高并发计数器LongAdder 实现原理与使用场景详解

// 基础值（无竞争时直接操作）
transient volatile long base;
// 分片单元数组（应对高并发）
transient volatile Cell[] cells;
// 分片单元结构（避免伪共享）
@jdk.internal.vm.annotation.Contended
static final class Cell {
    volatile long value;
    Cell(long x) { value = x; }
}

| Cell1 (64字节) | Cell2 (64字节) | ... |

public void add(long x) {
    Cell[] cs; long b, v; int m; Cell c;
    if ((cs = cells) != null || 
        !casBase(b = base, b + x)) {
        boolean uncontended = true;
        if (cs == null || (m = cs.length - 1) < 0 ||
            (c = cs[getProbe() & m]) == null ||
            !(uncontended = c.cas(v = c.value, v + x)))
            longAccumulate(x, null, uncontended);
    }
}

public long sum() {
    Cell[] cs = cells;
    long sum = base;
    if (cs != null) {
        for (Cell c : cs)
            if (c != null) sum += c.value;
    }
    return sum;
}

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.LongAdder;
public class SimpleLongAdderExample {
    public static void main(String[] args) throws InterruptedException {
        // 1. 创建LongAdder实例
        LongAdder counter = new LongAdder();
        // 2. 创建线程池（模拟并发请求）
        ExecutorService executor = Executors.newFixedThreadPool(10);
        // 3. 提交100个累加任务
        for (int i = 0; i < 100; i++) {
            executor.submit(() -> {
                // 每个任务累加1000次
                for (int j = 0; j < 1000; j++) {
                    counter.increment(); // 等同于add(1)
                }
            });
        }
        // 4. 关闭线程池并等待任务完成
        executor.shutdown();
        executor.awaitTermination(1, TimeUnit.MINUTES);
        // 5. 输出最终结果
        System.out.println("最终计数: " + counter.sum()); // 应输出100000
    }
}