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} |
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|
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/** Create table array and set the per-segment threshold **/ |
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private Entry<K,V>[] newTable(int capacity) { |
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threshold = (int)(capacity * loadFactor / CONCURRENCY_LEVEL) + 1; |
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return new Entry<K,V>[capacity]; |
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} |
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|
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/** |
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* Constructs a new, empty map with the specified initial |
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* capacity and the specified load factor. |
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* |
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* @param initialCapacity the initial capacity. |
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* The actual initial capacity is rounded up to the nearest power of two. |
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* @param loadFactor the load factor threshold, used to control resizing. |
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* This value is used in an approximate way: When at least |
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* a quarter of the segments of the table reach per-segment threshold, or |
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* one of the segments itself exceeds overall threshold, |
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* the table is doubled. |
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* This will on average cause resizing when the table-wide |
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* load factor is slightly less than the threshold. If you'd like |
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* to avoid resizing, you can set this to a ridiculously large |
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* value. |
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* @throws IllegalArgumentException if the load factor is nonpositive. |
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*/ |
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public ConcurrentHashMap(int initialCapacity, float loadFactor) { |
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if (!(loadFactor > 0)) |
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throw new IllegalArgumentException("Illegal Load factor: "+ loadFactor); |
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this.loadFactor = loadFactor; |
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for (int i = 0; i < segments.length; ++i) |
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segments[i] = new Segment(); |
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int cap = p2capacity(initialCapacity); |
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table = newTable(cap); |
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} |
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|
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/** |
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* Constructs a new, empty map with the specified initial |
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* capacity and default load factor. |
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* |
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* @param initialCapacity the initial capacity of the |
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* ConcurrentHashMap. |
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* @throws IllegalArgumentException if the initial maximum number |
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* of elements is less |
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* than zero. |
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*/ |
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public ConcurrentHashMap(int initialCapacity) { |
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this(initialCapacity, DEFAULT_LOAD_FACTOR); |
324 |
} |
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|
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/** |
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* Constructs a new, empty map with a default initial capacity |
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* and default load factor. |
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*/ |
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public ConcurrentHashMap() { |
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this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR); |
