類結構圖
image
Node介紹
static class Node<K,V> implements Map.Entry<K,V> {
//key的hash值
final int hash;
//key
final K key;
//value
volatile V val;
//下一個node節點
volatile Node<K,V> next;
//構造函數
Node(int hash, 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; }
//重寫hashcode()；key的hashcode異或val的hashcode
public final int hashCode() { return key.hashCode() ^ val.hashCode(); }
public final String toString(){ return key + "=" + val; }
//不支持setValue操作
public final V setValue(V value) {
throw new UnsupportedOperationException();
}
//重寫equals
public final boolean equals(Object o) {
Object k, v, u; Map.Entry<?,?> e;
return ((o instanceof Map.Entry) &&
(k = (e = (Map.Entry<?,?>)o).getKey()) != null &&
(v = e.getValue()) != null &&
(k == key || k.equals(key)) &&
(v == (u = val) || v.equals(u)));
}
/**
* 這個方法用來支持map.get()方法。大概邏輯就是遍歷table尋找node
*/
Node<K,V> find(int h, Object k) {
//當前對象
Node<K,V> e = this;
if (k != null) {
//遍歷數組，根據key查找
do {
K ek;
if (e.hash == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
return e;
} while ((e = e.next) != null);
}
return null;
}
}
ForwardingNode介紹
//該類僅僅用在map擴容
static final class ForwardingNode<K,V> extends Node<K,V> {
//nextTable表示擴容之后的數組。當一個線程訪問到ForwardingNode對象，就知道當前正在進行擴容操作，當前這個線程會幫助擴容
final Node<K,V>[] nextTable;
ForwardingNode(Node<K,V>[] tab) {
super(MOVED, null, null, null);
this.nextTable = tab;
}
Node<K,V> find(int h, Object k) {
//遍歷新的數組
outer: for (Node<K,V>[] tab = nextTable;;) {
Node<K,V> e; int n;
//檢驗k，tabl數組是否為空，如果為空直接返回null
if (k == null || tab == null || (n = tab.length) == 0 || (e = tabAt(tab, (n - 1) & h)) == null)
return null;
for (;;) {
int eh; K ek;
//判斷bin上的第一個元素是否等于k，等于直接返回
if ((eh = e.hash) == h &&
((ek = e.key) == k || (ek != null && k.equals(ek))))
return e;
//eh<0并且是ForwardingNode 那么繼續循環
if (eh < 0) {
if (e instanceof ForwardingNode) {
tab = ((ForwardingNode<K,V>)e).nextTable;
continue outer;
}
else
//否則調用父類Node的find返回
return e.find(h, k);
}
if ((e = e.next) == null)
return null;
}
}
}
}
成員變量
/**
*最大容量：2^30=1073741824
*/
private static final int MAXIMUM_CAPACITY = 1 << 30;
/**
* 默認容量 16
*/
private static final int DEFAULT_CAPACITY = 16;
/**
* 沒有用到，注釋解釋的用來兼容啥玩意的。看代碼只在序列化反序列化用到啦
*/
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
/**
* 加載因子
*/
private static final float LOAD_FACTOR = 0.75f;
/**
* 轉為紅黑樹判斷條件之一 bin數量大于8
*/
static final int TREEIFY_THRESHOLD = 8;
/**
* 由樹轉換成鏈表的閾值UNTREEIFY_THRESHOLD當執行resize操作時
* 當桶中bin的數量少于UNTREEIFY_THRESHOLD時使用鏈表來代替樹。默認值是6
*/
static final int UNTREEIFY_THRESHOLD = 6;
/**
* 如果bin中的數量大于TREEIFY_THRESHOLD，但是capacity小于MIN_TREEIFY_CAPACITY，依然使用鏈表存儲。
* 此時會進行resize操作，如果capacity大于MIN_TREEIFY_CAPACITY進行樹化
*/
static final int MIN_TREEIFY_CAPACITY = 64;
/**
* 擴容線程每次最少要遷移16個hash桶
*/
private static final int MIN_TRANSFER_STRIDE = 16;
/**
* The number of bits used for generation stamp in sizeCtl.
