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();
}
}
}
}
}