457 lines
17 KiB
Java
457 lines
17 KiB
Java
/*
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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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* This file is available under and governed by the GNU General Public
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* License version 2 only, as published by the Free Software Foundation.
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* However, the following notice accompanied the original version of this
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* file:
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*
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* Written by Doug Lea with assistance from members of JCP JSR-166
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* Expert Group and released to the public domain, as explained at
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* http://creativecommons.org/publicdomain/zero/1.0/
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*/
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package java.util.concurrent.locks;
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/**
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* Basic thread blocking primitives for creating locks and other
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* synchronization classes.
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*
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* <p>This class associates, with each thread that uses it, a permit
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* (in the sense of the {@link java.util.concurrent.Semaphore
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* Semaphore} class). A call to {@code park} will return immediately
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* if the permit is available, consuming it in the process; otherwise
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* it <em>may</em> block. A call to {@code unpark} makes the permit
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* available, if it was not already available. (Unlike with Semaphores
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* though, permits do not accumulate. There is at most one.)
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*
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* <p>Methods {@code park} and {@code unpark} provide efficient
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* means of blocking and unblocking threads that do not encounter the
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* problems that cause the deprecated methods {@code Thread.suspend}
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* and {@code Thread.resume} to be unusable for such purposes: Races
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* between one thread invoking {@code park} and another thread trying
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* to {@code unpark} it will preserve liveness, due to the
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* permit. Additionally, {@code park} will return if the caller's
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* thread was interrupted, and timeout versions are supported. The
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* {@code park} method may also return at any other time, for "no
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* reason", so in general must be invoked within a loop that rechecks
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* conditions upon return. In this sense {@code park} serves as an
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* optimization of a "busy wait" that does not waste as much time
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* spinning, but must be paired with an {@code unpark} to be
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* effective.
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*
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* <p>The three forms of {@code park} each also support a
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* {@code blocker} object parameter. This object is recorded while
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* the thread is blocked to permit monitoring and diagnostic tools to
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* identify the reasons that threads are blocked. (Such tools may
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* access blockers using method {@link #getBlocker(Thread)}.)
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* The use of these forms rather than the original forms without this
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* parameter is strongly encouraged. The normal argument to supply as
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* a {@code blocker} within a lock implementation is {@code this}.
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*
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* <p>These methods are designed to be used as tools for creating
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* higher-level synchronization utilities, and are not in themselves
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* useful for most concurrency control applications. The {@code park}
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* method is designed for use only in constructions of the form:
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*
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* <pre> {@code
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* while (!canProceed()) { ... LockSupport.park(this); }}</pre>
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*
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* where neither {@code canProceed} nor any other actions prior to the
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* call to {@code park} entail locking or blocking. Because only one
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* permit is associated with each thread, any intermediary uses of
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* {@code park} could interfere with its intended effects.
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*
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* <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
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* non-reentrant lock class:
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* <pre> {@code
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* class FIFOMutex {
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* private final AtomicBoolean locked = new AtomicBoolean(false);
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* private final Queue<Thread> waiters
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* = new ConcurrentLinkedQueue<Thread>();
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*
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* public void lock() {
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* boolean wasInterrupted = false;
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* Thread current = Thread.currentThread();
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* waiters.add(current);
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*
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* // Block while not first in queue or cannot acquire lock
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* while (waiters.peek() != current ||
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* !locked.compareAndSet(false, true)) {
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* LockSupport.park(this);
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* if (Thread.interrupted()) // ignore interrupts while waiting
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* wasInterrupted = true;
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* }
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*
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* waiters.remove();
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* if (wasInterrupted) // reassert interrupt status on exit
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* current.interrupt();
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* }
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*
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* public void unlock() {
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* locked.set(false);
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* LockSupport.unpark(waiters.peek());
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* }
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* }}</pre>
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*/
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public class LockSupport {
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private LockSupport() {} // Cannot be instantiated.
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private static void setBlocker(Thread t, Object arg) {
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t.parkBlocker = arg;
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}
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private static final int PARK_STATE_RUNNING = 0;
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private static final int PARK_STATE_PERMIT = 1;
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private static final int PARK_STATE_PARKED = 2;
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// these native methods are all implemented in map.xml
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private static native int cmpxchgParkState(Thread t, int newValue, int comparand);
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private static native Object getParkLock(Thread t);
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private static native void setParkLock(Thread t, Object obj);
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/**
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* Makes available the permit for the given thread, if it
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* was not already available. If the thread was blocked on
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* {@code park} then it will unblock. Otherwise, its next call
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* to {@code park} is guaranteed not to block. This operation
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* is not guaranteed to have any effect at all if the given
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* thread has not been started.
