Java并发编程规则：同步容器与并发容器
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同步性和并发性都是线程安全的知识，只要同时满足条件就可以编写支持并发线程安全的程序。
同步容器
首先，同步容器是线程安全的。Java中设计了同步容器的数据结构对象，如：Vector和HashTable。但必须说明的是，同步容器在复合操作（迭代、运算、逻辑处理等）时如果没有线程同步策略，那么程序就不是线程安全的。Java定义Vector是线程安全的。但是在多线程环境下，多个线程进行remove、get操作就会出现ArrayIndexOutOfBoundsException问题。修复此类问题的最好方式就是加锁。
复合操作不安全示例：
import java.util.Vector;public class VectorC1 {public static Object getLast(Vector vector){int index=vector.size()-1;return vector.get(index);} public static void deleteLast(Vector vector){ int index=vector.size()-1;vector.remove(index);} }修复复合操作示例：
import java.util.Vector;public class VectorC2 {public static Object getLast(Vector vector){synchronized (vector) {int index=vector.size()-1;return vector.get(index);}} public static void deleteLast(Vector vector){ synchronized (vector) { int index=vector.size()-1;vector.remove(index);} } }迭代操作不安全示例：
import java.util.Vector;public class VectorFor1 {public static void doService(Vector vector){for (int i = 0,j=vector.size(); i < j; i++) {doSomething(vector.get(i)); }} public static void doSomething(Object obj){ } }修复迭代操作不安全示例：
import java.util.Vector;public class VectorFor2 {public static void doService(Vector vector){synchronized (vector) {for (int i = 0,j=vector.size(); i < j; i++) {doSomething(vector.get(i)); }}} public static void doSomething(Object obj){ } }对Collection进行迭代的标准方式是使用Iterator。如果容器在迭代期间发生了改变，其他线程执行操作类似contains*,remove*(隐藏迭代器)等方法都有可能抛出ConcurrentModificationException异常。同步容器所有状态是串行访问，从而实现了线程安全，但是削弱了并发性，使程序的吞吐量降低。并发容器
Java5.0通过几种并发容器来改进同步容器。用以替代同步哈希Map的设计，如：ConcurrentHashMap和ConcurrentMap。CopyOnWriteArrayList是List相应的同步实现。用并发容器替代同步容器，以很小的风险带了了扩展性显著的提高。
Java5.0同样提供了两个新的容器类型：Queue(null)和BlockingQueue(阻塞队列)。（Queue用于临时存储下一步执行等待的执行单元逻辑元素；FIFO先进先出的ConcurrentLinkedQueue；PriorityQueue优先顺序级别的队列。）
Java6.0中基于哈希Map的设计还新增了ConcurrentSkipListMap和ConcurrentSkipListSet，用来作为同步的SortedMap和SortedSet的替代品。
ConcurrentHashMap
ConcurrentHashMap具备更加细致的锁机制(分离锁)，并且相比于HashTable和synchornizedMap，ConcurrentHashMap几乎没有什么劣势，因此大多数情况下用ConcurrentHashMap取代同步Map可以获得更好的可伸缩性。只有当你的程序需要独占访问加锁时，ConcurrentHashMap才无法胜任。
ConcurrentMap接口：
package java.util.concurrent;import java.util.Map;/** * A {@link java.util.Map} providing additional atomic * <tt>putIfAbsent</tt>, <tt>remove</tt>, and <tt>replace</tt> methods. * * <p>Memory consistency effects: As with other concurrent * collections, actions in a thread prior to placing an object into a * {@code ConcurrentMap} as a key or value * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> * actions subsequent to the access or removal of that object from * the {@code ConcurrentMap} in another thread. * * <p>This interface is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values */public interface ConcurrentMap<K, V> extends Map<K, V> { /** * If the specified key is not already associated * with a value, associate it with the given value. * This is equivalent to * <pre> * if (!map.containsKey(key)) * return map.put(key, value); * else * return map.get(key);</pre> * except that the action is performed atomically. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with the specified key, or * <tt>null</tt> if there was no mapping for the key. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with the key, * if the implementation supports null values.) * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by this map * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of the specified key * or value prevents it from being stored in this map * */ V putIfAbsent(K key, V value); /** * Removes the entry for a key only if currently mapped to a given value. * This is equivalent to * <pre> * if (map.containsKey(key) && map.get(key).equals(value)) { * map.remove(key); * return true; * } else return false;</pre> * except that the action is performed atomically. * * @param key key with which the specified value is associated * @param value value expected to be associated with the specified key * @return <tt>true</tt> if the value was removed * @throws UnsupportedOperationException if the <tt>remove</tt> operation * is not supported by this map * @throws ClassCastException if the key or value is of an inappropriate * type for this map * (<a href="../Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * (<a href="../Collection.html#optional-restrictions">optional</a>) */ boolean remove(Object key, Object value); /** * Replaces the entry for a key only if currently mapped to a given value. * This is equivalent to * <pre> * if (map.containsKey(key) && map.get(key).equals(oldValue)) { * map.put(key, newValue); * return true; * } else return false;</pre> * except that the action is performed atomically. * * @param key key with which the specified value is associated * @param oldValue value expected to be associated with the specified key * @param newValue value to be associated with the specified key * @return <tt>true</tt> if the value was replaced * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by this map * @throws ClassCastException if the class of a specified key or value * prevents it from being stored in this map * @throws NullPointerException if a specified key or value is null, * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of a specified key * or value prevents it from being stored in this map */ boolean replace(K key, V oldValue, V newValue); /** * Replaces the entry for a key only if currently mapped to some value. * This is equivalent to * <pre> * if (map.containsKey(key)) { * return map.put(key, value); * } else return null;</pre> * except that the action is performed atomically. * * @param key key with which the specified value is associated * @param value value to be associated with the specified key * @return the previous value associated with the specified key, or * <tt>null</tt> if there was no mapping for the key. * (A <tt>null</tt> return can also indicate that the map * previously associated <tt>null</tt> with the key, * if the implementation supports null values.) * @throws UnsupportedOperationException if the <tt>put</tt> operation * is not supported by this map * @throws ClassCastException if the class of the specified key or value * prevents it from being stored in this map * @throws NullPointerException if the specified key or value is null, * and this map does not permit null keys or values * @throws IllegalArgumentException if some property of the specified key * or value prevents it from being stored in this map */ V replace(K key, V value);}ConcurrentMap的ConcurrentHashMap实现：
package java.util.concurrent;import java.util.concurrent.locks.*;import java.util.*;import java.io.Serializable;import java.io.IOException;import java.io.ObjectInputStream;import java.io.ObjectOutputStream;import java.io.ObjectStreamField;/** * A hash table supporting full concurrency of retrievals and * adjustable expected concurrency for updates. This class obeys the * same functional specification as {@link java.util.Hashtable}, and * includes versions of methods corresponding to each method of * <tt>Hashtable</tt>. However, even though all operations are * thread-safe, retrieval operations do <em>not</em> entail locking, * and there is <em>not</em> any support for locking the entire table * in a way that prevents all access. This class is fully * interoperable with <tt>Hashtable</tt> in programs that rely on its * thread safety but not on its synchronization details. * * <p> Retrieval operations (including <tt>get</tt>) generally do not * block, so may overlap with update operations (including * <tt>put</tt> and <tt>remove</tt>). Retrievals reflect the results * of the most recently <em>completed</em> update operations holding * upon their onset. For aggregate operations such as <tt>putAll</tt> * and <tt>clear</tt>, concurrent retrievals may reflect insertion or * removal of only some entries. Similarly, Iterators and * Enumerations return elements reflecting the state of the hash table * at some point at or since the creation of the iterator/enumeration. * They do <em>not</em> throw {@link ConcurrentModificationException}. * However, iterators are designed to be used by only one thread at a time. * * <p> The allowed concurrency among update operations is guided by * the optional <tt>concurrencyLevel</tt> constructor argument * (default <tt>16</tt>), which is used as a hint for internal sizing. The * table is internally partitioned to try to permit the indicated * number of concurrent updates without contention. Because placement * in hash tables is essentially random, the actual concurrency will * vary. Ideally, you should choose a value to accommodate as many * threads as will ever concurrently modify the table. Using a * significantly higher value than you need can waste space and time, * and a significantly lower value can lead to thread contention. But * overestimates and underestimates within an order of magnitude do * not usually have much noticeable impact. A value of one is * appropriate when it is known that only one thread will modify and * all others will only read. Also, resizing this or any other kind of * hash table is a relatively slow operation, so, when possible, it is * a good idea to provide estimates of expected table sizes in * constructors. * * <p>This class and its views and iterators implement all of the * <em>optional</em> methods of the {@link Map} and {@link Iterator} * interfaces. * * <p> Like {@link Hashtable} but unlike {@link HashMap}, this class * does <em>not</em> allow <tt>null</tt> to be used as a key or value. * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <K> the type of keys maintained by this map * @param <V> the type of mapped values */public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable { private static final long serialVersionUID = 7249069246763182397L; /* * The basic strategy is to subdivide the table among Segments, * each of which itself is a concurrently readable hash table. To * reduce footprint, all but one segments are constructed only * when first needed (see ensureSegment). To maintain visibility * in the presence of lazy construction, accesses to segments as * well as elements of segment's table must use volatile access, * which is done via Unsafe within methods segmentAt etc * below. These provide the functionality of AtomicReferenceArrays * but reduce the levels of indirection. Additionally, * volatile-writes of table elements and entry "next" fields * within locked operations use the cheaper "lazySet" forms of * writes (via putOrderedObject) because these writes are always * followed by lock releases that maintain sequential consistency * of table updates. * * Historical note: The previous version of this class relied * heavily on "final" fields, which avoided some volatile reads at * the expense of a large initial footprint. Some remnants of * that design (including forced construction of segment 0) exist * to ensure serialization compatibility. */ /* ---------------- Constants -------------- */ /** * The default initial capacity for this table, * used when not otherwise specified in a constructor. */ static final int DEFAULT_INITIAL_CAPACITY = 16; /** * The default load factor for this table, used when not * otherwise specified in a constructor. */ static final float DEFAULT_LOAD_FACTOR = 0.75f; /** * The default concurrency level for this table, used when not * otherwise specified in a constructor. */ static final int DEFAULT_CONCURRENCY_LEVEL = 16; /** * The maximum capacity, used if a higher value is implicitly * specified by either of the constructors with arguments. MUST * be a power of two <= 1<<30 to ensure that entries are indexable * using ints. */ static final int MAXIMUM_CAPACITY = 1 << 30; /** * The minimum capacity for per-segment tables. Must be a power * of two, at least two to avoid immediate resizing on next use * after lazy construction. */ static final int MIN_SEGMENT_TABLE_CAPACITY = 2; /** * The maximum number of segments to allow; used to bound * constructor arguments. Must be power of two less than 1 << 24. */ static final int MAX_SEGMENTS = 1 << 16; // slightly conservative /** * Number of unsynchronized retries in size and containsValue * methods before resorting to locking. This is used to avoid * unbounded retries if tables undergo continuous modification * which would make it impossible to obtain an accurate result. */ static final int RETRIES_BEFORE_LOCK = 2; /* ---------------- Fields -------------- */ /** * holds values which can't be initialized until after VM is booted. */ private static class Holder { /** * Enable alternative hashing of String keys? * * <p>Unlike the other hash map implementations we do not implement a * threshold for regulating whether alternative hashing is used for * String keys. Alternative hashing is either enabled for all instances * or disabled for all instances. */ static final boolean ALTERNATIVE_HASHING; static { // Use the "threshold" system property even though our threshold // behaviour is "ON" or "OFF". String altThreshold = java.security.AccessController.doPrivileged( new sun.security.action.GetPropertyAction( "jdk.map.althashing.threshold")); int threshold; try { threshold = (null != altThreshold) ? Integer.parseInt(altThreshold) : Integer.MAX_VALUE; // disable alternative hashing if -1 if (threshold == -1) { threshold = Integer.MAX_VALUE; } if (threshold < 0) { throw new IllegalArgumentException("value must be positive integer."); } } catch(IllegalArgumentException failed) { throw new Error("Illegal value for 'jdk.map.althashing.threshold'", failed); } ALTERNATIVE_HASHING = threshold <= MAXIMUM_CAPACITY; } } /** * A randomizing value associated with this instance that is applied to * hash code of keys to make hash collisions harder to find. */ private transient final int hashSeed = randomHashSeed(this); private static int randomHashSeed(ConcurrentHashMap instance) { if (sun.misc.VM.isBooted() && Holder.ALTERNATIVE_HASHING) { return sun.misc.Hashing.randomHashSeed(instance); } return 0; } /** * Mask value for indexing into segments. The upper bits of a * key's hash code are used to choose the segment. */ final int segmentMask; /** * Shift value for indexing within segments. */ final int segmentShift; /** * The segments, each of which is a specialized hash table. */ final Segment<K,V>[] segments; transient Set<K> keySet; transient Set<Map.Entry<K,V>> entrySet; transient Collection<V> values; /** * ConcurrentHashMap list entry. Note that this is never exported * out as a user-visible Map.Entry. */ static final class HashEntry<K,V> { final int hash; final K key; volatile V value; volatile HashEntry<K,V> next; HashEntry(int hash, K key, V value, HashEntry<K,V> next) { this.hash = hash; this.key = key; this.value = value; this.next = next; } /** * Sets next field with volatile write semantics. (See above * about use of putOrderedObject.) */ final void setNext(HashEntry<K,V> n) { UNSAFE.putOrderedObject(this, nextOffset, n); } // Unsafe mechanics static final sun.misc.Unsafe UNSAFE; static final long nextOffset; static { try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class k = HashEntry.class; nextOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("next")); } catch (Exception e) { throw new Error(e); } } } /** * Gets the ith element of given table (if nonnull) with volatile * read semantics. Note: This is manually integrated into a few * performance-sensitive methods to reduce call overhead. */ @SuppressWarnings("unchecked") static final <K,V> HashEntry<K,V> entryAt(HashEntry<K,V>[] tab, int i) { return (tab == null) ? null : (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)i << TSHIFT) + TBASE); } /** * Sets the ith element of given table, with volatile write * semantics. (See above about use of putOrderedObject.) */ static final <K,V> void setEntryAt(HashEntry<K,V>[] tab, int i, HashEntry<K,V> e) { UNSAFE.putOrderedObject(tab, ((long)i << TSHIFT) + TBASE, e); } /** * Applies a supplemental hash function to a given hashCode, which * defends against poor quality hash functions. This is critical * because ConcurrentHashMap uses power-of-two length hash tables, * that otherwise encounter collisions for hashCodes that do not * differ in lower or upper bits. */ private int hash(Object k) { int h = hashSeed; if ((0 != h) && (k instanceof String)) { return sun.misc.Hashing.stringHash32((String) k); } h ^= k.hashCode(); // Spread bits to regularize both segment and index locations, // using variant of single-word Wang/Jenkins hash. h += (h << 15) ^ 0xffffcd7d; h ^= (h >>> 10); h += (h << 3); h ^= (h >>> 6); h += (h << 2) + (h << 14); return h ^ (h >>> 16); } /** * Segments are specialized versions of hash tables. This * subclasses from ReentrantLock opportunistically, just to * simplify some locking and avoid separate construction. */ static final class Segment<K,V> extends ReentrantLock implements Serializable { /* * Segments maintain a table of entry lists that are always * kept in a consistent state, so can be read (via volatile * reads of segments and tables) without locking. This * requires replicating nodes when necessary during table * resizing, so the old lists can be traversed by readers * still using old version of table. * * This class defines only mutative methods requiring locking. * Except as noted, the methods of this class perform the * per-segment versions of ConcurrentHashMap methods. (Other * methods are integrated directly into ConcurrentHashMap * methods.) These mutative methods use a form of controlled * spinning on contention via methods scanAndLock and * scanAndLockForPut. These intersperse tryLocks with * traversals to locate nodes. The main benefit is to absorb * cache misses (which are very common for hash tables) while * obtaining locks so that traversal is faster once * acquired. We do not actually use the found nodes since they * must be re-acquired under lock anyway to ensure sequential * consistency of updates (and in any case may be undetectably * stale), but they will normally be much faster to re-locate. * Also, scanAndLockForPut speculatively creates a fresh node * to use in put if no node is found. */ private static final long serialVersionUID = 2249069246763182397L; /** * The maximum number of times to tryLock in a prescan before * possibly blocking on acquire in preparation for a locked * segment operation. On multiprocessors, using a bounded * number of retries maintains cache acquired while locating * nodes. */ static final int MAX_SCAN_RETRIES = Runtime.getRuntime().availableProcessors() > 1 ? 64 : 1; /** * The per-segment table. Elements are accessed via * entryAt/setEntryAt providing volatile semantics. */ transient volatile HashEntry<K,V>[] table; /** * The number of elements. Accessed only either within locks * or among other volatile reads that maintain visibility. */ transient int count; /** * The total number of mutative operations in this segment. * Even though this may overflows 32 bits, it provides * sufficient accuracy for stability checks in CHM isEmpty() * and size() methods. Accessed only either within locks or * among other volatile reads that maintain visibility. */ transient int modCount; /** * The table is rehashed when its size exceeds this threshold. * (The value of this field is always <tt>(int)(capacity * * loadFactor)</tt>.) */ transient int threshold; /** * The load factor for the hash table. Even though this value * is same for all segments, it is replicated to avoid needing * links to outer object. * @serial */ final float loadFactor; Segment(float lf, int threshold, HashEntry<K,V>[] tab) { this.loadFactor = lf; this.threshold = threshold; this.table = tab; } final V put(K key, int hash, V value, boolean onlyIfAbsent) { HashEntry<K,V> node = tryLock() ? null : scanAndLockForPut(key, hash, value); V oldValue; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; HashEntry<K,V> first = entryAt(tab, index); for (HashEntry<K,V> e = first;;) { if (e != null) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; if (!onlyIfAbsent) { e.value = value; ++modCount; } break; } e = e.next; } else { if (node != null) node.setNext(first); else node = new HashEntry<K,V>(hash, key, value, first); int c = count + 1; if (c > threshold && tab.length < MAXIMUM_CAPACITY) rehash(node); else setEntryAt(tab, index, node); ++modCount; count = c; oldValue = null; break; } } } finally { unlock(); } return oldValue; } /** * Doubles size of table and repacks entries, also adding the * given node to new table */ @SuppressWarnings("unchecked") private void rehash(HashEntry<K,V> node) { /* * Reclassify nodes in each list to new table. Because we * are using power-of-two expansion, the elements from * each bin must either stay at same index, or move with a * power of two offset. We eliminate unnecessary node * creation by catching cases where old nodes can be * reused because their next fields won't change. * Statistically, at the default threshold, only about * one-sixth of them need cloning when a table * doubles. The nodes they replace will be garbage * collectable as soon as they are no longer referenced by * any reader thread that may be in the midst of * concurrently traversing table. Entry accesses use plain * array indexing because they are followed by volatile * table write. */ HashEntry<K,V>[] oldTable = table; int oldCapacity = oldTable.length; int newCapacity = oldCapacity << 1; threshold = (int)(newCapacity * loadFactor); HashEntry<K,V>[] newTable = (HashEntry<K,V>[]) new HashEntry[newCapacity]; int sizeMask = newCapacity - 1; for (int i = 0; i < oldCapacity ; i++) { HashEntry<K,V> e = oldTable[i]; if (e != null) { HashEntry<K,V> next = e.next; int idx = e.hash & sizeMask; if (next == null) // Single node on list newTable[idx] = e; else { // Reuse consecutive sequence at same slot HashEntry<K,V> lastRun = e; int lastIdx = idx; for (HashEntry<K,V> last = next; last != null; last = last.next) { int k = last.hash & sizeMask; if (k != lastIdx) { lastIdx = k; lastRun = last; } } newTable[lastIdx] = lastRun; // Clone remaining nodes for (HashEntry<K,V> p = e; p != lastRun; p = p.next) { V v = p.value; int h = p.hash; int k = h & sizeMask; HashEntry<K,V> n = newTable[k]; newTable[k] = new HashEntry<K,V>(h, p.key, v, n); } } } } int nodeIndex = node.hash & sizeMask; // add the new node node.setNext(newTable[nodeIndex]); newTable[nodeIndex] = node; table = newTable; } /** * Scans for a node containing given key while trying to * acquire lock, creating and returning one if not found. Upon * return, guarantees that lock is held. UNlike in most * methods, calls to method equals are not screened: Since * traversal speed doesn't matter, we might as well help warm * up the associated code and accesses as well. * * @return a new node if key not found, else null */ private HashEntry<K,V> scanAndLockForPut(K key, int hash, V value) { HashEntry<K,V> first = entryForHash(this, hash); HashEntry<K,V> e = first; HashEntry<K,V> node = null; int retries = -1; // negative while locating node while (!tryLock()) { HashEntry<K,V> f; // to recheck first below if (retries < 0) { if (e == null) { if (node == null) // speculatively create node node = new HashEntry<K,V>(hash, key, value, null); retries = 0; } else if (key.equals(e.key)) retries = 0; else e = e.next; } else if (++retries > MAX_SCAN_RETRIES) { lock(); break; } else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) { e = first = f; // re-traverse if entry changed retries = -1; } } return node; } /** * Scans for a node containing the given key while trying to * acquire lock for a remove or replace operation. Upon * return, guarantees that lock is held. Note that we must * lock even if the key is not found, to ensure sequential * consistency of updates. */ private void scanAndLock(Object key, int hash) { // similar to but simpler than scanAndLockForPut HashEntry<K,V> first = entryForHash(this, hash); HashEntry<K,V> e = first; int retries = -1; while (!tryLock()) { HashEntry<K,V> f; if (retries < 0) { if (e == null || key.equals(e.key)) retries = 0; else e = e.next; } else if (++retries > MAX_SCAN_RETRIES) { lock(); break; } else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) { e = first = f; retries = -1; } } } /** * Remove; match on key only if value null, else match both. */ final V remove(Object key, int hash, Object value) { if (!tryLock()) scanAndLock(key, hash); V oldValue = null; try { HashEntry<K,V>[] tab = table; int index = (tab.length - 1) & hash; HashEntry<K,V> e = entryAt(tab, index); HashEntry<K,V> pred = null; while (e != null) { K k; HashEntry<K,V> next = e.next; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { V v = e.value; if (value == null || value == v || value.equals(v)) { if (pred == null) setEntryAt(tab, index, next); else pred.setNext(next); ++modCount; --count; oldValue = v; } break; } pred = e; e = next; } } finally { unlock(); } return oldValue; } final boolean replace(K key, int hash, V oldValue, V newValue) { if (!tryLock()) scanAndLock(key, hash); boolean replaced = false; try { HashEntry<K,V> e; for (e = entryForHash(this, hash); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { if (oldValue.equals(e.value)) { e.value = newValue; ++modCount; replaced = true; } break; } } } finally { unlock(); } return replaced; } final V replace(K key, int hash, V value) { if (!tryLock()) scanAndLock(key, hash); V oldValue = null; try { HashEntry<K,V> e; for (e = entryForHash(this, hash); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == hash && key.equals(k))) { oldValue = e.value; e.value = value; ++modCount; break; } } } finally { unlock(); } return oldValue; } final void clear() { lock(); try { HashEntry<K,V>[] tab = table; for (int i = 0; i < tab.length ; i++) setEntryAt(tab, i, null); ++modCount; count = 0; } finally { unlock(); } } } // Accessing segments /** * Gets the jth element of given segment array (if nonnull) with * volatile element access semantics via Unsafe. (The null check * can trigger harmlessly only during deserialization.) Note: * because each element of segments array is set only once (using * fully ordered writes), some performance-sensitive methods rely * on this method only as a recheck upon null reads. */ @SuppressWarnings("unchecked") static final <K,V> Segment<K,V> segmentAt(Segment<K,V>[] ss, int j) { long u = (j << SSHIFT) + SBASE; return ss == null ? null : (Segment<K,V>) UNSAFE.getObjectVolatile(ss, u); } /** * Returns the segment for the given index, creating it and * recording in segment table (via CAS) if not already present. * * @param k the index * @return the segment */ @SuppressWarnings("unchecked") private Segment<K,V> ensureSegment(int k) { final Segment<K,V>[] ss = this.segments; long u = (k << SSHIFT) + SBASE; // raw offset Segment<K,V> seg; if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { Segment<K,V> proto = ss[0]; // use segment 0 as prototype int cap = proto.table.length; float lf = proto.loadFactor; int threshold = (int)(cap * lf); HashEntry<K,V>[] tab = (HashEntry<K,V>[])new HashEntry[cap]; if ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { // recheck Segment<K,V> s = new Segment<K,V>(lf, threshold, tab); while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) { if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s)) break; } } } return seg; } // Hash-based segment and entry accesses /** * Get the segment for the given hash */ @SuppressWarnings("unchecked") private Segment<K,V> segmentForHash(int h) { long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; return (Segment<K,V>) UNSAFE.getObjectVolatile(segments, u); } /** * Gets the table entry for the given segment and hash */ @SuppressWarnings("unchecked") static final <K,V> HashEntry<K,V> entryForHash(Segment<K,V> seg, int h) { HashEntry<K,V>[] tab; return (seg == null || (tab = seg.table) == null) ? null : (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); } /* ---------------- Public operations -------------- */ /** * Creates a new, empty map with the specified initial * capacity, load factor and concurrency level. * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @param concurrencyLevel the estimated number of concurrently * updating threads. The implementation performs internal sizing * to try to accommodate this many threads. * @throws IllegalArgumentException if the initial capacity is * negative or the load factor or concurrencyLevel are * nonpositive. */ @SuppressWarnings("unchecked") public ConcurrentHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) { if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0) throw new IllegalArgumentException(); if (concurrencyLevel > MAX_SEGMENTS) concurrencyLevel = MAX_SEGMENTS; // Find power-of-two sizes best matching arguments int sshift = 0; int ssize = 1; while (ssize < concurrencyLevel) { ++sshift; ssize <<= 1; } this.segmentShift = 32 - sshift; this.segmentMask = ssize - 1; if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY; int c = initialCapacity / ssize; if (c * ssize < initialCapacity) ++c; int cap = MIN_SEGMENT_TABLE_CAPACITY; while (cap < c) cap <<= 1; // create segments and segments[0] Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]); Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize]; UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0] this.segments = ss; } /** * Creates a new, empty map with the specified initial capacity * and load factor and with the default concurrencyLevel (16). * * @param initialCapacity The implementation performs internal * sizing to accommodate this many elements. * @param loadFactor the load factor threshold, used to control resizing. * Resizing may be performed when the average number of elements per * bin exceeds this threshold. * @throws IllegalArgumentException if the initial capacity of * elements is negative or the load factor is nonpositive * * @since 1.6 */ public ConcurrentHashMap(int initialCapacity, float loadFactor) { this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new, empty map with the specified initial capacity, * and with default load factor (0.75) and concurrencyLevel (16). * * @param initialCapacity the initial capacity. The implementation * performs internal sizing to accommodate this many elements. * @throws IllegalArgumentException if the initial capacity of * elements is negative. */ public ConcurrentHashMap(int initialCapacity) { this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new, empty map with a default initial capacity (16), * load factor (0.75) and concurrencyLevel (16). */ public ConcurrentHashMap() { this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); } /** * Creates a new map with the same mappings as the given map. * The map is created with a capacity of 1.5 times the number * of mappings in the given map or 16 (whichever is greater), * and a default load factor (0.75) and concurrencyLevel (16). * * @param m the map */ public ConcurrentHashMap(Map<? extends K, ? extends V> m) { this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL); putAll(m); } /** * Returns <tt>true</tt> if this map contains no key-value mappings. * * @return <tt>true</tt> if this map contains no key-value mappings */ public boolean isEmpty() { /* * Sum per-segment modCounts to avoid mis-reporting when * elements are concurrently added and removed in one segment * while checking another, in which case the table was never * actually empty at any point. (The sum ensures accuracy up * through at least 1<<31 per-segment modifications before * recheck.) Methods size() and containsValue() use similar * constructions for stability checks. */ long sum = 0L; final Segment<K,V>[] segments = this.segments; for (int j = 0; j < segments.length; ++j) { Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { if (seg.count != 0) return false; sum += seg.modCount; } } if (sum != 0L) { // recheck unless no modifications for (int j = 0; j < segments.length; ++j) { Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { if (seg.count != 0) return false; sum -= seg.modCount; } } if (sum != 0L) return false; } return true; } /** * Returns the number of key-value mappings in this map. If the * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns * <tt>Integer.MAX_VALUE</tt>. * * @return the number of key-value mappings in this map */ public int size() { // Try a few times to get accurate count. On failure due to // continuous async changes in table, resort to locking. final Segment<K,V>[] segments = this.segments; int size; boolean overflow; // true if size overflows 32 bits long sum; // sum of modCounts long last = 0L; // previous sum int retries = -1; // first iteration isn't retry try { for (;;) { if (retries++ == RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) ensureSegment(j).lock(); // force creation } sum = 0L; size = 0; overflow = false; for (int j = 0; j < segments.length; ++j) { Segment<K,V> seg = segmentAt(segments, j); if (seg != null) { sum += seg.modCount; int c = seg.count; if (c < 0 || (size += c) < 0) overflow = true; } } if (sum == last) break; last = sum; } } finally { if (retries > RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) segmentAt(segments, j).unlock(); } } return overflow ? Integer.MAX_VALUE : size; } /** * Returns the value to which the specified key is mapped, * or {@code null} if this map contains no mapping for the key. * * <p>More formally, if this map contains a mapping from a key * {@code k} to a value {@code v} such that {@code key.equals(k)}, * then this method returns {@code v}; otherwise it returns * {@code null}. (There can be at most one such mapping.) * * @throws NullPointerException if the specified key is null */ public V get(Object key) { Segment<K,V> s; // manually integrate access methods to reduce overhead HashEntry<K,V>[] tab; int h = hash(key); long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return e.value; } } return null; } /** * Tests if the specified object is a key in this table. * * @param key possible key * @return <tt>true</tt> if and only if the specified object * is a key in this table, as determined by the * <tt>equals</tt> method; <tt>false</tt> otherwise. * @throws NullPointerException if the specified key is null */ @SuppressWarnings("unchecked") public boolean containsKey(Object key) { Segment<K,V> s; // same as get() except no need for volatile value read HashEntry<K,V>[] tab; int h = hash(key); long u = (((h >>> segmentShift) & segmentMask) << SSHIFT) + SBASE; if ((s = (Segment<K,V>)UNSAFE.getObjectVolatile(segments, u)) != null && (tab = s.table) != null) { for (HashEntry<K,V> e = (HashEntry<K,V>) UNSAFE.getObjectVolatile (tab, ((long)(((tab.length - 1) & h)) << TSHIFT) + TBASE); e != null; e = e.next) { K k; if ((k = e.key) == key || (e.hash == h && key.equals(k))) return true; } } return false; } /** * Returns <tt>true</tt> if this map maps one or more keys to the * specified value. Note: This method requires a full internal * traversal of the hash table, and so is much slower than * method <tt>containsKey</tt>. * * @param value value whose presence in this map is to be tested * @return <tt>true</tt> if this map maps one or more keys to the * specified value * @throws NullPointerException if the specified value is null */ public boolean containsValue(Object value) { // Same idea as size() if (value == null) throw new NullPointerException(); final Segment<K,V>[] segments = this.segments; boolean found = false; long last = 0; int retries = -1; try { outer: for (;;) { if (retries++ == RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) ensureSegment(j).lock(); // force creation } long hashSum = 0L; int sum = 0; for (int j = 0; j < segments.length; ++j) { HashEntry<K,V>[] tab; Segment<K,V> seg = segmentAt(segments, j); if (seg != null && (tab = seg.table) != null) { for (int i = 0 ; i < tab.length; i++) { HashEntry<K,V> e; for (e = entryAt(tab, i); e != null; e = e.next) { V v = e.value; if (v != null && value.equals(v)) { found = true; break outer; } } } sum += seg.modCount; } } if (retries > 0 && sum == last) break; last = sum; } } finally { if (retries > RETRIES_BEFORE_LOCK) { for (int j = 0; j < segments.length; ++j) segmentAt(segments, j).unlock(); } } return found; } /** * Legacy method testing if some key maps into the specified value * in this table. This method is identical in functionality to * {@link #containsValue}, and exists solely to ensure * full compatibility with class {@link java.util.Hashtable}, * which supported this method prior to introduction of the * Java Collections framework. * @param value a value to search for * @return <tt>true</tt> if and only if some key maps to the * <tt>value</tt> argument in this table as * determined by the <tt>equals</tt> method; * <tt>false</tt> otherwise * @throws NullPointerException if the specified value is null */ public boolean contains(Object value) { return containsValue(value); } /** * Maps the specified key to the specified value in this table. * Neither the key nor the value can be null. * * <p> The value can be retrieved by calling the <tt>get</tt> method * with a key that is equal to the original key. * * @param key key with which the specified value is to be associated * @param value value to be associated with the specified key * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key or value is null */ @SuppressWarnings("unchecked") public V put(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key); int j = (hash >>> segmentShift) & segmentMask; if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck (segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment s = ensureSegment(j); return s.put(key, hash, value, false); } /** * {@inheritDoc} * * @return the previous value associated with the specified key, * or <tt>null</tt> if there was no mapping for the key * @throws NullPointerException if the specified key or value is null */ @SuppressWarnings("unchecked") public V putIfAbsent(K key, V value) { Segment<K,V> s; if (value == null) throw new NullPointerException(); int hash = hash(key); int j = (hash >>> segmentShift) & segmentMask; if ((s = (Segment<K,V>)UNSAFE.getObject (segments, (j << SSHIFT) + SBASE)) == null) s = ensureSegment(j); return s.put(key, hash, value, true); } /** * Copies all of the mappings from the specified map to this one. * These mappings replace any mappings that this map had for any of the * keys currently in the specified map. * * @param m mappings to be stored in this map */ public void putAll(Map<? extends K, ? extends V> m) { for (Map.Entry<? extends K, ? extends V> e : m.entrySet()) put(e.getKey(), e.getValue()); } /** * Removes the key (and its corresponding value) from this map. * This method does nothing if the key is not in the map. * * @param key the key that needs to be removed * @return the previous value associated with <tt>key</tt>, or * <tt>null</tt> if there was no mapping for <tt>key</tt> * @throws NullPointerException if the specified key is null */ public V remove(Object key) { int hash = hash(key); Segment<K,V> s = segmentForHash(hash); return s == null ? null : s.remove(key, hash, null); } /** * {@inheritDoc} * * @throws NullPointerException if the specified key is null */ public boolean remove(Object key, Object value) { int hash = hash(key); Segment<K,V> s; return value != null && (s = segmentForHash(hash)) != null && s.remove(key, hash, value) != null; } /** * {@inheritDoc} * * @throws NullPointerException if any of the arguments are null */ public boolean replace(K key, V oldValue, V newValue) { int hash = hash(key); if (oldValue == null || newValue == null) throw new NullPointerException(); Segment<K,V> s = segmentForHash(hash); return s != null && s.replace(key, hash, oldValue, newValue); } /** * {@inheritDoc} * * @return the previous value associated with the specified key, * or <tt>null</tt> if there was no mapping for the key * @throws NullPointerException if the specified key or value is null */ public V replace(K key, V value) { int hash = hash(key); if (value == null) throw new NullPointerException(); Segment<K,V> s = segmentForHash(hash); return s == null ? null : s.replace(key, hash, value); } /** * Removes all of the mappings from this map. */ public void clear() { final Segment<K,V>[] segments = this.segments; for (int j = 0; j < segments.length; ++j) { Segment<K,V> s = segmentAt(segments, j); if (s != null) s.clear(); } } /** * Returns a {@link Set} view of the keys contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. The set supports element * removal, which removes the corresponding mapping from this map, * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or * <tt>addAll</tt> operations. * * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator * that will never throw {@link ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. */ public Set<K> keySet() { Set<K> ks = keySet; return (ks != null) ? ks : (keySet = new KeySet()); } /** * Returns a {@link Collection} view of the values contained in this map. * The collection is backed by the map, so changes to the map are * reflected in the collection, and vice-versa. The collection * supports element removal, which removes the corresponding * mapping from this map, via the <tt>Iterator.remove</tt>, * <tt>Collection.remove</tt>, <tt>removeAll</tt>, * <tt>retainAll</tt>, and <tt>clear</tt> operations. It does not * support the <tt>add</tt> or <tt>addAll</tt> operations. * * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator * that will never throw {@link ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. */ public Collection<V> values() { Collection<V> vs = values; return (vs != null) ? vs : (values = new Values()); } /** * Returns a {@link Set} view of the mappings contained in this map. * The set is backed by the map, so changes to the map are * reflected in the set, and vice-versa. The set supports element * removal, which removes the corresponding mapping from the map, * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>, * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt> * operations. It does not support the <tt>add</tt> or * <tt>addAll</tt> operations. * * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator * that will never throw {@link ConcurrentModificationException}, * and guarantees to traverse elements as they existed upon * construction of the iterator, and may (but is not guaranteed to) * reflect any modifications subsequent to construction. */ public Set<Map.Entry<K,V>> entrySet() { Set<Map.Entry<K,V>> es = entrySet; return (es != null) ? es : (entrySet = new EntrySet()); } /** * Returns an enumeration of the keys in this table. * * @return an enumeration of the keys in this table * @see #keySet() */ public Enumeration<K> keys() { return new KeyIterator(); } /** * Returns an enumeration of the values in this table. * * @return an enumeration of the values in this table * @see #values() */ public Enumeration<V> elements() { return new ValueIterator(); } /* ---------------- Iterator Support -------------- */ abstract class HashIterator { int nextSegmentIndex; int nextTableIndex; HashEntry<K,V>[] currentTable; HashEntry<K, V> nextEntry; HashEntry<K, V> lastReturned; HashIterator() { nextSegmentIndex = segments.length - 1; nextTableIndex = -1; advance(); } /** * Set nextEntry to first node of next non-empty table * (in backwards order, to simplify checks). */ final void advance() { for (;;) { if (nextTableIndex >= 0) { if ((nextEntry = entryAt(currentTable, nextTableIndex--)) != null) break; } else if (nextSegmentIndex >= 0) { Segment<K,V> seg = segmentAt(segments, nextSegmentIndex--); if (seg != null && (currentTable = seg.table) != null) nextTableIndex = currentTable.length - 1; } else break; } } final HashEntry<K,V> nextEntry() { HashEntry<K,V> e = nextEntry; if (e == null) throw new NoSuchElementException(); lastReturned = e; // cannot assign until after null check if ((nextEntry = e.next) == null) advance(); return e; } public final boolean hasNext() { return nextEntry != null; } public final boolean hasMoreElements() { return nextEntry != null; } public final void remove() { if (lastReturned == null) throw new IllegalStateException(); ConcurrentHashMap.this.remove(lastReturned.key); lastReturned = null; } } final class KeyIterator extends HashIterator implements Iterator<K>, Enumeration<K> { public final K next() { return super.nextEntry().key; } public final K nextElement() { return super.nextEntry().key; } } final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V> { public final V next() { return super.nextEntry().value; } public final V nextElement() { return super.nextEntry().value; } } /** * Custom Entry class used by EntryIterator.next(), that relays * setValue changes to the underlying map. */ final class WriteThroughEntry extends AbstractMap.SimpleEntry<K,V> { WriteThroughEntry(K k, V v) { super(k,v); } /** * Set our entry's value and write through to the map. The * value to return is somewhat arbitrary here. Since a * WriteThroughEntry does not necessarily track asynchronous * changes, the most recent "previous" value could be * different from what we return (or could even have been * removed in which case the put will re-establish). We do not * and cannot guarantee more. */ public V setValue(V value) { if (value == null) throw new NullPointerException(); V v = super.setValue(value); ConcurrentHashMap.this.put(getKey(), value); return v; } } final class EntryIterator extends HashIterator implements Iterator<Entry<K,V>> { public Map.Entry<K,V> next() { HashEntry<K,V> e = super.nextEntry(); return new WriteThroughEntry(e.key, e.value); } } final class KeySet extends AbstractSet<K> { public Iterator<K> iterator() { return new KeyIterator(); } public int size() { return ConcurrentHashMap.this.size(); } public boolean isEmpty() { return ConcurrentHashMap.this.isEmpty(); } public boolean contains(Object o) { return ConcurrentHashMap.this.containsKey(o); } public boolean remove(Object o) { return ConcurrentHashMap.this.remove(o) != null; } public void clear() { ConcurrentHashMap.this.clear(); } } final class Values extends AbstractCollection<V> { public Iterator<V> iterator() { return new ValueIterator(); } public int size() { return ConcurrentHashMap.this.size(); } public boolean isEmpty() { return ConcurrentHashMap.this.isEmpty(); } public boolean contains(Object o) { return ConcurrentHashMap.this.containsValue(o); } public void clear() { ConcurrentHashMap.this.clear(); } } final class EntrySet extends AbstractSet<Map.Entry<K,V>> { public Iterator<Map.Entry<K,V>> iterator() { return new EntryIterator(); } public boolean contains(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> e = (Map.Entry<?,?>)o; V v = ConcurrentHashMap.this.get(e.getKey()); return v != null && v.equals(e.getValue()); } public boolean remove(Object o) { if (!(o instanceof Map.Entry)) return false; Map.Entry<?,?> e = (Map.Entry<?,?>)o; return ConcurrentHashMap.this.remove(e.getKey(), e.getValue()); } public int size() { return ConcurrentHashMap.this.size(); } public boolean isEmpty() { return ConcurrentHashMap.this.isEmpty(); } public void clear() { ConcurrentHashMap.this.clear(); } } /* ---------------- Serialization Support -------------- */ /** * Save the state of the <tt>ConcurrentHashMap</tt> instance to a * stream (i.e., serialize it). * @param s the stream * @serialData * the key (Object) and value (Object) * for each key-value mapping, followed by a null pair. * The key-value mappings are emitted in no particular order. */ private void writeObject(java.io.ObjectOutputStream s) throws IOException { // force all segments for serialization compatibility for (int k = 0; k < segments.length; ++k) ensureSegment(k); s.defaultWriteObject(); final Segment<K,V>[] segments = this.segments; for (int k = 0; k < segments.length; ++k) { Segment<K,V> seg = segmentAt(segments, k); seg.lock(); try { HashEntry<K,V>[] tab = seg.table; for (int i = 0; i < tab.length; ++i) { HashEntry<K,V> e; for (e = entryAt(tab, i); e != null; e = e.next) { s.writeObject(e.key); s.writeObject(e.value); } } } finally { seg.unlock(); } } s.writeObject(null); s.writeObject(null); } /** * Reconstitute the <tt>ConcurrentHashMap</tt> instance from a * stream (i.e., deserialize it). * @param s the stream */ @SuppressWarnings("unchecked") private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException { // Don't call defaultReadObject() ObjectInputStream.GetField oisFields = s.readFields(); final Segment<K,V>[] oisSegments = (Segment<K,V>[])oisFields.get("segments", null); final int ssize = oisSegments.length; if (ssize < 1 || ssize > MAX_SEGMENTS || (ssize & (ssize-1)) != 0 ) // ssize not power of two throw new java.io.InvalidObjectException("Bad number of segments:" + ssize); int sshift = 0, ssizeTmp = ssize; while (ssizeTmp > 1) { ++sshift; ssizeTmp >>>= 1; } UNSAFE.putIntVolatile(this, SEGSHIFT_OFFSET, 32 - sshift); UNSAFE.putIntVolatile(this, SEGMASK_OFFSET, ssize - 1); UNSAFE.putObjectVolatile(this, SEGMENTS_OFFSET, oisSegments); // set hashMask UNSAFE.putIntVolatile(this, HASHSEED_OFFSET, randomHashSeed(this)); // Re-initialize segments to be minimally sized, and let grow. int cap = MIN_SEGMENT_TABLE_CAPACITY; final Segment<K,V>[] segments = this.segments; for (int k = 0; k < segments.length; ++k) { Segment<K,V> seg = segments[k]; if (seg != null) { seg.threshold = (int)(cap * seg.loadFactor); seg.table = (HashEntry<K,V>[]) new HashEntry[cap]; } } // Read the keys and values, and put the mappings in the table for (;;) { K key = (K) s.readObject(); V value = (V) s.readObject(); if (key == null) break; put(key, value); } } // Unsafe mechanics private static final sun.misc.Unsafe UNSAFE; private static final long SBASE; private static final int SSHIFT; private static final long TBASE; private static final int TSHIFT; private static final long HASHSEED_OFFSET; private static final long SEGSHIFT_OFFSET; private static final long SEGMASK_OFFSET; private static final long SEGMENTS_OFFSET; static { int ss, ts; try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class tc = HashEntry[].class; Class sc = Segment[].class; TBASE = UNSAFE.arrayBaseOffset(tc); SBASE = UNSAFE.arrayBaseOffset(sc); ts = UNSAFE.arrayIndexScale(tc); ss = UNSAFE.arrayIndexScale(sc); HASHSEED_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("hashSeed")); SEGSHIFT_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("segmentShift")); SEGMASK_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("segmentMask")); SEGMENTS_OFFSET = UNSAFE.objectFieldOffset( ConcurrentHashMap.class.getDeclaredField("segments")); } catch (Exception e) { throw new Error(e); } if ((ss & (ss-1)) != 0 || (ts & (ts-1)) != 0) throw new Error("data type scale not a power of two"); SSHIFT = 31 - Integer.numberOfLeadingZeros(ss); TSHIFT = 31 - Integer.numberOfLeadingZeros(ts); }}CopyOnWriteArrayList
CopyOnWriteArrayList是同步List的并发替代品，它提供了更好的并发性，并避免了在迭代期间加锁和复制。
List实现类CopyOnWriteArrayList：
package java.util.concurrent;import java.util.*;import java.util.concurrent.locks.*;import sun.misc.Unsafe;/** * A thread-safe variant of {@link java.util.ArrayList} in which all mutative * operations (<tt>add</tt>, <tt>set</tt>, and so on) are implemented by * making a fresh copy of the underlying array. * * <p> This is ordinarily too costly, but may be <em>more</em> efficient * than alternatives when traversal operations vastly outnumber * mutations, and is useful when you cannot or don't want to * synchronize traversals, yet need to preclude interference among * concurrent threads. The "snapshot" style iterator method uses a * reference to the state of the array at the point that the iterator * was created. This array never changes during the lifetime of the * iterator, so interference is impossible and the iterator is * guaranteed not to throw <tt>ConcurrentModificationException</tt>. * The iterator will not reflect additions, removals, or changes to * the list since the iterator was created. Element-changing * operations on iterators themselves (<tt>remove</tt>, <tt>set</tt>, and * <tt>add</tt>) are not supported. These methods throw * <tt>UnsupportedOperationException</tt>. * * <p>All elements are permitted, including <tt>null</tt>. * * <p>Memory consistency effects: As with other concurrent * collections, actions in a thread prior to placing an object into a * {@code CopyOnWriteArrayList} * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a> * actions subsequent to the access or removal of that element from * the {@code CopyOnWriteArrayList} in another thread. * * <p>This class is a member of the * <a href="{@docRoot}/../technotes/guides/collections/index.html"> * Java Collections Framework</a>. * * @since 1.5 * @author Doug Lea * @param <E> the type of elements held in this collection */public class CopyOnWriteArrayList<E> implements List<E>, RandomAccess, Cloneable, java.io.Serializable { private static final long serialVersionUID = 8673264195747942595L; /** The lock protecting all mutators */ transient final ReentrantLock lock = new ReentrantLock(); /** The array, accessed only via getArray/setArray. */ private volatile transient Object[] array; /** * Gets the array. Non-private so as to also be accessible * from CopyOnWriteArraySet class. */ final Object[] getArray() { return array; } /** * Sets the array. */ final void setArray(Object[] a) { array = a; } /** * Creates an empty list. */ public CopyOnWriteArrayList() { setArray(new Object[0]); } /** * Creates a list containing the elements of the specified * collection, in the order they are returned by the collection's * iterator. * * @param c