标签：should mapping try shm 防止 back 数组 available 产生
/**
* Returns the number of mappings. This method should be used
* instead of {@link #size} because a ConcurrentHashMap may
* contain more mappings than can be represented as an int. The
* value returned is an estimate; the actual count may differ if
* there are concurrent insertions or removals.
*
* @return the number of mappings
* @since 1.8
*/
public long mappingCount() {
long n = sumCount();
return (n < 0L) ? 0L : n; // ignore transient negative values
}
/**
* map长度超过Integer.MAX_VALUE? 惊人
* {@inheritDoc}
*/
public int size() {
long n = sumCount();
return ((n < 0L) ? 0 :
(n > (long)Integer.MAX_VALUE) ? Integer.MAX_VALUE :
(int)n);
}
final long sumCount() {
/* 取base+counterCells */
CounterCell[] as = counterCells; CounterCell a;
long sum = baseCount;
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
sum += a.value;
}
}
return sum;
}
/**
* Adds to count, and if table is too small and not already
* resizing, initiates transfer. If already resizing, helps
* perform transfer if work is available. Rechecks occupancy
* after a transfer to see if another resize is already needed
* because resizings are lagging additions.
*
* @param x the count to add
* @param check if <0, don't check resize, if <= 1 only check if uncontended
*/
private final void addCount(long x, int check) {
/* CounterCell改造自Striped64，用以在计数过程中降低并发冲突，因为是数组，内存位置相邻，所以使用jdk1.8中的@sun.misc.Contended 进行填充防止伪共享 */
CounterCell[] as; long b, s;
/* 已经有counterCells直接使用，否则操作baseCount，如果失败则进入if */
if ((as = counterCells) != null ||
/* 先读到baseCount到本地变量，防止ABA */
!U.compareAndSwapLong(this, BASECOUNT, b = baseCount, s = b + x)) {
CounterCell a; long v; int m;
/* 有没有在CounterCell上发生竞争 */
boolean uncontended = true;
if (as == null || (m = as.length - 1) < 0 ||
/* 对当前线程的probe用CounterCell[]长度取模得到CounterCell[]的一项，如果ThreadLocalRandom未初始化，则取第一项，如果此项为null，则认为找到的 位置未被使用，进入fullAddCount，否则在此存在的cell上进行CAS，如果CAS发生冲突，则进入fullAddCount, 继续进行size的count计数。 */
(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<K,V>[] 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 fullAddCount(long x, boolean wasUncontended) {
int h;
if ((h = ThreadLocalRandom.getProbe()) == 0) {
/* 确定是初始化后产生的随机数，用随机数对CounterCell[]长度取模(hash)来减少碰撞 */
ThreadLocalRandom.localInit(); // force initialization
h = ThreadLocalRandom.getProbe();
wasUncontended = true;
}
boolean collide = false; // True if last slot nonempty
for (;;) {
CounterCell[] as; CounterCell a; int n; long v;
if ((as = counterCells) != null && (n = as.length) > 0) {
if ((a = as[(n - 1) & h]) == null) {
if (cellsBusy == 0) { // Try to attach new Cell
CounterCell r = new CounterCell(x); // Optimistic create
/* 此处的LoadLoad屏障保证其它线程创建的CounterCell[]元素可见性 */
if (cellsBusy == 0 &&
/*使用CAS volatile的CELLSBUSY的方式加锁操作CounterCell[], 失败则用for自旋*/
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
boolean created = false;
try { // Recheck under lock
CounterCell[] rs; int m, j;
if ((rs = counterCells) != null &&
(m = rs.length) > 0 &&
rs[j = (m - 1) & h] == null) {
rs[j] = r;
created = true;
}
} finally {
/* 此处的StoreStore屏障保证数组中的新CounterCell引用可见性 */
cellsBusy = 0;
}
if (created)
break;
continue; // Slot is now non-empty
}
}
collide = false;
}
else if (!wasUncontended) // CAS already known to fail
wasUncontended = true; // Continue after rehash
else if (U.compareAndSwapLong(a, CELLVALUE, v = a.value, v + x))
break;
else if (counterCells != as || n >= NCPU)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (cellsBusy == 0 &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
/* 冲突太厉害，就扩容cells */
try {
if (counterCells == as) {// Expand table unless stale
CounterCell[] rs = new CounterCell[n << 1];
for (int i = 0; i < n; ++i)
rs[i] = as[i];
counterCells = rs;
}
} finally {
cellsBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
/* 冲突严重换个位置 */
h = ThreadLocalRandom.advanceProbe(h);
}
else if (cellsBusy == 0 && counterCells == as &&
U.compareAndSwapInt(this, CELLSBUSY, 0, 1)) {
boolean init = false;
try { // Initialize table
if (counterCells == as) {
/* 初始容量为2 */
CounterCell[] rs = new CounterCell[2];
rs[h & 1] = new CounterCell(x);
counterCells = rs;
init = true;
}
} finally {
cellsBusy = 0;
}
if (init)
break;
}
/* 初始化有竞争时，再试下baseCount */
else if (U.compareAndSwapLong(this, BASECOUNT, v = baseCount, v + x))
break; // Fall back on using base
}
}标签：should mapping try shm 防止 back 数组 available 产生
原文地址：https://www.cnblogs.com/niulang85/p/12502893.html