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} |
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|
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/** |
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* Constructs a new map with the same mappings as the given map. The |
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* map is created with a capacity of twice the number of mappings in |
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* the given map or 32 (whichever is greater), and a default load factor. |
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*/ |
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public <A extends K, B extends V> ConcurrentHashMap(Map<A,B> t) { |
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this(Math.max((int) (t.size() / DEFAULT_LOAD_FACTOR) + 1, |
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MINIMUM_CAPACITY), |
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DEFAULT_LOAD_FACTOR); |
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putAll(t); |
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} |
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|
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/** |
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* Returns the number of key-value mappings in this map. |
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* |
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* @return the number of key-value mappings in this map. |
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*/ |
351 |
public int size() { |
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int c = 0; |
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for (int i = 0; i < segments.length; ++i) |
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c += segments[i].getCount(); |
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return c; |
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} |
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|
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/** |
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* Returns <tt>true</tt> if this map contains no key-value mappings. |
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* |
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* @return <tt>true</tt> if this map contains no key-value mappings. |
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*/ |
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public boolean isEmpty() { |
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for (int i = 0; i < segments.length; ++i) |
365 |
if (segments[i].getCount() != 0) |
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return false; |
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return true; |
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} |
369 |
|
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|
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/** |
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* Returns the value to which the specified key is mapped in this table. |
373 |
* |
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* @param key a key in the table. |
375 |
* @return the value to which the key is mapped in this table; |
376 |
* <code>null</code> if the key is not mapped to any value in |
377 |
* this table. |
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* @exception NullPointerException if the key is |
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* <code>null</code>. |
380 |
* @see #put(Object, Object) |
381 |
*/ |
382 |
public V get(K key) { |
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int hash = hash(key); // throws null pointer exception if key null |
384 |
|
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// Try first without locking... |
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Entry<K,V>[] tab = table; |
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int index = hash & (tab.length - 1); |
388 |
Entry<K,V> first = tab[index]; |
389 |
Entry<K,V> e; |
390 |
|
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for (e = first; e != null; e = e.next) { |
392 |
if (e.hash == hash && eq(key, e.key)) { |
393 |
V value = e.value; |
394 |
if (value != null) |
395 |
return value; |
396 |
else |
397 |
break; |
398 |
} |
399 |
} |
400 |
|
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// Recheck under synch if key apparently not there or interference |
402 |
Segment seg = segments[hash & SEGMENT_MASK]; |
403 |
seg.lock(); |
404 |
try { |
405 |
tab = table; |
406 |
index = hash & (tab.length - 1); |
407 |
Entry<K,V> newFirst = tab[index]; |
408 |
if (e != null || first != newFirst) { |
409 |
for (e = newFirst; e != null; e = e.next) { |
410 |
if (e.hash == hash && eq(key, e.key)) |
411 |
return e.value; |
412 |
} |
413 |
} |
414 |
return null; |
