《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：

  1 // 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?

  2 public class FirstRun implements Runnable {

  3     private FooBar fooBar;

  4     

  5     public FirstRun(FooBar fooBar) {

  6         // TODO Auto-generated constructor stub

  7         this.fooBar = fooBar;

  8     }

  9     

 10     @Override

 11     public void run() {

 12         // TODO Auto-generated method stub

 13         fooBar.first();

 14     }

 15 }

 16 

 17 // -----------------------------------------------------------------------------

 18 public class SecondRun implements Runnable {

 19     private FooBar fooBar;

 20     

 21     public SecondRun(FooBar fooBar) {

 22         // TODO Auto-generated constructor stub

 23         this.fooBar = fooBar;

 24     }

 25     

 26     @Override

 27     public void run() {

 28         // TODO Auto-generated method stub

 29         fooBar.second();

 30     }

 31 }

 32 

 33 // -----------------------------------------------------------------------------

 34 public class ThirdRun implements Runnable {

 35     private FooBar fooBar;

 36     

 37     public ThirdRun(FooBar fooBar) {

 38         // TODO Auto-generated constructor stub

 39         this.fooBar = fooBar;

 40     }

 41     

 42     @Override

 43     public void run() {

 44         // TODO Auto-generated method stub

 45         fooBar.third();

 46     }

 47 }

 48 

 49 // -----------------------------------------------------------------------------

 50 import java.util.concurrent.Semaphore;

 51 

 52 public class FooBar {

 53     private Semaphore sem1;

 54     private Semaphore sem2;

 55     private Semaphore sem3;

 56     

 57     public FooBar() {

 58         // TODO Auto-generated constructor stub

 59         sem1 = new Semaphore(1);

 60         sem2 = new Semaphore(1);

 61         sem3 = new Semaphore(1);

 62         

 63         try {

 64             sem1.acquire();

 65             sem2.acquire();

 66             sem3.acquire();

 67         } catch (InterruptedException e) {

 68             // TODO Auto-generated catch block

 69             e.printStackTrace();

 70         }

 71     }

 72 

 73     public void first() {

 74         System.out.println("first");

 75         

 76         sem1.release();

 77     }

 78 

 79     public void second() {

 80         try {

 81             sem1.acquire();

 82         } catch (InterruptedException e) {

 83             // TODO Auto-generated catch block

 84             e.printStackTrace();

 85         }

 86         sem1.release();

 87         System.out.println("second");

 88         sem2.release();

 89     }

 90 

 91     public void third() {

 92         try {

 93             sem2.acquire();

 94         } catch (InterruptedException e) {

 95             // TODO Auto-generated catch block

 96             e.printStackTrace();

 97         }

 98         sem2.release();

 99         System.out.println("third");

100         sem3.release();

101     }

102 

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

104         FooBar fooBar = new FooBar();

105         Thread t1 = new Thread(new FirstRun(fooBar));

106         Thread t2 = new Thread(new SecondRun(fooBar));

107         Thread t3 = new Thread(new ThirdRun(fooBar));

108         

109         t3.start();

110         t1.start();

111         t2.start();

112     }

113 }