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 sun.misc.ManagedLocalsThread;
  29 import sun.misc.Unsafe;
  30 import 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 
  39 abstract 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 }