《Cracking the Coding Interview》——第16章：线程与锁——题目5

2014-04-27 20:16

题目：假设一个类Foo有三个公有的成员方法first()、second()、third()。请用锁的方法来控制调用行为，使得他们的执行循序总是遵从first、second、third的顺序。

解法：你应该想到了用lock的方法类阻塞，不过这里面有个概念问题使得直接用ReentrantLock不能通过编译（对于一个锁对象，不同在A线程中锁定，又在B线程中解锁，不允许这样的归属关系），可以用Semaphore来达到相同的目的。请看下面的代码。

代码：

// 16.5 There're three methods in a class FooBar, how would you make sure that they're executed in a fixed order, in whichever order they're called?
public class FirstRun implements Runnable {
    private FooBar fooBar;
    
    public FirstRun(FooBar fooBar) {
        // TODO Auto-generated constructor stub
        this.fooBar = fooBar;
    }
    
    @Override
    public void run() {
        // TODO Auto-generated method stub
        fooBar.first();
    }
}

// -----------------------------------------------------------------------------
public class SecondRun implements Runnable {
    private FooBar fooBar;
    
    public SecondRun(FooBar fooBar) {
        // TODO Auto-generated constructor stub
        this.fooBar = fooBar;
    }
    
    @Override
    public void run() {
        // TODO Auto-generated method stub
        fooBar.second();
    }
}

// -----------------------------------------------------------------------------
public class ThirdRun implements Runnable {
    private FooBar fooBar;
    
    public ThirdRun(FooBar fooBar) {
        // TODO Auto-generated constructor stub
        this.fooBar = fooBar;
    }
    
    @Override
    public void run() {
        // TODO Auto-generated method stub
        fooBar.third();
    }
}

// -----------------------------------------------------------------------------
import java.util.concurrent.Semaphore;

public class FooBar {
    private Semaphore sem1;
    private Semaphore sem2;
    private Semaphore sem3;
    
    public FooBar() {
        // TODO Auto-generated constructor stub
        sem1 = new Semaphore(1);
        sem2 = new Semaphore(1);
        sem3 = new Semaphore(1);
        
        try {
            sem1.acquire();
            sem2.acquire();
            sem3.acquire();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public void first() {
        System.out.println("first");
        
        sem1.release();
    }

    public void second() {
        try {
            sem1.acquire();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        sem1.release();
        System.out.println("second");
        sem2.release();
    }

    public void third() {
        try {
            sem2.acquire();
        } catch (InterruptedException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        sem2.release();
        System.out.println("third");
        sem3.release();
    }

    public static void main(String[] args) {
        FooBar fooBar = new FooBar();
        Thread t1 = new Thread(new FirstRun(fooBar));
        Thread t2 = new Thread(new SecondRun(fooBar));
        Thread t3 = new Thread(new ThirdRun(fooBar));
        
        t3.start();
        t1.start();
        t2.start();
    }
}