* Must be at least 6 for 32bit arrays.
*/
private static int RESIZE_STAMP_BITS = 16;
/**
* 幫助擴容線程最大值65535
*/
private static final int MAX_RESIZERS = (1 << (32 - RESIZE_STAMP_BITS)) - 1;
/**
* The bit shift for recording size stamp in sizeCtl.
*/
private static final int RESIZE_STAMP_SHIFT = 32 - RESIZE_STAMP_BITS;
//當前位置的Node是一個ForwardingNode節點
static final int MOVED = -1;
//當前位置的Node為一個TreeBin節點
static final int TREEBIN = -2;
//暫存態，即這個節點沒有真正初始化完畢
static final int RESERVED = -3;
static final int HASH_BITS = 0x7fffffff;
/** 可用處理器（cpu）數量 */
static final int NCPU=Runtime.getRuntime().availableProcessors();
/**
* 桶數組，用來存儲Node元素的。默認為null，只在第一次put操作的進行初始化，該數組的長度永遠為2的n次方。
*/
transient volatile Node<K,V>[] table;
/**
* 默認為null，當不為null，表示當前正在進行擴容操作，這個數組就是擴容之后的數組，長度為原數組的兩倍。
*/
private transient volatile Node<K,V>[] nextTable;
/**
* map中元素個數，由于是多線程操作，baseCount記錄的不準確，所以要結合counterCells 來使用保證記錄的正確性。map的元素個數 = baseCount + 所有的cell的value值。
*/
private transient volatile long baseCount;
/**
* 表初始化和擴容的控制位。
* -1表示當前table數組正在被初始化；
* -N表示有N-1個線程在進行擴容操作；
* 0（默認值）表示當前table還未使用；此時table為null；
* 正整數時，表示table的容量，默認是table大小的0.75倍，(n - (n>>>2))的方式來計算0.75
*/
private transient volatile int sizeCtl;
/**
* 用來拆分table的，在擴容的時候
*/
private transient volatile int transferIndex;
/**
* 用來實現cellsBusy鎖的，0無鎖，1鎖z
*/
private transient volatile int cellsBusy;
/**
* @sun.misc.Contended 用來避免偽共享 counterCells用來記錄出現并發的次數
*/
private transient volatile CounterCell[] counterCells;
// views
private transient KeySetView<K,V> keySet;
private transient ValuesView<K,V> values;
private transient EntrySetView<K,V> entrySet;
構造方法
//initialCapacity 初始容量
//loadFactor加載因子
//concurrencyLevel預估并發線程
public ConcurrentHashMap(int initialCapacity,float loadFactor, int concurrencyLevel) {
//校驗參數
if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new IllegalArgumentException();
//如果容量小于預估并發線程數。則使用concurrencyLevel
if (initialCapacity < concurrencyLevel)
initialCapacity = concurrencyLevel;
long size = (long)(1.0 + (long)initialCapacity / loadFactor);
//tableSizeFor((int)size) 找到大于等于size的最小2的冪；得到數組容量
int cap = (size >= (long)MAXIMUM_CAPACITY) ?
MAXIMUM_CAPACITY : tableSizeFor((int)size);
this.sizeCtl = cap;
}
//傳入一個map
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
this.sizeCtl = DEFAULT_CAPACITY;
putAll(m);
}
//根據初始容量初始化ConcurrentHashMap
public ConcurrentHashMap(int initialCapacity) {
if (initialCapacity < 0)
throw new IllegalArgumentException();
int cap = ((initialCapacity >= (MAXIMUM_CAPACITY >>> 1)) ?
