jdk1.8并发容器:ConcurrentHashMap源码分析

时间:2021-10-03 17:21:52

下面第一部分转载自http://www.cnblogs.com/everSeeker/p/5601861.html,该博客对jdk1.7与jdk1.8的设计与性能作了部分比较,可作为参考学习。
先简单看下ConcurrentHashMap类在jdk1.7中的设计,其基本结构如图所示:
jdk1.8并发容器:ConcurrentHashMap源码分析

每一个segment都是一个HashEntry

public class ConcurrentHashMap<K, V> extends AbstractMap<K, V>
implements ConcurrentMap<K, V>, Serializable {


// 将整个hashmap分成几个小的map,每个segment都是一个锁;与hashtable相比,这么设计的目的是对于put, remove等操作,可以减少并发冲突,对
// 不属于同一个片段的节点可以并发操作,大大提高了性能
final Segment<K,V>[] segments;

// 本质上Segment类就是一个小的hashmap,里面table数组存储了各个节点的数据,继承了ReentrantLock, 可以作为互拆锁使用
static final class Segment<K,V> extends ReentrantLock implements Serializable {
transient volatile HashEntry<K,V>[] table;
transient int count;
}

// 基本节点,存储Key, Value值
static final class HashEntry<K,V> {
final int hash;
final K key;
volatile V value;
volatile HashEntry<K,V> next;
}
}

  接下来的这部分转载文章,原博客地址为:ConcurrentHashMap源码分析–Java8

  jdk1.8相对于jdk1.7出现了许多不同的变化,不再采用分段锁的方法,这篇文章分析的比较详细,转载留存。

  本文首写于有道云笔记,并在小组分享会分享,先整理发布,希望和大家交流探讨。云笔记地址

概述:

  1. 设计首要目的:维护并发可读性(get、迭代相关);次要目的:使空间消耗比HashMap相同或更好,且支持多线程高效率的初始插入(empty table)。
  2. HashTable线程安全,但采用synchronized,多线程下效率低下。线程1put时,线程2无法put或get。

java8以前版本实现原理:

  锁分离:

  在HashMap的基础上,将数据分段存储,ConcurrentHashMap由多个Segment组成,每个Segment都有把锁。Segment下包含很多Node,也就是我们的键值对了。

如果还停留在锁分离、segment,那就out了!Segment虽保留,但已经简化了属性,仅仅为了兼容旧版本。

  1. CAS算法;unsafe.compareAndSwapInt(this, valueOffset, expect, update); CAS(Compare And Swap),意思是如果valueOffset位置包含的值与expect值相同,则更新valueOffset位置的值为update,并返回true,否则不更新,返回false。
  2. 与Java8的HashMap有相通之处,底层依然由“数组”+链表+红黑树
  3. 底层结构存放的是TreeBin对象,而不是TreeNode对象;
  4. CAS作为知名无锁算法,那ConcurrentHashMap就没用锁了么?当然不是,hash值相同的链表的头结点还是会synchronized上锁。
    private static final int MAXIMUM_CAPACITY = 1 << 30;    // 2的30次方=1073741824
private static final int DEFAULT_CAPACITY = 16;
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;// MAX_VALUE=2^31-1=2147483647
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
private static final float LOAD_FACTOR = 0.75f;
static final int TREEIFY_THRESHOLD = 8;// 链表转树阀值,大于8时

//树转链表阀值,小于等于6(tranfer时,lc、hc=0两个计数器分别++记录原bin、新binTreeNode数量,<=UNTREEIFY_THRESHOLD 则untreeify(lo))。
// 【仅在扩容tranfer时才可能树转链表】
static final int UNTREEIFY_THRESHOLD = 6;
static final int MIN_TREEIFY_CAPACITY = 64;
private static final int MIN_TRANSFER_STRIDE = 16;
private static int RESIZE_STAMP_BITS = 16;

// 2^15-1,help resize的最大线程数
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
// 32-16=16,sizeCtl中记录size大小的偏移量
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;


static final int MOVED = -1;// hash for forwarding nodes(forwarding nodes的hash值)、标示位
static final int TREEBIN = -2;// hash for roots of trees(树根节点的hash值)
static final int RESERVED = -3;// hash for transient reservations(ReservationNode的hash值)
static final int HASH_BITS = 0x7fffffff;
// 可用处理器数量
static final int NCPU = Runtime.getRuntime().availableProcessors();
private transient volatile int sizeCtl;

sizeCtl是控制标识符,不同的值表示不同的意义。

  1. 负数代表正在进行初始化或扩容操作
  2. -1代表正在初始化
  3. -N表示有N-1个线程正在进行扩容操作
  4. 正数或0代表hash表还没有被初始化,这个数值表示初始化或下一次进行扩容的大小,类似于扩容阈值。它的值始终是当前ConcurrentHashMap容量的0.75倍,这与loadfactor是对应的。实际容量>=sizeCtl,则扩容。

