一道Java多线程面试题的解答

最近看到一道面试题，比较有意思：

有三个线程ID分别是A、B、C,请有多线编程实现，在屏幕上循环打印10次ABCABC…

由于线程执行的不确定性，要保证这样有序的输出，必须控制好多线程的同步。

线程同步有两种基本方法：

（1）    synchronized

（2）    wait,notify,notifyAll

现在分别采用这两种方法来解答这道题目。

/**

* @author陈新汉 http://www.blogjava.net/hankchen

 * 2009-12-28 下午01:57:04

 */

/**

 *采用多线程技术打印10次“ABC”，即“ＡＢＣＡＢＣ...”

 * 实现方式（一）利用synchronized关键字实现

 */

public class XunleiInterviewMultithread {

       /**

        * @param args

        */

       public static void main(String[] args) {

              XunleiLock lock = new XunleiLock();

              new Thread(new XunleiPrinter("A", lock)).start();

              new Thread(new XunleiPrinter("B", lock)).start();

              new Thread(new XunleiPrinter("C", lock)).start();

       }

}

class XunleiPrinter implements Runnable {

       private String name = "";

       private XunleiLock lock = null;

       private int count=10;

       public XunleiPrinter(String name, XunleiLock lock) {

              this.name = name;

              this.lock = lock;

       }

       @Override

       public void run() {

              while(count>0) {

                     synchronized (lock) {

                            if (lock.getName().equalsIgnoreCase(this.name)) {

                                   System.out.print(name);

                                   count--;

                                   if (this.name.equals("A")) {

                                          lock.setName("B");

                                   } elseif (this.name.equals("B")) {

                                          lock.setName("C");

                                   } elseif (this.name.equals("C")) {

                                          lock.setName("A");

                                   }

                            }

                     }

              }

       }

}

class XunleiLock

{

       public String name = "A";

       public String getName() {

              return name;

       }

       public void setName(String name) {

              this.name = name;

       }

}

方法（二）线程类修改如下，其他类一样：

class XunleiPrinter2 implements Runnable {

       private String name = "";

       private XunleiLock lock = null;

       private int count=10;

       public XunleiPrinter2(String name, XunleiLock lock) {

              this.name = name;

              this.lock = lock;

       }

       @Override

       public void run() {

              while(count>0) {

                     synchronized (lock) {

                            while(!lock.getName().equalsIgnoreCase(this.name)) {

                                   try{

                                          lock.wait();

                                   }catch(InterruptedException e){

                                          e.printStackTrace();

                                   }

                            }

                            System.out.print(name);

                            count--;

                            if (this.name.equals("A")) {

                                   lock.setName("B");

                            } elseif (this.name.equals("B")) {

                                   lock.setName("C");

                            } elseif (this.name.equals("C")) {

                                   lock.setName("A");

                            }

                            lock.notifyAll();

                     }

              }

       }

}

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