concurrentHashMap 是用的最多的一个concurrent包数据结构，了解内部设计对高并发有帮助。
ConcurrentHashMap:非阻塞Map
要点
- 1.7采用分段锁的机制
- 1.8取消分段锁机制，减少了锁竞争
- 效率：1.8>1.7
1.8 源码：
- 抛弃了Segment分段锁机制，直接用Node<K,V>来保存数据(数组+链表+红黑树)
- CAS+Synchronized来保证并发更新的安全.
- 属性
- sizeCtl：node控制标识符，用来控制table的初始化和扩容的操作，不同的值有不同的含义
- 当为负数时：-1代表正在初始化，-N代表有N-1个线程正在 进行扩容
- 当为0时：代表当时的table还没有被初始化
- 当为正数时：表示初始化或者下一次进行扩容的大
- sizeCtl：node控制标识符，用来控制table的初始化和扩容的操作，不同的值有不同的含义
public class ConcurrentHashMap<K,V> extends AbstractMap<K,V>
implements ConcurrentMap<K,V>, Serializable {
'node容器大小'
// node数组最大容量：2^30=1073741824
private static final int MAXIMUM_CAPACITY = 1 << 30;
// 默认初始值，必须是2的幕数
private static final int DEFAULT_CAPACITY = 16;
'数组'
//数组可能最大值，需要与toArray（）相关方法关联
static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;
//并发级别，遗留下来的，为兼容以前的版本
private static final int DEFAULT_CONCURRENCY_LEVEL = 16;
// 负载因子
private static final float LOAD_FACTOR = 0.75f;
// 链表转红黑树阀值,> 8 链表转换为红黑树
static final int TREEIFY_THRESHOLD = 8; '链表转树'
//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)）
static final int UNTREEIFY_THRESHOLD = 6; '树转链表'
static final int MIN_TREEIFY_CAPACITY = 64; //转树后的最小值
private static final int MIN_TRANSFER_STRIDE = 16; '转义时的最小值'
private static int RESIZE_STAMP_BITS = 16; '在sizeCtl上使用的二进制位的数量'
// 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;
// forwarding nodes的hash值
static final int MOVED = -1;
// 树根节点的hash值
static final int TREEBIN = -2;
// ReservationNode的hash值
static final int RESERVED = -3;
// 可用处理器数量
static final int NCPU = Runtime.getRuntime().availableProcessors();
//存放node的数组
transient volatile Node<K,V>[] table;
/*控制标识符，用来控制table的初始化和扩容的操作，不同的值有不同的含义
*当为负数时：-1代表正在初始化，-N代表有N-1个线程正在 进行扩容
*当为0时：代表当时的table还没有被初始化
*当为正数时：表示初始化或者下一次进行扩容的大小
*/
private transient volatile int sizeCtl;
}
- 节点：与hashMap一样，有写法上的小区别
'node<K,V>直接继承了Map'
static class Node<K,V> implements Map.Entry<K,V> {
final int hash;
final K key;
volatile V val;
volatile Node<K,V> next;
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; }
public final int hashCode() { return key.hashCode() ^ val.hashCode(); }
public final String toString(){ return key + "=" + val; }
public final V setValue(V value) {
throw new UnsupportedOperationException();
}
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)));
}
/**
* Virtualized support for map.get(); overridden in subclasses.
*/
Node<K,V> find(int h, Object k) {
Node<K,V> e = this;
if (k != null) {
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;
}
}
- 红黑树属性：不直接转，将这些节点包装成TreeNode放到TreeBin中，再由TreeBin来转化红黑树
'红黑树'
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; // 需要在删除时取消链接
boolean red;
TreeNode(int hash, K key, V val, Node<K,V> next,
TreeNode<K,V> parent) {
super(hash, key, val, next);
this.parent = parent;
}
Node<K,V> find(int h, Object k) {
return findTreeNode(h, k, null);
}
/**
* Returns the TreeNode (or null if not found) for the given key
* starting at given root.
