ConcurrentHashMap结构图如下：
ConcurrentHashMap实现类图如下：
segment的结构图如下：
package concurrentMy.juc_collections.hashMap;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.Serializable;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
/**
*
* (ConcurrentHashMap 原理理解)
*
* <p>
* 修改历史: <br>
* 修改日期 修改人员 版本 修改内容<br>
* -------------------------------------------------<br>
* 2016年8月5日 下午3:37:13 user 1.0 初始化创建<br>
* </p>
*
* @author Peng.Li
* @version 1.0
* @since JDK1.7
*
*
*
*
*
* 0.HashMap是非线程安全的哈希表，常用于单线程程序中。
*
* 1.HashTable容器在激烈的并发环境下面效率低的原因：强一致性
* （1）HashTable通过synchronized来保证线程安全的，当一个线程进行put到HashTable添加元素时，线程2不但不能put方法添加元素，也不能通过get获取元素。
* （2） 访问HashTable的线程都必须竞争同一把锁
* 2.ConcurrentHashMap效率高的原因：弱一致性
* （1）容器中有多把锁，每一把锁锁住的是容器中的一部分数据，当多个线程访问容器中的不同的数据段的时候，由于获取的是不同的锁，
* 所以不存在竞争的问题，从而提高并发访问效率。
* （2）采用锁分段技术：首先将数据分成一段一段的存储，然后给每一段数据配置一把锁，当一个线程占有锁访问其中一个段数据的时候，其他段的数据也能被其他线程访问。
* （3）多线程对于同一个段数据的访问，是互斥的；但是对于不同片段的访问，却是可以同步进行的。
* 3.结构原理
* （1）ConcurrentHashMap是由Segment数组接口和HashEntry数组结构组成。
* Segment是一种可重入锁ReentrantLock，在ConcurrentHashMap扮演锁的角色；
* HashEntry用于存储键值对数据。
* (2) Segment的结构和HashMap类似，是一组数组和链表结构。一个Segment包含一个HashEntry数组，每个HashEntry是一个链表结构的元素，
* 每个Segment守护者一个HashEntry数组里面的元素，当对HashEntry的数组进行修改的时候，首先需要获得HashEntry数组对应的数据段的Segment锁。
*
* (3) 读不加锁：定义成volatile的变量，能够在线程之间保持可见性，能够被多线程同时读，并且保证不会读到过期的值，但是只能被单线程写（有一种情况可以被多线程写，就是写入的值不依赖于原值），
* 在get操作里只需要读写 transient volatile HashEntry<K,V>[] table;所以可以不用加锁。之所以不会读到过期的值，是根据java内存模型的happen before原则，
* 对volatile字段的写入操作先于读操作，即使两个线程同时修改和获取volatile变量，get操作也能拿到最新的值，这是用volatile替换锁的经典应用场景。
*
*
(4) 从图中可以看到，ConcurrentHashMap内部分为很多个Segment，每一个Segment拥有一把锁，然后每个Segment（继承ReentrantLock）下面包含很多个HashEntry列表数组。对于一个key，
需要经过三次（为什么要hash三次下文会详细讲解）hash操作，才能最终定位这个元素的位置，这三次hash分别为：
对于一个key，先进行一次hash操作，得到hash值h1，也即h1 = hash1(key)；
将得到的h1的高几位进行第二次hash，得到hash值h2，也即h2 = hash2(h1高几位)，通过h2能够确定该元素的放在哪个Segment；
将得到的h1进行第三次hash，得到hash值h3，也即h3 = hash3(h1)，通过h3能够确定该元素放置在哪个HashEntry。
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
import java.util.concurrent.locks.ReentrantLock;
/**
* 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 ConcurrentHashMapSourceCode<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（不受限制的） retries if tables undergo （经受）continuous（连续的） 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(ConcurrentHashMapSourceCode 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(补充的，追加的) hash function to a given hashCode, which
* defends against (对抗) poor quality （质量差的） hash functions. This is critical(爱挑剔的)
* because ConcurrentHashMap uses power-of-two length hash tables,
* that otherwise encounter（遭遇） collisions（hash碰撞） for hashCodes that do not
* differ in lower or upper bits.
