1.简介
LongAdder是Java8中新的并发包类，相比较之前的原子类AtomicXXX,LongAdder在低并发的情况下性能和原子类基本持平，但在高并发的情况下性能优于原子类
2.源码分析
/**
* One or more variables that together maintain an initially zero
* {@code long} sum. When updates (method {@link #add}) are contended
* across threads, the set of variables may grow dynamically to reduce
* contention. Method {@link #sum} (or, equivalently, {@link
* #longValue}) returns the current total combined across the
* variables maintaining the sum.
*
* <p>This class is usually preferable to {@link AtomicLong} when
* multiple threads update a common sum that is used for purposes such
* as collecting statistics, not for fine-grained synchronization
* control. Under low update contention, the two classes have similar
* characteristics. But under high contention, expected throughput of
* this class is significantly higher, at the expense of higher space
* consumption.
* @since 1.8
* @author Doug Lea
*/
public class LongAdder extends Striped64 implements Serializable
注释的意思是，1个或多个值的和被保存在一个初始化值为0的"sum"中，当调用add()方法的并发量很大时，那么存储值的容器会动态扩容以降低碰撞几率(类似hash碰撞)，方法sum()返回并发操作后得到的最终结果。
通常情况下，LongAdder在统计数据更新时比AtomicLong更适用。但不应该用于细粒化的同步控制。LongAdder在低并发的情况下性能和AtomicLong基本持平，但在高并发的情况下性能优于AtomicLong。
那我们就来看看add(long x)和sum()方法的具体实现吧！
public void add(long x) {
Cell[] as; long b, v; int m; Cell a;
//尝试更新base,若并发不高，能更新成功，退出条件(cas更新)
if ((as = cells) != null || !casBase(b = base, b + x)) {
boolean uncontended = true;
/**
* 尝试第二次更新值,取得Cell[]中的一个值，getProbe() & m是
* 取得一个随机整数然后和m(数组大小-1)做与运算，得出的值在
* 0～(as.length - 1)之间,更新成功则退出条件
*/
if (as == null || (m = as.length - 1) < 0 ||
(a = as[getProbe() & m]) == null ||
!(uncontended = a.cas(v = a.value, v + x)))
//更新失败后进入
longAccumulate(x, null, uncontended);
}
}
//这是存储实体Cell的源码，对CAS不了解的同学可以看看Atomic包
@sun.misc.Contended static final class Cell {
volatile long value;
Cell(long x) { value = x; }
final boolean cas(long cmp, long val) {
return UNSAFE.compareAndSwapLong(this, valueOffset, cmp, val);
}
// Unsafe mechanics
private static final sun.misc.Unsafe UNSAFE;
private static final long valueOffset;
static {
try {
UNSAFE = sun.misc.Unsafe.getUnsafe();
Class<?> ak = Cell.class;
valueOffset = UNSAFE.objectFieldOffset
(ak.getDeclaredField("value"));
} catch (Exception e) {
throw new Error(e);
}
}
}
final void longAccumulate(long x, LongBinaryOperator fn,
boolean wasUncontended) {
int h;
// 取得一个随机整数，设置是否并发竞争为true
if ((h = getProbe()) == 0) {
ThreadLocalRandom.current();
h = getProbe();
wasUncontended = true;
}
//是否碰撞，参考hashmap
boolean collide = false;
for (;;) {
Cell[] as; Cell a; int n; long v;
if ((as = cells) != null && (n = as.length) > 0) {
//如果数组指定位置为空，则赋值
if ((a = as[(n - 1) & h]) == null) {
if (cellsBusy == 0) {
Cell r = new Cell(x);
//创建Cell时,casCellsBusy()锁住数组
if (cellsBusy == 0 && casCellsBusy()) {
boolean created = false;
try {
Cell[] rs; int m, j;
if ((rs = cells) != null &&
(m = rs.length) > 0 &&
rs[j = (m - 1) & h] == null) {
rs[j] = r;
created = true;
}
} finally {
cellsBusy = 0;
}
if (created)
break;
continue;
}
}
//指定数组位置为null则说明没有碰撞
collide = false;
}
else if (!wasUncontended)
wasUncontended = true;
/**
* a不为null,更新a的value,成功则退出。更新失败则
* 说明多个线程同时更新一个Cell，并发量大，碰撞几率高
* 可能需要扩容
*/
else if (a.cas(v = a.value, ((fn == null) ? v + x :
fn.applyAsLong(v, x))))
break;
/**
* 若数组大小大于CPU个数，则说明碰撞不是由于数组过小导致
* 则重新尝试更新数据
*/
else if (n >= NCPU || cells != as)
collide = false;
else if (!collide)
collide = true;
//扩容数组,扩容时锁住数组
else if (cellsBusy == 0 && casCellsBusy()) {
try {
if (cells == as) {
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;
}
h = advanceProbe(h);
}
//若cell[]为空，初始化
else if (cellsBusy == 0 && cells == as && casCellsBusy()) {
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
}
}
//加锁方法
final boolean casCellsBusy() {
return UNSAFE.compareAndSwapInt(this, CELLSBUSY, 0, 1);
}
/**
* 返回各个线程操作总和，LongAdder在统计的时候如果有并发更新
* 可能导致统计的数据有误差，因为volatile并不能保证并发安全
*/
public long sum() {
Cell[] as = cells; Cell a;
long sum = base;
if (as != null) {
for (int i = 0; i < as.length; ++i) {
if ((a = as[i]) != null)
sum += a.value;
}
}
return sum;
}
3.总结
LongAdder在AtomicLong的基础上将单点的更新压力分散到各个节点，在低并发的时候通过对base的直接更新可以很好的保障和AtomicLong的性能基本保持一致，而在高并发的时候通过分散，让各个线程操作不同的节点，提高了性能。
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