*/
final TreeNode<K,V> findTreeNode(int h, Object k, Class<?> kc) {
if (k != null) {
TreeNode<K,V> p = this;
do {
int ph, dir; K pk; TreeNode<K,V> q;
TreeNode<K,V> pl = p.left, pr = p.right;
if ((ph = p.hash) > h)
p = pl;
else if (ph < h)
p = pr;
else if ((pk = p.key) / k || (pk != null && k.equals(pk)))
return p;
else if (pl / null)
p = pr;
else if (pr / null)
p = pl;
else if ((kc != null ||
(kc = comparableClassFor(k)) != null) &&
(dir = compareComparables(kc, k, pk)) != 0)
p = (dir < 0) ? pl : pr;
else if ((q = pr.findTreeNode(h, k, kc)) != null)
return q;
else
p = pl;
} while (p != null);
}
return null;
}
}
- ForwardingNode节点：在传输操作期间插入到treebin头部的一个节点
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);
this.nextTable = tab;
}
Node<K,V> find(int h, Object k) {
// loop to avoid arbitrarily deep recursion on forwarding nodes
outer: for (Node<K,V>[] tab = nextTable;;) {
Node<K,V> e; int n;
if (k / null || tab / null || (n = tab.length) / 0 ||
(e = tabAt(tab, (n - 1) & h)) / null)
return null;
for (;;) {
int eh; K ek;
if ((eh = e.hash) / h &&
((ek = e.key) / k || (ek != null && k.equals(ek))))
return e;
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;
}
}
}
}
- 初始化
/**
* Creates a new, empty map with the default initial table size (16).
*/
public ConcurrentHashMap() {
}
'initialCapacity:node节点初始化个数'
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;
}
public ConcurrentHashMap(Map<? extends K, ? extends V> m) {
this.sizeCtl = DEFAULT_CAPACITY;
putAll(m);
}
public ConcurrentHashMap(int initialCapacity, float loadFactor) {
this(initialCapacity, loadFactor, 1);
}
'兼容旧的构造器'
public ConcurrentHashMap(int initialCapacity,
float loadFactor, int concurrencyLevel) {
if (!(loadFactor > 0.0f) || initialCapacity < 0 || concurrencyLevel <= 0)
throw new 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;
}
- 添加获取
- 如果槽中的链表头或者树根为null，则直接用cas存放
- 如果有链表头或树根，则用synchronized将起加锁
'插入数据'
public V put(K key, V value) {
return putVal(key, value, false);
}
/** Implementation for put and putIfAbsent */
final V putVal(K key, V value, boolean onlyIfAbsent) {
if (key / null || value / null) throw new NullPointerException();
int hash = spread(key.hashCode()); "获取hash值"
int binCount = 0;
for (Node<K,V>[] tab = table;;) {
Node<K,V> f; int n, i, fh;
if (tab / null || (n = tab.length) / 0)
tab = initTable(); '如果table为null，则初始化'
else if ((f = tabAt(tab, i = (n - 1) & hash)) / null) { '从table中找出链表头或树的根都不存在,直接存放'
if (casTabAt(tab, i, null,new Node<K,V>(hash, key, value, null))) 'CAS：无锁添加'
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) / MOVED) 'f是ForwardingNode节点'
tab = helpTransfer(tab, f); '一起进行扩容操作'
else {
V oldVal = null;
synchronized (f) { '添加同步锁'
if (tabAt(tab, i) / f) {
if (fh >= 0) { '如果是链表头'
binCount = 1;
for (Node<K,V> e = f;; ++binCount) {
K ek;
'找到hah值相同，key相同'
if (e.hash / hash && ( (ek = e.key) / key ||(ek != null && key.equals(ek))) ) {
oldVal = e.val;
if (!onlyIfAbsent) '已存在'
e.val = value;
break;
}
'不存在，则添加'
Node<K,V> pred = e;
if ((e = e.next) / null) {
pred.next = new Node<K,V>(hash, key,value, null);
break;
}
}
}
else if (f instanceof TreeBin) { '如果是树根'
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) '如果节点数>8，则转成树'
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount); '增加个数，并检测是否需要扩容'
return null;
}
'初始化table：使用sizeCtl的大小'
private final Node<K,V>[] initTable() {
Node<K,V>[] tab; int sc;
while ((tab = table) / null || tab.length / 0) {
if ((sc = sizeCtl) < 0) '如果sizeCtl<0'
Thread.yield(); //线程等待
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) { //cas获取锁，然后开始初始化
try {
if ((tab = table) / null || tab.length / 0) {