415 |
} |
416 |
finally { |
417 |
seg.unlock(); |
418 |
} |
419 |
} |
420 |
|
421 |
/** |
422 |
* Tests if the specified object is a key in this table. |
423 |
* |
424 |
* @param key possible key. |
425 |
* @return <code>true</code> if and only if the specified object |
426 |
* is a key in this table, as determined by the |
427 |
* <tt>equals</tt> method; <code>false</code> otherwise. |
428 |
* @exception NullPointerException if the key is |
429 |
* <code>null</code>. |
430 |
* @see #contains(Object) |
431 |
*/ |
432 |
public boolean containsKey(Object key) { |
433 |
// Annoyingly, for now, duplicate get, since can't call |
434 |
// because different signatures. |
435 |
|
436 |
int hash = hash(key); // throws null pointer exception if key null |
437 |
|
438 |
// Try first without locking... |
439 |
Entry<K,V>[] tab = table; |
440 |
int index = hash & (tab.length - 1); |
441 |
Entry<K,V> first = tab[index]; |
442 |
Entry<K,V> e; |
443 |
|
444 |
for (e = first; e != null; e = e.next) { |
445 |
if (e.hash == hash && eq(key, e.key)) { |
446 |
V value = e.value; |
447 |
if (value != null) |
448 |
return true; |
449 |
else |
450 |
break; |
451 |
} |
452 |
} |
453 |
|
454 |
// Recheck under synch if key apparently not there or interference |
455 |
Segment seg = segments[hash & SEGMENT_MASK]; |
456 |
seg.lock(); |
457 |
try { |
458 |
tab = table; |
459 |
index = hash & (tab.length - 1); |
460 |
Entry<K,V> newFirst = tab[index]; |
461 |
if (e != null || first != newFirst) { |
462 |
for (e = newFirst; e != null; e = e.next) { |
463 |
if (e.hash == hash && eq(key, e.key)) |
464 |
return true; |
465 |
} |
466 |
} |
467 |
return false; |
468 |
} |
469 |
finally { |
470 |
seg.unlock(); |
471 |
} |
472 |
} |
473 |
|
474 |
|
475 |
/** |
476 |
* Maps the specified <code>key</code> to the specified |
477 |
* <code>value</code> in this table. Neither the key nor the |
478 |
* value can be <code>null</code>. (Note that this policy is |
479 |
* the same as for java.util.Hashtable, but unlike java.util.HashMap, |
480 |
* which does accept nulls as valid keys and values.)<p> |
481 |
* |
482 |
* The value can be retrieved by calling the <code>get</code> method |
483 |
* with a key that is equal to the original key. |
484 |
* |
485 |
* @param key the table key. |
486 |
* @param value the value. |
487 |
* @return the previous value of the specified key in this table, |
488 |
* or <code>null</code> if it did not have one. |
489 |
* @exception NullPointerException if the key or value is |
490 |
* <code>null</code>. |
491 |
* @see Object#equals(Object) |
492 |
* @see #get(Object) |
493 |
*/ |
494 |
public V put(K key, V value) { |
495 |
if (value == null) |
496 |
throw new NullPointerException(); |
497 |
|
498 |
int hash = hash(key); |
499 |
Segment seg = segments[hash & SEGMENT_MASK]; |
500 |
int segcount; |
501 |
Entry<K,V>[] tab; |
502 |
int votes; |
503 |
|
504 |
seg.lock(); |
505 |
try { |
506 |
tab = table; |
507 |
int index = hash & (tab.length-1); |
508 |
Entry<K,V> first = tab[index]; |
509 |
|
510 |
for (Entry<K,V> e = first; e != null; e = e.next) { |
511 |
if (e.hash == hash && eq(key, e.key)) { |
512 |
V oldValue = e.value; |
513 |
e.value = value; |
514 |
return oldValue; |
515 |
} |
516 |
} |
517 |
|
518 |
// Add to front of list |
519 |
Entry<K,V> newEntry = new Entry<K,V>(hash, key, value, first); |
520 |
tab[index] = newEntry; |
521 |
|
522 |
if ((segcount = ++seg.count) < threshold) |
523 |
return null; |
524 |
|
525 |
int bit = (1 << (hash & SEGMENT_MASK)); |
526 |
votes = votesForResize; |
527 |
if ((votes & bit) == 0) |
528 |
votes = votesForResize |= bit; |
529 |
} |
530 |
finally { |
531 |
seg.unlock(); |
532 |
} |
533 |
|
534 |
// Attempt resize if 1/4 segs vote, |
535 |
// or if this seg itself reaches the overall threshold. |
536 |
// (The latter check is just a safeguard to avoid pathological cases.) |
537 |
if (bitcount(votes) >= CONCURRENCY_LEVEL / 4 || |
538 |
segcount > (threshold * CONCURRENCY_LEVEL)) |
539 |
resize(tab); |
540 |
|
541 |
return null; |
542 |
} |
543 |
|
544 |
public V putIfAbsent(K key, V value) { |
545 |
if (value == null) |
546 |
throw new NullPointerException(); |
547 |
|
548 |
int hash = hash(key); |
549 |
Segment seg = segments[hash & SEGMENT_MASK]; |
550 |
int segcount; |
551 |
Entry<K,V>[] tab; |
552 |
int votes; |
553 |
|
554 |
seg.lock(); |
555 |
try { |
556 |
tab = table; |
557 |
int index = hash & (tab.length-1); |
558 |
Entry<K,V> first = tab[index]; |
559 |
|
560 |
for (Entry<K,V> e = first; e != null; e = e.next) { |
561 |
if (e.hash == hash && eq(key, e.key)) { |
562 |
V oldValue = e.value; |
563 |
return oldValue; |
564 |
} |
565 |
} |
566 |
|
567 |
// Add to front of list |
568 |
Entry<K,V> newEntry = new Entry<K,V>(hash, key, value, first); |
569 |
tab[index] = newEntry; |
570 |
|
571 |
if ((segcount = ++seg.count) < threshold) |
