Careercup - Microsoft面试题 - 6751316000899072

2014-05-12 07:10

题目链接

原题：

Write a thread safe data structure such that there could be only one writer at a time but there could be n readers reading the data. You can consider that incrementing or decrementing a variable is an atomic operation. If more than one threads try to write simultaneously then just select one randomly and let others wait

题目：写一个线程安全的数据结构，允许单线程写，n线程读。如果多个线程尝试写数据，随机选择一个，让其他线程等待。

解法：我用信号量试着写了一个，不过不太清楚“随机选择”这个要如何表示。

代码：

 // http://www.careercup.com/question?id=6751316000899072
 import java.util.concurrent.Semaphore;

 public class FooBar {
     public int n = 100;
     private Semaphore sharedSemaphore;
     private Semaphore exclusiveSemaphore;

     public FooBar(int n) {
         // TODO Auto-generated constructor stub
         this.n = n;
         this.sharedSemaphore = new Semaphore(n);
         this.exclusiveSemaphore = new Semaphore(1);
     }

     public void reader() {
         // The reader here is not to return a value, but to perform read()
         // action. Thus it is 'void reader()'.
         while (exclusiveSemaphore.availablePermits() < 1) {
             try {
                 Thread.sleep(50);
             } catch (InterruptedException e) {
                 // TODO Auto-generated catch block
                 e.printStackTrace();
             }
         }

         try {
             sharedSemaphore.acquire();
             System.out.println("Performing read() operation.");
             sharedSemaphore.release();
         } catch (InterruptedException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
     }

     public void writer() {
         while (exclusiveSemaphore.availablePermits() < 1
                 && sharedSemaphore.availablePermits() < n) {
             try {
                 Thread.sleep(50);
             } catch (InterruptedException e) {
                 // TODO Auto-generated catch block
                 e.printStackTrace();
             }
         }

         try {
             exclusiveSemaphore.acquire();
             System.out.println("Performing write() operation.");
             exclusiveSemaphore.release();
         } catch (InterruptedException e) {
             // TODO Auto-generated catch block
             e.printStackTrace();
         }
     }

     public static void main(String[] args) {
         FooBar fooBar = new FooBar(100);

         fooBar.reader();
         fooBar.writer();
     }
 }