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12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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15  * accompanied this code).
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24  */
25 
26 package sun.nio.fs;
27 
28 import sun.misc.Unsafe;
29 import java.util.concurrent.ExecutionException;
30 
31 /**
32  * Base implementation of a task (typically native) that polls a memory location
33  * during execution so that it may be aborted/cancelled before completion. The
34  * task is executed by invoking the {@link runInterruptibly} method defined
35  * here and cancelled by invoking Thread.interrupt.
36  */
37 
38 abstract class Cancellable implements Runnable {
39     private static final Unsafe unsafe = Unsafe.getUnsafe();
40 
41     private final long pollingAddress;
42     private final Object lock = new Object();
43 
44     // the following require lock when examining or changing
45     private boolean completed;
46     private Throwable exception;
47 
Cancellable()48     protected Cancellable() {
49         pollingAddress = unsafe.allocateMemory(4);
50         unsafe.putIntVolatile(null, pollingAddress, 0);
51     }
52 
53     /**
54      * Returns the memory address of a 4-byte int that should be polled to
55      * detect cancellation.
56      */
addressToPollForCancel()57     protected long addressToPollForCancel() {
58         return pollingAddress;
59     }
60 
61     /**
62      * The value to write to the polled memory location to indicate that the
63      * task has been cancelled. If this method is not overridden then it
64      * defaults to MAX_VALUE.
65      */
cancelValue()66     protected int cancelValue() {
67         return Integer.MAX_VALUE;
68     }
69 
70     /**
71      * "cancels" the task by writing bits into memory location that it polled
72      * by the task.
73      */
cancel()74     final void cancel() {
75         synchronized (lock) {
76             if (!completed) {
77                 unsafe.putIntVolatile(null, pollingAddress, cancelValue());
78             }
79         }
80     }
81 
82     /**
83      * Returns the exception thrown by the task or null if the task completed
84      * successfully.
85      */
exception()86     private Throwable exception() {
87         synchronized (lock) {
88             return exception;
89         }
90     }
91 
92     @Override
run()93     public final void run() {
94         try {
95             implRun();
96         } catch (Throwable t) {
97             synchronized (lock) {
98                 exception = t;
99             }
100         } finally {
101             synchronized (lock) {
102                 completed = true;
103                 unsafe.freeMemory(pollingAddress);
104             }
105         }
106     }
107 
108     /**
109      * The task body. This should periodically poll the memory location
110      * to check for cancellation.
111      */
implRun()112     abstract void implRun() throws Throwable;
113 
114     /**
115      * Invokes the given task in its own thread. If this (meaning the current)
116      * thread is interrupted then an attempt is make to cancel the background
117      * thread by writing into the memory location that it polls cooperatively.
118      */
runInterruptibly(Cancellable task)119     static void runInterruptibly(Cancellable task) throws ExecutionException {
120         Thread t = new Thread(task);
121         t.start();
122         boolean cancelledByInterrupt = false;
123         while (t.isAlive()) {
124             try {
125                 t.join();
126             } catch (InterruptedException e) {
127                 cancelledByInterrupt = true;
128                 task.cancel();
129             }
130         }
131         if (cancelledByInterrupt)
132             Thread.currentThread().interrupt();
133         Throwable exc = task.exception();
134         if (exc != null)
135             throw new ExecutionException(exc);
136     }
137 }
138