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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, as explained at |
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* http://creativecommons.org/licenses/publicdomain |
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*/ |
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|
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package java.util.concurrent; |
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import java.util.concurrent.locks.*; |
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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 hash table supporting full concurrency of retrievals and |
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* adjustable expected concurrency for updates. This class obeys the |
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* same functional specification as {@link java.util.Hashtable}, and |
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* includes versions of methods corresponding to each method of |
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* <tt>Hashtable</tt>. However, even though all operations are |
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* thread-safe, retrieval operations do <em>not</em> entail locking, |
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* and there is <em>not</em> any support for locking the entire table |
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* in a way that prevents all access. This class is fully |
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* interoperable with <tt>Hashtable</tt> in programs that rely on its |
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* thread safety but not on its synchronization details. |
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* |
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* <p> Retrieval operations (including <tt>get</tt>) generally do not |
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* block, so may overlap with update operations (including |
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* <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results |
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* of the most recently <em>completed</em> update operations holding |
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* upon their onset. For aggregate operations such as <tt>putAll</tt> |
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* and <tt>clear</tt>, concurrent retrievals may reflect insertion or |
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* removal of only some entries. Similarly, Iterators and |
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* Enumerations return elements reflecting the state of the hash table |
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* at some point at or since the creation of the iterator/enumeration. |
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* They do <em>not</em> throw {@link ConcurrentModificationException}. |
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* However, iterators are designed to be used by only one thread at a time. |
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* |
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* <p> The allowed concurrency among update operations is guided by |
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* the optional <tt>concurrencyLevel</tt> constructor argument |
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* (default <tt>16</tt>), which is used as a hint for internal sizing. The |
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* table is internally partitioned to try to permit the indicated |
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* number of concurrent updates without contention. Because placement |
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* in hash tables is essentially random, the actual concurrency will |
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* vary. Ideally, you should choose a value to accommodate as many |
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* threads as will ever concurrently modify the table. Using a |
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* significantly higher value than you need can waste space and time, |
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* and a significantly lower value can lead to thread contention. But |
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* overestimates and underestimates within an order of magnitude do |
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* not usually have much noticeable impact. A value of one is |
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* appropriate when it is known that only one thread will modify and |
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* all others will only read. Also, resizing this or any other kind of |
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* hash table is a relatively slow operation, so, when possible, it is |
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* a good idea to provide estimates of expected table sizes in |
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* constructors. |
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* |
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* <p>This class and its views and iterators implement all of the |
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* <em>optional</em> methods of the {@link Map} and {@link Iterator} |
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* interfaces. |
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* |
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* <p> Like {@link Hashtable} but unlike {@link HashMap}, this class |
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* does <em>not</em> allow <tt>null</tt> to be used as a key or value. |
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* |
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* <p>This class is a member of the |
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* <a href="{@docRoot}/../guide/collections/index.html"> |
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* Java Collections Framework</a>. |
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* |
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* @since 1.5 |
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* @author Doug Lea |
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* @param <K> the type of keys maintained by this map |
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* @param <V> the type of mapped values |
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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>, Serializable { |
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private static final long serialVersionUID = 7249069246763182397L; |
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|
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/* |
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* The basic strategy is to subdivide the table among Segments, |
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* each of which itself is a concurrently readable hash table. |
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*/ |
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|
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/* ---------------- Constants -------------- */ |
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|
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/** |
