本文基于jdk1.8版本进行源码解读
其实HashMap最主要的几个方法分别是
- put（设置值）
- get（获取值）
- initTable（初始化hash表）
- resize（扩容）
- treeifyBin（树化）
- untreeify（退化为链表）
所以ConcurrentHashMap的源码我们也基于这些进行分析
- put方法
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) {
// key 和 value 不能为null
if (key == null || value == null) throw new NullPointerException();
// hash (pow 16 ^ low 16) & (2^32 - 1)
int hash = spread(key.hashCode());
int binCount = 0;
// 获取table
for (Node[] tab = table;;) {
Node f; int n, i, fh;
// 如果table未初始化
if (tab == null || (n = tab.length) == 0)
tab = initTable();
// 获取hash槽node,==null代表可以直接插入了
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
// cas插入
if (casTabAt(tab, i, null,
new Node(hash, key, value, null)))
break; // no lock when adding to empty bin
}
// 正在resize
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
// node 不为null
V oldVal = null;
// 可以看出，以头节点上锁
synchronized (f) {
// 防止首节点被改变
if (tabAt(tab, i) == f) {
// 链表
if (fh >= 0) {
binCount = 1;
//遍历链表
for (Node e = f;; ++binCount) {
K ek;
if (e.hash == hash &&
((ek = e.key) == key ||
(ek != null && key.equals(ek)))) {
oldVal = e.val;
if (!onlyIfAbsent)
e.val = value;
break;
}
Node pred = e;
if ((e = e.next) == null) {
pred.next = new Node(hash, key,
value, null);
break;
}
}
}
// 红黑树
else if (f instanceof TreeBin) {
Node p;
binCount = 2;
if ((p = ((TreeBin)f).putTreeVal(hash, key,
value)) != null) {
oldVal = p.val;
if (!onlyIfAbsent)
p.val = value;
}
}
}
}
if (binCount != 0) {
// 转换为红黑树
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
- initTable方法
private final Node[] initTable() {
Node[] tab; int sc;
while ((tab = table) == null || tab.length == 0) {
// 如果sizeCtl小于0,有线程在尝试初始化table
// sizeCtl = 0 ，若使用capacity初始化map，sizeCtl = capacity
if ((sc = sizeCtl) < 0)
Thread.yield(); // lost initialization race; just spin
// cas设置sizeCtl = -1,SIZECTL为sizeCtl字段的头地址
else if (U.compareAndSwapInt(this, SIZECTL, sc, -1)) {
try {
// 双重检查锁
if ((tab = table) == null || tab.length == 0) {
// sc = sizeCtl = 0 | capacity
int n = (sc > 0) ? sc : DEFAULT_CAPACITY;
@SuppressWarnings("unchecked")
Node[] nt = (Node[])new Node[n];
table = tab = nt;
// n - (n/4) = 0.75n, sc = threshold
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc;
}
break;
}
}
return tab;
}
- resize 方法
ConcurrentHashMap中resize方法名为transfer,在put方法中的addCount里有使用
private final void addCount(long x, int check) {
CounterCell[] as; long b, s;
// cas 增加count
if ((as = counterCells) != null ||
!U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
CounterCell a; long v; int m;
boolean uncontended = true;
if (as == null || (m = as.length - 1) < 0 ||
(a = as[ThreadLocalRandom.getProbe() & m]) == null ||
!(uncontended =
U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))) {
fullAddCount(x, uncontended);
return;
}
if (check <= 1)
return;
s = sumCount();
}
if (check >= 0) {
Node[] tab, nt; int n, sc;
//
while (s >= (long)(sc = sizeCtl) && (tab = table) != null &&
(n = tab.length) < MAXIMUM_CAPACITY) {
int rs = resizeStamp(n);
if (sc < 0) {
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);
s = sumCount();
}
}
}
private final void transfer(Node[] tab, Node[] nextTab) {
int n = tab.length, stride;
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[] nt = (Node[])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 fwd = new ForwardingNode(nextTab);
boolean advance = true;
boolean finishing = false; // to ensure sweep before committing nextTab
for (int i = 0, bound = 0;;) {
Node 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 ln, hn;
if (fh >= 0) {
int runBit = fh & n;
Node lastRun = f;
for (Node 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 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(ph, pk, pv, ln);
else
hn = new Node(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 t = (TreeBin)f;
TreeNode lo = null, loTail = null;
TreeNode hi = null, hiTail = null;
int lc = 0, hc = 0;
for (Node e = t.first; e != null; e = e.next) {
int h = e.hash;
TreeNode p = new TreeNode
(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(lo) : t;
hn = (hc <= UNTREEIFY_THRESHOLD) ? untreeify(hi) :
(lc != 0) ? new TreeBin(hi) : t;
setTabAt(nextTab, i, ln);
setTabAt(nextTab, i + n, hn);
setTabAt(tab, i, fwd);
advance = true;
}
}
}
}
}
}
- treeifyBin方法
在hash槽中的链表长度到一定的时候对链表执行树化操作
private final void treeifyBin(Node[] tab, int index) {
Node b; int n, sc;
if (tab != null) {
// 转换为红黑树还有整个map大小限制
if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
// 扩容
tryPresize(n << 1);
else if ((b = tabAt(tab, index)) != null && b.hash >= 0) {
synchronized (b) {
if (tabAt(tab, index) == b) {
TreeNode hd = null, tl = null;
for (Node e = b; e != null; e = e.next) {
TreeNode p =
new TreeNode(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(hd));
}
}
}
}
}
- unTreeifyBin
在remove的情况下，树节点小于一定值的时候，将树退化为链表
static Node untreeify(Node b) {
Node hd = null, tl = null;
for (Node q = b; q != null; q = q.next) {
Node p = new Node(q.hash, q.key, q.val, null);
if (tl == null)
hd = p;
else
tl.next = p;
tl = p;
}
return hd;
}
- 注意点
ConcurrentHashMap比起HashMap，就是多了加锁操作，cas和synchronized,当然，synchronized大家都熟悉，而cas使用是unsafe里面的cas方法，这种方法是直接操作内存的，有兴趣的可以自行了解下