MAXIMUM_CAPACITY :
tableSizeFor(initialCapacity + (initialCapacity >>> 1) + 1));
this.sizeCtl = cap;
}
操作table的方法
//原子操作，返回table指定位置的元素
static final <K,V> Node<K,V> tabAt(Node<K,V>[] tab, int i) {
return (Node<K,V>)U.getObjectVolatile(tab, ((long)i << ASHIFT) + ABASE);
}
//cas操作，在指定位置賦值
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);
}
//原子操作，在指定位置賦值
static final <K,V> void setTabAt(Node<K,V>[] tab, int i, Node<K,V> v) {
U.putObjectVolatile(tab, ((long)i << ASHIFT) + ABASE, v);
}
initTable()解析
//初始化table
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
//table等于null或者長度為0則初始化table
while ((tab = table) == null || tab.length == 0) {
//sizeCtl<0說明當前數組正在初始化。則當前讓出cpu
if ((sc = sizeCtl) < 0)
Thread.yield();
//否則通過cas把SIZECTL修改成-1，表示當前正在初始化
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
if ((tab = table) == null || tab.length == 0) {
//判斷初始化map的時候是否指定容量，沒有使用DEFAULT_CAPACITY
int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
@SuppressWarnings("unchecked")
//構造一個node數組指定長度
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n];
table = tab = nt;
//根據node數組長度重新計算sizeCtl。其實就是n*0.75得到下次擴容的閾值
sc = n - (n >>> 2);
}
} finally {
//重新賦值sizeCtl
sizeCtl = sc;
}
break;
}
}
return tab;
}
put()解析
public V put(K key, V value) {
return putVal(key, value, false);
}
//onlyIfAbsent true不改變存在的值；false改變存在的值
final V putVal(K key, V value, boolean onlyIfAbsent) {
//檢驗key,value不能為空
if (key == null || value == null) throw new NullPointerException();
//計算hash值。高16位異或低16位與HASH_BITS
int hash = spread(key.hashCode());
int binCount = 0;
//遍歷數組
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
//如果node數組為空，則初始化table
if (tab == null || (n = tab.length) == 0)
tab = initTable();
//返回(n - 1) & hash=index 位置的元素
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//通過cas賦值。
if (casTabAt(tab, i, null,new Node<K,V>(hash, key, value, null)))
//這里操作不需要鎖，即使multi thread add 那么只會有一個執行成功。casTabAt是原子操作
break;
}
//f.hash == MOVED表示當前數組正在擴容。則進行幫助擴容
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
//注意，這里針對數組的某一個桶加鎖
synchronized (f) {
//校驗f
if (tabAt(tab, i) == f) {
//fh >= 0得到的節點就是hash值相同的節點組成的鏈表的頭節點
if (fh >= 0) {
binCount = 1;
//遍歷數組
for (Node<K,V> e = f;; ++binCount) {
K ek;
//根據hash找到key，判斷是否可以覆蓋原來的值
//onlyIfAbsent=false覆蓋
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node<K,V> pred = e;
if ((e = e.next) == null) {
//說明遍歷到鏈表的尾節點還沒找到元素，直接構建元素，跟鏈表連接上
pred.next = new Node<K,V>(hash, key, value, null);
break;
}
}
}
//說明鏈表是紅黑樹
else if (f instanceof TreeBin) {
Node<K,V> p;
binCount = 2;
//把當前元素加入樹
if ((p = ((TreeBin<K,V>)f).putTreeVal(hash, key,value)) != null) {
//根據onlyIfAbsent判斷是否覆蓋
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
//binCount >= TREEIFY_THRESHOLD說明要把鏈表轉為紅黑樹
if (binCount >= TREEIFY_THRESHOLD)
//鏈表轉樹
treeifyBin(tab, i);
//老值不為空，返回原來的值
if (oldVal != null)
return oldVal;
break;
}
}
}
//map的容量加1，檢查map是否需要擴容
addCount(1L, binCount);
return null;
}
//計算hash值
static final int spread(int h) {
return (h ^ (h >>> 16)) & HASH_BITS;
}
addCount()解析
//如果想要看懂這個方法，那么需要先去了解LongAddr實現的原理。
