Atomic包

• AtomicXXX：CAS、unsafe.compareAndSwapInt

@ThreadSafe
@Slf4j
public class SafeCountAtomic {
    public static int clientTotal = 5000;
    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.get()); // 5000
    }

    private static void add() {
        count.incrementAndGet();
        // count.getAndIncrement();
    }

}

原始碼分析 AtomicInteger.class

public final int incrementAndGet() {
    return unsafe.getAndAddInt(this, valueOffset, 1) + 1;
}

Unsafe.class

// 傳入物件，
public final int getAndAddInt(Object var1, long var2, int var4) {
        int var5;
        do {
           // 從主記憶體中獲取到值
            var5 = this.getIntVolatile(var1, var2);
            // 將var1物件，工作記憶體中的值（var2）和主記憶體中的值（var5）比較，
            // 如果相等，則將該值設定為（ var5 + var4），
            // 如果不相等，迴圈直到相等。
        } while(!this.compareAndSwapInt(var1, var2, var5, var5 + var4));
	
	// 返回主記憶體中的值
        return var5;
}
// JVM底層方法
public final native boolean compareAndSwapInt(Object var1, long var2, int var4, int var5);