部分构造函数:

public ConcurrentHashMap(int initialCapacity,  
float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
thrownew IllegalArgumentException();
if (initialCapacity < concurrencyLevel) // Use at least as many bins
initialCapacity = concurrencyLevel; // as estimated threads
long size = (long)(1.0 + (long)initialCapacity / loadFactor);
int cap = (size >= (long)MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY : tableSizeFor((int)size);
this.sizeCtl = cap;
}

concurrencyLevel:
   concurrencyLevel,能够同时更新ConccurentHashMap且不产生锁竞争的最大线程数,在Java8之前实际上就是ConcurrentHashMap中的分段锁个数,即Segment[]的数组长度。
   正确地估计很重要,当低估,数据结构将根据额外的竞争,从而导致线程试图写入当前锁定的段时阻塞;相反,如果高估了并发级别,你遇到过大的膨胀,由于段的不必要的数量; 这种膨胀可能会导致性能下降,由于高数缓存未命中。
在Java8里,仅仅是为了兼容旧版本而保留。唯一的作用就是保证构造map时初始容量不小于concurrencyLevel。
  源码122行:
Also, for compatibility with previous versions of this class, constructors may optionally specify an expected {@code concurrencyLevel} as an additional hint for internal sizing.
  源码482行:
Mainly: We leave untouched but unused constructor arguments refering to concurrencyLevel .……

1、重要属性:
1.1 Node:

static class Node<K,V> implements Map.Entry<K,V> {  
final int hash;
final K key;
volatile V val; // Java8增加volatile,保证可见性
volatile Node<K,V> next;

Node(inthash, K key, V val, Node<K,V> next) {
this.hash = hash;
this.key = key;
this.val = val;
this.next = next;
}

public final K getKey() { return key; }
public final V getValue() { return val; }
// HashMap调用Objects.hashCode(),最终也是调用Object.hashCode();效果一样
public final int hashCode() { returnkey.hashCode() ^ val.hashCode(); }
public final String toString(){ returnkey + "=" + val; }
public final V setValue(V value) { // 不允许修改value值,HashMap允许
throw new UnsupportedOperationException();
}
// HashMap使用if (o == this),且嵌套if;concurrent使用&&
public final boolean equals(Object o) {
Object k, v, u; Map.Entry<?,?> e;
return ((oinstanceof Map.Entry) &&
(k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
(v = e.getValue()) != null &&
(k == key || k.equals(key)) &&
(v == (u = val) || v.equals(u)));
}

/**
* Virtualized support for map.get(); overridden in subclasses.
*/

Node<K,V> find(inth, Object k) { // 增加find方法辅助get方法
Node<K,V> e = this;
if (k != null) {
do {
K ek;
if (e.hash == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
returne;
} while ((e = e.next) != null);
}
returnnull;
}
}

1.2 TreeNode

// Nodes for use in TreeBins,链表>8,才可能转为TreeNode. 
// HashMap的TreeNode继承至LinkedHashMap.Entry;而这里继承至自己实现的Node,将带有next指针,便于treebin访问。
static final class TreeNode<K,V> extends Node<K,V> {
TreeNode<K,V> parent; // red-black tree links
TreeNode<K,V> left;
TreeNode<K,V> right;
TreeNode<K,V> prev; // needed to unlink next upon deletion
boolean red;

TreeNode(inthash, K key, V val, Node<K,V> next,
TreeNode<K,V> parent) {
super(hash, key, val, next);
this.parent = parent;
}

