1 import sun.misc.Unsafe;
2 
3 import java.lang.reflect.Field;
4 
5 public class Main {
6     private static Unsafe UNSAFE;
7 
main(String[] args)8     public static void main(String[] args) throws Exception {
9         setUp();
10 
11         ParkTester test = new ParkTester();
12 
13         System.out.println("Test starting");
14 
15         test.start();
16         UNSAFE.unpark(test);
17         clearStack(10);
18 
19         System.out.println("GC'ing");
20         System.gc();
21         System.runFinalization();
22         System.gc();
23 
24         System.out.println("Asking thread to park");
25         test.parkNow = true;
26 
27         try {
28             Thread.sleep(1500);
29         } catch (InterruptedException ex) {
30             // Ignore it.
31         }
32 
33         if (test.success) {
34             System.out.println("Test succeeded!");
35         } else {
36             System.out.println("Test failed.");
37         }
38     }
39 
40     /**
41      * Set up {@link #UNSAFE}.
42      */
setUp()43     public static void setUp() throws Exception{
44         /*
45          * Subvert the access check to get the unique Unsafe instance.
46          * We can do this because there's no security manager
47          * installed when running the test.
48          */
49         Field field = null;
50         try {
51             field = Unsafe.class.getDeclaredField("THE_ONE");
52         } catch (NoSuchFieldException e1) {
53             try {
54                 field = Unsafe.class.getDeclaredField("theUnsafe");
55             } catch (NoSuchFieldException e2) {
56                 throw new RuntimeException("Failed to find THE_ONE or theUnsafe");
57             }
58         }
59         field.setAccessible(true);
60         UNSAFE = (Unsafe) field.get(null);
61     }
62 
63     /**
64      * Scribbles on the stack to help ensure we don't have a fake
65      * pointer that would keep would-be garbage alive.
66      */
clearStack(int depth)67     private static void clearStack(int depth) {
68         int a = 0;
69         int b = 0;
70         int c = 0;
71         int d = 0;
72         int e = 0;
73         int f = 0;
74         int g = 0;
75         int h = 0;
76         int i = 0;
77         int j = 0;
78 
79         if (depth > 0) {
80             clearStack(depth - 1);
81         }
82     }
83 
84     private static class ParkTester extends Thread {
85         public volatile boolean parkNow = false;
86         public volatile boolean success = false;
87 
run()88         public void run() {
89             while (!parkNow) {
90                 try {
91                     Thread.sleep(500);
92                 } catch (InterruptedException ex) {
93                     // Ignore it.
94                 }
95             }
96 
97             long start = System.currentTimeMillis();
98             UNSAFE.park(false, 500 * 1000000); // 500 msec
99             long elapsed = System.currentTimeMillis() - start;
100 
101             if (elapsed > 200) {
102                 System.out.println("park()ed for " + elapsed + " msec");
103                 success = false;
104             } else {
105                 System.out.println("park() returned quickly");
106                 success = true;
107             }
108         }
109     }
110 }
111