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* The default initial capacity for this table, |
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* used when not otherwise specified in a constructor. |
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*/ |
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static final int DEFAULT_INITIAL_CAPACITY = 16; |
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|
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/** |
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* The default load factor for this table, used when not |
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* otherwise specified in a constructor. |
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*/ |
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static final float DEFAULT_LOAD_FACTOR = 0.75f; |
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|
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/** |
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* The default concurrency level for this table, used when not |
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* otherwise specified in a constructor. |
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*/ |
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static final int DEFAULT_CONCURRENCY_LEVEL = 16; |
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|
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/** |
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* The maximum capacity, used if a higher value is implicitly |
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* specified by either of the constructors with arguments. MUST |
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* be a power of two <= 1<<30 to ensure that entries are indexable |
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* using ints. |
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*/ |
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static final int MAXIMUM_CAPACITY = 1 << 30; |
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|
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/** |
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* The maximum number of segments to allow; used to bound |
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* constructor arguments. |
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*/ |
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static final int MAX_SEGMENTS = 1 << 16; // slightly conservative |
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|
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/** |
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* Number of unsynchronized retries in size and containsValue |
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* methods before resorting to locking. This is used to avoid |
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* unbounded retries if tables undergo continuous modification |
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* which would make it impossible to obtain an accurate result. |
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*/ |
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static final int RETRIES_BEFORE_LOCK = 2; |
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|
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/* ---------------- Fields -------------- */ |
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|
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/** |
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* Mask value for indexing into segments. The upper bits of a |
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* key's hash code are used to choose the segment. |
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*/ |
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final int segmentMask; |
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|
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/** |
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* Shift value for indexing within segments. |
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*/ |
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final int segmentShift; |
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|
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/** |
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* The segments, each of which is a specialized hash table |
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*/ |
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final Segment<K,V>[] segments; |
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|
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transient Set<K> keySet; |
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transient Set<Map.Entry<K,V>> entrySet; |
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transient Collection<V> values; |
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|
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/* ---------------- Small Utilities -------------- */ |
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|
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/** |
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* Returns a hash code for non-null Object x. |
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* Uses the same hash code spreader as most other java.util hash tables. |
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* @param x the object serving as a key |
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* @return the hash code |
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*/ |
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static int hash(Object x) { |
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int h = x.hashCode(); |
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h += ~(h << 9); |
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h ^= (h >>> 14); |
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h += (h << 4); |
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h ^= (h >>> 10); |
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return h; |
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} |
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|
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/** |
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* Returns the segment that should be used for key with given hash |
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* @param hash the hash code for the key |
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* @return the segment |
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*/ |
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final Segment<K,V> segmentFor(int hash) { |
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return segments[(hash >>> segmentShift) & segmentMask]; |
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} |
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|
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/* ---------------- Inner Classes -------------- */ |
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|
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/** |
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* ConcurrentHashMap list entry. Note that this is never exported |
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* out as a user-visible Map.Entry. |
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* |
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* Because the value field is volatile, not final, it is legal wrt |