572 |
return null; |
573 |
|
574 |
int bit = (1 << (hash & SEGMENT_MASK)); |
575 |
votes = votesForResize; |
576 |
if ((votes & bit) == 0) |
577 |
votes = votesForResize |= bit; |
578 |
} |
579 |
finally { |
580 |
seg.unlock(); |
581 |
} |
582 |
|
583 |
// Attempt resize if 1/4 segs vote, |
584 |
// or if this seg itself reaches the overall threshold. |
585 |
// (The latter check is just a safeguard to avoid pathological cases.) |
586 |
if (bitcount(votes) >= CONCURRENCY_LEVEL / 4 || |
587 |
segcount > (threshold * CONCURRENCY_LEVEL)) |
588 |
resize(tab); |
589 |
|
590 |
return value; |
591 |
} |
592 |
|
593 |
/** |
594 |
* Gather all locks in order to call rehash, by |
595 |
* recursing within synch blocks for each segment index. |
596 |
* @param index the current segment. initially call value must be 0 |
597 |
* @param assumedTab the state of table on first call to resize. If |
598 |
* this changes on any call, the attempt is aborted because the |
599 |
* table has already been resized by another thread. |
600 |
*/ |
601 |
private void resize(Entry<K,V>[] assumedTab) { |
602 |
boolean ok = true; |
603 |
int lastlocked = 0; |
604 |
for (int i = 0; i < segments.length; ++i) { |
605 |
segments[i].lock(); |
606 |
lastlocked = i; |
607 |
if (table != assumedTab) { |
608 |
ok = false; |
609 |
break; |
610 |
} |
611 |
} |
612 |
try { |
613 |
if (ok) |
614 |
rehash(); |
615 |
} |
616 |
finally { |
617 |
for (int i = lastlocked; i >= 0; --i) |
618 |
segments[i].unlock(); |
619 |
} |
620 |
} |
621 |
|
622 |
/** |
623 |
* Rehashes the contents of this map into a new table |
624 |
* with a larger capacity. |
625 |
*/ |
626 |
private void rehash() { |
627 |
votesForResize = 0; // reset |
628 |
|
629 |
Entry<K,V>[] oldTable = table; |
630 |
int oldCapacity = oldTable.length; |
631 |
|
632 |
if (oldCapacity >= MAXIMUM_CAPACITY) { |
633 |
threshold = Integer.MAX_VALUE; // avoid retriggering |
634 |
return; |
635 |
} |
636 |
|
637 |
int newCapacity = oldCapacity << 1; |
638 |
Entry<K,V>[] newTable = newTable(newCapacity); |
639 |
int mask = newCapacity - 1; |
640 |
|
641 |
/* |
642 |
* Reclassify nodes in each list to new Map. Because we are |
643 |
* using power-of-two expansion, the elements from each bin |
644 |
* must either stay at same index, or move to |
645 |
* oldCapacity+index. We also eliminate unnecessary node |
646 |
* creation by catching cases where old nodes can be reused |
647 |
* because their next fields won't change. Statistically, at |
648 |
* the default threshhold, only about one-sixth of them need |
649 |
* cloning. (The nodes they replace will be garbage |
650 |
* collectable as soon as they are no longer referenced by any |
651 |
* reader thread that may be in the midst of traversing table |
652 |
* right now.) |
653 |
*/ |
654 |
|
655 |
for (int i = 0; i < oldCapacity ; i++) { |
656 |
// We need to guarantee that any existing reads of old Map can |
657 |
// proceed. So we cannot yet null out each bin. |
658 |
Entry<K,V> e = oldTable[i]; |
659 |
|
660 |
if (e != null) { |
661 |
int idx = e.hash & mask; |
662 |
Entry<K,V> next = e.next; |
663 |
|
664 |
// Single node on list |
665 |
if (next == null) |
666 |
newTable[idx] = e; |
667 |
|
668 |
else { |
669 |
// Reuse trailing consecutive sequence of all same bit |
670 |
Entry<K,V> lastRun = e; |
671 |
int lastIdx = idx; |
672 |
for (Entry<K,V> last = next; last != null; last = last.next) { |
673 |
int k = last.hash & mask; |
674 |
if (k != lastIdx) { |
675 |
lastIdx = k; |
676 |
lastRun = last; |
677 |
} |
678 |
} |
679 |
newTable[lastIdx] = lastRun; |
680 |
|
681 |
// Clone all remaining nodes |
682 |
for (Entry<K,V> p = e; p != lastRun; p = p.next) { |
683 |
int k = p.hash & mask; |
684 |
newTable[k] = new Entry<K,V>(p.hash, p.key, |
685 |
p.value, newTable[k]); |
686 |
} |
687 |
} |
688 |
} |
689 |
} |
690 |
|
691 |
table = newTable; |
692 |
} |
693 |
|
694 |
|
695 |
/** |
696 |
* Removes the key (and its corresponding value) from this |
697 |
* table. This method does nothing if the key is not in the table. |
698 |
* |
699 |
* @param key the key that needs to be removed. |
700 |
* @return the value to which the key had been mapped in this table, |
701 |
* or <code>null</code> if the key did not have a mapping. |
702 |
* @exception NullPointerException if the key is |
703 |
* <code>null</code>. |
704 |
*/ |
705 |
public V remove(Object key) { |
706 |
return remove(key, null); |
707 |
} |
708 |
|
709 |
|
710 |
/** |
711 |
* Removes the (key, value) pair from this |
712 |
* table. This method does nothing if the key is not in the table, |
713 |
* or if the key is associated with a different value. This method |
714 |
* is needed by EntrySet. |
715 |
* |
716 |
* @param key the key that needs to be removed. |
717 |
* @param value the associated value. If the value is null, |
718 |
* it means "any value". |
