JUC-LongAdder

冷不防 2022-09-15 06:04 137阅读 0赞

LongAdder只能用来计算加法，且从零开始计算

LongAccumulator提供了自定义的操作函数

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16][]

普通int+sychronized、AtomicInteger和LongAdder的性能比较

public class LongAdderCalcDemo {
    
        int num = 0;
        public synchronized void add_synchronized() {
            num++;
        }
    
        AtomicInteger atomicInteger = new AtomicInteger();
        public void add_AtomicInteger() {
            atomicInteger.incrementAndGet();
        }
    
        LongAdder longAdder = new LongAdder();
        public void add_LongAdder() {
            longAdder.increment();
        }
    
        public static void main(String[] args) throws InterruptedException {
            int count = 50;
            int size = 1000000;
            LongAdderCalcDemo demo = new LongAdderCalcDemo();
            CountDownLatch countDownLatch1 = new CountDownLatch(count);
            CountDownLatch countDownLatch2 = new CountDownLatch(count);
            CountDownLatch countDownLatch3 = new CountDownLatch(count);
            long start;
            long end;
    
            start = System.currentTimeMillis();
            for (int i = 0; i < count; i++) {
                new Thread(()->{
                    try {
                        for (int j = 0; j < size; j++) {
                            demo.add_synchronized();
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    } finally {
                        countDownLatch1.countDown();
                    }
                }).start();
            }
            countDownLatch1.await();
            end = System.currentTimeMillis();
            System.out.println("add_synchronized costTime = " + (end - start) + " 毫秒, 结果 = " + demo.num);
    
            start = System.currentTimeMillis();
            for (int i = 0; i < count; i++) {
                new Thread(()->{
                    try {
                        for (int j = 0; j < size; j++) {
                            demo.add_AtomicInteger();
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    } finally {
                        countDownLatch2.countDown();
                    }
                }).start();
            }
            countDownLatch2.await();
            end = System.currentTimeMillis();
            System.out.println("add_AtomicInteger costTime = " + (end - start) + " 毫秒, 结果 = " + demo.atomicInteger.get());
    
            start = System.currentTimeMillis();
            for (int i = 0; i < count; i++) {
                new Thread(()->{
                    try {
                        for (int j = 0; j < size; j++) {
                            demo.add_LongAdder();
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    } finally {
                        countDownLatch3.countDown();
                    }
                }).start();
            }
            countDownLatch3.await();
            end = System.currentTimeMillis();
            System.out.println("add_LongAdder costTime = " + (end - start) + " 毫秒, 结果 = " + demo.longAdder.sum());
        }
    }

> add\_synchronized costTime = 1880 毫秒, 结果 = 50000000  
> add\_AtomicInteger costTime = 1123 毫秒, 结果 = 50000000  
> add\_LongAdder costTime = 115 毫秒, 结果 = 500000

**原理**

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 1][]![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 2][]

** Striped64中有2个重要的属性，long base和Cell\[\] cells数组**

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 3][]

Cell是Striped64的内部类

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 4][]

**LongAdder在无竞争的情况下，跟AtomicLong一样，对同一个base进行操作，当出现竞争关系时采用化整为零的做法，用空间换时间，****分散热点，将value值分散到一个cell数组中，不同线程会命中到数组中的不同槽中，各个线程只对自己槽中的那个值进行CAS操作，这样热点就被分散了，冲突的概率就小很多。如果要获取真正的long值，只要将各个槽中的变量值累加返回。**

**sum()会将所有cell数组中的value和base累加作为返回值，核心思想就是将之前AtomicLong一个value的更新压力分散到多个cell中去，从而降级更新热点。**

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 5][]

longAdder.sum()返回当前值，在没有并发更新value的情况下，sum()会返回一个精确值，在存在并发的情况下，sum()不保证返回精确值。

**sum()执行时，并没有限制对base和cells的更新，所以LongAdder不是强一致性，它是最终一致性的。**

### 区别 ###

![watermark_type_ZHJvaWRzYW5zZmFsbGJhY2s_shadow_50_text_Q1NETiBA5Yqq5Yqb5aWL5paXR08_size_20_color_FFFFFF_t_70_g_se_x_16 6][]

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JUC-LongAdder

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