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：

 1 // http://www.careercup.com/question?id=6751316000899072

 2 import java.util.concurrent.Semaphore;

 3 

 4 public class FooBar {

 5     public int n = 100;

 6     private Semaphore sharedSemaphore;

 7     private Semaphore exclusiveSemaphore;

 8 

 9     public FooBar(int n) {

10         // TODO Auto-generated constructor stub

11         this.n = n;

12         this.sharedSemaphore = new Semaphore(n);

13         this.exclusiveSemaphore = new Semaphore(1);

14     }

15 

16     public void reader() {

17         // The reader here is not to return a value, but to perform read()

18         // action. Thus it is 'void reader()'.

19         while (exclusiveSemaphore.availablePermits() < 1) {

20             try {

21                 Thread.sleep(50);

22             } catch (InterruptedException e) {

23                 // TODO Auto-generated catch block

24                 e.printStackTrace();

25             }

26         }

27 

28         try {

29             sharedSemaphore.acquire();

30             System.out.println("Performing read() operation.");

31             sharedSemaphore.release();

32         } catch (InterruptedException e) {

33             // TODO Auto-generated catch block

34             e.printStackTrace();

35         }

36     }

37 

38     public void writer() {

39         while (exclusiveSemaphore.availablePermits() < 1

40                 && sharedSemaphore.availablePermits() < n) {

41             try {

42                 Thread.sleep(50);

43             } catch (InterruptedException e) {

44                 // TODO Auto-generated catch block

45                 e.printStackTrace();

46             }

47         }

48 

49         try {

50             exclusiveSemaphore.acquire();

51             System.out.println("Performing write() operation.");

52             exclusiveSemaphore.release();

53         } catch (InterruptedException e) {

54             // TODO Auto-generated catch block

55             e.printStackTrace();

56         }

57     }

58 

59     public static void main(String[] args) {

60         FooBar fooBar = new FooBar(100);

61 

62         fooBar.reader();

63         fooBar.writer();

64     }

65 }