*
* 用到了Wang/Jenkins算法变种，主要的目的为了减少hash冲突，使元素能够均匀的分布到不同的Segment上，从而提高容器的存取的效率。
* 假如哈希的质量差到极点，所有的元素都在同一个Segment中，不仅存取缓慢，分段锁也会失去意义。
*
* System.out.println(Integer.parseInt("0001111", 2) & 15);
System.out.println(Integer.parseInt("0011111", 2) & 15);
System.out.println(Integer.parseInt("0111111", 2) & 15);
System.out.println(Integer.parseInt("1111111", 2) & 15);
上面的结果全都是15，通过这个例子发现如果不进行再hash，hash冲突非常严重，因为只要低位一样，无论高位是什么，与15做&操作都为15。
发生冲突的几率还是很大的，但是如果我们先把上例中的二进制数字使用hash()函数先进行一次预hash，得到的结果是这样的：
0100｜0111｜0110｜0111｜1101｜1010｜0100｜1110
1111｜0111｜0100｜0011｜0000｜0001｜1011｜1000
0111｜0111｜0110｜1001｜0100｜0110｜0011｜1110
1000｜0011｜0000｜0000｜1100｜1000｜0001｜1010
可以看到每一位的数据都散开了，并且ConcurrentHashMap中是使用预hash值的高位参与运算的。比如之前说的先将hash值向右按位移动28位，
再与15做&运算，得到的结果都别为：4，15，7，8，没有冲突！
*/
public 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(调整；使合法化) both segment and index locations（位置，地点）,
// using variant(不同的) 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.
*
* 实现原理：
* ConcurrentHashMap使用分段锁技术，将数据分成一段一段的存储，然后给每一段数据配一把锁，当一个线程占用锁访问其中一个段数据的时候，
* 其他段的数据也能被其他线程访问，能够实现真正的并发访问。如下图是ConcurrentHashMap的内部结构图：
*
* 1.initialCapacity 表示新建的这个ConcurrentHashMap的初始容量，也就是上线结构图中的Entry数量。
* 默认值为static final int DEFAULT_INITIAL_CAPACITY = 16;
*
* 2.loadFactor表示负载因子，就是当ConcurrentHashMap中的元素个数大于loadFactor * 最大容量时候就需要rehash和扩容。
* 默认值为static final float DEFAULT_LOAD_FACTOR = 0.75f;
*
* 3.concurrencyLevel表示并发级别，这个值用来确定segment的个数，segment的个数大于等于concurrencyLevel的第一个2的n次方的数。
* 比如，如果concurrencyLevel为12，13，14，15，16，则Segment的数目为16(2的4次方)。
*
* 4.理想情况下ConcurrentHashMap真正的访问量能够达到concurrencyLevel，因为有concurrencyLevel个Segment，
* 假如有concurrencyLevel个线程要访问Map，并且需要访问的数据都恰好分别落在不同的segment中，则这些线程能够无竞
* 争的自由访问（因为不需要竞争同一把锁）达到同时访问的效果。这也是这个concurrencyLevel参数为什么起名为“并发级别”的原因。
*
*
*/
@SuppressWarnings("unchecked")
public ConcurrentHashMapSourceCode(int initialCapacity,
float loadFactor, int concurrencyLevel) {
//1.验证参数的合法性，如果不合法，直接抛出异常
if (!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
//2.concurrencyLevel也就是Segment的个数不能超过最大Segment的个数，最大个数MAX_SEGMENTS默认值为 2 << 16,如果超过这个值，设置这个值。
if (concurrencyLevel > MAX_SEGMENTS)
concurrencyLevel = MAX_SEGMENTS;
// Find power-of-two sizes best matching arguments
//比如concurrencyLevel=16默认值，则ssize也会等于16（2的4次方，sshift=4），如果concurrencyLevel=18，则ssize=32（也就是2的5次方，sshift=5），
//3.这段代码的使用循环找到>=concurrencyLevel的第一个2的n次方的数ssize，这个数ssize就是Segment数组的大小;并记录一共向左按位移动的次数sshift。
int sshift = 0;
int ssize = 1;
while (ssize < concurrencyLevel) {
//sshift记录ssize向左移动的次数
++sshift;
//ssize就是Segment数组的大小
ssize <<= 1;
}
//segmentShift 默认的情况下为28
this.segmentShift = 32 - sshift;
//segmentMask 默认情况下为15，segmentMask的各个二进制位都为1，目的是之后可以通过key的hash值与这个值做&运算确定Segment的索引。
this.segmentMask = ssize - 1;
//4 检查给的容量值是否大于允许的最大容量，如果大于MAXIMUM_CAPACITY，就设置为该值。initialCapacity默认值也为16。static final int MAXIMUM_CAPACITY = 1 << 30;
if (initialCapacity > MAXIMUM_CAPACITY)
initialCapacity = MAXIMUM_CAPACITY;
//5 计算每个Segment平均应该放置多少元素，这个值c是向上取整的值。比如初始容量initialCapacity=15，Segment数组的大小为16，Segment的个数为4，则每个Segment平均需要放置4个元素。