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* the Java Memory Model for an unsynchronized reader to see null |
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* instead of initial value when read via a data race. Although a |
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* reordering leading to this is not likely to ever actually |
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* occur, the Segment.readValueUnderLock method is used as a |
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* backup in case a null (pre-initialized) value is ever seen in |
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* an unsynchronized access method. |
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*/ |
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static final class HashEntry<K,V> { |
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final K key; |
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final int hash; |
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volatile V value; |
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final HashEntry<K,V> next; |
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|
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HashEntry(K key, int hash, HashEntry<K,V> next, V value) { |
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this.key = key; |
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this.hash = hash; |
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this.next = next; |
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this.value = value; |
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} |
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|
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@SuppressWarnings("unchecked") |
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static final <K,V> HashEntry<K,V>[] newArray(int i) { |
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return new HashEntry[i]; |
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} |
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} |
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|
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/** |
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* Segments are specialized versions of hash tables. This |
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* subclasses from ReentrantLock opportunistically, just to |
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* simplify some locking and avoid separate construction. |
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*/ |
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static final class Segment<K,V> extends ReentrantLock implements Serializable { |
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/* |
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* Segments maintain a table of entry lists that are ALWAYS |
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* kept in a consistent state, so can be read without locking. |
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* Next fields of nodes are immutable (final). All list |
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* additions are performed at the front of each bin. This |
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* makes it easy to check changes, and also fast to traverse. |
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* When nodes would otherwise be changed, new nodes are |
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* created to replace them. This works well for hash tables |
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* since the bin lists tend to be short. (The average length |
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* is less than two for the default load factor threshold.) |
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* |
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* Read operations can thus proceed without locking, but rely |
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* on selected uses of volatiles to ensure that completed |
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* write operations performed by other threads are |
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* noticed. For most purposes, the "count" field, tracking the |
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* number of elements, serves as that volatile variable |
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* ensuring visibility. This is convenient because this field |
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* needs to be read in many read operations anyway: |
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* |
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* - All (unsynchronized) read operations must first read the |
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* "count" field, and should not look at table entries if |
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* it is 0. |
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* |
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* - All (synchronized) write operations should write to |
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* the "count" field after structurally changing any bin. |
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* The operations must not take any action that could even |
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* momentarily cause a concurrent read operation to see |
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* inconsistent data. This is made easier by the nature of |
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* the read operations in Map. For example, no operation |
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* can reveal that the table has grown but the threshold |
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* has not yet been updated, so there are no atomicity |
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* requirements for this with respect to reads. |
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* |
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* As a guide, all critical volatile reads and writes to the |
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* count field are marked in code comments. |
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*/ |
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|
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private static final long serialVersionUID = 2249069246763182397L; |
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|
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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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transient volatile int count; |
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|
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/** |
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* Number of updates that alter the size of the table. This is |
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* used during bulk-read methods to make sure they see a |
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* consistent snapshot: If modCounts change during a traversal |
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* of segments computing size or checking containsValue, then |
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* we might have an inconsistent view of state so (usually) |
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* must retry. |