private final void addCount(long x, int check) {
CounterCell[] as; long b, s;
// counterCells!=null，或者通過cas修改baseCount失敗則進入if
if ((as = counterCells) != null ||
!U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
//執行到這里，說明counterCells=null并且cas修改失敗（修改失敗說明出現競爭）
CounterCell a; long v; int m;
//是否出現競爭，true沒有出現競爭
boolean uncontended = true;
//如果as=null，或者數組隨機一個node為null，或者cas修改CELLVALUE值失敗
if (as == null || (m = as.length - 1) < 0 ||
(a = as[ThreadLocalRandom.getProbe() & m]) == null ||
!(uncontended = U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
//這個方法的原理跟longAccumulate()一模一樣。不懂的朋友可以去看完LongAddr源碼解析的文章
fullAddCount(x, uncontended);
return;
}
//如果check小于等于0不檢查是否需要擴容
if (check <= 1)
return;
//獲取map的大小
s = sumCount();
}
//檢查是否要擴容
if (check >= 0) {
Node<K,V>[] tab, nt; int n, sc;
//根據map的容量s跟sizeCtl比較，判斷是否要擴容
while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&(n = tab.length) < MAXIMUM_CAPACITY) {
//返回擴容以后的標記位
int rs = resizeStamp(n);
//說明正在擴容，幫助擴容
if (sc < 0) {
//判斷是否需要幫助擴容
if ((sc >>> RESIZE_STAMP_SHIFT) != rs || sc == rs + 1 || sc == rs + MAX_RESIZERS || (nt = nextTable) == null ||
transferIndex <= 0)
break;
//擴容線程數加1
if (U.compareAndSwapInt(this, SIZECTL, sc, sc + 1)) //幫助擴容
transfer(tab, nt);
}
//當前線程是唯一的或是第一個發起擴容的線程 此時nextTable=null
//sizeCtl = (resizeStamp(n) << RESIZE_STAMP_SHIFT) + 2表示只有一個線程擴容
else if (U.compareAndSwapInt(this, SIZECTL, sc,
(rs <<RESIZE_STAMP_SHIFT) + 2))
transfer(tab, null);
s = sumCount();
}
}
}
helpTransfer()解析
//這個邏輯跟上面if(check>=0)差不多。幫助擴容
final Node<K,V>[] helpTransfer(Node<K,V>[] tab, Node<K,V> f) {
Node<K,V>[] nextTab; int sc;
if (tab != null && (f instanceof ForwardingNode) &&
(nextTab = ((ForwardingNode<K,V>)f).nextTable) != null) {
int rs = resizeStamp(tab.length);
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);
break;
}
}
return nextTab;
}
return table;
}
transfer()解析
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
int n = tab.length, stride;
// 擴容線程每次最少要遷移16個hash桶
if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
stride = MIN_TRANSFER_STRIDE;
//第一次擴容的時候nextTab=null
if (nextTab == null) {
try {
@SuppressWarnings("unchecked")
//數組長度擴大2倍
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n << 1];
nextTab = nt;
} catch (Throwable ex) { // try to cope with OOME
sizeCtl = Integer.MAX_VALUE;
return;
}
nextTable = nextTab;
//賦值transferIndex
transferIndex = n;
}
//獲取新數組的長度
int nextn = nextTab.length;
//如果已經處理（遷移）就設置為fwd節點
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab);
//是否繼續向前查找的標志位
boolean advance = true;
//擴容操作是否完成標識。完成之前重新掃描一邊數組
boolean finishing = false;
// 自旋，i表示數組下標，bound表示當前線程可以處理的當前桶區間最小下標
for (int i = 0, bound = 0;;) {
Node<K,V> f; int fh;
while (advance) {
int nextIndex, nextBound;
//第一次不會進入這個if finishing=true說明擴容完成了
if (--i >= bound || finishing)
advance = false;
//transferIndex<=0桶已經處理完了，不需要別的線程在處理
else if ((nextIndex = transferIndex) <= 0) {
i = -1;
advance = false;
}
else if (U.compareAndSwapInt
(this, TRANSFERINDEX, nextIndex,
nextBound = (nextIndex > stride ?
