1 /*
2 * Copyright (c) 2008, 2009, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package sun.nio.fs;
27
28 import jdk.internal.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
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 */
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 */
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 */
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 */
86 private Throwable exception() {
87 synchronized (lock) {
88 return exception;
89 }
90 }
91
92 @Override
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 */
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 */
119 static void runInterruptibly(Cancellable task) throws ExecutionException {
120 Thread t = new Thread(null, task, "NIO-Task", 0, false);
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 }
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