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*/ |
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transient int modCount; |
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|
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/** |
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* The table is rehashed when its size exceeds this threshold. |
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* (The value of this field is always <tt>(int)(capacity * |
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* loadFactor)</tt>.) |
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*/ |
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transient int threshold; |
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|
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/** |
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* The per-segment table. |
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*/ |
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transient volatile HashEntry<K,V>[] table; |
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|
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/** |
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* The load factor for the hash table. Even though this value |
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* is same for all segments, it is replicated to avoid needing |
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* links to outer object. |
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* @serial |
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*/ |
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final float loadFactor; |
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|
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Segment(int initialCapacity, float lf) { |
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loadFactor = lf; |
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setTable(HashEntry.<K,V>newArray(initialCapacity)); |
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} |
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|
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@SuppressWarnings("unchecked") |
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static final <K,V> Segment<K,V>[] newArray(int i) { |
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return new Segment[i]; |
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} |
294 |
|
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/** |
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* Sets table to new HashEntry array. |
297 |
* Call only while holding lock or in constructor. |
298 |
*/ |
299 |
void setTable(HashEntry<K,V>[] newTable) { |
300 |
threshold = (int)(newTable.length * loadFactor); |
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table = newTable; |
302 |
} |
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|
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/** |
305 |
* Returns properly casted first entry of bin for given hash. |
306 |
*/ |
307 |
HashEntry<K,V> getFirst(int hash) { |
308 |
HashEntry<K,V>[] tab = table; |
309 |
return tab[hash & (tab.length - 1)]; |
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} |
311 |
|
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/** |
313 |
* Reads value field of an entry under lock. Called if value |
314 |
* field ever appears to be null. This is possible only if a |
315 |
* compiler happens to reorder a HashEntry initialization with |
316 |
* its table assignment, which is legal under memory model |
317 |
* but is not known to ever occur. |
318 |
*/ |
319 |
V readValueUnderLock(HashEntry<K,V> e) { |
320 |
lock(); |
321 |
try { |
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return e.value; |
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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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/* Specialized implementations of map methods */ |
329 |
|
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V get(Object key, int hash) { |
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if (count != 0) { // read-volatile |
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HashEntry<K,V> e = getFirst(hash); |
333 |
while (e != null) { |
334 |
if (e.hash == hash && key.equals(e.key)) { |
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V v = e.value; |
336 |
if (v != null) |
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return v; |
338 |
return readValueUnderLock(e); // recheck |
339 |
} |
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e = e.next; |
341 |
} |
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} |
343 |
return null; |
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} |
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|
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boolean containsKey(Object key, int hash) { |
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if (count != 0) { // read-volatile |
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HashEntry<K,V> e = getFirst(hash); |
349 |
while (e != null) { |
350 |
if (e.hash == hash && key.equals(e.key)) |
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return true; |
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e = e.next; |
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} |
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} |
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return false; |
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} |
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|
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boolean containsValue(Object value) { |
359 |
if (count != 0) { // read-volatile |
360 |
HashEntry<K,V>[] tab = table; |
361 |
int len = tab.length; |
362 |
for (int i = 0 ; i < len; i++) { |
363 |
for (HashEntry<K,V> e = tab[i]; e != null; e = e.next) { |
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V v = e.value; |
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if (v == null) // recheck |
366 |
v = readValueUnderLock(e); |
367 |
if (value.equals(v)) |
368 |
return true; |
369 |
} |
370 |
} |
371 |
} |
372 |
return false; |
373 |
} |
374 |
|
375 |
boolean replace(K key, int hash, V oldValue, V newValue) { |
376 |
lock(); |
377 |
try { |
378 |
HashEntry<K,V> e = getFirst(hash); |
379 |
while (e != null && (e.hash != hash || !key.equals(e.key))) |
380 |
e = e.next; |
381 |
|
382 |
boolean replaced = false; |
383 |
if (e != null && oldValue.equals(e.value)) { |
384 |
replaced = true; |
385 |
e.value = newValue; |
386 |
} |
387 |
return replaced; |
388 |
} finally { |
389 |
unlock(); |
390 |
} |
391 |
} |
392 |
|
393 |
V replace(K key, int hash, V newValue) { |
394 |
lock(); |
395 |
try { |
396 |
HashEntry<K,V> e = getFirst(hash); |
397 |
while (e != null && (e.hash != hash || !key.equals(e.key))) |
398 |
e = e.next; |
399 |
|
400 |
V oldValue = null; |
401 |
if (e != null) { |
402 |
oldValue = e.value; |
403 |
e.value = newValue; |
404 |
} |
405 |
return oldValue; |
406 |
} finally { |
407 |
unlock(); |
408 |
} |
409 |
} |
410 |
|
411 |
|
412 |
V put(K key, int hash, V value, boolean onlyIfAbsent) { |
413 |
lock(); |
414 |
try { |
415 |
int c = count; |
416 |
if (c++ > threshold) // ensure capacity |
417 |
rehash(); |
418 |
HashEntry<K,V>[] tab = table; |
419 |
int index = hash & (tab.length - 1); |
420 |
HashEntry<K,V> first = tab[index]; |
421 |
HashEntry<K,V> e = first; |
422 |
while (e != null && (e.hash != hash || !key.equals(e.key))) |
423 |
e = e.next; |