Node<K,V> find(inth, Object k) {
return findTreeNode(h, k, null);
}

/**
* Returns the TreeNode (or null if not found) for the given key
* starting at given root.
*/
// 查找hash为h,key为k的节点
final TreeNode<K,V> findTreeNode(int h, Object k, Class<?> kc) {
if (k != null) { // 比HMap增加判空
TreeNode<K,V> p = this;
do {
intph, dir; K pk; TreeNode<K,V> q;
TreeNode<K,V> pl = p.left, pr = p.right;
if ((ph = p.hash) > h)
p = pl;
elseif (ph < h)
p = pr;
elseif ((pk = p.key) == k || (pk != null && k.equals(pk)))
returnp;
elseif (pl == null)
p = pr;
elseif (pr == null)
p = pl;
elseif ((kc != null ||
(kc = comparableClassFor(k)) != null) &&
(dir = compareComparables(kc, k, pk)) != 0)
p = (dir < 0) ? pl : pr;
elseif ((q = pr.findTreeNode(h, k, kc)) != null)
returnq;
else
p = pl;
} while (p != null);
}
return null;
}
}
// 和HashMap相比,这里的TreeNode相当简洁;ConcurrentHashMap链表转树时,并不会直接转,正如注释(Nodes for use in TreeBins)所说,只是把这些节点包装成TreeNode放到TreeBin中,再由TreeBin来转化红黑树。

1.3 TreeBin

// TreeBin用于封装维护TreeNode,包含putTreeVal、lookRoot、UNlookRoot、remove、balanceInsetion、balanceDeletion等方法,这里只分析其构造函数。 
// 当链表转树时,用于封装TreeNode,也就是说,ConcurrentHashMap的红黑树存放的时TreeBin,而不是treeNode。
TreeBin(TreeNode<K,V> b) {
super(TREEBIN, null, null, null);//hash值为常量TREEBIN=-2,表示roots of trees
this.first = b;
TreeNode<K,V> r = null;
for (TreeNode<K,V> x = b, next; x != null; x = next) {
next = (TreeNode<K,V>)x.next;
x.left = x.right = null;
if (r == null) {
x.parent = null;
x.red = false;
r = x;
}
else {
K k = x.key;
inth = x.hash;
Class<?> kc = null;
for (TreeNode<K,V> p = r;;) {
intdir, ph;
K pk = p.key;
if ((ph = p.hash) > h)
dir = -1;
elseif (ph < h)
dir = 1;
elseif ((kc == null &&
(kc = comparableClassFor(k)) == null) ||
(dir = compareComparables(kc, k, pk)) == 0)
dir = tieBreakOrder(k, pk);
TreeNode<K,V> xp = p;
if ((p = (dir <= 0) ? p.left : p.right) == null) {
x.parent = xp;
if (dir <= 0)
xp.left = x;
else
xp.right = x;
r = balanceInsertion(r, x);
break;
}
}
}
}
this.root = r;
assert checkInvariants(root);
}

1.4 treeifyBin

/** 
* Replaces all linked nodes in bin at given index unless table is
* too small, in which case resizes instead.链表转树
*/

private final void treeifyBin(Node<K,V>[] tab, int index) {
Node<K,V> b; intn, sc;
if (tab != null) {
if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
tryPresize(n << 1); // 容量<64,则table两倍扩容,不转树了
else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
synchronized (b) { // 读写锁
if (tabAt(tab, index) == b) {
TreeNode<K,V> hd = null, tl = null;
for (Node<K,V> e = b; e != null; e = e.next) {
TreeNode<K,V> p =
new TreeNode<K,V>(e.hash, e.key, e.val,
null, null);
if ((p.prev = tl) == null)
hd = p;
else
tl.next = p;
tl = p;
}
setTabAt(tab, index, new TreeBin<K,V>(hd));
}
}
}
}
}

1.5 ForwardingNode

// A node inserted at head of bins during transfer operations.连接两个table 
// 并不是我们传统的包含key-value的节点,只是一个标志节点,并且指向nextTable,提供find方法而已。生命周期:仅存活于扩容操作且bin不为null时,一定会出现在每个bin的首位。
static final class ForwardingNode<K,V> extends Node<K,V> {
final Node<K,V>[] nextTable;
ForwardingNode(Node<K,V>[] tab) {
super(MOVED, null, null, null); // 此节点hash=-1,key、value、next均为null
this.nextTable = tab;
}

Node<K,V> find(int h, Object k) {
// 查nextTable节点,outer避免深度递归
outer: for (Node<K,V>[] tab = nextTable;;) {
Node<K,V> e; intn;
if (k == null || tab == null || (n = tab.length) == 0 ||
(e = tabAt(tab, (n - 1) & h)) == null)
returnnull;
for (;;) { // CAS算法多和死循环搭配!直到查到或null
int eh; K ek;
if ((eh = e.hash) == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
returne;
if (eh < 0) {
if (e instanceof ForwardingNode) {
tab = ((ForwardingNode<K,V>)e).nextTable;
continue outer;
}
else
return e.find(h, k);
}
if ((e = e.next) == null)
return null;
}
}
}
}

