Cancellable.java revision 13050:98f89fe2b722 
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
26package sun.nio.fs;
27
28import sun.misc.ManagedLocalsThread;
29import jdk.internal.misc.Unsafe;
30import java.util.concurrent.ExecutionException;
31
32/**
33 * Base implementation of a task (typically native) that polls a memory location
34 * during execution so that it may be aborted/cancelled before completion. The
35 * task is executed by invoking the {@link runInterruptibly} method defined
36 * here and cancelled by invoking Thread.interrupt.
37 */
38
39abstract class Cancellable implements Runnable {
40    private static final Unsafe unsafe = Unsafe.getUnsafe();
41
42    private final long pollingAddress;
43    private final Object lock = new Object();
44
45    // the following require lock when examining or changing
46    private boolean completed;
47    private Throwable exception;
48
49    protected Cancellable() {
50        pollingAddress = unsafe.allocateMemory(4);
51        unsafe.putIntVolatile(null, pollingAddress, 0);
52    }
53
54    /**
55     * Returns the memory address of a 4-byte int that should be polled to
56     * detect cancellation.
57     */
58    protected long addressToPollForCancel() {
59        return pollingAddress;
60    }
61
62    /**
63     * The value to write to the polled memory location to indicate that the
64     * task has been cancelled. If this method is not overridden then it
65     * defaults to MAX_VALUE.
66     */
67    protected int cancelValue() {
68        return Integer.MAX_VALUE;
69    }
70
71    /**
72     * "cancels" the task by writing bits into memory location that it polled
73     * by the task.
74     */
75    final void cancel() {
76        synchronized (lock) {
77            if (!completed) {
78                unsafe.putIntVolatile(null, pollingAddress, cancelValue());
79            }
80        }
81    }
82
83    /**
84     * Returns the exception thrown by the task or null if the task completed
85     * successfully.
86     */
87    private Throwable exception() {
88        synchronized (lock) {
89            return exception;
90        }
91    }
92
93    @Override
94    public final void run() {
95        try {
96            implRun();
97        } catch (Throwable t) {
98            synchronized (lock) {
99                exception = t;
100            }
101        } finally {
102            synchronized (lock) {
103                completed = true;
104                unsafe.freeMemory(pollingAddress);
105            }
106        }
107    }
108
109    /**
110     * The task body. This should periodically poll the memory location
111     * to check for cancellation.
112     */
113    abstract void implRun() throws Throwable;
114
115    /**
116     * Invokes the given task in its own thread. If this (meaning the current)
117     * thread is interrupted then an attempt is make to cancel the background
118     * thread by writing into the memory location that it polls cooperatively.
119     */
120    static void runInterruptibly(Cancellable task) throws ExecutionException {
121        Thread t = new ManagedLocalsThread(task);
122        t.start();
123        boolean cancelledByInterrupt = false;
124        while (t.isAlive()) {
125            try {
126                t.join();
127            } catch (InterruptedException e) {
128                cancelledByInterrupt = true;
129                task.cancel();
130            }
131        }
132        if (cancelledByInterrupt)
133            Thread.currentThread().interrupt();
134        Throwable exc = task.exception();
135        if (exc != null)
136            throw new ExecutionException(exc);
137    }
138}
139