424 |
|
425 |
V oldValue; |
426 |
if (e != null) { |
427 |
oldValue = e.value; |
428 |
if (!onlyIfAbsent) |
429 |
e.value = value; |
430 |
} |
431 |
else { |
432 |
oldValue = null; |
433 |
++modCount; |
434 |
tab[index] = new HashEntry<K,V>(key, hash, first, value); |
435 |
count = c; // write-volatile |
436 |
} |
437 |
return oldValue; |
438 |
} finally { |
439 |
unlock(); |
440 |
} |
441 |
} |
442 |
|
443 |
void rehash() { |
444 |
HashEntry<K,V>[] oldTable = table; |
445 |
int oldCapacity = oldTable.length; |
446 |
if (oldCapacity >= MAXIMUM_CAPACITY) |
447 |
return; |
448 |
|
449 |
/* |
450 |
* Reclassify nodes in each list to new Map. Because we are |
451 |
* using power-of-two expansion, the elements from each bin |
452 |
* must either stay at same index, or move with a power of two |
453 |
* offset. We eliminate unnecessary node creation by catching |
454 |
* cases where old nodes can be reused because their next |
455 |
* fields won't change. Statistically, at the default |
456 |
* threshold, only about one-sixth of them need cloning when |
457 |
* a table doubles. The nodes they replace will be garbage |
458 |
* collectable as soon as they are no longer referenced by any |
459 |
* reader thread that may be in the midst of traversing table |
460 |
* right now. |
461 |
*/ |
462 |
|
463 |
HashEntry<K,V>[] newTable = HashEntry.newArray(oldCapacity<<1); |
464 |
threshold = (int)(newTable.length * loadFactor); |
465 |
int sizeMask = newTable.length - 1; |
466 |
for (int i = 0; i < oldCapacity ; i++) { |
467 |
// We need to guarantee that any existing reads of old Map can |
468 |
// proceed. So we cannot yet null out each bin. |
469 |
HashEntry<K,V> e = oldTable[i]; |
470 |
|
471 |
if (e != null) { |
472 |
HashEntry<K,V> next = e.next; |
473 |
int idx = e.hash & sizeMask; |
474 |
|
475 |
// Single node on list |
476 |
if (next == null) |
477 |
newTable[idx] = e; |
478 |
|
479 |
else { |
480 |
// Reuse trailing consecutive sequence at same slot |
481 |
HashEntry<K,V> lastRun = e; |
482 |
int lastIdx = idx; |
483 |
for (HashEntry<K,V> last = next; |
484 |
last != null; |
485 |
last = last.next) { |
486 |
int k = last.hash & sizeMask; |
487 |
if (k != lastIdx) { |
488 |
lastIdx = k; |
489 |
lastRun = last; |
490 |
} |
491 |
} |
492 |
newTable[lastIdx] = lastRun; |
493 |
|
494 |
// Clone all remaining nodes |
495 |
for (HashEntry<K,V> p = e; p != lastRun; p = p.next) { |
496 |
int k = p.hash & sizeMask; |
497 |
HashEntry<K,V> n = newTable[k]; |
498 |
newTable[k] = new HashEntry<K,V>(p.key, p.hash, |
499 |
n, p.value); |
500 |
} |
501 |
} |
502 |
} |
503 |
} |
504 |
table = newTable; |
505 |
} |
506 |
|
507 |
/** |
508 |
* Remove; match on key only if value null, else match both. |
509 |
*/ |
510 |
V remove(Object key, int hash, Object value) { |
511 |
lock(); |
512 |
try { |
513 |
int c = count - 1; |
514 |
HashEntry<K,V>[] tab = table; |
515 |
int index = hash & (tab.length - 1); |
516 |
HashEntry<K,V> first = tab[index]; |
517 |
HashEntry<K,V> e = first; |
518 |
while (e != null && (e.hash != hash || !key.equals(e.key))) |
519 |
e = e.next; |
520 |
|
521 |
V oldValue = null; |
522 |
if (e != null) { |
523 |
V v = e.value; |
524 |
if (value == null || value.equals(v)) { |
525 |
oldValue = v; |
526 |
// All entries following removed node can stay |
527 |
// in list, but all preceding ones need to be |
528 |
// cloned. |
529 |
++modCount; |
530 |
HashEntry<K,V> newFirst = e.next; |
531 |
for (HashEntry<K,V> p = first; p != e; p = p.next) |
532 |
newFirst = new HashEntry<K,V>(p.key, p.hash, |
533 |
newFirst, p.value); |
534 |
tab[index] = newFirst; |
535 |
count = c; // write-volatile |
536 |
} |
537 |
} |
538 |
return oldValue; |
539 |
} finally { |
540 |
unlock(); |
541 |
} |
542 |
} |
543 |
|
544 |
void clear() { |
545 |
if (count != 0) { |
546 |
lock(); |
547 |
try { |
548 |
HashEntry<K,V>[] tab = table; |
549 |
for (int i = 0; i < tab.length ; i++) |
550 |
tab[i] = null; |
551 |
++modCount; |
552 |
count = 0; // write-volatile |
553 |
} finally { |
554 |
unlock(); |
555 |
} |
556 |
} |
557 |
} |
558 |
} |
559 |
|
560 |
|
561 |
|
562 |
/* ---------------- Public operations -------------- */ |
563 |
|
564 |
/** |
565 |
* Creates a new, empty map with the specified initial |
566 |
* capacity, load factor and concurrency level. |
567 |
* |
568 |
* @param initialCapacity the initial capacity. The implementation |
569 |
* performs internal sizing to accommodate this many elements. |
570 |
* @param loadFactor the load factor threshold, used to control resizing. |
571 |
* Resizing may be performed when the average number of elements per |
572 |
* bin exceeds this threshold. |
573 |
* @param concurrencyLevel the estimated number of concurrently |
574 |
* updating threads. The implementation performs internal sizing |
575 |
* to try to accommodate this many threads. |
576 |
* @throws IllegalArgumentException if the initial capacity is |
577 |
* negative or the load factor or concurrencyLevel are |
578 |
* nonpositive. |
579 |
*/ |
580 |
public ConcurrentHashMap(int initialCapacity, |
581 |
float loadFactor, int concurrencyLevel) { |
582 |
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) |
583 |
throw new IllegalArgumentException(); |
584 |
|
585 |
if (concurrencyLevel > MAX_SEGMENTS) |
586 |
concurrencyLevel = MAX_SEGMENTS; |
587 |
|
588 |
// Find power-of-two sizes best matching arguments |
589 |
int sshift = 0; |
590 |
int ssize = 1; |
591 |
while (ssize < concurrencyLevel) { |
592 |
++sshift; |
593 |
ssize <<= 1; |
594 |
} |
595 |
segmentShift = 32 - sshift; |
596 |
segmentMask = ssize - 1; |
597 |
this.segments = Segment.newArray(ssize); |
598 |
|
599 |
if (initialCapacity > MAXIMUM_CAPACITY) |
600 |
initialCapacity = MAXIMUM_CAPACITY; |
601 |
int c = initialCapacity / ssize; |
602 |
if (c * ssize < initialCapacity) |
603 |
++c; |
604 |
int cap = 1; |
605 |
while (cap < c) |
606 |
cap <<= 1; |
607 |
|
608 |
for (int i = 0; i < this.segments.length; ++i) |
609 |
this.segments[i] = new Segment<K,V>(cap, loadFactor); |
610 |
} |
611 |
|
612 |
/** |
613 |
* Creates a new, empty map with the specified initial capacity |
614 |
* and load factor and with the default concurrencyLevel (16). |
615 |
* |
616 |
* @param initialCapacity The implementation performs internal |
617 |
* sizing to accommodate this many elements. |
618 |
* @param loadFactor the load factor threshold, used to control resizing. |
619 |
* Resizing may be performed when the average number of elements per |
620 |
* bin exceeds this threshold. |
621 |
* @throws IllegalArgumentException if the initial capacity of |
622 |
* elements is negative or the load factor is nonpositive |
623 |
* |
624 |
* @since 1.6 |
625 |
*/ |
626 |
public ConcurrentHashMap(int initialCapacity, float loadFactor) { |
627 |
this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL); |
628 |
} |
629 |
|
630 |
/** |
631 |
* Creates a new, empty map with the specified initial capacity, |
632 |
* and with default load factor (0.75) and concurrencyLevel (16). |
633 |
* |
634 |
* @param initialCapacity the initial capacity. The implementation |
635 |
* performs internal sizing to accommodate this many elements. |
636 |
* @throws IllegalArgumentException if the initial capacity of |
637 |
* elements is negative. |
638 |
*/ |
639 |
public ConcurrentHashMap(int initialCapacity) { |
640 |
this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); |
641 |
} |
642 |
|
643 |
/** |
644 |
* Creates a new, empty map with a default initial capacity (16), |
645 |
* load factor (0.75) and concurrencyLevel (16). |
646 |
*/ |
647 |
