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/*
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* Copyright (c) 2001, 2002, Oracle and/or its affiliates. All rights reserved.
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* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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*
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* This code is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License version 2 only, as
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* published by the Free Software Foundation. Oracle designates this
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* particular file as subject to the "Classpath" exception as provided
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* by Oracle in the LICENSE file that accompanied this code.
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*
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* This code is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* version 2 for more details (a copy is included in the LICENSE file that
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* accompanied this code).
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*
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* You should have received a copy of the GNU General Public License version
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* 2 along with this work; if not, write to the Free Software Foundation,
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* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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*
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* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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* or visit www.oracle.com if you need additional information or have any
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* questions.
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*/
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/*
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File: Sync.java
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Originally written by Doug Lea and released into the public domain.
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This may be used for any purposes whatsoever without acknowledgment.
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Thanks for the assistance and support of Sun Microsystems Labs,
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and everyone contributing, testing, and using this code.
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History:
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Date Who What
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11Jun1998 dl Create public version
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5Aug1998 dl Added some convenient time constants
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*/
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package com.sun.corba.se.impl.orbutil.concurrent;
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/**
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* Main interface for locks, gates, and conditions.
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* <p>
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* Sync objects isolate waiting and notification for particular
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* logical states, resource availability, events, and the like that are
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* shared across multiple threads. Use of Syncs sometimes
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* (but by no means always) adds flexibility and efficiency
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* compared to the use of plain java monitor methods
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* and locking, and are sometimes (but by no means always)
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* simpler to program with.
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* <p>
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*
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* Most Syncs are intended to be used primarily (although
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* not exclusively) in before/after constructions such as:
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* <pre>
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* class X {
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* Sync gate;
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* // ...
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*
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* public void m() {
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* try {
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* gate.acquire(); // block until condition holds
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* try {
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* // ... method body
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* }
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* finally {
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* gate.release()
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* }
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* }
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* catch (InterruptedException ex) {
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* // ... evasive action
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* }
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* }
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*
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* public void m2(Sync cond) { // use supplied condition
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* try {
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* if (cond.attempt(10)) { // try the condition for 10 ms
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* try {
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* // ... method body
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* }
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* finally {
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* cond.release()
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* }
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* }
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* }
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* catch (InterruptedException ex) {
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* // ... evasive action
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* }
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* }
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* }
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* </pre>
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* Syncs may be used in somewhat tedious but more flexible replacements
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* for built-in Java synchronized blocks. For example:
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* <pre>
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* class HandSynched {
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* private double state_ = 0.0;
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* private final Sync lock; // use lock type supplied in constructor
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* public HandSynched(Sync l) { lock = l; }
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*
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* public void changeState(double d) {
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* try {
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* lock.acquire();
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* try { state_ = updateFunction(d); }
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* finally { lock.release(); }
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* }
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* catch(InterruptedException ex) { }
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* }
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*
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* public double getState() {
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* double d = 0.0;
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* try {
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* lock.acquire();
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* try { d = accessFunction(state_); }
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* finally { lock.release(); }
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* }
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* catch(InterruptedException ex){}
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* return d;
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* }
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* private double updateFunction(double d) { ... }
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* private double accessFunction(double d) { ... }
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* }
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* </pre>
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* If you have a lot of such methods, and they take a common
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* form, you can standardize this using wrappers. Some of these
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* wrappers are standardized in LockedExecutor, but you can make others.
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* For example:
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* <pre>
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* class HandSynchedV2 {
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* private double state_ = 0.0;
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* private final Sync lock; // use lock type supplied in constructor
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* public HandSynchedV2(Sync l) { lock = l; }
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*
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* protected void runSafely(Runnable r) {
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* try {
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* lock.acquire();
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* try { r.run(); }
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* finally { lock.release(); }
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* }
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* catch (InterruptedException ex) { // propagate without throwing
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* Thread.currentThread().interrupt();
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* }
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* }
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*
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* public void changeState(double d) {
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* runSafely(new Runnable() {
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* public void run() { state_ = updateFunction(d); }
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* });
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* }
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* // ...
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* }
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* </pre>
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* <p>
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* One reason to bother with such constructions is to use deadlock-
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* avoiding back-offs when dealing with locks involving multiple objects.
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* For example, here is a Cell class that uses attempt to back-off
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* and retry if two Cells are trying to swap values with each other
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* at the same time.
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* <pre>
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* class Cell {
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* long value;
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* Sync lock = ... // some sync implementation class
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* void swapValue(Cell other) {
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* for (;;) {
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* try {
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* lock.acquire();
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* try {
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* if (other.lock.attempt(100)) {
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* try {
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* long t = value;
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* value = other.value;
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* other.value = t;
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* return;
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* }
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* finally { other.lock.release(); }
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* }
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* }
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* finally { lock.release(); }
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* }
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* catch (InterruptedException ex) { return; }
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* }
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* }
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* }
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*</pre>
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* <p>
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* Here is an even fancier version, that uses lock re-ordering
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* upon conflict:
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* <pre>
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* class Cell {
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* long value;
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* Sync lock = ...;
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* private static boolean trySwap(Cell a, Cell b) {
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* a.lock.acquire();
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* try {
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* if (!b.lock.attempt(0))
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* return false;
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* try {
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* long t = a.value;
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* a.value = b.value;
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* b.value = t;
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* return true;
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* }
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* finally { other.lock.release(); }
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* }
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* finally { lock.release(); }
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* return false;
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* }
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*
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* void swapValue(Cell other) {
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* try {
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* while (!trySwap(this, other) &&
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* !tryswap(other, this))
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* Thread.sleep(1);
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* }
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* catch (InterruptedException ex) { return; }
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* }
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*}
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*</pre>
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* <p>
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* Interruptions are in general handled as early as possible.