1.6 3个原子操作(调用频率很高)

@SuppressWarnings("unchecked") // ASHIFT等均为private static final 
static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) { // 获取索引i处Node
return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
}
// 利用CAS算法设置i位置上的Node节点(将c和table[i]比较,相同则插入v)。
static final <K,V> boolean casTabAt(Node<K,V>[] tab, int i,
Node<K,V> c, Node<K,V> v) {
return U.compareAndSwapObject(tab, ((long)i << ASHIFT) + ABASE, c, v);
}
// 设置节点位置的值,仅在上锁区被调用
static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v) {
U.putObjectVolatile(tab, ((long)i << ASHIFT) + ABASE, v);
}

1.7 Unsafe

//在源码的6277行到最后,有着ConcurrentHashMap中极为重要的几个属性(SIZECTL),unsafe静态块控制其修改行为。Java8中,大量运用CAS进行变量、属性的无锁修改,大大提高性能。 
// Unsafe mechanics
private static final sun.misc.Unsafe U;
private static final long SIZECTL;
private static final long TRANSFERINDEX;
private static final long BASECOUNT;
private static final long CELLSBUSY;
private static final long CELLVALUE;
private static final long ABASE;
private static final int ASHIFT;

static {
try {
U = sun.misc.Unsafe.getUnsafe();
Class<?> k = ConcurrentHashMap.class;
SIZECTL = U.objectFieldOffset (k.getDeclaredField("sizeCtl"));
TRANSFERINDEX=U.objectFieldOffset(k.getDeclaredField("transferIndex"));
BASECOUNT = U.objectFieldOffset (k.getDeclaredField("baseCount"));
CELLSBUSY = U.objectFieldOffset (k.getDeclaredField("cellsBusy"));
Class<?> ck = CounterCell.class;
CELLVALUE = U.objectFieldOffset (ck.getDeclaredField("value"));
Class<?> ak = Node[].class;
ABASE = U.arrayBaseOffset(ak);
intscale = U.arrayIndexScale(ak);
if ((scale & (scale - 1)) != 0)
thrownew Error("data type scale not a power of two");
ASHIFT = 31 - Integer.numberOfLeadingZeros(scale);
} catch (Exception e) {
thrownew Error(e);
}
}

1.8 扩容相关

tryPresize在putAll以及treeifyBin中调用

private final void tryPresize(int size) {  
// 给定的容量若>=MAXIMUM_CAPACITY的一半,直接扩容到允许的最大值,否则调用函数扩容
int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
tableSizeFor(size + (size >>> 1) + 1);
int sc;
while ((sc = sizeCtl) >= 0) { //没有正在初始化或扩容,或者说表还没有被初始化
Node<K,V>[] tab = table; int n;
if(tab == null || (n = tab.length) == 0) {
n = (sc > c) ? sc : c; // 扩容阀值取较大者
// 期间没有其他线程对表操作,则CAS将SIZECTL状态置为-1,表示正在进行初始化
if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if (table == tab) {
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = nt;
sc = n - (n >>> 2); //无符号右移2位,此即0.75*n
}
} finally {
sizeCtl = sc; // 更新扩容阀值
}
}
}// 若欲扩容值不大于原阀值,或现有容量>=最值,什么都不用做了
else if (c <= sc || n >= MAXIMUM_CAPACITY)
break;
else if (tab == table) { // table不为空,且在此期间其他线程未修改table
int rs = resizeStamp(n);
if (sc < 0) {
Node<K,V>[] nt;//RESIZE_STAMP_SHIFT=16,MAX_RESIZERS=2^15-1
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
transferIndex <= 0)
break;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1))
transfer(tab, nt);
}
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs << RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
}
}
}
private static final int tableSizeFor(int c){//和HashMap一样,返回>=n的最小2的自然数幂 
int n = c - 1;
n |= n >>> 1;
n |= n >>> 2;
n |= n >>> 4;
n |= n >>> 8;
n |= n >>> 16;
return (n < 0) ? 1 : (n >= MAXIMUM_CAPACITY) ? MAXIMUM_CAPACITY : n + 1;
}
/** 
* Returns the stamp bits for resizing a table of size n.
* Must be negative when shifted left by RESIZE_STAMP_SHIFT.
*/

static final int resizeStamp(int n) { // 返回一个标志位
return Integer.numberOfLeadingZeros(n) | (1 << (RESIZE_STAMP_BITS - 1));
}// numberOfLeadingZeros返回n对应32位二进制数左侧0的个数,如9(1001)返回28
// RESIZE_STAMP_BITS=16,(左侧0的个数)|(2^15)