the collection of initially held elements * @throws NullPointerException if the specified collection is null */ public CopyOnWriteArrayList(Collection<? extends E> c) { Object[] elements = c.toArray(); // c.toArray might (incorrectly) not return Object[] (see 6260652) if (elements.getClass() != Object[].class) elements = Arrays.copyOf(elements, elements.length, Object[].class); setArray(elements); } /** * Creates a list holding a copy of the given array. * * @param toCopyIn the array (a copy of this array is used as the * internal array) * @throws NullPointerException if the specified array is null */ public CopyOnWriteArrayList(E[] toCopyIn) { setArray(Arrays.copyOf(toCopyIn, toCopyIn.length, Object[].class)); } /** * Returns the number of elements in this list. * * @return the number of elements in this list */ public int size() { return getArray().length; } /** * Returns <tt>true</tt> if this list contains no elements. * * @return <tt>true</tt> if this list contains no elements */ public boolean isEmpty() { return size() == 0; } /** * Test for equality, coping with nulls. */ private static boolean eq(Object o1, Object o2) { return (o1 == null ? o2 == null : o1.equals(o2)); } /** * static version of indexOf, to allow repeated calls without * needing to re-acquire array each time. * @param o element to search for * @param elements the array * @param index first index to search * @param fence one past last index to search * @return index of element, or -1 if absent */ private static int indexOf(Object o, Object[] elements, int index, int fence) { if (o == null) { for (int i = index; i < fence; i++) if (elements[i] == null) return i; } else { for (int i = index; i < fence; i++) if (o.equals(elements[i])) return i; } return -1; } /** * static version of lastIndexOf. * @param o element to search for * @param elements the array * @param index first index to search * @return index of element, or -1 if absent */ private static int lastIndexOf(Object o, Object[] elements, int index) { if (o == null) { for (int i = index; i >= 0; i--) if (elements[i] == null) return i; } else { for (int i = index; i >= 0; i--) if (o.equals(elements[i])) return i; } return -1; } /** * Returns <tt>true</tt> if this list contains the specified element. * More formally, returns <tt>true</tt> if and only if this list contains * at least one element <tt>e</tt> such that * <tt>(o==null ? e==null : o.equals(e))</tt>. * * @param o element whose presence in this list is to be tested * @return <tt>true</tt> if this list contains the specified element */ public boolean contains(Object o) { Object[] elements = getArray(); return indexOf(o, elements, 0, elements.length) >= 0; } /** * {@inheritDoc} */ public int indexOf(Object o) { Object[] elements = getArray(); return indexOf(o, elements, 0, elements.length); } /** * Returns the index of the first occurrence of the specified element in * this list, searching forwards from <tt>index</tt>, or returns -1 if * the element is not found. * More formally, returns the lowest index <tt>i</tt> such that * <tt>(i >= index && (e==null ? get(i)==null : e.equals(get(i))))</tt>, * or -1 if there is no such index. * * @param e element to search for * @param index index to start searching from * @return the index of the first occurrence of the element in * this list at position <tt>index</tt> or later in the list; * <tt>-1</tt> if the element is not found. * @throws IndexOutOfBoundsException if the specified index is negative */ public int indexOf(E e, int index) { Object[] elements = getArray(); return indexOf(e, elements, index, elements.length); } /** * {@inheritDoc} */ public int lastIndexOf(Object o) { Object[] elements = getArray(); return lastIndexOf(o, elements, elements.length - 1); } /** * Returns the index of the last occurrence of the specified element in * this list, searching backwards from <tt>index</tt>, or returns -1 if * the element is not found. * More formally, returns the highest index <tt>i</tt> such that * <tt>(i <= index && (e==null ? get(i)==null : e.equals(get(i))))</tt>, * or -1 if there is no such index. * * @param e element to search for * @param index index to start searching backwards from * @return the index of the last occurrence of the element at position * less than or equal to <tt>index</tt> in this list; * -1 if the element is not found. * @throws IndexOutOfBoundsException if the specified index is greater * than or equal to the current size of this list */ public int lastIndexOf(E e, int index) { Object[] elements = getArray(); return lastIndexOf(e, elements, index); } /** * Returns a shallow copy of this list. (The elements themselves * are not copied.) * * @return a clone of this list */ public Object clone() { try { CopyOnWriteArrayList c = (CopyOnWriteArrayList)(super.clone()); c.resetLock(); return c; } catch (CloneNotSupportedException e) { // this shouldn't happen, since we are Cloneable throw new InternalError(); } } /** * Returns an array containing all of the elements in this list * in proper sequence (from first to last element). * * <p>The returned array will be "safe" in that no references to it are * maintained by this list. (In other words, this method must allocate * a new array). The caller is thus free to modify the returned array. * * <p>This method acts as bridge between array-based and collection-based * APIs. * * @return an array containing all the elements in this list */ public Object[] toArray() { Object[] elements = getArray(); return Arrays.copyOf(elements, elements.length); } /** * Returns an array containing all of the elements in this list in * proper sequence (from first to last element); the runtime type of * the returned array is that of the specified array. If the list fits * in the specified array, it is returned therein. Otherwise, a new * array is allocated with the runtime type of the specified array and * the size of this list. * * <p>If this list fits in the specified array with room to spare * (i.e., the array has more elements than this list), the element in * the array immediately following the end of the list is set to * <tt>null</tt>. (This is useful in determining the length of this * list <i>only</i> if the caller knows that this list does not contain * any null elements.) * * <p>Like the {@link #toArray()} method, this method acts as bridge between * array-based and collection-based APIs. Further, this method allows * precise control over the runtime type of the output array, and may, * under certain circumstances, be used to save allocation costs. * * <p>Suppose <tt>x</tt> is a list known to contain only strings. * The following code can be used to dump the list into a newly * allocated array of <tt>String</tt>: * * <pre> * String[] y = x.toArray(new String[0]);</pre> * * Note that <tt>toArray(new Object[0])</tt> is identical in function to * <tt>toArray()</tt>. * * @param a the array into which the elements of the list are to * be stored, if it is big enough; otherwise, a new array of the * same runtime type is allocated for this purpose. * @return an array containing all the elements in this list * @throws ArrayStoreException if the runtime type of the specified array * is not a supertype of the runtime type of every element in * this list * @throws NullPointerException if the specified array is null */ @SuppressWarnings("unchecked") public <T> T[] toArray(T a[]) { Object[] elements = getArray(); int len = elements.length; if (a.length < len) return (T[]) Arrays.copyOf(elements, len, a.getClass()); else { System.arraycopy(elements, 0, a, 0, len); if (a.length > len) a[len] = null; return a; } } // Positional Access Operations @SuppressWarnings("unchecked") private E get(Object[] a, int index) { return (E) a[index]; } /** * {@inheritDoc} * * @throws IndexOutOfBoundsException {@inheritDoc} */ public E get(int index) { return get(getArray(), index); } /** * Replaces the element at the specified position in this list with the * specified element. * * @throws IndexOutOfBoundsException {@inheritDoc} */ public E set(int index, E element) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); E oldValue = get(elements, index); if (oldValue != element) { int len = elements.length; Object[] newElements = Arrays.copyOf(elements, len); newElements[index] = element; setArray(newElements); } else { // Not quite a no-op; ensures volatile write semantics setArray(elements); } return oldValue; } finally { lock.unlock(); } } /** * Appends the specified element to the end of this list. * * @param e element to be appended to this list * @return <tt>true</tt> (as specified by {@link Collection#add}) */ public boolean add(E e) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; Object[] newElements = Arrays.copyOf(elements, len + 1); newElements[len] = e; setArray(newElements); return true; } finally { lock.unlock(); } } /** * Inserts the specified element at the specified position in this * list. Shifts the element currently at that position (if any) and * any subsequent elements to the right (adds one to their indices). * * @throws IndexOutOfBoundsException {@inheritDoc} */ public void add(int index, E element) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; if (index > len || index < 0) throw new IndexOutOfBoundsException("Index: "+index+ ", Size: "+len); Object[] newElements; int numMoved = len - index; if (numMoved == 0) newElements = Arrays.copyOf(elements, len + 1); else { newElements = new Object[len + 1]; System.arraycopy(elements, 0, newElements, 0, index); System.arraycopy(elements, index, newElements, index + 1, numMoved); } newElements[index] = element; setArray(newElements); } finally { lock.unlock(); } } /** * Removes the element at the specified position in this list. * Shifts any subsequent elements to the left (subtracts one from their * indices). Returns the element that was removed from the list. * * @throws IndexOutOfBoundsException {@inheritDoc} */ public E remove(int index) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; E oldValue = get(elements, index); int numMoved = len - index - 1; if (numMoved == 0) setArray(Arrays.copyOf(elements, len - 1)); else { Object[] newElements = new Object[len - 1]; System.arraycopy(elements, 0, newElements, 0, index); System.arraycopy(elements, index + 1, newElements, index, numMoved); setArray(newElements); } return oldValue; } finally { lock.unlock(); } } /** * Removes the first occurrence of the specified element from this list, * if it is present. If this list does not contain the element, it is * unchanged. More formally, removes the element with the lowest index * <tt>i</tt> such that * <tt>(o==null ? get(i)==null : o.equals(get(i)))</tt> * (if such an element exists). Returns <tt>true</tt> if this list * contained the specified element (or equivalently, if this list * changed as a result of the call). * * @param o element to be removed from this list, if present * @return <tt>true</tt> if this list contained the specified element */ public boolean remove(Object o) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; if (len != 0) { // Copy while searching for element to remove // This wins in the normal case of element being present int newlen = len - 1; Object[] newElements = new Object[newlen]; for (int i = 0; i < newlen; ++i) { if (eq(o, elements[i])) { // found one; copy remaining and exit for (int k = i + 1; k < len; ++k) newElements[k-1] = elements[k]; setArray(newElements); return true; } else newElements[i] = elements[i]; } // special handling for last cell if (eq(o, elements[newlen])) { setArray(newElements); return true; } } return false; } finally { lock.unlock(); } } /** * Removes from this list all of the elements whose index is between * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive. * Shifts any succeeding elements to the left (reduces their index). * This call shortens the list by <tt>(toIndex - fromIndex)</tt> elements. * (If <tt>toIndex==fromIndex</tt>, this operation has no effect.) * * @param fromIndex index of first element to be removed * @param toIndex index after last element to be removed * @throws IndexOutOfBoundsException if fromIndex or toIndex out of range * ({@code{fromIndex < 0 || toIndex > size() || toIndex < fromIndex}) */ private void removeRange(int fromIndex, int toIndex) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; if (fromIndex < 0 || toIndex > len || toIndex < fromIndex) throw new IndexOutOfBoundsException(); int newlen = len - (toIndex - fromIndex); int numMoved = len - toIndex; if (numMoved == 0) setArray(Arrays.copyOf(elements, newlen)); else { Object[] newElements = new Object[newlen]; System.arraycopy(elements, 0, newElements, 0, fromIndex); System.arraycopy(elements, toIndex, newElements, fromIndex, numMoved); setArray(newElements); } } finally { lock.unlock(); } } /** * Append the element if not present. * * @param e element to be added to this list, if absent * @return <tt>true</tt> if the element was added */ public boolean addIfAbsent(E e) { final ReentrantLock lock = this.lock; lock.lock(); try { // Copy while checking if already present. // This wins in the most common case where it is not present Object[] elements = getArray(); int len = elements.length; Object[] newElements = new Object[len + 1]; for (int i = 0; i < len; ++i) { if (eq(e, elements[i])) return false; // exit, throwing away copy else newElements[i] = elements[i]; } newElements[len] = e; setArray(newElements); return true; } finally { lock.unlock(); } } /** * Returns <tt>true</tt> if this list contains all of the elements of the * specified collection. * * @param c collection to be checked for containment in this list * @return <tt>true</tt> if this list contains all of the elements of the * specified collection * @throws NullPointerException if the specified collection is null * @see #contains(Object) */ public boolean containsAll(Collection<?> c) { Object[] elements = getArray(); int len = elements.length; for (Object e : c) { if (indexOf(e, elements, 0, len) < 0) return false; } return true; } /** * Removes from this list all of its elements that are contained in * the specified collection. This is a particularly expensive operation * in this class because of the need for an internal temporary array. * * @param c collection containing elements to be removed from this list * @return <tt>true</tt> if this list changed as a result of the call * @throws ClassCastException if the class of an element of this list * is incompatible with the specified collection * (<a href="../Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if this list contains a null element and the * specified collection does not permit null elements * (<a href="../Collection.html#optional-restrictions">optional</a>), * or if the specified collection is null * @see #remove(Object) */ public boolean removeAll(Collection<?> c) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; if (len != 0) { // temp array holds those elements we know we want to keep int newlen = 0; Object[] temp = new Object[len]; for (int i = 0; i < len; ++i) { Object element = elements[i]; if (!c.contains(element)) temp[newlen++] = element; } if (newlen != len) { setArray(Arrays.copyOf(temp, newlen)); return true; } } return false; } finally { lock.unlock(); } } /** * Retains only the elements in this list that are contained in the * specified collection. In other words, removes from this list all of * its elements that are not contained in the specified collection. * * @param c collection containing elements to be retained in this list * @return <tt>true</tt> if this list changed as a result of the call * @throws ClassCastException if the class of an element of this list * is incompatible with the specified collection * (<a href="../Collection.html#optional-restrictions">optional</a>) * @throws NullPointerException if this list contains a null element and the * specified collection does not permit null elements * (<a href="../Collection.html#optional-restrictions">optional</a>), * or if the specified collection is null * @see #remove(Object) */ public boolean retainAll(Collection<?> c) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; if (len != 0) { // temp array holds those elements we know we want to keep int newlen = 0; Object[] temp = new Object[len]; for (int i = 0; i < len; ++i) { Object element = elements[i]; if (c.contains(element)) temp[newlen++] = element; } if (newlen != len) { setArray(Arrays.copyOf(temp, newlen)); return true; } } return false; } finally { lock.unlock(); } } /** * Appends all of the elements in the specified collection that * are not already contained in this list, to the end of * this list, in the order that they are returned by the * specified collection's iterator. * * @param c collection containing elements to be added to this list * @return the number of elements added * @throws NullPointerException if the specified collection is null * @see #addIfAbsent(Object) */ public int addAllAbsent(Collection<? extends E> c) { Object[] cs = c.toArray(); if (cs.length == 0) return 0; Object[] uniq = new Object[cs.length]; final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; int added = 0; for (int i = 0; i < cs.length; ++i) { // scan for duplicates Object e = cs[i]; if (indexOf(e, elements, 0, len) < 0 && indexOf(e, uniq, 0, added) < 0) uniq[added++] = e; } if (added > 0) { Object[] newElements = Arrays.copyOf(elements, len + added); System.arraycopy(uniq, 0, newElements, len, added); setArray(newElements); } return added; } finally { lock.unlock(); } } /** * Removes all of the elements from this list. * The list will be empty after this call returns. */ public void clear() { final ReentrantLock lock = this.lock; lock.lock(); try { setArray(new Object[0]); } finally { lock.unlock(); } } /** * Appends all of the elements in the specified collection to the end * of this list, in the order that they are returned by the specified * collection's iterator. * * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws NullPointerException if the specified collection is null * @see #add(Object) */ public boolean addAll(Collection<? extends E> c) { Object[] cs = c.toArray(); if (cs.length == 0) return false; final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; Object[] newElements = Arrays.copyOf(elements, len + cs.length); System.arraycopy(cs, 0, newElements, len, cs.length); setArray(newElements); return true; } finally { lock.unlock(); } } /** * Inserts all of the elements in the specified collection into this * list, starting at the specified position. Shifts the element * currently at that position (if any) and any subsequent elements to * the right (increases their indices). The new elements will appear * in this list in the order that they are returned by the * specified collection's iterator. * * @param index index at which to insert the first element * from the specified collection * @param c collection containing elements to be added to this list * @return <tt>true</tt> if this list changed as a result of the call * @throws IndexOutOfBoundsException {@inheritDoc} * @throws NullPointerException if the specified collection is null * @see #add(int,Object) */ public boolean addAll(int index, Collection<? extends E> c) { Object[] cs = c.toArray(); final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; if (index > len || index < 0) throw new IndexOutOfBoundsException("Index: "+index+ ", Size: "+len); if (cs.length == 0) return false; int numMoved = len - index; Object[] newElements; if (numMoved == 0) newElements = Arrays.copyOf(elements, len + cs.length); else { newElements = new Object[len + cs.length]; System.arraycopy(elements, 0, newElements, 0, index); System.arraycopy(elements, index, newElements, index + cs.length, numMoved); } System.arraycopy(cs, 0, newElements, index, cs.length); setArray(newElements); return true; } finally { lock.unlock(); } } /** * Saves the state of the list to a stream (that is, serializes it). * * @serialData The length of the array backing the list is emitted * (int), followed by all of its elements (each an Object) * in the proper order. * @param s the stream */ private void writeObject(java.io.ObjectOutputStream s) throws java.io.IOException{ s.defaultWriteObject(); Object[] elements = getArray(); // Write out array length s.writeInt(elements.length); // Write out all elements in the proper order. for (Object element : elements) s.writeObject(element); } /** * Reconstitutes the list from a stream (that is, deserializes it). * * @param s the stream */ private void readObject(java.io.ObjectInputStream s) throws java.io.IOException, ClassNotFoundException { s.defaultReadObject(); // bind to new lock resetLock(); // Read in array length and allocate array int len = s.readInt(); Object[] elements = new Object[len]; // Read in all elements in the proper order. for (int i = 0; i < len; i++) elements[i] = s.readObject(); setArray(elements); } /** * Returns a string representation of this list. The string * representation consists of the string representations of the list's * elements in the order they are returned by its iterator, enclosed in * square brackets (<tt>"[]"</tt>). Adjacent elements are separated by * the characters <tt>", "</tt> (comma and space). Elements are * converted to strings as by {@link String#valueOf(Object)}. * * @return a string representation of this list */ public String toString() { return Arrays.toString(getArray()); } /** * Compares the specified object with this list for equality. * Returns {@code true} if the specified object is the same object * as this object, or if it is also a {@link List} and the sequence * of elements returned by an {@linkplain List#iterator() iterator} * over the specified list is the same as the sequence returned by * an iterator over this list. The two sequences are considered to * be the same if they have the same length and corresponding * elements at the same position in the sequence are <em>equal</em>. * Two elements {@code e1} and {@code e2} are considered * <em>equal</em> if {@code (e1==null ? e2==null : e1.equals(e2))}. * * @param o the object to be compared for equality with this list * @return {@code true} if the specified object is equal to this list */ public boolean equals(Object o) { if (o == this) return true; if (!