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* Normally, InterruptionExceptions are thrown
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* in acquire and attempt(msec) if interruption
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* is detected upon entry to the method, as well as in any
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* later context surrounding waits.
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* However, interruption status is ignored in release();
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* <p>
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* Timed versions of attempt report failure via return value.
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* If so desired, you can transform such constructions to use exception
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* throws via
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* <pre>
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* if (!c.attempt(timeval)) throw new TimeoutException(timeval);
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* </pre>
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* <p>
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* The TimoutSync wrapper class can be used to automate such usages.
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* <p>
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* All time values are expressed in milliseconds as longs, which have a maximum
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* value of Long.MAX_VALUE, or almost 300,000 centuries. It is not
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* known whether JVMs actually deal correctly with such extreme values.
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* For convenience, some useful time values are defined as static constants.
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* <p>
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* All implementations of the three Sync methods guarantee to
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* somehow employ Java <code>synchronized</code> methods or blocks,
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* and so entail the memory operations described in JLS
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* chapter 17 which ensure that variables are loaded and flushed
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* within before/after constructions.
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* <p>
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* Syncs may also be used in spinlock constructions. Although
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* it is normally best to just use acquire(), various forms
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* of busy waits can be implemented. For a simple example
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* (but one that would probably never be preferable to using acquire()):
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* <pre>
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* class X {
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* Sync lock = ...
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* void spinUntilAcquired() throws InterruptedException {
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* // Two phase.
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* // First spin without pausing.
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* int purespins = 10;
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* for (int i = 0; i < purespins; ++i) {
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* if (lock.attempt(0))
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* return true;
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* }
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* // Second phase - use timed waits
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* long waitTime = 1; // 1 millisecond
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* for (;;) {
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* if (lock.attempt(waitTime))
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* return true;
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* else
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* waitTime = waitTime * 3 / 2 + 1; // increase 50%
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* }
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* }
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* }
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* </pre>
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* <p>
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* In addition pure synchronization control, Syncs
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* may be useful in any context requiring before/after methods.
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* For example, you can use an ObservableSync
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* (perhaps as part of a LayeredSync) in order to obtain callbacks
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* before and after each method invocation for a given class.
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*
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* <p>[<a href="http://gee.cs.oswego.edu/dl/classes/EDU/oswego/cs/dl/util/concurrent/intro.html"> Introduction to this package. </a>]
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**/
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public interface Sync {
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/**
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* Wait (possibly forever) until successful passage.
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* Fail only upon interuption. Interruptions always result in
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* `clean' failures. On failure, you can be sure that it has not
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* been acquired, and that no
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* corresponding release should be performed. Conversely,
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* a normal return guarantees that the acquire was successful.
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**/
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public void acquire() throws InterruptedException;
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/**
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* Wait at most msecs to pass; report whether passed.
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* <p>
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* The method has best-effort semantics:
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* The msecs bound cannot
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* be guaranteed to be a precise upper bound on wait time in Java.
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* Implementations generally can only attempt to return as soon as possible
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* after the specified bound. Also, timers in Java do not stop during garbage
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* collection, so timeouts can occur just because a GC intervened.
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* So, msecs arguments should be used in
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* a coarse-grained manner. Further,
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* implementations cannot always guarantee that this method
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* will return at all without blocking indefinitely when used in
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* unintended ways. For example, deadlocks may be encountered
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* when called in an unintended context.
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* <p>
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* @param msecs the number of milleseconds to wait.
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* An argument less than or equal to zero means not to wait at all.
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* However, this may still require
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* access to a synchronization lock, which can impose unbounded
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* delay if there is a lot of contention among threads.
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* @return true if acquired
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**/
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public boolean attempt(long msecs) throws InterruptedException;
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/**
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* Potentially enable others to pass.
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* <p>
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* Because release does not raise exceptions,
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* it can be used in `finally' clauses without requiring extra
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* embedded try/catch blocks. But keep in mind that
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* as with any java method, implementations may
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* still throw unchecked exceptions such as Error or NullPointerException
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* when faced with uncontinuable errors. However, these should normally
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* only be caught by higher-level error handlers.
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**/
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public void release();
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/** One second, in milliseconds; convenient as a time-out value **/
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public static final long ONE_SECOND = 1000;
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/** One minute, in milliseconds; convenient as a time-out value **/
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public static final long ONE_MINUTE = 60 * ONE_SECOND;
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/** One hour, in milliseconds; convenient as a time-out value **/
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public static final long ONE_HOUR = 60 * ONE_MINUTE;
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/** One day, in milliseconds; convenient as a time-out value **/
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public static final long ONE_DAY = 24 * ONE_HOUR;
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/** One week, in milliseconds; convenient as a time-out value **/
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public static final long ONE_WEEK = 7 * ONE_DAY;
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/** One year in milliseconds; convenient as a time-out value **/
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// Not that it matters, but there is some variation across
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// standard sources about value at msec precision.
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// The value used is the same as in java.util.GregorianCalendar
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public static final long ONE_YEAR = (long)(365.2425 * ONE_DAY);
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/** One century in milliseconds; convenient as a time-out value **/
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public static final long ONE_CENTURY = 100 * ONE_YEAR;
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}
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