719 |
* @return the value to which the key had been mapped in this table, |
720 |
* or <code>null</code> if the key did not have a mapping. |
721 |
* @exception NullPointerException if the key is |
722 |
* <code>null</code>. |
723 |
*/ |
724 |
private V remove(Object key, V value) { |
725 |
/* |
726 |
Find the entry, then |
727 |
1. Set value field to null, to force get() to retry |
728 |
2. Rebuild the list without this entry. |
729 |
All entries following removed node can stay in list, but |
730 |
all preceeding ones need to be cloned. Traversals rely |
731 |
on this strategy to ensure that elements will not be |
732 |
repeated during iteration. |
733 |
*/ |
734 |
|
735 |
int hash = hash(key); |
736 |
Segment seg = segments[hash & SEGMENT_MASK]; |
737 |
|
738 |
seg.lock(); |
739 |
try { |
740 |
Entry<K,V>[] tab = table; |
741 |
int index = hash & (tab.length-1); |
742 |
Entry<K,V> first = tab[index]; |
743 |
Entry<K,V> e = first; |
744 |
|
745 |
for (;;) { |
746 |
if (e == null) |
747 |
return null; |
748 |
if (e.hash == hash && eq(key, e.key)) |
749 |
break; |
750 |
e = e.next; |
751 |
} |
752 |
|
753 |
V oldValue = e.value; |
754 |
if (value != null && !value.equals(oldValue)) |
755 |
return null; |
756 |
|
757 |
e.value = null; |
758 |
|
759 |
Entry<K,V> head = e.next; |
760 |
for (Entry<K,V> p = first; p != e; p = p.next) |
761 |
head = new Entry<K,V>(p.hash, p.key, p.value, head); |
762 |
tab[index] = head; |
763 |
seg.count--; |
764 |
return oldValue; |
765 |
} |
766 |
finally { |
767 |
seg.unlock(); |
768 |
} |
769 |
} |
770 |
|
771 |
|
772 |
/** |
773 |
* Returns <tt>true</tt> if this map maps one or more keys to the |
774 |
* specified value. Note: This method requires a full internal |
775 |
* traversal of the hash table, and so is much slower than |
776 |
* method <tt>containsKey</tt>. |
777 |
* |
778 |
* @param value value whose presence in this map is to be tested. |
779 |
* @return <tt>true</tt> if this map maps one or more keys to the |
780 |
* specified value. |
781 |
* @exception NullPointerException if the value is <code>null</code>. |
782 |
*/ |
783 |
public boolean containsValue(Object value) { |
784 |
|
785 |
if (value == null) throw new NullPointerException(); |
786 |
|
787 |
for (int s = 0; s < segments.length; ++s) { |
788 |
Segment seg = segments[s]; |
789 |
Entry<K,V>[] tab; |
790 |
seg.lock(); |
791 |
try { |
792 |
tab = table; |
793 |
} |
794 |
finally { |
795 |
seg.unlock(); |
796 |
} |
797 |
for (int i = s; i < tab.length; i+= segments.length) { |
798 |
for (Entry<K,V> e = tab[i]; e != null; e = e.next) |
799 |
if (value.equals(e.value)) |
800 |
return true; |
801 |
} |
802 |
} |
803 |
return false; |
804 |
} |
805 |
|
806 |
/** |
807 |
* Tests if some key maps into the specified value in this table. |
808 |
* This operation is more expensive than the <code>containsKey</code> |
809 |
* method.<p> |
810 |
* |
811 |
* Note that this method is identical in functionality to containsValue, |
812 |
* (which is part of the Map interface in the collections framework). |
813 |
* |
814 |
* @param value a value to search for. |
815 |
* @return <code>true</code> if and only if some key maps to the |
816 |
* <code>value</code> argument in this table as |
817 |
* determined by the <tt>equals</tt> method; |
818 |
* <code>false</code> otherwise. |
819 |
* @exception NullPointerException if the value is <code>null</code>. |
820 |
* @see #containsKey(Object) |
821 |
* @see #containsValue(Object) |
822 |
* @see Map |
823 |
*/ |
824 |
public boolean contains(V value) { |
825 |
return containsValue(value); |
826 |
} |
827 |
|
828 |
/** |
829 |
* Copies all of the mappings from the specified map to this one. |
830 |
* |
831 |
* These mappings replace any mappings that this map had for any of the |
832 |
* keys currently in the specified Map. |
833 |
* |
834 |
* @param t Mappings to be stored in this map. |
835 |
*/ |
836 |
public <A extends K, B extends V> void putAll(Map<A, B> t) { |
837 |
int n = t.size(); |
838 |
if (n == 0) |
839 |
return; |
840 |
|
841 |
// Expand enough to hold at least n elements without resizing. |
842 |
// We can only resize table by factor of two at a time. |
843 |
// It is faster to rehash with fewer elements, so do it now. |
844 |
for(;;) { |
845 |
Entry<K,V>[] tab; |
846 |
int max; |
847 |
// must synch on some segment. pick 0. |
848 |
segments[0].lock(); |
849 |
try { |
850 |
tab = table; |
851 |
max = threshold * CONCURRENCY_LEVEL; |
852 |
} |
853 |
finally { |
854 |
segments[0].unlock(); |
855 |
} |
856 |
if (n < max) |
857 |
break; |
858 |
resize(tab); |
859 |
} |
860 |
|
861 |
for (Iterator<Map.Entry<A,B>> it = t.entrySet().iterator(); it.hasNext();) { |
862 |
Map.Entry<A,B> entry = (Map.Entry<A,B>) it.next(); |
863 |
put(entry.getKey(), entry.getValue()); |
864 |
} |
865 |
} |
866 |
|
867 |
/** |
868 |
* Removes all mappings from this map. |
