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 }