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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package sun.nio.fs;

import jdk.internal.misc.Unsafe;
import java.util.concurrent.ExecutionException;

Base implementation of a task (typically native) that polls a memory location during execution so that it may be aborted/cancelled before completion. The task is executed by invoking the runInterruptibly method defined here and cancelled by invoking Thread.interrupt. /**
 * Base implementation of a task (typically native) that polls a memory location
 * during execution so that it may be aborted/cancelled before completion. The
 * task is executed by invoking the {@link runInterruptibly} method defined
 * here and cancelled by invoking Thread.interrupt.
 */

abstract class Cancellable implements Runnable {
    private static final Unsafe unsafe = Unsafe.getUnsafe();

    private final long pollingAddress;
    private final Object lock = new Object();

    // the following require lock when examining or changing
    private boolean completed;
    private Throwable exception;

    protected Cancellable() {
        pollingAddress = unsafe.allocateMemory(4);
        unsafe.putIntVolatile(null, pollingAddress, 0);
    }

    
Returns the memory address of a 4-byte int that should be polled to
detect cancellation.
/**
     * Returns the memory address of a 4-byte int that should be polled to
     * detect cancellation.
     */
    protected long addressToPollForCancel() {
        return pollingAddress;
    }

    
The value to write to the polled memory location to indicate that the
task has been cancelled. If this method is not overridden then it
defaults to MAX_VALUE.
/**
     * The value to write to the polled memory location to indicate that the
     * task has been cancelled. If this method is not overridden then it
     * defaults to MAX_VALUE.
     */
    protected int cancelValue() {
        return Integer.MAX_VALUE;
    }

    
"cancels" the task by writing bits into memory location that it polled
by the task.
/**
     * "cancels" the task by writing bits into memory location that it polled
     * by the task.
     */
    final void cancel() {
        synchronized (lock) {
            if (!completed) {
                unsafe.putIntVolatile(null, pollingAddress, cancelValue());
            }
        }
    }

    
Returns the exception thrown by the task or null if the task completed
successfully.
/**
     * Returns the exception thrown by the task or null if the task completed
     * successfully.
     */
    private Throwable exception() {
        synchronized (lock) {
            return exception;
        }
    }

    @Override
    public final void run() {
        try {
            implRun();
        } catch (Throwable t) {
            synchronized (lock) {
                exception = t;
            }
        } finally {
            synchronized (lock) {
                completed = true;
                unsafe.freeMemory(pollingAddress);
            }
        }
    }

    
The task body. This should periodically poll the memory location
to check for cancellation.
/**
     * The task body. This should periodically poll the memory location
     * to check for cancellation.
     */
    abstract void implRun() throws Throwable;

    
Invokes the given task in its own thread. If this (meaning the current)
thread is interrupted then an attempt is make to cancel the background
thread by writing into the memory location that it polls cooperatively.
/**
     * Invokes the given task in its own thread. If this (meaning the current)
     * thread is interrupted then an attempt is make to cancel the background
     * thread by writing into the memory location that it polls cooperatively.
     */
    static void runInterruptibly(Cancellable task) throws ExecutionException {
        Thread t = new Thread(null, task, "NIO-Task", 0, false);
        t.start();
        boolean cancelledByInterrupt = false;
        while (t.isAlive()) {
            try {
                t.join();
            } catch (InterruptedException e) {
                cancelledByInterrupt = true;
                task.cancel();
            }
        }
        if (cancelledByInterrupt)
            Thread.currentThread().interrupt();
        Throwable exc = task.exception();
        if (exc != null)
            throw new ExecutionException(exc);
    }
}