nextIndex - stride : 0))) {
//這里可以得到當前線程處理數組的最小區間
bound = nextBound;
//得到線程處理數組的最大區間
//那么這個線程處理的區間就是[bound,i]
i = nextIndex - 1;
advance = false;
}
}
//i<0數組遍歷完成；i目前看到只等于n；i + n >= nextn擴容完成
if (i < 0 || i >= n || i + n >= nextn) {
int sc;
if (finishing) {
//用于擴容table
nextTable = null;
//擴容以后的心table
table = nextTab;
//設置sizeCtl為擴容后的0.75
sizeCtl = (n << 1) - (n >>> 1);
return;
}
/**
第一個擴容的線程，執行transfer方法會設置 sizeCtl = (resizeStamp(n) << RESIZE_STAMP_SHIFT) + 2)；
幫助擴容的線程，執行transfer會設置 sizeCtl = sizeCtl+1退出transfer的方法的線程會設置 sizeCtl = sizeCtl-1；
最后一個線程退出sc == (resizeStamp(n) <<RESIZE_STAMP_SHIFT) + 2)，即 (sc - 2) == resizeStamp(n) << RESIZE_STAMP_SHIFT
*/
if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
//不相等，說明不到最后一個線程，直接退出transfer方法
if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
return;
//最后一個線程，擴容完成
finishing = advance = true;
//重新檢查
i = n;
}
}
//獲取指定位置元素
else if ((f = tabAt(tab, i)) == null)
//指定位置插入fwd節點
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) == MOVED)
//已經處理
advance = true;
else {
//遷移數據
synchronized (f) {
//再次校驗f
if (tabAt(tab, i) == f) {
//ln=lowNode 低位 hn=hignNode 高位
Node<K,V> ln, hn;
//fh>=0說明是鏈表。這里涉及到鏈表的反轉。可能反轉一部分，可能全反轉。ln等于null那么ln鏈表全反轉
if (fh >= 0) {
//把鏈表數據分為2類，0和1
int runBit = fh & n;
//lastRun記錄的是最后一個hash值變化的Node。
Node<K,V> lastRun = f;
//遍歷當前桶位置的鏈表，得到最后一個hash值變化的Node
for (Node<K,V> p = f.next; p != null; p = p.next) {
int b = p.hash & n;
if (b != runBit) {
runBit = b;
lastRun = p;
}
}
//保持當前位置
if (runBit == 0) {
ln = lastRun;
hn = null;
}else {
//遷移位置old+n
hn = lastRun;
ln = null;
}
//遷移節點
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)
ln = new Node<K,V>(ph, pk, pv, ln);
else
hn = new Node<K,V>(ph, pk, pv, hn);
}
setTabAt(nextTab, i, ln);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
//判斷是否是樹，關于樹就不講了，紅黑樹我也沒吃透
else if (f instanceof TreeBin) {
TreeBin<K,V> t = (TreeBin<K,V>)f;
TreeNode<K,V> lo = null, loTail = null;
TreeNode<K,V> hi = null, hiTail = null;
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;
}
}
//擴容完以后判斷是否樹是否要轉為鏈表
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;
}
}
}
}
}
}
get()方法解析
public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
//獲取key的hash值
int h = spread(key.hashCode());
//table不等于null，計算index獲取元素不等于null
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
//判斷第一個節點是否相等
if ((eh = e.hash) == h) {
if ((ek = e.key) == key || (ek != null && key.equals(ek)))
return e.val;
}
//小于0說明在擴容。需要調用forwoard的find()
else if (eh < 0)
return (p = e.find(h, key)) != null ? p.val : null;
//遍歷鏈表獲取
while ((e = e.next) != null) {
if (e.hash == h &&
((ek = e.key) == key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
閱讀更多文章
image.png