int c = initialCapacity / ssize;
if (c * ssize < initialCapacity)
++c;
int cap = MIN_SEGMENT_TABLE_CAPACITY;
while (cap < c)
cap <<= 1;
//6 创建一个Segment的实例，将其当做Segment数组的第一个元素。
// create segments and segments[0]，cap * loadFactor = 1.5，cap=2
Segment<K,V> s0 =
new Segment<K,V>(loadFactor, (int)(cap * loadFactor),
(HashEntry<K,V>[])new HashEntry[cap]);
// ssize默认=16，表示Segment数组的大小
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 ConcurrentHashMapSourceCode(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 ConcurrentHashMapSourceCode(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 ConcurrentHashMapSourceCode() {
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 ConcurrentHashMapSourceCode(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
*
*1. size 操作和put与get的区别在于，size操作需要遍历所有的segment才能算出整个map的大小，而put和get操作只需要关心一个segment；
*2. 假设我们当前遍历的Segment为SA，那么在遍历SA过程中，其他的Segment比如SB可能会被修改，那么这一次计算出来的size值并不是Map的当前真正大小。
* 所以一个比较简单的办法是就是计算Map大小的时候所有的segment都lock住，不能更新数据（put 和 remove，计算完之后unlock；
*
*3. 作者Doug Lea 想出一个更好的idea：先给3次机会（retries初始化为-1，一直重试到RETRIES_BEFORE_LOCK值为2 ，不锁定lock所有Segment；
* 遍历所有的segment，累加各个segment的大小得到整个Map的大小。
*
*4.如果某相邻的2次计算获取的所有Segment的所有更新次数（每个Segment都有一个变量modCount变量，这个变量在Segment的Entry被修改的时候会加1
* 通过这个值可以得到每个Segment的更新操作的次数）是一样的，说明在计算的过程中没有更新操作，直接结束循环，返回当前的size；
*
* 5. 如果重试3次计算的结果中，Map的更新次数和前一次不一致，则之后的计算先对所有的Segment加锁，遍历所有segment计算map的大小，最后当重试计算>3次后再解锁所有的
* segment。
*
* 6.例子：
*
* 假如一个Map有4个segment，标记S1,S2,S3,S4,现在我们要获取Map的Size；
* 计算过程是这样的：
* 第一次计算不对segment S1,S2,S3,S4加锁，遍历所有的segment，假设这次每个segment的大小变成了1，2，3，4；更新次数分别为2，2，3，1；则这次计算可以得到Map的总大小为1+2+3+4=10，总更新次数modCount=2+2+3+1=8；
* 第二次计算，不对S1,S2,S3,S4加锁，遍历所有的Segment，假设这次每个segment的大小变成了2，2，3，4;更新次数变为了3，2，3，1； 则Map的size=2+2+3+4=11；modCount=9
* 那么第一次和第二次计算得到的更新次数不一致，第一次是8，第二次是9；则可以判定这段时间Map的数据被更新；因此必须进行第3次重试计算；
* 第三次计算，不对S1,S2,S3,S4加锁，遍历所有的Segment，假设每个Segment的更新次数还是为3，2，3，1；则因为第2次计算和第3次计算的得到的Map的modCount次数是一致的，则说明这段时间内第2次和第3次这段时间内Map的数据没有被更新
* 此时可以返回第3次计算的Map大小；最坏的情况：第3次计算得到的计算结果和第2次不一致，则只能先对所有的Segment加锁再计算，最后解锁。
*/
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 (;;) {
// 如果重试次数为3次，锁定segment
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) {
//遍历所有的Segment
Segment<K,V> seg = segmentAt(segments, j);
if (seg != null) {
//累加修改的次数
sum += seg.modCount;
//c代表segment的
int c = seg.count;
if (c < 0 || (size += c) < 0)
overflow = true;
}
}
//如果和前一次计算的Map的size一致，结束循环，返回最终的size值
if (sum == last)
break;
last = sum;
}
} finally {
// 如果重试次数>3次则，释放segment锁
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;
//1 和put操作一样，先通过key进行两次hash确定取哪个segment中的数据
int h = hash(key);
//2 使用UNSAFE方法获取对应的Segment，然后再进行一次&运算得到HashEntry链表的位置，然后从链表头开始遍历整个链表。
//（由于hash会碰撞，所以用一个链表保存），如果找到对应的key，则返回对应的value值，如果链表遍历完都没有找到对应的key，
// 则说明map中不包含该key，返回null
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 (;;) {
//重试3次，计算size后才给所有segment加锁，计算Map的size
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;
//遍历所有的Segment
Segment<K,V> seg = segmentAt(segments, j);
if (seg != null && (tab = seg.table) != null) {
