一个有趣的多线程实验程序及其结果

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什么也不说了，看程序，然后测试一下：

1. import java.util.concurrent.atomic.*;
   
2. import static java.lang.System.*;
   
3. 
   
4. public class T1 implements Runnable {
   
5.    private static final AtomicInteger ai = new AtomicInteger();
   
6.    private static final ThreadLocal<Integer> v =
   
7.         new ThreadLocal<Integer>() {
   
8.             public Integer initialValue() {
   
9.                 return ai.getAndIncrement();
   
10.             }
    
11.         };
    
12.     private static int lastValue = -1;
    
13. 
    
14.     public T1() {
    
15.     }
    
16. 
    
17.     public void run() {
    
18.         v.get(); //initialize thread local value
    
19.         doit();
    
20.     }
    
21. 
    
22.     private synchronized void doit() {
    
23.         int x = v.get();
    
24.         if(lastValue!=-1 && lastValue != x + 1)
    
25.             out.println("last = " + lastValue + ", cur  = " + x);
    
26.         lastValue = x;
    
27. 
    
28.         //out.println(v.get() + ": start");
    
29.         try{ Thread.sleep(1000); } catch(InterruptedException ie){}
    
30.         //out.println(v.get() + ": stop");
    
31.     }
    
32. 
    
33.     public static void main(String [] args){
    
34.         Thread[] th = new Thread[1000];
    
35.         T1 t1 = new T1();
    
36. 
    
37.         for(int k=0; k<th.length; ++k) {
    
38.             th[k] = new Thread(t1);
    
39.         }
    
40. 
    
41.         for(int k=0; k<th.length; ++k) {
    
42.             th[k].start();
    
43.         }
    
44. 
    
45.     }
    
46. }

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在winxp下的某次运行结果是：

1. $ java T1
   
2. last = 0, cur  = 999
   
3. last = 999, cur  = 997
   
4. last = 997, cur  = 998
   
5. last = 998, cur  = 996
   
6. last = 890, cur  = 888
   
7. last = 888, cur  = 889
   
8. last = 889, cur  = 887
   
9. last = 817, cur  = 814
   
10. last = 814, cur  = 815
    
11. last = 815, cur  = 816
    
12. last = 816, cur  = 813
    
13. last = 805, cur  = 802
    
14. last = 802, cur  = 803
    
15. last = 803, cur  = 804
    
16. last = 804, cur  = 801
    
17. last = 793, cur  = 791
    
18. last = 791, cur  = 792
    
19. last = 792, cur  = 790
    
20. last = 727, cur  = 725
    
21. last = 725, cur  = 726
    
22. last = 726, cur  = 724
    
23. last = 672, cur  = 670
    
24. last = 670, cur  = 671
    
25. last = 671, cur  = 669
    
26. last = 659, cur  = 657
    
27. last = 657, cur  = 658
    
28. last = 658, cur  = 656
    
29. last = 532, cur  = 530
    
30. last = 530, cur  = 531
    
31. last = 531, cur  = 529
    
32. last = 479, cur  = 477
    
33. last = 477, cur  = 478
    
34. last = 478, cur  = 476
    
35. last = 398, cur  = 396
    
36. last = 396, cur  = 397
    
37. last = 397, cur  = 395
    
38. last = 329, cur  = 327
    
39. last = 327, cur  = 328
    
40. last = 328, cur  = 326
    
41. last = 321, cur  = 319
    
42. last = 319, cur  = 320
    
43. last = 320, cur  = 318
    
44. last = 254, cur  = 251
    
45. last = 251, cur  = 252
    
46. last = 252, cur  = 253
    
47. last = 253, cur  = 250
    
48. last = 215, cur  = 213
    
49. last = 213, cur  = 214
    
50. last = 214, cur  = 212
    
51. last = 201, cur  = 199
    
52. last = 199, cur  = 200
    
53. last = 200, cur  = 198
    
54. last = 198, cur  = 196
    
55. last = 196, cur  = 197
    
56. last = 197, cur  = 195
    
57. last = 171, cur  = 169
    
58. last = 169, cur  = 170
    
59. last = 170, cur  = 168
    
60. last = 93, cur  = 90
    
61. last = 90, cur  = 92
    
62. last = 91, cur  = 89
    
63. last = 63, cur  = 61
    
64. last = 61, cur  = 62
    
65. last = 62, cur  = 60
    
66. last = 42, cur  = 21
    
67. last = 21, cur  = 41
    
68. last = 22, cur  = 20
    
69. last = 12, cur  = 1
    
70. last = 1, cur  = 11

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本实验的结果具有随机性，但从统计的角度来看，Java在线程竞争和执行上有一定的bias