ConcurrentHashMap无锁多线程扩容,减少扩容时的时间消耗。

transfer扩容操作:单线程构建两倍容量的nextTable;允许多线程复制原table元素到nextTable。

  1. 为每个内核均分任务,并保证其不小于16;
  2. 若nextTable为null,则初始化其为原table的2倍;
  3. 死循环遍历,直到finishing。

    • 节点为空,则插入ForwardingNode;
    • 链表节点(fh>=0),分别插入nextTable的i和i+n的位置;【逆序链表??】
    • TreeBin节点(fh<0),判断是否需要untreefi,分别插入nextTable的i和i+n的位置;【逆序数??】
    • finishing时,nextTable赋给table,更新sizeCtl为新容量的0.75倍,完成扩容。

以上说的都是单线程,多线程又是如何实现的呢?
  遍历到forwardingNode节点((fh = f.hash)==MOVED),说明此节点被处理过了,直接跳过。这是控制并发控容的核心。由于给节点上了锁,只允许当前线程完成此节点的操作,处理完毕后,将对应值设为ForwardingNode(fwd),其他线程看到forward,直接向后遍历。如此便完成了多线程的复制工作,也解决了线程安全问题。

    private transient volatile Node<K,V>[] nextTable; //仅仅在扩容使用,并且此时非空
// 将table每一个bin(桶位)的Node移动或复制到nextTable 
// 只在addCount(long x, int check)、helpTransfer、tryPresize中调用
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
int n = tab.length, stride;
// 每核处理的量小于16,则强制赋值16
if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
stride = MIN_TRANSFER_STRIDE; // subdivide range
if (nextTab == null) { // initiating
try {
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1]; //两倍
nextTab = nt;
} catch (Throwable ex) { // try to cope with OOME
sizeCtl = Integer.MAX_VALUE;
return;
}
nextTable = nextTab;
transferIndex = n;
}
int nextn = nextTab.length;
//连节点指针,标志位,fwd的hash值为-1,fwd.nextTable=nextTab。
ForwardingNode<K,V> fwd= new ForwardingNode<K,V>(nextTab);
boolean advance= true;//并发扩容的关键属性,等于true,说明此节点已经处理过
boolean finishing = false; // to ensure sweep before committing nextTab
for (int i = 0, bound = 0;;) { // 死循环
Node<K,V> f; int fh;
while (advance) { // 控制--i,遍历原hash表中的节点
int nextIndex, nextBound;
if (--i >= bound || finishing)
advance = false;
else if ((nextIndex = transferIndex) <= 0) {
i = -1;
advance = false;
}//TRANSFERINDEX 即用CAS计算得到的transferIndex
else if (U.compareAndSwapInt
(this, TRANSFERINDEX, nextIndex,
nextBound = (nextIndex > stride ?
nextIndex - stride : 0))) {
bound = nextBound;
i = nextIndex - 1;
advance = false;
}
}
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
if (finishing) { // 所有节点复制完毕
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1); //扩容阀值设为原来的1.5倍,即现在的0.75倍
return; // 仅有的2个跳出死循环出口之一
}//CAS更新扩容阈值,sc-1表明新加入一个线程参与扩容
if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
return;// 仅有的2个跳出死循环出口之一
finishing = advance = true;
i = n; // recheck before commit
}
}
else if ((f = tabAt(tab, i)) == null) //该节点为空,则插入ForwardingNode
advance = casTabAt(tab, i, null, fwd);
//遍历到ForwardingNode节点,说明此节点被处理过了,直接跳过。这是控制并发扩容的核心
else if ((fh = f.hash) == MOVED) // MOVED=-1,hash for fwd
advance = true; // already processed
else {
synchronized (f) { //上锁
if (tabAt(tab, i) == f) {
Node<K,V> ln, hn; //ln原位置节点,hn新位置节点
if (fh >= 0) { // 链表
int runBit = fh & n; // f.hash & n
Node<K,V> lastRun = f; // lastRun和p两个链表,逆序??
for (Node<K,V> p = f.next; p != null; p = p.next) {
int b = p.hash & n; // f.next.hash & n
if (b != runBit) {
runBit = b;
lastRun = p;
}
}
if (runBit == 0) {
ln = lastRun;
hn = null;
}
else {
hn = lastRun;
ln = null;
}
for (Node<K,V> p = f; p != lastRun; p = p.next) {
int ph = p.hash; K pk = p.key; V pv = p.val;
if ((ph & n) == 0) // 和HashMap确定扩容后的节点位置一样
ln = new Node<K,V>(ph, pk, pv, ln);
else
hn = new Node<K,V>(ph, pk, pv, hn); //新位置节点
}//类似HashMap,为何i+n?参见HashMap的笔记
setTabAt(nextTab, i, ln);//在nextTable[i]插入原节点
setTabAt(nextTab, i + n, hn);//在nextTable[i+n]插入新节点
//在nextTable[i]插入forwardNode节点,表示已经处理过该节点
setTabAt(tab, i, fwd);
//设置advance为true 返回到上面的while循环中 就可以执行--i操作
advance = true;
}
else if (f instanceof TreeBin) { //树
TreeBin<K,V> t = (TreeBin<K,V>)f;
TreeNode<K,V> lo = null, loTail = null;
TreeNode<K,V> hi = null, hiTail = null;
//lc、hc=0两计数器分别++记录原、新bin中TreeNode数量
int lc = 0, hc = 0;
for (Node<K,V> e = t.first; e != null; e = e.next) {
int h = e.hash;
TreeNode<K,V> p = new TreeNode<K,V>
(h, e.key, e.val, null, null);
if ((h & n) == 0) {
if ((p.prev = loTail) == null)
lo = p;
else
loTail.next = p;
loTail = p;
++lc;
}
else {
if ((p.prev = hiTail) == null)
hi = p;
else
hiTail.next = p;
hiTail = p;
++hc;
}
}//扩容后树节点个数若<=6,将树转链表
ln = (lc <= UNTREEIFY_THRESHOLD) ? untreeify(lo) :
(hc != 0) ? new TreeBin<K,V>(lo) : t;
hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
(lc != 0) ? new TreeBin<K,V>(hi) : t;
setTabAt(nextTab, i, ln);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
}
}
}
}
}
// 协助扩容方法。多线程下,当前线程检测到其他线程正进行扩容操作,则协助其一起扩容;(只有这种情况会被调用)从某种程度上说,其“优先级”很高,只要检测到扩容,就会放下其他工作,先扩容。  
// 调用之前,nextTable一定已存在。