//遍历每个Segment里面的HashEntry
for (int i = 0 ; i < tab.length; i++) {
HashEntry<K,V> e;
for (e = entryAt(tab, i); e != null; e = e.next) {
//获取value值，并且与入参value进行比较
V v = e.value;
//相同返回，found=true，退出循环
if (v != null && value.equals(v)) {
found = true;
break outer;
}
}
}
//累加各个segment的更新次数
sum += seg.modCount;
}
}
//前一次计算的更新次数modCount和当前计算的segment的更新次数进行比较，相同，退出循环，返回found = true
if (retries > 0 && sum == last)
break;
last = sum;
}
} finally {
//重试计算次数>3次后，释放segment锁
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;
//1.value值不能为空
if (value == null)
throw new NullPointerException();
//2.key通过一次hash运算得到一个hash值。(这个hash运算下文详说)
int hash = hash(key);
//3.将得到的hash值向右按位移动segmentShift位，然后再与segmentMask做&运算得到Segment的索引
//在初始化的时候，segmentShift的值等于32-sshift，例如concurrencyLevel等于16，则sshift等于4，那么segmentShift为28。
//hash值是一个32位的整数，将其向右移动28就变成这个样子：0000 0000 0000 0000 0000 0000 0000 XXXX,然后再用这个值与segmentMask
//做&运算，也就是说取最后四位的值。这个值确定Segment的索引。
int j = (hash >>> segmentShift) & segmentMask;
//4.使用UNSAFE的方式从Segment数组中获取该索引对应的Segment对象
if ((s = (Segment<K,V>)UNSAFE.getObject // nonvolatile; recheck
(segments, (j << SSHIFT) + SBASE)) == null) // in ensureSegment
s = ensureSegment(j);
//5.向这个Segment对象中put值，这个put操作也是一样的步骤
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();
ConcurrentHashMapSourceCode.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);
ConcurrentHashMapSourceCode.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 ConcurrentHashMapSourceCode.this.size();
}
public boolean isEmpty() {
return ConcurrentHashMapSourceCode.this.isEmpty();
}
public boolean contains(Object o) {
return ConcurrentHashMapSourceCode.this.containsKey(o);
}
public boolean remove(Object o) {
return ConcurrentHashMapSourceCode.this.remove(o) != null;
}
public void clear() {
ConcurrentHashMapSourceCode.this.clear();
}
}
final class Values extends AbstractCollection<V> {
public Iterator<V> iterator() {
return new ValueIterator();
}
public int size() {
return ConcurrentHashMapSourceCode.this.size();
}
public boolean isEmpty() {
return ConcurrentHashMapSourceCode.this.isEmpty();
}
public boolean contains(Object o) {
return ConcurrentHashMapSourceCode.this.containsValue(o);
}
public void clear() {
ConcurrentHashMapSourceCode.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 = ConcurrentHashMapSourceCode.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 ConcurrentHashMapSourceCode.this.remove(e.getKey(), e.getValue());
}
public int size() {
return ConcurrentHashMapSourceCode.this.size();
}
public boolean isEmpty() {
return ConcurrentHashMapSourceCode.this.isEmpty();
}
public void clear() {
ConcurrentHashMapSourceCode.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);
}
}
参考文章：
1.https://www.ibm.com/developerworks/cn/java/java-lo-concurrenthashmap/
2.http://www.infoq.com/cn/articles/ConcurrentHashMap
3.http://qifuguang.me/
4.阿里牛人：https://blog.csdn.net/justloveyou_/article/details/72783008
5.掘金面试总结：https://juejin.im/post/5ba591386fb9a05cd31eb85f