《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();
     }
 }