volatile修飾符

Volatile 修飾的成員變量在每次被線程訪問時，都強制從共享內存中從新讀取該成員變量的值。並且，當成員變量發生變化時，會強制線程將變化值回寫到共享內存。這樣在任什麼時候刻，兩個不一樣的線程老是看到某個成員變量的同一個值。

一個volatile對象引用多是null。

（1）Java程序代碼

package niukewang;

public class volatile_Test {

    public static void main(String args[]) {
        MyRunnable r = new MyRunnable();
        Thread t1 = new Thread(r);

        t1.start();
        
        try {
            Thread.sleep(2);
            r.stopThread();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }
}

class MyRunnable implements Runnable {
    private volatile boolean active;

    public void run() {
        active = true;
        while (active) // 第一行
        {
            // 代碼
            System.out.println("Print the line");
        }
    }

    public void stopThread() {
        System.out.println("Set to false");
        active = false; // 第二行
    }
}

只有當調用stopThread時候，設置了ative爲false的時候，而後線程獲取了false的值，會中止第一行中內容的輸出。

（2）輸出結果。

Print the line
Print the line
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Set to false
Print the line