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*
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* @param thread the thread to unpark, or {@code null}, in which case
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* this operation has no effect
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*/
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public static void unpark(Thread thread) {
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if (thread != null)
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{
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if (cmpxchgParkState(thread, PARK_STATE_PERMIT, PARK_STATE_RUNNING) == PARK_STATE_PARKED)
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{
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if (cmpxchgParkState(thread, PARK_STATE_RUNNING, PARK_STATE_PARKED) == PARK_STATE_PARKED)
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{
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// thread is currently blocking, so we have to release it
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Object lock = getParkLock(thread);
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synchronized (lock)
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{
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lock.notify();
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}
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}
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}
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}
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}
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private static void parkImpl(Thread currentThread, boolean deadline, long nanos)
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{
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if (cmpxchgParkState(currentThread, PARK_STATE_RUNNING, PARK_STATE_PERMIT) == PARK_STATE_PERMIT)
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{
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// we consumed a permit
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return;
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}
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Object lock = getParkLock(currentThread);
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if (lock == null)
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{
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// we lazily allocate the lock object
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lock = new Object();
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setParkLock(currentThread, lock);
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}
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synchronized (lock)
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{
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if (cmpxchgParkState(currentThread, PARK_STATE_PARKED, PARK_STATE_RUNNING) == PARK_STATE_PERMIT)
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{
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// entering the parked state failed because we got a permit after the previous permit test
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// release the permit and return
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cmpxchgParkState(currentThread, PARK_STATE_RUNNING, PARK_STATE_PERMIT);
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return;
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}
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if (deadline)
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{
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nanos -= System.currentTimeMillis() * 1000000;
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}
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if (nanos >= 0)
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{
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try
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{
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lock.wait(nanos / 1000000, (int)(nanos % 1000000));
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}
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catch (InterruptedException _)
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{
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currentThread.interrupt();
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}
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}
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cmpxchgParkState(currentThread, PARK_STATE_RUNNING, PARK_STATE_PARKED);
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}
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}
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/**
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* Disables the current thread for thread scheduling purposes unless the
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* permit is available.
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*
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* <p>If the permit is available then it is consumed and the call returns
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* immediately; otherwise
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* the current thread becomes disabled for thread scheduling
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* purposes and lies dormant until one of three things happens:
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*
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* <ul>
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* <li>Some other thread invokes {@link #unpark unpark} with the
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* current thread as the target; or
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*
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread; or
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*
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* <li>The call spuriously (that is, for no reason) returns.
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* </ul>
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*
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* <p>This method does <em>not</em> report which of these caused the
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* method to return. Callers should re-check the conditions which caused
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* the thread to park in the first place. Callers may also determine,
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* for example, the interrupt status of the thread upon return.
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*
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* @param blocker the synchronization object responsible for this
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* thread parking
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* @since 1.6
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*/
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public static void park(Object blocker) {
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Thread t = Thread.currentThread();
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setBlocker(t, blocker);
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parkImpl(t, false, 0L);
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setBlocker(t, null);
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}
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/**
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* Disables the current thread for thread scheduling purposes, for up to
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* the specified waiting time, unless the permit is available.
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*
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* <p>If the permit is available then it is consumed and the call
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* returns immediately; otherwise the current thread becomes disabled
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* for thread scheduling purposes and lies dormant until one of four
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* things happens:
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*
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* <ul>
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* <li>Some other thread invokes {@link #unpark unpark} with the
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* current thread as the target; or
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*
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread; or
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*
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* <li>The specified waiting time elapses; or
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*
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* <li>The call spuriously (that is, for no reason) returns.
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* </ul>
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*
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* <p>This method does <em>not</em> report which of these caused the
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* method to return. Callers should re-check the conditions which caused
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* the thread to park in the first place. Callers may also determine,
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* for example, the interrupt status of the thread, or the elapsed time
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* upon return.
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*
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* @param blocker the synchronization object responsible for this
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* thread parking
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* @param nanos the maximum number of nanoseconds to wait
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* @since 1.6
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*/
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public static void parkNanos(Object blocker, long nanos) {
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if (nanos > 0) {
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Thread t = Thread.currentThread();
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setBlocker(t, blocker);
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parkImpl(t, false, nanos);
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setBlocker(t, null);
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}
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}
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/**
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* Disables the current thread for thread scheduling purposes, until
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* the specified deadline, unless the permit is available.