public ConcurrentHashMap() { |
648 |
this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); |
649 |
} |
650 |
|
651 |
/** |
652 |
* Creates a new map with the same mappings as the given map. |
653 |
* The map is created with a capacity of 1.5 times the number |
654 |
* of mappings in the given map or 16 (whichever is greater), |
655 |
* and a default load factor (0.75) and concurrencyLevel (16). |
656 |
* |
657 |
* @param m the map |
658 |
*/ |
659 |
public ConcurrentHashMap(Map<? extends K, ? extends V> m) { |
660 |
this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, |
661 |
DEFAULT_INITIAL_CAPACITY), |
662 |
DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); |
663 |
putAll(m); |
664 |
} |
665 |
|
666 |
/** |
667 |
* Returns <tt>true</tt> if this map contains no key-value mappings. |
668 |
* |
669 |
* @return <tt>true</tt> if this map contains no key-value mappings |
670 |
*/ |
671 |
public boolean isEmpty() { |
672 |
final Segment<K,V>[] segments = this.segments; |
673 |
/* |
674 |
* We keep track of per-segment modCounts to avoid ABA |
675 |
* problems in which an element in one segment was added and |
676 |
* in another removed during traversal, in which case the |
677 |
* table was never actually empty at any point. Note the |
678 |
* similar use of modCounts in the size() and containsValue() |
679 |
* methods, which are the only other methods also susceptible |
680 |
* to ABA problems. |
681 |
*/ |
682 |
int[] mc = new int[segments.length]; |
683 |
int mcsum = 0; |
684 |
for (int i = 0; i < segments.length; ++i) { |
685 |
if (segments[i].count != 0) |
686 |
return false; |
687 |
else |
688 |
mcsum += mc[i] = segments[i].modCount; |
689 |
} |
690 |
// If mcsum happens to be zero, then we know we got a snapshot |
691 |
// before any modifications at all were made. This is |
692 |
// probably common enough to bother tracking. |
693 |
if (mcsum != 0) { |
694 |
for (int i = 0; i < segments.length; ++i) { |
695 |
if (segments[i].count != 0 || |
696 |
mc[i] != segments[i].modCount) |
697 |
return false; |
698 |
} |
699 |
} |
700 |
return true; |
701 |
} |
702 |
|
703 |
/** |
704 |
* Returns the number of key-value mappings in this map. If the |
705 |
* map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns |
706 |
* <tt>Integer.MAX_VALUE</tt>. |
707 |
* |
708 |
* @return the number of key-value mappings in this map |
709 |
*/ |
710 |
public int size() { |
711 |
final Segment<K,V>[] segments = this.segments; |
712 |
long sum = 0; |
713 |
long check = 0; |
714 |
int[] mc = new int[segments.length]; |
715 |
// Try a few times to get accurate count. On failure due to |
716 |
// continuous async changes in table, resort to locking. |
717 |
for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) { |
718 |
check = 0; |
719 |
sum = 0; |
720 |
int mcsum = 0; |
721 |
for (int i = 0; i < segments.length; ++i) { |
722 |
sum += segments[i].count; |
723 |
mcsum += mc[i] = segments[i].modCount; |
724 |
} |
725 |
if (mcsum != 0) { |
726 |
for (int i = 0; i < segments.length; ++i) { |
727 |
check += segments[i].count; |
728 |
if (mc[i] != segments[i].modCount) { |
729 |
check = -1; // force retry |
730 |
break; |
731 |
} |
732 |
} |
733 |
} |
734 |
if (check == sum) |
735 |
break; |
736 |
} |
737 |
if (check != sum) { // Resort to locking all segments |
738 |
sum = 0; |
739 |
for (int i = 0; i < segments.length; ++i) |
740 |
segments[i].lock(); |
741 |
for (int i = 0; i < segments.length; ++i) |
742 |
sum += segments[i].count; |
743 |
for (int i = 0; i < segments.length; ++i) |
744 |
segments[i].unlock(); |
745 |
} |
746 |
if (sum > Integer.MAX_VALUE) |
747 |
return Integer.MAX_VALUE; |
748 |
else |
749 |
return (int)sum; |
750 |
} |
751 |
|
752 |
/** |
753 |
* Returns the value to which the specified key is mapped, |
754 |
* or {@code null} if this map contains no mapping for the key. |
755 |
* |
756 |
* <p>More formally, if this map contains a mapping from a key |
757 |
* {@code k} to a value {@code v} such that {@code key.equals(k)}, |
758 |
* then this method returns {@code v}; otherwise it returns |
759 |
* {@code null}. (There can be at most one such mapping.) |
760 |
* |
761 |
* @throws NullPointerException if the specified key is null |
762 |
*/ |
763 |
public V get(Object key) { |
764 |
int hash = hash(key); // throws NullPointerException if key null |
765 |
return segmentFor(hash).get(key, hash); |
766 |
} |
767 |
|
768 |
/** |
769 |
* Tests if the specified object is a key in this table. |
770 |
* |
771 |
* @param key possible key |
772 |
* @return <tt>true</tt> if and only if the specified object |
773 |
* is a key in this table, as determined by the |
774 |
* <tt>equals</tt> method; <tt>false</tt> otherwise. |
775 |
* @throws NullPointerException if the specified key is null |
776 |
*/ |
777 |
public boolean containsKey(Object key) { |
778 |
int hash = hash(key); // throws NullPointerException if key null |
779 |
return segmentFor(hash).containsKey(key, hash); |
780 |
} |
781 |
|
782 |
/** |
783 |
* Returns <tt>true</tt> if this map maps one or more keys to the |
784 |
* specified value. Note: This method requires a full internal |
785 |
* traversal of the hash table, and so is much slower than |
786 |
* method <tt>containsKey</tt>. |
787 |
* |
788 |
* @param value value whose presence in this map is to be tested |
789 |
* @return <tt>true</tt> if this map maps one or more keys to the |
790 |
* specified value |
791 |
* @throws NullPointerException if the specified value is null |
792 |
*/ |
793 |
public boolean containsValue(Object value) { |
794 |
if (value == null) |
795 |
throw new NullPointerException(); |
796 |
|
797 |
// See explanation of modCount use above |
798 |
|
799 |
final Segment<K,V>[] segments = this.segments; |
800 |
int[] mc = new int[segments.length]; |
801 |
|
802 |
// Try a few times without locking |
803 |
for (int k = 0; k < RETRIES_BEFORE_LOCK; ++k) { |
804 |
int sum = 0; |
805 |
int mcsum = 0; |
806 |
for (int i = 0; i < segments.length; ++i) { |
807 |
int c = segments[i].count; |
808 |
mcsum += mc[i] = segments[i].modCount; |
809 |
if (segments[i].containsValue(value)) |
810 |
return true; |
811 |
} |
812 |
boolean cleanSweep = true; |
813 |
if (mcsum != 0) { |
814 |
for (int i = 0; i < segments.length; ++i) { |
815 |
int c = segments[i].count; |
816 |
if (mc[i] != segments[i].modCount) { |
817 |
cleanSweep = false; |
818 |
break; |
819 |
} |
820 |
} |
821 |
} |
822 |
if (cleanSweep) |
823 |
return false; |
824 |
} |
825 |
// Resort to locking all segments |
826 |
for (int i = 0; i < segments.length; ++i) |
827 |
segments[i].lock(); |
828 |
boolean found = false; |
829 |
try { |
830 |
for (int i = 0; i < segments.length; ++i) { |
831 |
if (segments[i].containsValue(value)) { |
832 |
found = true; |
833 |
break; |
834 |
} |
835 |
} |
836 |
} finally { |
837 |
for (int i = 0; i < segments.length; ++i) |
838 |
segments[i].unlock(); |
839 |
} |
840 |
return found; |
841 |
} |
842 |
|
843 |
/** |
844 |
* Legacy method testing if some key maps into the specified value |
845 |
* in this table. This method is identical in functionality to |
846 |
* {@link #containsValue}, and exists solely to ensure |
847 |
* full compatibility with class {@link java.util.Hashtable}, |
848 |
* which supported this method prior to introduction of the |
849 |
* Java Collections framework. |
850 |
|
851 |
* @param value a value to search for |
852 |
* @return <tt>true</tt> if and only if some key maps to the |
853 |
* <tt>value</tt> argument in this table as |
854 |
* determined by the <tt>equals</tt> method; |
855 |