final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
Node<K,V>[] nextTab; intsc;
if (tab != null && (finstanceof ForwardingNode) &&
(nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
intrs = resizeStamp(tab.length); //标志位
while (nextTab == nextTable && table == tab &&
(sc = sizeCtl) < 0) {
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 ||
sc == rs + MAX_RESIZERS || transferIndex <= 0)
break;
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) {
transfer(tab, nextTab);//调用扩容方法,直接进入复制阶段
break;
}
}
return nextTab;
}
return table;
}

2、 put相关

理解一下put的流程:
1. 判空:null直接抛出空指针异常;
2. hash:计算h=key.hashcode;调用spread计算hash=(h^(h>>>16))& HASH_BITS;
3. 遍历table
+ 若table为空,则初始化,仅设置相关参数;
+ @@@计算当前key存放位置,即table的下标i=(n-1)& hash;
+ 若待存放位置为null,casTabAt无锁插入;
+ 若是forwarding nodes(检测到正在扩容),则helpTransfer(帮助其扩容);
+ else(待插入位置非空且不是forward节点,即碰撞了),将头节点上锁(保证了线程安全):区分链表节点和树节点,分别插入(遇到hash值与key值都与新节点一致的情况,只需要更新value值即可。否则依次向后遍历,直到链表尾插入这个节点);
+ 若链表长度>8,则treeifyBin转树(Node:若length<64,直接tryPresize,两倍table.length;不转树)。
4. addCount(1L,binCount)

Node:

