ConcurrentHashMap线程安全的原理：
- 采用分段锁的方式，来实现不同段的并发访问。
- ConcurrentHashMap维护了一个Segment数组，Segment就是段的实现，每个Segment又维护了一个HashEntry数组，且Segment本身就是一把锁(继承了ReentrantLock)
- ConcurrentHashMap的扩容是通过Segment的扩容来实现的，Segment的数量(并发的级别)在创建ConcurrentHashMap的时候就确定了，之后不可以改变，默认为16。
ConcurrentHashMap在jdk8中的优化：
数据结构：
- jdk7：数组 + Segment + 链表
- jdk8：数组 + Node(链表 + 红黑树)
线程安全：
- jdk7：采用分段锁的方式，来实现不同段的并发访问。
- put方法：通过锁来保证线程安全(Segment继承了ReentrantLock)
- get方法：没有使用锁，根据key找到HashEntry后遍历链表来查询。注意：HashEntry的value和next是用volatile修饰的。
- jdk8：采用synchronized(Node) + cas操作 来保证线程安全，锁的粒度是Node(链表或红黑树)，比jdk7中锁的粒度更小，具有更好的并发性。
- put方法：使用synchronized代码块来保证线程安全。
- get方法：没有使用锁，根据key找到Node后遍历链表或红黑树来查询。注意：Node的val和next是用volatile修饰的。
size()方法：
- jdk7：首先以不加锁的方式获取3次(注:jdk6中是2次)，如果其中任意连续两次的modCounts相等，则直接返回，否则以加锁的方式重新获取并返回。
- jdk8：使用一个volatile变量(baseCount)来记录元素的个数，通过特定的方法(addCount())来更新这个变量的值。
源码：
JDK8源码：
ConcurrentHashMap(Java 8)_A__17的博客-优快云博客
JDK7源码：
package java.util.concurrent;
/**
* JDK7
*/
public class ConcurrentHashMap<K, V> extends AbstractMap<K, V> implements ConcurrentMap<K, V>, Serializable {
private static final long serialVersionUID = 7249069246763182397L;
/**
* 默认的初始容量是16
*/
static final int DEFAULT_INITIAL_CAPACITY = 16;
/**
* 默认的加载因子是0.75
*/
static final float DEFAULT_LOAD_FACTOR = 0.75f;
/**
* 默认的并发级别是16
*/
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;
/**
* 在计算数组的下标时会用到该值：hashValue & segmentMask
*
* segmentMask = segments.length - 1
*/
final int segmentMask;
/**
* Shift value for indexing within segments.
*/
final int segmentShift;
/**
* Segment数组，Segment的功能类似于HashTable。
*
*/
final Segment<K,V>[] segments;
/**
* ConcurrentHashMap的构造函数
* 参数：
* initialCapacity： ConcurrentHashMap的初始容量
* loadFactor： Segment的加载因子(Segment数组是不可以扩容的，ConcurrentHashMap的扩容是通过Segment的扩容实现的)
* concurrencyLevel： 并发级别，默认为16，根据该参数计算出Segment数组的长度，Segment数组的长度必须是2的整数次幂，并且一旦设定，不可改变。
* eg：指定concurrencyLevel为17，则Segment数组的长度为32。
*
*/
@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;
// 根据concurrencyLevel参数计算出一个2的整数次幂的数，作为Segment数组的长度。
// Find power-of-two sizes best matching arguments
int sshift = 0; // 2的指数
int ssize = 1; // Segment数组的长度：ssize=2^sshift
while (ssize < concurrencyLevel) {
++sshift;
ssize <<= 1;
}
this.segmentShift = 32 - sshift;
this.segmentMask = ssize - 1;
// 确定Segment数组中第一个Segment(s0)的HashEntry数组的长度。
if (initialCapacity > MAXIMUM_CAPACITY) initialCapacity = MAXIMUM_CAPACITY;
int c = initialCapacity / ssize;
if (c * ssize < initialCapacity) ++c;
int cap = MIN_SEGMENT_TABLE_CAPACITY; // HashEntry数组的长度，最小为2(最小值设为2，是为了避免插入一个元素后，就开始扩容)
while (cap < c)
cap <<= 1;
// new一个Segment对象
Segment<K,V> s0 = new Segment<K,V>(loadFactor, (int)(cap * loadFactor), (HashEntry<K,V>[])new HashEntry[cap]);
// new一个的Segment数组，大小为ssize
Segment<K,V>[] ss = (Segment<K,V>[])new Segment[ssize];
// 将S0放到Segment数组的第一个位置。Segment数组中其它位置的Segment在调用put()方法时会被初始化。
UNSAFE.putOrderedObject(ss, SBASE, s0); // ordered write of segments[0]
this.segments = ss;
}
/**
* key和value都不能为null，否则报空指针异常。
*
*/
@SuppressWarnings("unchecked")
public V put(K key, V value) {
Segment<K,V> s;
if (value == null) throw new NullPointerException();
// 根据key计算出Segment数组的下标j，计算方法与HashMap获取数组下标的方法类似，都是使用 hashVale & (2^n-1)。
int hash = hash(key);
int j = (hash >>> segmentShift) & segmentMask; // segmentMask = Segment数组的长度-1，此处类似于HashMap中的：h & (length-1);
// 对segments[j]进行初始化
if ((s = (Segment<K,V>)UNSAFE.getObject(segments, (j << SSHIFT) + SBASE)) == null) // nonvolatile; recheck; in ensureSegment
s = ensureSegment(j);
// 将key-value放到segments[j]的HashEntry数组的特定位置上。
return s.put(key, hash, value, false);
}
/**
* 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]; // Segment数组中的第一个Segment，即segments[0]
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
// 新建一个Segment对象
// 该对象的加载因子等于segments[0]的加载因子，该对象的HashEntry数组(table)的初始容量等于segments[0]的HashEntry数组(table)当前的容量。
// 注:此时，segments[0]可能已经扩容多次了。
Segment<K,V> s = new Segment<K,V>(lf, threshold, tab);
// 将新建的Segment对象添加到Segment数组(segments)指定的位置，通过循环和CAS来保证多线程环境下数据的安全
while ((seg = (Segment<K,V>)UNSAFE.getObjectVolatile(ss, u)) == null) {
if (UNSAFE.compareAndSwapObject(ss, u, null, seg = s))
break;
}
}
}
return seg;
}
/**
*
* Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key.