(o instanceof List)) return false; List<?> list = (List<?>)(o); Iterator<?> it = list.iterator(); Object[] elements = getArray(); int len = elements.length; for (int i = 0; i < len; ++i) if (!it.hasNext() || !eq(elements[i], it.next())) return false; if (it.hasNext()) return false; return true; } /** * Returns the hash code value for this list. * * <p>This implementation uses the definition in {@link List#hashCode}. * * @return the hash code value for this list */ public int hashCode() { int hashCode = 1; Object[] elements = getArray(); int len = elements.length; for (int i = 0; i < len; ++i) { Object obj = elements[i]; hashCode = 31*hashCode + (obj==null ? 0 : obj.hashCode()); } return hashCode; } /** * Returns an iterator over the elements in this list in proper sequence. * * <p>The returned iterator provides a snapshot of the state of the list * when the iterator was constructed. No synchronization is needed while * traversing the iterator. The iterator does <em>NOT</em> support the * <tt>remove</tt> method. * * @return an iterator over the elements in this list in proper sequence */ public Iterator<E> iterator() { return new COWIterator<E>(getArray(), 0); } /** * {@inheritDoc} * * <p>The returned iterator provides a snapshot of the state of the list * when the iterator was constructed. No synchronization is needed while * traversing the iterator. The iterator does <em>NOT</em> support the * <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods. */ public ListIterator<E> listIterator() { return new COWIterator<E>(getArray(), 0); } /** * {@inheritDoc} * * <p>The returned iterator provides a snapshot of the state of the list * when the iterator was constructed. No synchronization is needed while * traversing the iterator. The iterator does <em>NOT</em> support the * <tt>remove</tt>, <tt>set</tt> or <tt>add</tt> methods. * * @throws IndexOutOfBoundsException {@inheritDoc} */ public ListIterator<E> listIterator(final int index) { Object[] elements = getArray(); int len = elements.length; if (index<0 || index>len) throw new IndexOutOfBoundsException("Index: "+index); return new COWIterator<E>(elements, index); } private static class COWIterator<E> implements ListIterator<E> { /** Snapshot of the array */ private final Object[] snapshot; /** Index of element to be returned by subsequent call to next. */ private int cursor; private COWIterator(Object[] elements, int initialCursor) { cursor = initialCursor; snapshot = elements; } public boolean hasNext() { return cursor < snapshot.length; } public boolean hasPrevious() { return cursor > 0; } @SuppressWarnings("unchecked") public E next() { if (! hasNext()) throw new NoSuchElementException(); return (E) snapshot[cursor++]; } @SuppressWarnings("unchecked") public E previous() { if (! hasPrevious()) throw new NoSuchElementException(); return (E) snapshot[--cursor]; } public int nextIndex() { return cursor; } public int previousIndex() { return cursor-1; } /** * Not supported. Always throws UnsupportedOperationException. * @throws UnsupportedOperationException always; <tt>remove</tt> * is not supported by this iterator. */ public void remove() { throw new UnsupportedOperationException(); } /** * Not supported. Always throws UnsupportedOperationException. * @throws UnsupportedOperationException always; <tt>set</tt> * is not supported by this iterator. */ public void set(E e) { throw new UnsupportedOperationException(); } /** * Not supported. Always throws UnsupportedOperationException. * @throws UnsupportedOperationException always; <tt>add</tt> * is not supported by this iterator. */ public void add(E e) { throw new UnsupportedOperationException(); } } /** * Returns a view of the portion of this list between * <tt>fromIndex</tt>, inclusive, and <tt>toIndex</tt>, exclusive. * The returned list is backed by this list, so changes in the * returned list are reflected in this list. * * <p>The semantics of the list returned by this method become * undefined if the backing list (i.e., this list) is modified in * any way other than via the returned list. * * @param fromIndex low endpoint (inclusive) of the subList * @param toIndex high endpoint (exclusive) of the subList * @return a view of the specified range within this list * @throws IndexOutOfBoundsException {@inheritDoc} */ public List<E> subList(int fromIndex, int toIndex) { final ReentrantLock lock = this.lock; lock.lock(); try { Object[] elements = getArray(); int len = elements.length; if (fromIndex < 0 || toIndex > len || fromIndex > toIndex) throw new IndexOutOfBoundsException(); return new COWSubList<E>(this, fromIndex, toIndex); } finally { lock.unlock(); } } /** * Sublist for CopyOnWriteArrayList. * This class extends AbstractList merely for convenience, to * avoid having to define addAll, etc. This doesn't hurt, but * is wasteful. This class does not need or use modCount * mechanics in AbstractList, but does need to check for * concurrent modification using similar mechanics. On each * operation, the array that we expect the backing list to use * is checked and updated. Since we do this for all of the * base operations invoked by those defined in AbstractList, * all is well. While inefficient, this is not worth * improving. The kinds of list operations inherited from * AbstractList are already so slow on COW sublists that * adding a bit more space/time doesn't seem even noticeable. */ private static class COWSubList<E> extends AbstractList<E> implements RandomAccess { private final CopyOnWriteArrayList<E> l; private final int offset; private int size; private Object[] expectedArray; // only call this holding l's lock COWSubList(CopyOnWriteArrayList<E> list, int fromIndex, int toIndex) { l = list; expectedArray = l.getArray(); offset = fromIndex; size = toIndex - fromIndex; } // only call this holding l's lock private void checkForComodification() { if (l.getArray() != expectedArray) throw new ConcurrentModificationException(); } // only call this holding l's lock private void rangeCheck(int index) { if (index<0 || index>=size) throw new IndexOutOfBoundsException("Index: "+index+ ",Size: "+size); } public E set(int index, E element) { final ReentrantLock lock = l.lock; lock.lock(); try { rangeCheck(index); checkForComodification(); E x = l.set(index+offset, element); expectedArray = l.getArray(); return x; } finally { lock.unlock(); } } public E get(int index) { final ReentrantLock lock = l.lock; lock.lock(); try { rangeCheck(index); checkForComodification(); return l.get(index+offset); } finally { lock.unlock(); } } public int size() { final ReentrantLock lock = l.lock; lock.lock(); try { checkForComodification(); return size; } finally { lock.unlock(); } } public void add(int index, E element) { final ReentrantLock lock = l.lock; lock.lock(); try { checkForComodification(); if (index<0 || index>size) throw new IndexOutOfBoundsException(); l.add(index+offset, element); expectedArray = l.getArray(); size++; } finally { lock.unlock(); } } public void clear() { final ReentrantLock lock = l.lock; lock.lock(); try { checkForComodification(); l.removeRange(offset, offset+size); expectedArray = l.getArray(); size = 0; } finally { lock.unlock(); } } public E remove(int index) { final ReentrantLock lock = l.lock; lock.lock(); try { rangeCheck(index); checkForComodification(); E result = l.remove(index+offset); expectedArray = l.getArray(); size--; return result; } finally { lock.unlock(); } } public boolean remove(Object o) { int index = indexOf(o); if (index == -1) return false; remove(index); return true; } public Iterator<E> iterator() { final ReentrantLock lock = l.lock; lock.lock(); try { checkForComodification(); return new COWSubListIterator<E>(l, 0, offset, size); } finally { lock.unlock(); } } public ListIterator<E> listIterator(final int index) { final ReentrantLock lock = l.lock; lock.lock(); try { checkForComodification(); if (index<0 || index>size) throw new IndexOutOfBoundsException("Index: "+index+ ", Size: "+size); return new COWSubListIterator<E>(l, index, offset, size); } finally { lock.unlock(); } } public List<E> subList(int fromIndex, int toIndex) { final ReentrantLock lock = l.lock; lock.lock(); try { checkForComodification(); if (fromIndex<0 || toIndex>size) throw new IndexOutOfBoundsException(); return new COWSubList<E>(l, fromIndex + offset, toIndex + offset); } finally { lock.unlock(); } } } private static class COWSubListIterator<E> implements ListIterator<E> { private final ListIterator<E> i; private final int index; private final int offset; private final int size; COWSubListIterator(List<E> l, int index, int offset, int size) { this.index = index; this.offset = offset; this.size = size; i = l.listIterator(index+offset); } public boolean hasNext() { return nextIndex() < size; } public E next() { if (hasNext()) return i.next(); else throw new NoSuchElementException(); } public boolean hasPrevious() { return previousIndex() >= 0; } public E previous() { if (hasPrevious()) return i.previous(); else throw new NoSuchElementException(); } public int nextIndex() { return i.nextIndex() - offset; } public int previousIndex() { return i.previousIndex() - offset; } public void remove() { throw new UnsupportedOperationException(); } public void set(E e) { throw new UnsupportedOperationException(); } public void add(E e) { throw new UnsupportedOperationException(); } } // Support for resetting lock while deserializing private void resetLock() { UNSAFE.putObjectVolatile(this, lockOffset, new ReentrantLock()); } private static final sun.misc.Unsafe UNSAFE; private static final long lockOffset; static { try { UNSAFE = sun.misc.Unsafe.getUnsafe(); Class k = CopyOnWriteArrayList.class; lockOffset = UNSAFE.objectFieldOffset (k.getDeclaredField("lock")); } catch (Exception e) { throw new Error(e); } }} 0 0
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