【高并发系列】4、volatile

1、volatile关键字不能代替锁来保证复合操作的原子性：

public class VolatileDemo {
	private static volatile int i = 0;
	public static class PlusTask implements Runnable {
		@Override
		public void run() {
			for (int k = 0; k < 10000; k++) {
				i++;
			}
		}
	}
	public static void main(String[] args) throws InterruptedException {
		Thread[] threads = new Thread[10];
		for (int i = 0; i < 10; i++) {
			threads[i] = new Thread(new PlusTask());
			threads[i].start();
		}
		for (int i = 0; i < 10; i++) {
			threads[i].join();
		}
		System.out.println(i);
	}
}

因为i++的操作不能保证原子性，i累加结果都会小于100000；

2、在server模式下，由于系统优化导致ReadThread线程无法感知到主线程中所做的修改，导致ReadThread无法退出，可以给ready变量增加volatile关键字，ReadThread对ready变量就可见了；

public class NoVisibility {
	private static boolean ready;
	private static int number;
	private static class ReadThread extends Thread {
		@Override
		public void run() {
			while (!ready) {
			}
			System.out.println(number);
		}
	}
	public static void main(String[] args) throws InterruptedException {
		new ReadThread().start();
		TimeUnit.SECONDS.sleep(1);
		number = 42;
		ready = true;
		TimeUnit.SECONDS.sleep(2);
	}
}

3、查看JDK的运行模式

java -version

java version "1.8.0_131"
Java(TM) SE Runtime Environment (build 1.8.0_131-b11)
Java HotSpot(TM) 64-Bit Server VM (build 25.131-b11, mixed mode)

其中Server VM表明是Server模式运行；