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.Unsafe;
29import 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
38abstract 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(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