int n = (sc > 0) ? sc : DEFAULT_CAPACITY; '数组大小:默认16'
@SuppressWarnings("unchecked")
Node<K,V>[] nt = (Node<K,V>[])new Node<?,?>[n]; '初始化table'
table = tab = nt;
sc = n - (n >>> 2); '右移2为:16-4'
}
} finally {
sizeCtl = sc; '将sizeCtl：12(下次扩容为12)'
}
break;
}
}
return tab;
}
'转义'
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;
}
- 扩容：
'列表转树:链表>8的时候调用'
private final void treeifyBin(Node<K,V>[] tab, int index) {
Node<K,V> b; int n, sc;
if (tab != null) {
if ((n = tab.length) < MIN_TREEIFY_CAPACITY) '如果数组.size<64'
tryPresize(n << 1); '扩大数组'
else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
synchronized (b) { '同步锁'
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));
}
}
}
}
}
'table.size<64'
private final void tryPresize(int size) {
int c = (size >= (MAXIMUM_CAPACITY >>> 1)) ? MAXIMUM_CAPACITY :
tableSizeFor(size + (size >>> 1) + 1);
int sc;
while ((sc = sizeCtl) >= 0) {
Node<K,V>[] tab = table; int n;
if (tab / null || (n = tab.length) / 0) {
n = (sc > c) ? sc : c;
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);
}
} finally {
sizeCtl = sc;
}
}
}
else if (c <= sc || n >= MAXIMUM_CAPACITY)
break;
else if (tab / table) { '真实的数组扩容'
int rs = resizeStamp(n);
if (sc < 0) {
Node<K,V>[] nt;
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);'调用扩容动作'
}
}
}
'扩容动作：将node复制、移动到新的数组中'
private final void transfer(Node<K,V>[] tab, Node<K,V>[] nextTab) {
int n = tab.length, stride;
if ((stride = (NCPU > 1) ? (n >>> 3) / NCPU : n) < MIN_TRANSFER_STRIDE)
stride = MIN_TRANSFER_STRIDE; // subdivide range
'新建一个2倍原数组的新数组'
if (nextTab / null) { // initiating
try {
@SuppressWarnings("unchecked")
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;
transferIndex = n;
}
int nextn = nextTab.length;
ForwardingNode<K,V> fwd = new ForwardingNode<K,V>(nextTab); '初始化ForwardingNode,保存了新数组nextTable的引用'
boolean advance = true;
boolean finishing = false;
''
for (int i = 0, bound = 0;;) {
Node<K,V> f; int fh;
//
while (advance) {
int nextIndex, nextBound;
if (--i >= bound || finishing)
advance = false;
else if ((nextIndex = transferIndex) <= 0) {
i = -1;
advance = false;
}
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) {
nextTable = null;
table = nextTab;
sizeCtl = (n << 1) - (n >>> 1);
return;
}
if (U.compareAndSwapInt(this, SIZECTL, sc = sizeCtl, sc - 1)) {
if ((sc - 2) != resizeStamp(n) << RESIZE_STAMP_SHIFT)
return;
finishing = advance = true;
i = n; // recheck before commit
}
}
else if ((f = tabAt(tab, i)) / null)
advance = casTabAt(tab, i, null, fwd);
else if ((fh = f.hash) / MOVED)
advance = true; // already processed
else {
synchronized (f) {
if (tabAt(tab, i) / f) {
Node<K,V> ln, hn;
if (fh >= 0) {
int runBit = fh & n;
Node<K,V> lastRun = f;
for (Node<K,V> p = f.next; p != null; p = p.next) {
int b = p.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)
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;
}
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;
}
}
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;
}
}
}
}
}
}
- 获取数据
'获取数据'
public V get(Object key) {
Node<K,V>[] tab; Node<K,V> e, p; int n, eh; K ek;
int h = spread(key.hashCode());
if ((tab = table) != null && (n = tab.length) > 0 &&
(e = tabAt(tab, (n - 1) & h)) != null) {
if ((eh = e.hash) / h) {
if ((ek = e.key) / key || (ek != null && key.equals(ek)))
return e.val;
}
else if (eh < 0)
return (p = e.find(h, key)) != null ? p.val : null;
while ((e = e.next) != null) {
if (e.hash / h &&
((ek = e.key) / key || (ek != null && key.equals(ek))))
return e.val;
}
}
return null;
}
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