*/
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;
}
// ************************************************ 补充：jdk1.6中ConcurrentHashMap的get方法 ************************************************
/**
* jdk1.6中ConcurrentHashMap的get方法：
* 1)首先根据key获取对应的HashEntry，若找不到对应的HashEntry，则直接返回null。
* 2)若找到了对应的HashEntry，则以不加锁的方式获取value(即HashEntry.value)，若value!=null，则直接返回。
* 注：HashEntry的value属性是volatile的，故value!=null时可直接返回value。
* 3)若value==null，则以加锁的方式来获取value并返回。
* 注：HashEntry!=null,但是HashEntry.value==null的情况是由于发生了指令重排序造成的。
*/
public V get(Object key) {
int hash = hash(key.hashCode());
return segmentFor(hash).get(key, hash);
}
/**
* ConcurrentHashMap.Segment的get方法：采用乐观锁的方式来保证数据的同步。
*
* Note：这里需要考虑到并发的情景：
* put方法中新建一个HashEntry的语句：tab[index] = new HashEntry<K,V>(key, hash, first, value);
* 1)这行代码可以分解为如下的3个步骤：
* ①类的加载、连接(验证->准备->解析)。
* ②初始化对象。 注：初始化后，类的加载就完成了。
* ③将tab[index]指向刚分配的内存地址。 注：这一步和类的加载过程没有任何关系
* 2)其中的②和③可能会被重排序：
* a compiler happens to reorder a HashEntry initialization with its table assignment
* 分配对象的内存空间 --> 将tab[index]指向刚分配的内存地址(即给tab[index]赋值) --> 初始化对象(给HashEntry的key、hash、next、value赋值)。
* 3)如果另一个线程执行put方法时，tab[index]已经被赋值，HashEntry的key、hash也已经被赋值，但是value还没来的及赋值，此时当前正在执行get方法的线程很可能会遇到：
* e(即tab[index]) != null 且 e.hash == hash && key.equals(e.key) 且 e.value = null 的情况，
* 故获取到e.value后需要判断一下e.value是否为空，如果e.value为空，则需要加锁重新读取。
*/
V get(Object key, int hash) {
if (count != 0) { // read-volatile (transient volatile int count;)
HashEntry<K,V> e = getFirst(hash);
while (e != null) {
if (e.hash == hash && key.equals(e.key)) { // 若key.equals(e.key)，说明此时找到了该key对应的HashEntry
V v = e.value;
if (v != null) // 判断是否为空。
return v;
return readValueUnderLock(e); // recheck 加锁重读
}
e = e.next;
}
}
return null;
}
/**
* ConcurrentHashMap.Segment的readValueUnderLock方法
*
* 【Reads value field of an entry under lock. Called if value field ever appears to be null.
* This is possible only if a compiler happens to reorder a HashEntry initialization with its table assignment, which is legal under memory model but is not known to ever occur.】
*/
V readValueUnderLock(HashEntry<K,V> e) {
lock();
try {
return e.value;
} finally {
unlock();
}
}
/**
* segmentFor的get方法
*/
final Segment<K,V> segmentFor(int hash) {
return segments[(hash >>> segmentShift) & segmentMask];
}
/**
* ConcurrentHashMap list entry. Note that this is never exported out as a user-visible Map.Entry.
*
* Because the value field is volatile, not final, it is legal wrt the Java Memory Model for an unsynchronized reader to see null instead of initial value when read via a data race.
* Although a reordering leading to this is not likely to ever actually occur,
* the Segment.readValueUnderLock method is used as a backup in case a null (pre-initialized) value is ever seen in an unsynchronized access method.
