《Cracking the Coding Interview》——第16章：线程与锁——题目3

2014-04-27 19:26

题目：哲学家吃饭问题，死锁问题经典模型（专门用来黑哲学家的？）。

解法：死锁四条件：1. 资源互斥。2. 请求保持。3. 非抢占。4. 循环等待。所以，某砖家拿起一只筷子后如果发现没有另一只了，就必须把手里这只筷子放下，这应该是通过破坏“请求保持”原则来防止死锁产生，请求资源失败时，连自己的资源也进一步释放，然后在下一轮里继续请求，直到成功执行。

代码：

 1 // This is the class for chopsticks.

 2 import java.util.concurrent.locks.Lock;

 3 import java.util.concurrent.locks.ReentrantLock;

 4 

 5 public class Chopstick {

 6     private Lock lock;

 7     

 8     public Chopstick() {

 9         lock = new ReentrantLock();

10     }

11     

12     public boolean pickUp() {

13         return lock.tryLock();

14     }

15     

16     public void putDown() {

17         lock.unlock();

18     }

19 }

20 

21 //------------------------------------I'm a delimiter------------------------------------

22 // This is the class for philosophers.

23 import java.util.Vector;

24 

25 public class Philosopher extends Thread {

26     private Chopstick left;

27     private Chopstick right;

28     private int id;

29     int appetite;

30 

31     final int FULL_APPETITE = 10;

32 

33     public Philosopher(Chopstick left, Chopstick right, int id) {

34         // TODO Auto-generated constructor stub

35         appetite = 0;

36         this.left = left;

37         this.right = right;

38         this.id = id;

39     }

40 

41     private boolean pickUp() {

42         if (!left.pickUp()) {

43             return false;

44         }

45         if (!right.pickUp()) {

46             left.putDown();

47             return false;

48         }

49         return true;

50     }

51 

52     private void putDown() {

53         left.putDown();

54         right.putDown();

55     }

56 

57     public boolean eat() {

58         while (appetite < FULL_APPETITE) {

59             if (!pickUp()) {

60                 return false;

61             }

62             System.out.println(id + ":chew~");

63             ++appetite;

64             putDown();

65         }

66         return appetite == FULL_APPETITE;

67     }

68 

69     @Override

70     public void run() {

71         // TODO Auto-generated method stub

72         super.run();

73         while (!eat()) {

74             // Not full yet.

75         }

76     }

77 

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

79         final int n = 6;

80         Vector<Chopstick> chopsticks = new Vector<Chopstick>();

81         Vector<Philosopher> philosophers = new Vector<Philosopher>();

82 

83         for (int i = 0; i < n; ++i) {

84             chopsticks.add(new Chopstick());

85         }

86         for (int i = 0; i < n; ++i) {

87             philosophers.add(new Philosopher(chopsticks.elementAt(i),

88                     chopsticks.elementAt((i + 1) % n), i + 1));

89         }

90         

91         for (int i = 0; i < n; ++i) {

92             philosophers.elementAt(i).start();

93         }

94     }

95 }