--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/src/share/classes/sun/nio/fs/Cancellable.java	Sun Feb 15 12:25:54 2009 +0000
@@ -0,0 +1,137 @@
+/*
+ * Copyright 2008-2009 Sun Microsystems, Inc.  All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * 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.  Sun designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+package sun.nio.fs;
+
+import sun.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 {@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.
+     */
+    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.
+     */
+    protected int cancelValue() {
+        return Integer.MAX_VALUE;
+    }
+
+    /**
+     * "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.
+     */
+    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.
+     */
+    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.
+     */
+    static void runInterruptibly(Cancellable task) throws ExecutionException {
+        Thread t = new Thread(task);
+        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);
+    }
+}