首先给出一个并发编程时,对于计数器进行累加时有问题的程序:
public class CountExample1 { // 请求总数 public static int clientTotal = 5000; // 同时并发执行的线程数 public static int threadTotal = 200; public static int count = 0; public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i< clientTotal; i ++) { executorService.execute(()->{ try { semaphore.acquire(); add(); semaphore.release(); } catch (InterruptedException e) { e.printStackTrace(); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("count:{}", count); } private static void add() { count ++; } }
上述代码,本意是运行之后,count的值为5000,实际上每次运行的结果都小于5000。
改进:
将上述的int改为AtomicInteger,如下:
public class CountExample2 { // 请求总数 public static int clientTotal = 5000; // 同时并发执行的线程数 public static int threadTotal = 200; public static AtomicInteger count = new AtomicInteger(0); public static void main(String[] args) throws InterruptedException { ExecutorService executorService = Executors.newCachedThreadPool(); final Semaphore semaphore = new Semaphore(threadTotal); final CountDownLatch countDownLatch = new CountDownLatch(clientTotal); for (int i = 0; i< clientTotal; i ++) { executorService.execute(()->{ try { semaphore.acquire(); add(); semaphore.release(); } catch (InterruptedException e) { e.printStackTrace(); } countDownLatch.countDown(); }); } countDownLatch.await(); executorService.shutdown(); log.info("count:{}", count); } private static void add() { count.incrementAndGet(); } }
删除代码中每次执行的结果都是5000,与我们预期的一致。
那么AtomicInteger的实现原理是什么呢?
查看count.incrementAndGet()函数的实现,如下:
public final int incrementAndGet() { return unsafe.getAndAddInt(this, valueOffset, 1) + 1; }
这个unsafe类提供的getAndAddInt(Object var1, long var2, int var4),查看该源码为:
public final int getAndAddInt(Object var1, long var2, int var4) { int var5; do { var5 = this.getIntVolatile(var1, var2); } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4)); return var5; }
分析上述方法:
var5通过获取本地中的变量(其实应该取的是
主存的值),然后比较当前的var2的值与本地变量var5是否相等,如果不相等则重复判断,直到相等才执行增加操作。这样就保证数据的一致性。这个也叫做CMS。