  1. put操作共计两次hash操作,再利用“与&”操作计算Node的存放位置。
  2. ConcurrentHashMap不允许key或value为null。
  3. addCount(longx,intcheck)方法:
    • 利用CAS快速更新baseCount的值;
    • check>=0.则检验是否需要扩容;if sizeCtl<0(正在进行初始化或扩容操作)【nexttable null等情况break;如果有线程正在扩容,则协助扩容】;else if 仅当前线程在扩容,调用协助扩容函数,注其参数nextTable为null。
public V put(K key, V value) {
return putVal(key, value, false);
}
final V <span style="background-color: rgb(255, 255, 51);">putVal</span>(K key, V value, boolean onlyIfAbsent) {  
// 不允许key、value为空
if (key == null || value == null) throw new NullPointerException();
int hash = spread(key.hashCode()); //返回(h^(h>>>16))&HASH_BITS
int binCount = 0;
for (Node<K,V>[] tab = table;;) { // 死循环,直到插入成功
Node<K,V> f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable(); // table为空,初始化table
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {// 索引处无值
if (casTabAt(tab, i, null,
new Node<K,V>(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED) // MOVED=-1;//hash for forwarding nodes
tab = helpTransfer(tab, f); //检测到正在扩容,则帮助其扩容
else {
V oldVal = null;
synchronized (f) { // 节点上锁(hash值相同的链表的头节点)
if (tabAt(tab, i) == f) {
if (fh >= 0) { // 链表节点
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;// hash和key相同,则修改value
if (e.hash == hash &&
((ek = e.key) == key ||(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent) //仅putIfAbsent()方法中onlyIfAbsent为true
e.val = value; //putIfAbsent()包含key则返回get,否则put并返回
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) { //已遍历到链表尾部,直接插入
pred.next = new Node<K,V>(hash, key, value, null);
break;
}
}
}
else if (f instanceof TreeBin) { // 树节点
Node<K,V> p;
binCount = 2;
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)//实则是>8,执行else,说明该桶位本就有Node
treeifyBin(tab, i);//若length<64,直接tryPresize,两倍table.length;不转树
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
// Initializes table, using the size recorded in sizeCtl.  
private final Node<K,V>[] <span style="background-color: rgb(255, 255, 51);">initTable</span>() { // 仅仅设置参数,并未实质初始化
Node<K,V>[] tab; intsc;
while ((tab = table) == null || tab.length == 0) {
if ((sc = sizeCtl) < 0) // 其他线程正在初始化,此线程挂起
Thread.yield(); // lost initialization race; just spin
//CAS方法把sizectl置为-1,表示本线程正在进行初始化
elseif (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if ((tab = table) == null || tab.length == 0) {
intn = (sc > 0) ? sc : DEFAULT_CAPACITY;//DEFAULT_CAPACITY=16
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
sc = n - (n >>> 2); // 扩容阀值,0.75*n
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}

3、 get,contains相关

public V <span style="background-color: rgb(255, 255, 51);">get</span>(Object key) {  
Node<K,V>[] tab; Node<K,V> e, p; intn, eh; K ek;
inth = spread(key.hashCode());
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {//tabAt(i),获取索引i处Node
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
returne.val;
}
elseif (eh < 0) // 树
return (p = e.find(h, key)) != null ? p.val : null;
while ((e = e.next) != null) { // 链表
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
returne.val;
}
}
return null;
}
    public boolean containsKey(Object key) {return get(key) != null;}  
public boolean containsValue(Object value) {}

理解一下get的流程:
1. spread计算hash值;
2. table不为空;
3. tabAt(i)处桶位不为空;
4. check first,是则返回当前Node的value;否则分别根据树、链表查询。

4、 Size相关

  由于ConcurrentHashMap在统计size时可能正被多个线程操作,而我们又不可能让他停下来让我们计算,所以只能计量一个估计值。

计数辅助:

    // Table of counter cells. When non-null, size is a power of 2
private transient volatile CounterCell[] counterCells;
    @sun.misc.Contended static final class CounterCell {
volatile long value;
CounterCell(long x) { value = x; }
}
    final long sumCount(){
CounterCell as[] = counterCells;
long sum = baseCount;
if(as != null){
for(int i = 0; i < as.length; i++){
CounterCell a;
if((a = as[i]) != null)
sum += a.value;
}
}
return sum;
}
    private final void fullAddCount(long x, boolean wasUncontended) {}
public int size() { // 旧版本方法,和推荐的mappingCount返回的值基本无区别
longn = sumCount();
return ((n < 0L) ? 0 :
(n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int)n);
}
    // 返回Mappings中的元素个数,官方建议用来替代size。此方法返回的是一个估计值;如果sumCount时有线程插入或删除,实际数量是和mappingCount不同的。since 1.8
public long mappingCount() {
longn = sumCount();
return (n < 0L) ? 0L : n; // ignore transient negative values
}
private transient volatile long baseCount; 
//ConcurrentHashMap中元素个数,基于CAS无锁更新,但返回的不一定是当前Map的真实元素个数。