*/
static final class HashEntry<K,V> {
final K key;
final int hash;
volatile V value; // value被volatile修饰：如果该HashEntry的value被其它线程修改了，volatile可以保证其它线程的get()方法获取到的value是最新的。
final HashEntry<K,V> next;
HashEntry(K key, int hash, HashEntry<K,V> next, V value) {
this.key = key;
this.hash = hash;
this.next = next;
this.value = value;
}
@SuppressWarnings("unchecked")
static final <K,V> HashEntry<K,V>[] newArray(int i) {
return new HashEntry[i];
}
}
// ************************************************ jdk1.6中ConcurrentHashMap的get方法 ************************************************
/**
* Segment类似一个HashTable
*
* 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 {
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;
/**
* entry数组，用来储存数据的
* The per-segment table. Elements are accessed via entryAt/setEntryAt providing volatile semantics.
*/
transient volatile HashEntry<K,V>[] table;
/**
* Segment中元素的数量
*
* The number of elements.
* Accessed only either within locks or among other volatile reads that maintain visibility.
*/
transient int count;
/**
* 对table的大小造成影响的操作(eg:put、remove)次数
*
* 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;
/**
* Segment的阀值，threshold = capacity * loadFactor
*/
transient int threshold;
/**
* Segment的负载因子
*/
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) {
// 获取Segment的独占锁，如果该key对应的node(HashEntry)存在，则node的值为null；如果node不存在，则new一个HashEntry并赋值给node。
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!=null说明该key对应的HashEntry之前不存在，此时node为scanAndLockForPut()方法中new的那个HashEntry
node.setNext(first);
else // node=null 说明该key对应的HashEntry之前就存在，故这里new一个HashEntry并赋值给node。
node = new HashEntry<K,V>(hash, key, value, first);
int c = count + 1;
if (c > threshold && tab.length < MAXIMUM_CAPACITY) // 若Segment的容量达到阀值，则扩容。
rehash(node);
else
setEntryAt(tab, index, node); // 若Segment的容量未达到阀值，则将node添加到链表的头部。
++modCount;
count = c;
oldValue = null;
break;
}
}
} finally {
// 释放Segment的独占锁
unlock();
}
return oldValue;
}
/**
* 寻找该key对应的HashEntry，如果找到则返回null；如果没有找到，则new一个HashEntry并返回。
* 在该方法返回前，当前线程必定已经持有该Segment的锁了。
*
* 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.
*
* @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); // 这里的this指当前的Segment
HashEntry<K,V> e = first;
HashEntry<K,V> node = null;
int retries = -1; // negative while locating node
while (!tryLock()) { // 循环tryLock()来确保获取到Segment的锁。
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;
}
// 如果遍历的次数(retries)超过了MAX_SCAN_RETRIES(单核时值为1，多核时值为64)，则使用lock()方法阻塞式的获取锁。
else if (++retries > MAX_SCAN_RETRIES) {
lock();
break;
}
// 如果有新的元素被添加到该链表(HashEntry)的头部，则重新遍历
else if ((retries & 1) == 0 && (f = entryForHash(this, hash)) != first) {
e = first = f; // re-traverse if entry changed
retries = -1;
}
}
return node;
}
/**
* 扩容为之前的2倍。
* 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
* 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; // 扩容为之前的2倍
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) // 如果该链表上只有一个元素
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 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;
}
}
}
}
/**
*
* value被volatile修饰：如果该HashEntry的value被其它线程修改了，volatile可以保证其它线程的get()方法获取到的value是最新的。
*
* ConcurrentHashMap list 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);
}
}
}
/**
*
* 首先以不加锁的方式获取3次(注:jdk6中是2次)，如果其中任意连续两次的modCounts相等，则直接返回，否则以加锁的方式重新获取并返回。
*
* Returns the number of key-value mappings in this map.
* If the map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns Integer.MAX_VALUE.
*/
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; // 记录上一次的sum
int retries = -1; // 记录获取的次数(0表示第一次，1表示第二次，2表示第三次)。
try {
for (;;) {
// 如果获取的次数超过3次，则给segments数组中的所有Segment加锁。
if (retries++ == RETRIES_BEFORE_LOCK) { // RETRIES_BEFORE_LOCK=2
for (int j = 0; j < segments.length; ++j)
ensureSegment(j).lock();
}
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; // map的modCount等于所有Segment的modCount相加
int c = seg.count;
if (c < 0 || (size += c) < 0) // map的size等于所有Segment的count相加 即：size += c
overflow = true;
}
}
// 判断本次获取的modCounts和上一次获取的modCounts是否相等，如果相等，则跳出循环。
if (sum == last) break;
last = sum;
}
} finally {
// 如果获取的次数超过3次，给segments数组中的所有Segment解锁。
if (retries > RETRIES_BEFORE_LOCK) {
for (int j = 0; j < segments.length; ++j)
segmentAt(segments, j).unlock();
}
}
return overflow ? Integer.MAX_VALUE : size;
}
// ...
}
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