LongAdder，LongAccumulator类了解
AtomicLong 通过cas提供了非阻塞的api，但是并发量高的时候，cas会导致大量线程自旋从而浪费cpu资源，所以开发者发明了LongAdder来减少自旋
测试代码
public class LongAdderTest {
public static void main(String[] args) throws InterruptedException {
LongAdder longAdder = new LongAdder();
ArrayList<Thread> threads = new ArrayList<>();
for (int i = 0; i < 100; i++) {
Thread thread =
new Thread(
() -> {
longAdder.add(1);
System.out.println(longAdder.longValue());
});
threads.add(thread);
}
threads.forEach(Thread::start);
threads.forEach((thread) -> {
try {
thread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
}
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longAdder.add方法代码解析
public void add(long x) {
Cell[] as; long b, v; int m; Cell a;
if ((as = cells) != null || !casBase(b = base, b + x)) {
boolean uncontended = true;
if (as == null || (m = as.length - 1) < 0 ||
(a = as[getProbe() & m]) == null ||
!(uncontended = a.cas(v = a.value, v + x)))
longAccumulate(x, null, uncontended);
}
}
/**
* Handles cases of updates involving initialization, resizing,
* creating new Cells, and/or contention. See above for
* explanation. This method suffers the usual non-modularity
* problems of optimistic retry code, relying on rechecked sets of
* reads.
*
* @param x the value
* @param fn the update function, or null for add (this convention
* avoids the need for an extra field or function in LongAdder).
* @param wasUncontended false if CAS failed before call
*/
final void longAccumulate(long x, LongBinaryOperator fn,
boolean wasUncontended) {
int h;
if ((h = getProbe()) == 0) {
ThreadLocalRandom.current(); // force initialization
h = getProbe();
wasUncontended = true;
}
boolean collide = false; // True if last slot nonempty
for (;;) {
Cell[] as; Cell a; int n; long v;
if ((as = cells) != null && (n = as.length) > 0) {
if ((a = as[(n - 1) & h]) == null) {
if (cellsBusy == 0) { // Try to attach new Cell
Cell r = new Cell(x); // Optimistically create
if (cellsBusy == 0 && casCellsBusy()) {
boolean created = false;
try { // Recheck under lock
Cell[] rs; int m, j;
if ((rs = cells) != null &&
(m = rs.length) > 0 &&
rs[j = (m - 1) & h] == null) { // (m - 1) & 位置的cell值为空，则赋值
rs[j] = r;
created = true;
}
} finally {
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 (a.cas(v = a.value, ((fn == null) ? v + x :
fn.applyAsLong(v, x)))) // (m - 1) & 位置的cell值不为空，则cas累加
break;
else if (n >= NCPU || cells != as)
collide = false; // At max size or stale
else if (!collide)
collide = true;
else if (cellsBusy == 0 && casCellsBusy()) { // 扩容
try {
if (cells == as) { // Expand table unless stale
Cell[] rs = new Cell[n << 1];
for (int i = 0; i < n; ++i)
rs[i] = as[i];
cells = rs;
}
} finally {
cellsBusy = 0;
}
collide = false;
continue; // Retry with expanded table
}
h = advanceProbe(h);
}
else if (cellsBusy == 0 && cells == as && casCellsBusy()) { // 初始化cell数组
boolean init = false;
try { // Initialize table
if (cells == as) {
Cell[] rs = new Cell[2];
rs[h & 1] = new Cell(x);
cells = rs;
init = true;
}
} finally {
cellsBusy = 0;
}
if (init)
break;
}
else if (casBase(v = base, ((fn == null) ? v + x :
fn.applyAsLong(v, x))))
break; // Fall back on using base
}
}
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LongAdder，LongAccumulator 一句话：线程过多，争夺同一个资源时，会将资源拆成一个数组，每个线程操作时会被随机分配到其中一个数组， 获取值时，将所有数组的值加起来即可，这样做法的优点就是降低了线程争抢共享资源的可能性，cas时间也会大大减小。 原理：
- 当线程较多，出现并发时，
- 内置的cells数组会被初始化，
- 如果cell数组上的值如果为null,就将add的值设置上去，
- 如果cell数组上的值不为nul,会进行cas操作，
- 如果设置值的时候cas成功
- 将add的值和原先的值设置到原来位置上
- 如果设置值的时候cas失败
- 如果cells数组大小小于cpu个数，则进行扩容
- 如果设置值的时候cas成功
- 内置的cells数组会被初始化，
上次更新: 2022/05/23, 19:12:51