5、 remove、clear相关

    public void clear() { // 移除所有元素 
long delta = 0L; // negative number of deletions
inti = 0;
Node<K,V>[] tab = table;
while (tab != null && i < tab.length) {
intfh;
Node<K,V> f = tabAt(tab, i);
if (f == null) // 为空,直接跳过
++i;
else if ((fh = f.hash) == MOVED) { //检测到其他线程正对其扩容
//则协助其扩容,然后重置计数器,重新挨个删除元素,避免删除了元素,其他线程又新增元素。

tab = helpTransfer(tab, f);
i = 0; // restart
}
else{
synchronized (f) { // 上锁
if (tabAt(tab, i) == f) { // 其他线程没有在此期间操作f
Node<K,V> p = (fh >= 0 ? f :
(finstanceof TreeBin) ?
((TreeBin<K,V>)f).first : null);
while (p != null) { // 首先删除链、树的末尾元素,避免产生大量垃圾
--delta;
p = p.next;
}
setTabAt(tab, i++, null); // 利用CAS无锁置null
}
}
}
}
if (delta != 0L)
addCount(delta, -1); // 无实际意义,参数check<=1,直接return。
}
    public V remove(Object key) { // key为null,将在计算hashCode时报空指针异常 
return replaceNode(key, null, null);
}
    public boolean remove(Object key, Object value) {  
if (key == null)
thrownew NullPointerException();
returnvalue != null && replaceNode(key, null, value) != null;
}
    // remove核心方法,注意,这里的cv才是key-value中的value! 
final V replaceNode(Object key, V value, Object cv) {
inthash = spread(key.hashCode());
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; intn, i, fh;
if (tab == null || (n = tab.length) == 0 ||
(f = tabAt(tab, i = (n - 1) & hash)) == null)
break; // 该桶位第一个元素为空,直接跳过
elseif ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f); // 先协助扩容再说
else {
V oldVal = null;
booleanvalidated = false;
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
validated = true;
//pred没看出来有什么用,全是别人赋值给他,他却不影响其他参数
for (Node<K,V> e = f, pred = null;;) {
K ek;
if (e.hash == hash &&((ek = e.key) == key ||
(ek != null && key.equals(ek)))){//hash且可以相等
V ev = e.val;
// value为null或value和查到的值相等
if (cv == null || cv == ev ||
(ev != null && cv.equals(ev))) {
oldVal = ev;
if (value != null) // replace中调用
e.val = value;
elseif (pred != null)
pred.next = e.next;
else
setTabAt(tab, i, e.next);
}
break;
}
pred = e;
if ((e = e.next) == null)
break;
}
}
elseif (finstanceof TreeBin) { // 以树的方式find、remove
validated = true;
TreeBin<K,V> t = (TreeBin<K,V>)f;
TreeNode<K,V> r, p;
if ((r = t.root) != null &&
(p = r.findTreeNode(hash, key, null)) != null) {
V pv = p.val;
if (cv == null || cv == pv ||
(pv != null && cv.equals(pv))) {
oldVal = pv;
if (value != null)
p.val = value;
elseif (t.removeTreeNode(p))
setTabAt(tab, i, untreeify(t.first));
}
}
}
}
}
if (validated) {
if (oldVal != null) {
if (value == null)
addCount(-1L, -1);
returnoldVal;
}
break;
}
}
}
return null;
}
    public boolean replace(K key, V oldValue, V newValue) {}

6、其它函数:

public boolean isEmpty() {
return sumCount() <= 0L; // ignore transient negative values
}

参考资料:

http://ifeve.com/concurrenthashmap/
http://ifeve.com/java-concurrent-hashmap-2/
、、、、、、、、、、、、、、、、、、、
http://ashkrit.blogspot.com/2014/12/what-is-new-in-java8-concurrenthashmap.html
http://blog.csdn.net/u010723709/article/details/48007881
http://yucchi.jp/blog/?p=2048
http://blog.csdn.net/q291611265/article/details/47985145
、、、、、、、、、、、、、、、、、、、、、
SynchronizedMap:http://blog.sina.com.cn/s/blog_5157093c0100hm3y.html
http://blog.csdn.net/yangfanend/article/details/7165742
http://blog.csdn.net/xuefeng0707/article/details/40797085

ArrayList源码分析(jdk1.8):http://blog.csdn.net/u010887744/article/details/49496093

HashMap源码分析(jdk1.8):http://write.blog.csdn.net/postedit/50346257

ConcurrentHashMap源码分析–Java8:http://blog.csdn.net/u010887744/article/details/50637030

史上最全Java集合关系图:http://blog.csdn.net/u010887744/article/details/50575735