869 |
*/ |
870 |
public void clear() { |
871 |
// We don't need all locks at once so long as locks |
872 |
// are obtained in low to high order |
873 |
for (int s = 0; s < segments.length; ++s) { |
874 |
Segment seg = segments[s]; |
875 |
seg.lock(); |
876 |
try { |
877 |
Entry<K,V>[] tab = table; |
878 |
for (int i = s; i < tab.length; i+= segments.length) { |
879 |
for (Entry<K,V> e = tab[i]; e != null; e = e.next) |
880 |
e.value = null; |
881 |
tab[i] = null; |
882 |
seg.count = 0; |
883 |
} |
884 |
} |
885 |
finally { |
886 |
seg.unlock(); |
887 |
} |
888 |
} |
889 |
} |
890 |
|
891 |
/** |
892 |
* Returns a shallow copy of this |
893 |
* <tt>ConcurrentHashMap</tt> instance: the keys and |
894 |
* values themselves are not cloned. |
895 |
* |
896 |
* @return a shallow copy of this map. |
897 |
*/ |
898 |
public Object clone() { |
899 |
// We cannot call super.clone, since it would share final segments array, |
900 |
// and there's no way to reassign finals. |
901 |
return new ConcurrentHashMap<K,V>(this); |
902 |
} |
903 |
|
904 |
// Views |
905 |
|
906 |
private transient Set<K> keySet = null; |
907 |
private transient Set<Map.Entry<K,V>> entrySet = null; |
908 |
private transient Collection<V> values = null; |
909 |
|
910 |
/** |
911 |
* Returns a set view of the keys contained in this map. The set is |
912 |
* backed by the map, so changes to the map are reflected in the set, and |
913 |
* vice-versa. The set supports element removal, which removes the |
914 |
* corresponding mapping from this map, via the <tt>Iterator.remove</tt>, |
915 |
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt>, and |
916 |
* <tt>clear</tt> operations. It does not support the <tt>add</tt> or |
917 |
* <tt>addAll</tt> operations. |
918 |
* |
919 |
* @return a set view of the keys contained in this map. |
920 |
*/ |
921 |
public Set<K> keySet() { |
922 |
Set<K> ks = keySet; |
923 |
return (ks != null)? ks : (keySet = new KeySet()); |
924 |
} |
925 |
|
926 |
private class KeySet extends AbstractSet<K> { |
927 |
public Iterator<K> iterator() { |
928 |
return new KeyIterator(); |
929 |
} |
930 |
public int size() { |
931 |
return ConcurrentHashMap.this.size(); |
932 |
} |
933 |
public boolean contains(Object o) { |
934 |
return ConcurrentHashMap.this.containsKey(o); |
935 |
} |
936 |
public boolean remove(Object o) { |
937 |
return ConcurrentHashMap.this.remove(o) != null; |
938 |
} |
939 |
public void clear() { |
940 |
ConcurrentHashMap.this.clear(); |
941 |
} |
942 |
} |
943 |
|
944 |
/** |
945 |
* Returns a collection view of the values contained in this map. The |
946 |
* collection is backed by the map, so changes to the map are reflected in |
947 |
* the collection, and vice-versa. The collection supports element |
948 |
* removal, which removes the corresponding mapping from this map, via the |
949 |
* <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>, |
950 |
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations. |
951 |
* It does not support the <tt>add</tt> or <tt>addAll</tt> operations. |
952 |
* |
953 |
* @return a collection view of the values contained in this map. |
954 |
*/ |
955 |
public Collection<V> values() { |
956 |
Collection<V> vs = values; |
957 |
return (vs != null)? vs : (values = new Values()); |
958 |
} |
959 |
|
960 |
private class Values extends AbstractCollection<V> { |
961 |
public Iterator<V> iterator() { |
962 |
return new ValueIterator(); |
963 |
} |
964 |
public int size() { |
965 |
return ConcurrentHashMap.this.size(); |
966 |
} |
967 |
public boolean contains(Object o) { |
968 |
return ConcurrentHashMap.this.containsValue(o); |
969 |
} |
970 |
public void clear() { |
971 |
ConcurrentHashMap.this.clear(); |
972 |
} |
973 |
} |
974 |
|
975 |
/** |
976 |
* Returns a collection view of the mappings contained in this map. Each |
977 |
* element in the returned collection is a <tt>Map.Entry</tt>. The |
978 |
* collection is backed by the map, so changes to the map are reflected in |
979 |
* the collection, and vice-versa. The collection supports element |
980 |
* removal, which removes the corresponding mapping from the map, via the |
981 |
* <tt>Iterator.remove</tt>, <tt>Collection.remove</tt>, |
982 |
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> operations. |
983 |
* It does not support the <tt>add</tt> or <tt>addAll</tt> operations. |
984 |
* |
985 |
* @return a collection view of the mappings contained in this map. |
986 |
*/ |
987 |
public Set<Map.Entry<K,V>> entrySet() { |
988 |
Set<Map.Entry<K,V>> es = entrySet; |
989 |
return (es != null) ? es : (entrySet = new EntrySet()); |
990 |
} |
991 |
|
992 |
private class EntrySet extends AbstractSet<Map.Entry<K,V>> { |
993 |
public Iterator<Map.Entry<K,V>> iterator() { |
994 |
return new EntryIterator(); |
995 |
} |
996 |
public boolean contains(Map.Entry<K,V> entry) { |