* <tt>false</tt> otherwise |
856 |
* @throws NullPointerException if the specified value is null |
857 |
*/ |
858 |
public boolean contains(Object value) { |
859 |
return containsValue(value); |
860 |
} |
861 |
|
862 |
/** |
863 |
* Maps the specified key to the specified value in this table. |
864 |
* Neither the key nor the value can be null. |
865 |
* |
866 |
* <p> The value can be retrieved by calling the <tt>get</tt> method |
867 |
* with a key that is equal to the original key. |
868 |
* |
869 |
* @param key key with which the specified value is to be associated |
870 |
* @param value value to be associated with the specified key |
871 |
* @return the previous value associated with <tt>key</tt>, or |
872 |
* <tt>null</tt> if there was no mapping for <tt>key</tt> |
873 |
* @throws NullPointerException if the specified key or value is null |
874 |
*/ |
875 |
public V put(K key, V value) { |
876 |
if (value == null) |
877 |
throw new NullPointerException(); |
878 |
int hash = hash(key); |
879 |
return segmentFor(hash).put(key, hash, value, false); |
880 |
} |
881 |
|
882 |
/** |
883 |
* {@inheritDoc} |
884 |
* |
885 |
* @return the previous value associated with the specified key, |
886 |
* or <tt>null</tt> if there was no mapping for the key |
887 |
* @throws NullPointerException if the specified key or value is null |
888 |
*/ |
889 |
public V putIfAbsent(K key, V value) { |
890 |
if (value == null) |
891 |
throw new NullPointerException(); |
892 |
int hash = hash(key); |
893 |
return segmentFor(hash).put(key, hash, value, true); |
894 |
} |
895 |
|
896 |
/** |
897 |
* Copies all of the mappings from the specified map to this one. |
898 |
* These mappings replace any mappings that this map had for any of the |
899 |
* keys currently in the specified map. |
900 |
* |
901 |
* @param m mappings to be stored in this map |
902 |
*/ |
903 |
public void putAll(Map<? extends K, ? extends V> m) { |
904 |
for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) |
905 |
put(e.getKey(), e.getValue()); |
906 |
} |
907 |
|
908 |
/** |
909 |
* Removes the key (and its corresponding value) from this map. |
910 |
* This method does nothing if the key is not in the map. |
911 |
* |
912 |
* @param key the key that needs to be removed |
913 |
* @return the previous value associated with <tt>key</tt>, or |
914 |
* <tt>null</tt> if there was no mapping for <tt>key</tt> |
915 |
* @throws NullPointerException if the specified key is null |
916 |
*/ |
917 |
public V remove(Object key) { |
918 |
int hash = hash(key); |
919 |
return segmentFor(hash).remove(key, hash, null); |
920 |
} |
921 |
|
922 |
/** |
923 |
* {@inheritDoc} |
924 |
* |
925 |
* @throws NullPointerException if the specified key is null |
926 |
*/ |
927 |
public boolean remove(Object key, Object value) { |
928 |
if (value == null) |
929 |
return false; |
930 |
int hash = hash(key); |
931 |
return segmentFor(hash).remove(key, hash, value) != null; |
932 |
} |
933 |
|
934 |
/** |
935 |
* {@inheritDoc} |
936 |
* |
937 |
* @throws NullPointerException if any of the arguments are null |
938 |
*/ |
939 |
public boolean replace(K key, V oldValue, V newValue) { |
940 |
if (oldValue == null || newValue == null) |
941 |
throw new NullPointerException(); |
942 |
int hash = hash(key); |
943 |
return segmentFor(hash).replace(key, hash, oldValue, newValue); |
944 |
} |
945 |
|
946 |
/** |
947 |
* {@inheritDoc} |
948 |
* |
949 |
* @return the previous value associated with the specified key, |
950 |
* or <tt>null</tt> if there was no mapping for the key |
951 |
* @throws NullPointerException if the specified key or value is null |
952 |
*/ |
953 |
public V replace(K key, V value) { |
954 |
if (value == null) |
955 |
throw new NullPointerException(); |
956 |
int hash = hash(key); |
957 |
return segmentFor(hash).replace(key, hash, value); |
958 |
} |
959 |
|
960 |
/** |
961 |
* Removes all of the mappings from this map. |
962 |
*/ |
963 |
public void clear() { |
964 |
for (int i = 0; i < segments.length; ++i) |
965 |
segments[i].clear(); |
966 |
} |
967 |
|
968 |
/** |
969 |
* Returns a {@link Set} view of the keys contained in this map. |
970 |
* The set is backed by the map, so changes to the map are |
971 |
* reflected in the set, and vice-versa. The set supports element |
972 |
* removal, which removes the corresponding mapping from this map, |
973 |
* via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, |
974 |
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> |
975 |
* operations. It does not support the <tt>add</tt> or |
976 |
* <tt>addAll</tt> operations. |
977 |
* |
978 |
* <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator |
979 |
* that will never throw {@link ConcurrentModificationException}, |
980 |
* and guarantees to traverse elements as they existed upon |
981 |
* construction of the iterator, and may (but is not guaranteed to) |
982 |
* reflect any modifications subsequent to construction. |
983 |
*/ |
984 |
public Set<K> keySet() { |
985 |
Set<K> ks = keySet; |
986 |
return (ks != null) ? ks : (keySet = new KeySet()); |
987 |
} |
988 |
|
989 |
/** |
990 |
* Returns a {@link Collection} view of the values contained in this map. |
991 |
* The collection is backed by the map, so changes to the map are |
992 |
* reflected in the collection, and vice-versa. The collection |
993 |
* supports element removal, which removes the corresponding |
994 |
* mapping from this map, via the <tt>Iterator.remove</tt>, |
995 |
* <tt>Collection.remove</tt>, <tt>removeAll</tt>, |
996 |
* <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not |
997 |
* support the <tt>add</tt> or <tt>addAll</tt> operations. |
998 |
* |
999 |
* <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator |
1000 |
* that will never throw {@link ConcurrentModificationException}, |
1001 |
* and guarantees to traverse elements as they existed upon |
1002 |
* construction of the iterator, and may (but is not guaranteed to) |
1003 |
* reflect any modifications subsequent to construction. |
1004 |
*/ |
1005 |
public Collection<V> values() { |
1006 |
Collection<V> vs = values; |
1007 |
return (vs != null) ? vs : (values = new Values()); |
1008 |
} |
1009 |
|
1010 |
/** |
1011 |
* Returns a {@link Set} view of the mappings contained in this map. |
1012 |
* The set is backed by the map, so changes to the map are |
1013 |
* reflected in the set, and vice-versa. The set supports element |
1014 |
* removal, which removes the corresponding mapping from the map, |
1015 |
* via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, |
1016 |
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> |
1017 |
* operations. It does not support the <tt>add</tt> or |
1018 |
* <tt>addAll</tt> operations. |
1019 |
* |
1020 |
* <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator |
1021 |
* that will never throw {@link ConcurrentModificationException}, |
1022 |
* and guarantees to traverse elements as they existed upon |
1023 |
* construction of the iterator, and may (but is not guaranteed to) |
1024 |
* reflect any modifications subsequent to construction. |
1025 |
*/ |
1026 |
public Set<Map.Entry<K,V>> entrySet() { |
1027 |
Set<Map.Entry<K,V>> es = entrySet; |
1028 |
return (es != null) ? es : (entrySet = new EntrySet()); |
1029 |
} |
1030 |
|
1031 |
/** |
1032 |
* Returns an enumeration of the keys in this table. |
1033 |
* |
1034 |
* @return an enumeration of the keys in this table |
1035 |
* @see #keySet |
1036 |
*/ |
1037 |
public Enumeration<K> keys() { |
1038 |
return new KeyIterator(); |
1039 |
} |
1040 |
|
1041 |
/** |
1042 |
