多线程环境下,hashmap是线程不安全.所以JDK给我们提供了HashTable和ConcurrentHashMap等同步方式.
HashTable的锁是通过Synchronized关键字实现的,这种重量级锁在线程竞争激烈的情况下,效率十分低下.当一个线程访问一个对象的一个同步方法时,那么其他线程就不能访问该对象的其他同步方法.所以HashTable的效率在并发场景下,将会十分低.
ConcurrentHashMap亮相
jdk1.7 锁分段技术
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ConcurrentHashMap将数据按照分桶(分段)之后,对每一个桶进行了同步.这样,对于多线程场景下,ConcurrentHashMap对象有着很多锁,只有在同一个桶下,才会出现线程之间竞争资源的问题.
jdk1.8采用了CAS技术来保证同步
看一下put()方法的实现:
final V putVal(K key, V value, boolean onlyIfAbsent) {\
//不允许key或者value为空
if (key == null || value == null) throw new NullPointerException();
//通过扰动函数,来获取hash,扰动函数将hashcode的值与低位的值异或并保证最高位为0.(保证最终结果为正整数)
int hash = spread(key.hashCode());
int binCount = 0;
//死循环保证插入成功
for (Node[] tab = table;;) {
Node f; int n, i, fh;
if (tab == null || (n = tab.length) == 0)
tab = initTable();
else if ((f = tabAt(tab, i = (n - 1) & hash)) == null) {
//头节点为null时,通过CAS操作来添加元素
if (casTabAt(tab, i, null,
new Node(hash, key, value, null)))
break; // no lock when adding to empty bin
}
else if ((fh = f.hash) == MOVED)
tab = helpTransfer(tab, f);
else {
V oldVal = null;
//f在这里是链表.
synchronized (f) {
if (tabAt(tab, i) == f) {
if (fh >= 0) {
binCount = 1;
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;
}
}
}
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) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
}
}
addCount(1L, binCount);
return null;
}
JDK1.8-ConcurrentHashMap数据结构
- TreeNode:树节点类,也是整个数据结构中的链表.
static class Node implements Map.Entry {
final int hash;
final K key;
volatile V val;
volatile Node next;
}
- TreeBin:该类是TreeNode链表的根节点.这是与HashMap不同的地方,HashMap整个链表结构都是通过TreeNode来实现的.而这里,数组里边存储的是一个TreeBin,TreeBin封装了TreeNode,并同时提供了链表转红黑树的实现.
TreeBin(TreeNode b) {
super(TREEBIN, null, null, null);
this.first = b;
TreeNode r = null;
for (TreeNode x = b, next; x != null; x = next) {
next = (TreeNode)x.next;
x.left = x.right = null;
if (r == null) {
x.parent = null;
x.red = false;
r = x;
}
else {
K k = x.key;
int h = x.hash;
Class kc = null;
for (TreeNode p = r;;) {
int dir, ph;
K pk = p.key;
if ((ph = p.hash) > h)
dir = -1;
else if (ph < h)
dir = 1;
else if ((kc == null &&
(kc = comparableClassFor(k)) == null) ||
(dir = compareComparables(kc, k, pk)) == 0)
dir = tieBreakOrder(k, pk);
TreeNode 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);
}
JDK1.8-ConcurrentHashMap扩容实现
在执行putVal方法时,如果链表长度大于8时,就会执行扩容触发转换红黑树的操作:
if (binCount != 0) {
if (binCount >= TREEIFY_THRESHOLD)
treeifyBin(tab, i);
if (oldVal != null)
return oldVal;
break;
}
private final void treeifyBin(Node[] tab, int index) {
Node b; int n, sc;
if (tab != null) {
//如果数组长度没有到达MIN_TREEIFY_CAPACITY(64)时,执行数组扩容
if ((n = tab.length) < MIN_TREEIFY_CAPACITY)
//扩容为当前容量的2倍
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));
}
}
}
}
}
数组扩容:
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[] 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[] nt = (Node[])new Node[n];
table = nt;
sc = n - (n >>> 2);
}
} finally {
sizeCtl = sc;
}
}
}
else if (c <= sc || n >= MAXIMUM_CAPACITY)
break;
else if (tab == table) {
int rs = resizeStamp(n);
if (sc < 0) {
Node[] 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);
}
}
}