997 |
V v = ConcurrentHashMap.this.get(entry.getKey()); |
998 |
return v != null && v.equals(entry.getValue()); |
999 |
} |
1000 |
public boolean remove(Map.Entry<K,V> e) { |
1001 |
return ConcurrentHashMap.this.remove(e.getKey(), e.getValue()) != null; |
1002 |
} |
1003 |
public int size() { |
1004 |
return ConcurrentHashMap.this.size(); |
1005 |
} |
1006 |
public void clear() { |
1007 |
ConcurrentHashMap.this.clear(); |
1008 |
} |
1009 |
} |
1010 |
|
1011 |
/** |
1012 |
* Returns an enumeration of the keys in this table. |
1013 |
* |
1014 |
* @return an enumeration of the keys in this table. |
1015 |
* @see Enumeration |
1016 |
* @see #elements() |
1017 |
* @see #keySet() |
1018 |
* @see Map |
1019 |
*/ |
1020 |
public Enumeration keys() { |
1021 |
return new KeyIterator(); |
1022 |
} |
1023 |
|
1024 |
/** |
1025 |
* Returns an enumeration of the values in this table. |
1026 |
* Use the Enumeration methods on the returned object to fetch the elements |
1027 |
* sequentially. |
1028 |
* |
1029 |
* @return an enumeration of the values in this table. |
1030 |
* @see java.util.Enumeration |
1031 |
* @see #keys() |
1032 |
* @see #values() |
1033 |
* @see Map |
1034 |
*/ |
1035 |
public Enumeration elements() { |
1036 |
return new ValueIterator(); |
1037 |
} |
1038 |
|
1039 |
/** |
1040 |
* ConcurrentHashMap collision list entry. |
1041 |
*/ |
1042 |
private static class Entry<K,V> implements Map.Entry<K,V> { |
1043 |
/* |
1044 |
The use of volatile for value field ensures that |
1045 |
we can detect status changes without synchronization. |
1046 |
The other fields are never changed, and are |
1047 |
marked as final. |
1048 |
*/ |
1049 |
|
1050 |
private final K key; |
1051 |
private volatile V value; |
1052 |
private final int hash; |
1053 |
private final Entry<K,V> next; |
1054 |
|
1055 |
Entry(int hash, K key, V value, Entry<K,V> next) { |
1056 |
this.value = value; |
1057 |
this.hash = hash; |
1058 |
this.key = key; |
1059 |
this.next = next; |
1060 |
} |
1061 |
|
1062 |
// Map.Entry Ops |
1063 |
|
1064 |
public K getKey() { |
1065 |
return key; |
1066 |
} |
1067 |
|
1068 |
/** |
1069 |
* Get the value. Note: In an entrySet or entrySet.iterator, |
1070 |
* unless you can guarantee lack of concurrent modification, |
1071 |
* <tt>getValue</tt> <em>might</em> return null, reflecting the |
1072 |
* fact that the entry has been concurrently removed. However, |
1073 |
* there are no assurances that concurrent removals will be |
1074 |
* reflected using this method. |
1075 |
* |
1076 |
* @return the current value, or null if the entry has been |
1077 |
* detectably removed. |
1078 |
**/ |
1079 |
public V getValue() { |
1080 |
return value; |
1081 |
} |
1082 |
|
1083 |
/** |
1084 |
* Set the value of this entry. Note: In an entrySet or |
1085 |
* entrySet.iterator), unless you can guarantee lack of concurrent |
1086 |
* modification, <tt>setValue</tt> is not strictly guaranteed to |
1087 |
* actually replace the value field obtained via the <tt>get</tt> |
1088 |
* operation of the underlying hash table in multithreaded |
1089 |
* applications. If iterator-wide synchronization is not used, |
1090 |
* and any other concurrent <tt>put</tt> or <tt>remove</tt> |
1091 |
* operations occur, sometimes even to <em>other</em> entries, |
1092 |
* then this change is not guaranteed to be reflected in the hash |
1093 |
* table. (It might, or it might not. There are no assurances |
1094 |
* either way.) |
1095 |
* |
1096 |
* @param value the new value. |
1097 |
* @return the previous value, or null if entry has been detectably |
1098 |
* removed. |
1099 |
* @exception NullPointerException if the value is <code>null</code>. |
1100 |
* |
1101 |
**/ |
1102 |
public V setValue(V value) { |
1103 |
if (value == null) |
1104 |
throw new NullPointerException(); |
1105 |
V oldValue = this.value; |
1106 |
this.value = value; |
1107 |
return oldValue; |
1108 |
} |
1109 |
|
1110 |
public boolean equals(Object o) { |
1111 |
if (!(o instanceof Map.Entry)) |
1112 |
return false; |
1113 |
Map.Entry e = (Map.Entry)o; |
1114 |
return (key.equals(e.getKey()) && value.equals(e.getValue())); |
1115 |
} |
1116 |
|
1117 |
public int hashCode() { |
1118 |
return key.hashCode() ^ value.hashCode(); |
1119 |
} |
1120 |
|
1121 |
public String toString() { |
1122 |
return key + "=" + value; |
1123 |
} |
1124 |
|
1125 |
} |
1126 |
|
1127 |
private abstract class HashIterator<T> implements Iterator<T>, Enumeration { |
1128 |
private final Entry<K,V>[] tab; // snapshot of table |
1129 |
private int index; // current slot |
1130 |
Entry<K,V> entry = null; // current node of slot |
1131 |
K currentKey; // key for current node |
1132 |
V currentValue; // value for current node |
1133 |
private Entry lastReturned = null; // last node returned by next |
1134 |
|
1135 |
private HashIterator() { |
1136 |
