volatile修飾全局變量，可以保證線程並發安全嗎？

 

今天被人問到volatile能不能保證並發安全？

呵，這能難倒我？

上代碼：（代碼太長建議跳到頁末）

//電腦太好，100線程起步~
public class ThreadTest {
    private static volatile int num = 0; public static void main(String[] args) throws InterruptedException { new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); new Thread(() -> { for (int i = 0; i < 100; i++) { num++; System.out.println(num); } }).start(); Thread.sleep(500); System.out.println(num); } }

期望最終值：
　　100 * 100 = 10000

輸出結果：
　　...
　　...
　　9998
　　9998
　

分析：

　　100個線程對volatilei修飾的num++，會被編譯成以下三步：
　　 1.獲取i的值；2.執行i+1；3.將結果賦值給i。
　　volatile只能保證可見性，並不能保證原子性。

結論：
　　volatile只能保證這3步在編譯后指令不會被重新排序，並不能保證線程並發的數據安全。建議搭配上synchronized或其他Lock鎖使用。