一、 LongAccumulator类原理探究
LongAdder类是LongAccumulator的一个特例，LongAccumulator提供了比LongAdder更强大的功能，如下构造函数其中accumulatorFunction一个双目运算器接口，根据输入的两个参数返回一个计算值，identity则是LongAccumulator累加器的初始值。
public LongAccumulator(LongBinaryOperator accumulatorFunction,
long identity) {
this.function = accumulatorFunction;
base = this.identity = identity;
}
public interface LongBinaryOperator {
//根据两个参数计算返回一个值
long applyAsLong(long left, long right);
}
LongAdder其实是LongAccumulator的一个特例，调用LongAdder相当使用下面的方式调用LongAccumulator。
LongAdder adder = new LongAdder();
LongAccumulator accumulator = new LongAccumulator(new LongBinaryOperator() {
@Override
public long applyAsLong(long left, long right) {
return left + right;
}
}, 0);
LongAccumulator相比于LongAdder可以提供累加器初始非0值，后者只能默认为0，另外前者还可以指定累加规则比如不是累加而是相乘，只需要构造LongAccumulator时候传入自定义双面运算器就OK，后者则内置累加的规则。
从下面代码知道LongAccumulator相比于LongAdder不同在于casBase时候后者传递的是b+x,前者则是调用了r = function.applyAsLong(b = base, x)来计算。
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);
}
}
public void accumulate(long x) {
Cell[] as; long b, v, r; int m; Cell a;
if ((as = cells) != null ||
(r = function.applyAsLong(b = base, x)) != b && !casBase(b, r)) {
boolean uncontended = true;
if (as == null || (m = as.length - 1) < 0 ||
(a = as[getProbe() & m]) == null ||
!(uncontended =
(r = function.applyAsLong(v = a.value, x)) == v ||
a.cas(v, r)))
longAccumulate(x, function, uncontended);
}
}
另外前者调用longAccumulate时候传递到是function,而后者是null，从下面代码可知当fn为null时候就是使用v+x加法运算这时候就等价于LongAdder，fn不为null时候则使用传递的fn函数计算，如果fn为加法则等价于LongAdder；
else if (casBase(v = base, ((fn == null) ? v + x :
fn.applyAsLong(v, x))))
break; // Fall back on using base
更详细的说明敬请期待 Java并发编程基础之并发包源码剖析 一书的出版
