由一个多线程共享Integer类变量问题引起的。。。

    xiaoxiao2021-03-25  288

     最近看到一个多线程面试题,有三个线程分别打印A、B、C,请用多线程编程实现,在屏幕上循环打印10次ABCABC… 

      看到这个题目,首先想到的是解决方法是定义一个Integer类对象,初始化为0,由3个线程共享,如果Integer对象取余3之后等于0,则打印A,同时进行加1操作;如果Integer对象取3之后等于1,则打印B,同时进行加1操作;如果Integer对象取3之后等于1,则打印C,如果循环打印了10次的话,就退出线程。

    /** * ThreeThread * 3个线程测试 */ public class ThreeThread { public static void main(String[] args) throws InterruptedException { Integer gData = 0; Thread thread1 = new MyTask(gData, 0, "A"); Thread thread2 = new MyTask(gData, 1, "B"); Thread thread3 = new MyTask(gData, 2, "C"); thread1.start(); thread2.start(); thread3.start(); thread1.join(); thread2.join(); thread3.join(); } } class MyTask extends Thread { private Integer gData; private int n; private String info; public MyTask(Integer gData, int n, String info) { super("thread " + info); this.gData = gData; this.n = n; this.info = info; } public void run() { int i = 0; while (true) { synchronized (gData) { if (gData % 3 == n) { System.out.print(info + " "); gData++; i++; } } if (i == 10) { break; } else { Thread.yield(); try { sleep(10); } catch (InterruptedException e) { e.printStackTrace(); } } } } }

    运行程序结果如下:

      发现只有A线程打印了"A",并没有发现B线程和C线程打印字符串:(。难道是A线程更改了Integer对象的值,而B线程和C线程并没有“看到”更新后的值?于是,在线程类的run方法的while循环中增加代码如下:

    while (true) { System.out.println(Thread.currentThread().getName() + " " + gData); synchronized (gData) { if (gData % 3 == n) { System.out.print(info + " "); gData++; i++; } } ... }

    运行程序结果如下:

      由运行结果可知,刚开始A、B、C线程都拥有Integer类变量,并且初值为0。当A线程更改Integer类变量为1时,但是B和C线程中的Integer类变量的值仍然为0,因此,结果肯定不会打印出ABCABC....

      通过阅读Integer类源码,可知Integer类中存放int值的变量类型是final的:

    /** * The value of the {@code Integer}. * * @serial */ private final int value;

      也就是说,Integer类对象的值每更新一次,就会创建一个新的Integer对象。运行程序结果只打印出了"A",表示刚开始A、B、C线程都拥有同一个Integer类变量,并且初值为0,但是当A线程更新Integer对象的值后,A线程中的Integer对象和B/C线程中的Integer对象已经不是同一个对象了。

      为了能够正常打印出ABCABC字符串,可以把Integer对象类型改为AtomicInteger,代码如下:

    /** * ThreeThread * 3个线程测试 */ public class ThreeThread { public static void main(String[] args) throws InterruptedException { AtomicInteger gData = new AtomicInteger(0); Thread thread1 = new MyTask(gData, 0, "A"); Thread thread2 = new MyTask(gData, 1, "B"); Thread thread3 = new MyTask(gData, 2, "C"); thread1.start(); thread2.start(); thread3.start(); thread1.join(); thread2.join(); thread3.join(); } } class MyTask extends Thread { private AtomicInteger gData; private int n; private String info; public MyTask(AtomicInteger gData, int n, String info) { super("thread " + info); this.gData = gData; this.n = n; this.info = info; } public void run() { int i = 0; while (true) { synchronized (gData) { if (gData.get() % 3 == n) { System.out.print(info + " "); gData.incrementAndGet(); i++; } } if (i == 10) { break; } else { Thread.yield(); try { sleep(10); } catch (InterruptedException e) { e.printStackTrace(); } } } } }

     

      第二种打印ABCABC...字符串的解决方法是使用wait/notify函数,示例代码如下:

    /** * ThreeThread2 * 三个线程依次输出A B C,使用线程同步方式 */ public class ThreeThread2 { public static void main(String[] args) throws InterruptedException { Object A = new Object(); Object B = new Object(); Object C = new Object(); MyThread myThread1 = new MyThread(C, A, "A"); MyThread myThread2 = new MyThread(A, B, "B"); MyThread myThread3 = new MyThread(B, C, "C"); myThread1.start(); Thread.sleep(10); myThread2.start(); Thread.sleep(10); myThread3.start(); try { myThread1.join(); myThread2.join(); myThread3.join(); } catch (InterruptedException e) { e.printStackTrace(); } } } class MyThread extends Thread { private Object prev; private Object curr; private String info; public MyThread(Object prev, Object curr, String info) { this.prev = prev; this.curr = curr; this.info = info; } public void run() { int cnt = 10; while (cnt-- > 0) { synchronized (prev) { synchronized (curr) { System.out.print(info + " "); curr.notify(); } try { prev.wait(); } catch (InterruptedException e) { e.printStackTrace(); } } } } }

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