Careercup

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();
    }
}