Careercup

2014-05-10 22:30

题目链接

原题：

Design database locks to allow r/w concurrency and data consistency.

题目：设计一个支持并行读写并保证数据一致性的数据库锁。

解法：这是什么范畴的面试题？对于我这种完全没有相关项目经验的人，还真是无从下手。翻了书之后顺便重新复习了一下共享锁、互斥锁的概念。说白了，这些都是课本上的基础知识，可是毕业没多久就忘光了。看来知识不用，保质期比水果还短。然后我琢磨着锁的模型，写了个模拟的代码。这个代码和SQL毫无关系，可能也和正确答案相去甚远。姑妄言之，姑妄听之吧。

代码：

// http://www.careercup.com/question?id=6366101810184192
import java.util.concurrent.Semaphore;

public class FooBar {
    public static final int MAX_CONCURRENT_READ = 100;
    private Semaphore sharedSemaphore;
    private Semaphore exclusiveSemaphore;

    public FooBar() {
        // TODO Auto-generated constructor stub
        this.sharedSemaphore = new Semaphore(MAX_CONCURRENT_READ);
        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() < MAX_CONCURRENT_READ) {
            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();

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