// force all segments to synch |
1137 |
for (int i = 0; i < segments.length; ++i) { |
1138 |
segments[i].lock(); |
1139 |
segments[i].unlock(); |
1140 |
} |
1141 |
tab = table; |
1142 |
index = tab.length - 1; |
1143 |
} |
1144 |
|
1145 |
public boolean hasMoreElements() { return hasNext(); } |
1146 |
public Object nextElement() { return next(); } |
1147 |
|
1148 |
public boolean hasNext() { |
1149 |
/* |
1150 |
currentkey and currentValue are set here to ensure that next() |
1151 |
returns normally if hasNext() returns true. This avoids |
1152 |
surprises especially when final element is removed during |
1153 |
traversal -- instead, we just ignore the removal during |
1154 |
current traversal. |
1155 |
*/ |
1156 |
|
1157 |
while (true) { |
1158 |
if (entry != null) { |
1159 |
V v = entry.value; |
1160 |
if (v != null) { |
1161 |
currentKey = entry.key; |
1162 |
currentValue = v; |
1163 |
return true; |
1164 |
} |
1165 |
else |
1166 |
entry = entry.next; |
1167 |
} |
1168 |
|
1169 |
while (entry == null && index >= 0) |
1170 |
entry = tab[index--]; |
1171 |
|
1172 |
if (entry == null) { |
1173 |
currentKey = null; |
1174 |
currentValue = null; |
1175 |
return false; |
1176 |
} |
1177 |
} |
1178 |
} |
1179 |
|
1180 |
abstract T returnValueOfNext(); |
1181 |
|
1182 |
public T next() { |
1183 |
if (currentKey == null && !hasNext()) |
1184 |
throw new NoSuchElementException(); |
1185 |
|
1186 |
T result = returnValueOfNext(); |
1187 |
lastReturned = entry; |
1188 |
currentKey = null; |
1189 |
currentValue = null; |
1190 |
entry = entry.next; |
1191 |
return result; |
1192 |
} |
1193 |
|
1194 |
public void remove() { |
1195 |
if (lastReturned == null) |
1196 |
throw new IllegalStateException(); |
1197 |
ConcurrentHashMap.this.remove(lastReturned.key); |
1198 |
lastReturned = null; |
1199 |
} |
1200 |
|
1201 |
} |
1202 |
|
1203 |
private class KeyIterator extends HashIterator<K> { |
1204 |
K returnValueOfNext() { return currentKey; } |
1205 |
public K next() { return super.next(); } |
1206 |
} |
1207 |
|
1208 |
private class ValueIterator extends HashIterator<V> { |
1209 |
V returnValueOfNext() { return currentValue; } |
1210 |
public V next() { return super.next(); } |
1211 |
} |
1212 |
|
1213 |
private class EntryIterator extends HashIterator<Map.Entry<K,V>> { |
1214 |
Map.Entry<K,V> returnValueOfNext() { return entry; } |
1215 |
public Map.Entry<K,V> next() { return super.next(); } |
1216 |
} |
1217 |
|
1218 |
/** |
1219 |
* Save the state of the <tt>ConcurrentHashMap</tt> |
1220 |
* instance to a stream (i.e., |
1221 |
* serialize it). |
1222 |
* |
1223 |
* @serialData |
1224 |
* An estimate of the table size, followed by |
1225 |
* the key (Object) and value (Object) |
1226 |
* for each key-value mapping, followed by a null pair. |
1227 |
* The key-value mappings are emitted in no particular order. |
1228 |
*/ |
1229 |
private void writeObject(java.io.ObjectOutputStream s) throws IOException { |
1230 |
// Write out the loadfactor, and any hidden stuff |
1231 |
s.defaultWriteObject(); |
1232 |
|
1233 |
// Write out capacity estimate. It is OK if this |
1234 |
// changes during the write, since it is only used by |
1235 |
// readObject to set initial capacity, to avoid needless resizings. |
1236 |
|
1237 |
int cap; |
1238 |
segments[0].lock(); |
1239 |
try { |
1240 |
cap = table.length; |
1241 |
} |
1242 |
finally { |
1243 |
segments[0].unlock(); |
1244 |
} |
1245 |
s.writeInt(cap); |
1246 |
|
1247 |
// Write out keys and values (alternating) |
1248 |
for (int k = 0; k < segments.length; ++k) { |
1249 |
Segment seg = segments[k]; |
1250 |
Entry[] tab; |
1251 |
seg.lock(); |
1252 |
try { |
1253 |
tab = table; |
1254 |
} |
1255 |
finally { |
1256 |
seg.unlock(); |
1257 |
} |
1258 |
for (int i = k; i < tab.length; i+= segments.length) { |
1259 |
for (Entry e = tab[i]; e != null; e = e.next) { |
1260 |
s.writeObject(e.key); |
1261 |
s.writeObject(e.value); |
1262 |
} |
1263 |
} |
1264 |
} |
1265 |
|
1266 |
s.writeObject(null); |
1267 |
s.writeObject(null); |
1268 |
} |
1269 |
|
1270 |
/** |
1271 |
* Reconstitute the <tt>ConcurrentHashMap</tt> |
1272 |
* instance from a stream (i.e., |
1273 |
* deserialize it). |
1274 |
*/ |
1275 |
private void readObject(java.io.ObjectInputStream s) |
1276 |
throws IOException, ClassNotFoundException { |
1277 |
|
1278 |
// Read in the threshold, loadfactor, and any hidden stuff |
1279 |
s.defaultReadObject(); |
1280 |
|
1281 |
int cap = s.readInt(); |
1282 |
table = newTable(cap); |
1283 |
for (int i = 0; i < segments.length; ++i) |
1284 |
segments[i] = new Segment(); |
1285 |
|
1286 |
|
1287 |
// Read the keys and values, and put the mappings in the table |
1288 |
while (true) { |
1289 |
K key = (K) s.readObject(); |
1290 |
V value = (V) s.readObject(); |
1291 |
if (key == null) |
1292 |
break; |
1293 |
put(key, value); |
1294 |
} |
1295 |
} |
1296 |
} |