* Returns an enumeration of the values in this table. |
1043 |
* |
1044 |
* @return an enumeration of the values in this table |
1045 |
* @see #values |
1046 |
*/ |
1047 |
public Enumeration<V> elements() { |
1048 |
return new ValueIterator(); |
1049 |
} |
1050 |
|
1051 |
/* ---------------- Iterator Support -------------- */ |
1052 |
|
1053 |
abstract class HashIterator { |
1054 |
int nextSegmentIndex; |
1055 |
int nextTableIndex; |
1056 |
HashEntry<K,V>[] currentTable; |
1057 |
HashEntry<K, V> nextEntry; |
1058 |
HashEntry<K, V> lastReturned; |
1059 |
|
1060 |
HashIterator() { |
1061 |
nextSegmentIndex = segments.length - 1; |
1062 |
nextTableIndex = -1; |
1063 |
advance(); |
1064 |
} |
1065 |
|
1066 |
public boolean hasMoreElements() { return hasNext(); } |
1067 |
|
1068 |
final void advance() { |
1069 |
if (nextEntry != null && (nextEntry = nextEntry.next) != null) |
1070 |
return; |
1071 |
|
1072 |
while (nextTableIndex >= 0) { |
1073 |
if ( (nextEntry = currentTable[nextTableIndex--]) != null) |
1074 |
return; |
1075 |
} |
1076 |
|
1077 |
while (nextSegmentIndex >= 0) { |
1078 |
Segment<K,V> seg = segments[nextSegmentIndex--]; |
1079 |
if (seg.count != 0) { |
1080 |
currentTable = seg.table; |
1081 |
for (int j = currentTable.length - 1; j >= 0; --j) { |
1082 |
if ( (nextEntry = currentTable[j]) != null) { |
1083 |
nextTableIndex = j - 1; |
1084 |
return; |
1085 |
} |
1086 |
} |
1087 |
} |
1088 |
} |
1089 |
} |
1090 |
|
1091 |
public boolean hasNext() { return nextEntry != null; } |
1092 |
|
1093 |
HashEntry<K,V> nextEntry() { |
1094 |
if (nextEntry == null) |
1095 |
throw new NoSuchElementException(); |
1096 |
lastReturned = nextEntry; |
1097 |
advance(); |
1098 |
return lastReturned; |
1099 |
} |
1100 |
|
1101 |
public void remove() { |
1102 |
if (lastReturned == null) |
1103 |
throw new IllegalStateException(); |
1104 |
ConcurrentHashMap.this.remove(lastReturned.key); |
1105 |
lastReturned = null; |
1106 |
} |
1107 |
} |
1108 |
|
1109 |
final class KeyIterator |
1110 |
extends HashIterator |
1111 |
implements Iterator<K>, Enumeration<K> |
1112 |
{ |
1113 |
public K next() { return super.nextEntry().key; } |
1114 |
public K nextElement() { return super.nextEntry().key; } |
1115 |
} |
1116 |
|
1117 |
final class ValueIterator |
1118 |
extends HashIterator |
1119 |
implements Iterator<V>, Enumeration<V> |
1120 |
{ |
1121 |
public V next() { return super.nextEntry().value; } |
1122 |
public V nextElement() { return super.nextEntry().value; } |
1123 |
} |
1124 |
|
1125 |
/** |
1126 |
* Custom Entry class used by EntryIterator.next(), that relays |
1127 |
* setValue changes to the underlying map. |
1128 |
*/ |
1129 |
final class WriteThroughEntry |
1130 |
extends AbstractMap.SimpleEntry<K,V> |
1131 |
{ |
1132 |
WriteThroughEntry(K k, V v) { |
1133 |
super(k,v); |
1134 |
} |
1135 |
|
1136 |
/** |
1137 |
* Set our entry's value and write through to the map. The |
1138 |
* value to return is somewhat arbitrary here. Since a |
1139 |
* WriteThroughEntry does not necessarily track asynchronous |
1140 |
* changes, the most recent "previous" value could be |
1141 |
* different from what we return (or could even have been |
1142 |
* removed in which case the put will re-establish). We do not |
1143 |
* and cannot guarantee more. |
1144 |
*/ |
1145 |
public V setValue(V value) { |
1146 |
if (value == null) throw new NullPointerException(); |
1147 |
V v = super.setValue(value); |
1148 |
ConcurrentHashMap.this.put(getKey(), value); |
1149 |
return v; |
1150 |
} |
1151 |
} |
1152 |
|
1153 |
final class EntryIterator |
1154 |
extends HashIterator |
1155 |
implements Iterator<Entry<K,V>> |
1156 |
{ |
1157 |
public Map.Entry<K,V> next() { |
1158 |
HashEntry<K,V> e = super.nextEntry(); |
1159 |
return new WriteThroughEntry(e.key, e.value); |
1160 |
} |
1161 |
} |
1162 |
|
1163 |
final class KeySet extends AbstractSet<K> { |
1164 |
public Iterator<K> iterator() { |
1165 |
return new KeyIterator(); |
1166 |
} |
1167 |
public int size() { |
1168 |
return ConcurrentHashMap.this.size(); |
1169 |
} |
1170 |
public boolean contains(Object o) { |
1171 |
return ConcurrentHashMap.this.containsKey(o); |
1172 |
} |
1173 |
public boolean remove(Object o) { |
1174 |
return ConcurrentHashMap.this.remove(o) != null; |
1175 |
} |
1176 |
public void clear() { |
1177 |
ConcurrentHashMap.this.clear(); |
1178 |
} |
1179 |
} |
1180 |
|
1181 |
final class Values extends AbstractCollection<V> { |
1182 |
public Iterator<V> iterator() { |
1183 |
return new ValueIterator(); |
1184 |
} |
1185 |
public int size() { |
1186 |
return ConcurrentHashMap.this.size(); |
1187 |
} |
1188 |
public boolean contains(Object o) { |
1189 |
return ConcurrentHashMap.this.containsValue(o); |
1190 |
} |
1191 |
public void clear() { |
1192 |
ConcurrentHashMap.this.clear(); |
1193 |
} |
1194 |
} |
1195 |
|
1196 |
final class EntrySet extends AbstractSet<Map.Entry<K,V>> { |
1197 |
public Iterator<Map.Entry<K,V>> iterator() { |
1198 |
return new EntryIterator(); |
1199 |
} |
1200 |
public boolean contains(Object o) { |
1201 |
if (!(o instanceof Map.Entry)) |
1202 |
return false; |
1203 |
Map.Entry<?,?> e = (Map.Entry<?,?>)o; |
1204 |
V v = ConcurrentHashMap.this.get(e.getKey()); |
1205 |
return v != null && v.equals(e.getValue()); |
1206 |
} |
1207 |
public boolean remove(Object o) { |
1208 |
if (!(o instanceof Map.Entry)) |
1209 |
return false; |
1210 |
Map.Entry<?,?> e = (Map.Entry<?,?>)o; |
1211 |
return ConcurrentHashMap.this.remove(e.getKey(), e.getValue()); |
1212 |
} |
1213 |
public int size() { |
1214 |
return ConcurrentHashMap.this.size(); |
1215 |
} |
1216 |
public void clear() { |
1217 |
ConcurrentHashMap.this.clear(); |
1218 |
} |
1219 |
} |
1220 |
|
1221 |
/* ---------------- Serialization Support -------------- */ |
1222 |
|
1223 |
/** |
1224 |
* Save the state of the <tt>ConcurrentHashMap</tt> instance to a |
1225 |
* stream (i.e., serialize it). |
1226 |
* @param s the stream |
1227 |
* @serialData |
1228 |
* the key (Object) and value (Object) |
1229 |
* for each key-value mapping, followed by a null pair. |
1230 |
* The key-value mappings are emitted in no particular order. |
1231 |
*/ |
1232 |
private void writeObject(java.io.ObjectOutputStream s) throws IOException { |
1233 |
s.defaultWriteObject(); |
1234 |
|
1235 |
for (int k = 0; k < segments.length; ++k) { |
1236 |
Segment<K,V> seg = segments[k]; |
1237 |
seg.lock(); |
1238 |
try { |
1239 |
HashEntry<K,V>[] tab = seg.table; |
1240 |
for (int i = 0; i < tab.length; ++i) { |
1241 |
for (HashEntry<K,V> e = tab[i]; e != null; e = e.next) { |
1242 |
s.writeObject(e.key); |
1243 |
s.writeObject(e.value); |
1244 |
} |
1245 |
} |
1246 |
} finally { |
1247 |
seg.unlock(); |
1248 |
} |
1249 |
} |
1250 |
s.writeObject(null); |
1251 |
s.writeObject(null); |
1252 |
} |
1253 |
|
1254 |
/** |
1255 |
* Reconstitute the <tt>ConcurrentHashMap</tt> instance from a |
1256 |
* stream (i.e., deserialize it). |
1257 |
* @param s the stream |
1258 |
*/ |
1259 |
private void readObject(java.io.ObjectInputStream s) |
1260 |
throws IOException, ClassNotFoundException { |
1261 |
s.defaultReadObject(); |
1262 |
|
1263 |
// Initialize each segment to be minimally sized, and let grow. |
1264 |
for (int i = 0; i < segments.length; ++i) { |
1265 |
segments[i].setTable(new HashEntry[1]); |
1266 |
} |
1267 |
|
1268 |
// Read the keys and values, and put the mappings in the table |
1269 |
for (;;) { |
1270 |
K key = (K) s.readObject(); |
1271 |
V value = (V) s.readObject(); |
1272 |
if (key == null) |
1273 |
break; |
1274 |
put(key, value); |
1275 |
} |
1276 |
} |
1277 |
} |