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*
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* <p>If the permit is available then it is consumed and the call
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* returns immediately; otherwise the current thread becomes disabled
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* for thread scheduling purposes and lies dormant until one of four
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* things happens:
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*
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* <ul>
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* <li>Some other thread invokes {@link #unpark unpark} with the
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* current thread as the target; or
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*
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* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
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* current thread; or
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*
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* <li>The specified deadline passes; or
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*
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* <li>The call spuriously (that is, for no reason) returns.
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* </ul>
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*
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* <p>This method does <em>not</em> report which of these caused the
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* method to return. Callers should re-check the conditions which caused
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* the thread to park in the first place. Callers may also determine,
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* for example, the interrupt status of the thread, or the current time
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* upon return.
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*
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* @param blocker the synchronization object responsible for this
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* thread parking
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* @param deadline the absolute time, in milliseconds from the Epoch,
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* to wait until
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* @since 1.6
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*/
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public static void parkUntil(Object blocker, long deadline) {
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Thread t = Thread.currentThread();
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setBlocker(t, blocker);
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parkImpl(t, true, deadline * 1000000);
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setBlocker(t, null);
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}
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/**
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* Returns the blocker object supplied to the most recent
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* invocation of a park method that has not yet unblocked, or null
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* if not blocked. The value returned is just a momentary
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* snapshot -- the thread may have since unblocked or blocked on a
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* different blocker object.
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*
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* @param t the thread
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* @return the blocker
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* @throws NullPointerException if argument is null
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* @since 1.6
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*/
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public static Object getBlocker(Thread t) {
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if (t == null)
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throw new NullPointerException();
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return t.parkBlocker;
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}
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/**
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* Disables the current thread for thread scheduling purposes unless the
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* permit is available.
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*
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* <p>If the permit is available then it is consumed and the call
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* returns immediately; otherwise the current thread becomes disabled
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* for thread scheduling purposes and lies dormant until one of three
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* things happens:
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*
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* <ul>
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*
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* <li>Some other thread invokes {@link #unpark unpark} with the
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* current thread as the target; or
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*
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread; or
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*
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* <li>The call spuriously (that is, for no reason) returns.
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* </ul>
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*
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* <p>This method does <em>not</em> report which of these caused the
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* method to return. Callers should re-check the conditions which caused
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* the thread to park in the first place. Callers may also determine,
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* for example, the interrupt status of the thread upon return.
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*/
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public static void park() {
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parkImpl(Thread.currentThread(), false, 0L);
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}
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/**
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* Disables the current thread for thread scheduling purposes, for up to
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* the specified waiting time, unless the permit is available.
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*
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* <p>If the permit is available then it is consumed and the call
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* returns immediately; otherwise the current thread becomes disabled
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* for thread scheduling purposes and lies dormant until one of four
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* things happens:
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*
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* <ul>
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* <li>Some other thread invokes {@link #unpark unpark} with the
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* current thread as the target; or
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*
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread; or
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*
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* <li>The specified waiting time elapses; or
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*
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* <li>The call spuriously (that is, for no reason) returns.
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* </ul>
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*
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* <p>This method does <em>not</em> report which of these caused the
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* method to return. Callers should re-check the conditions which caused
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* the thread to park in the first place. Callers may also determine,
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* for example, the interrupt status of the thread, or the elapsed time
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* upon return.
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*
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* @param nanos the maximum number of nanoseconds to wait
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*/
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public static void parkNanos(long nanos) {
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if (nanos > 0)
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parkImpl(Thread.currentThread(), false, nanos);
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}
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/**
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* Disables the current thread for thread scheduling purposes, until
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* the specified deadline, unless the permit is available.
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*
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* <p>If the permit is available then it is consumed and the call
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* returns immediately; otherwise the current thread becomes disabled
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* for thread scheduling purposes and lies dormant until one of four
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* things happens:
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*
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* <ul>
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* <li>Some other thread invokes {@link #unpark unpark} with the
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* current thread as the target; or
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*
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread; or
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*
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* <li>The specified deadline passes; or
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*
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* <li>The call spuriously (that is, for no reason) returns.
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* </ul>
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*
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* <p>This method does <em>not</em> report which of these caused the
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* method to return. Callers should re-check the conditions which caused
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* the thread to park in the first place. Callers may also determine,
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* for example, the interrupt status of the thread, or the current time
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* upon return.
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*
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* @param deadline the absolute time, in milliseconds from the Epoch,
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* to wait until
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*/
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public static void parkUntil(long deadline) {
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parkImpl(Thread.currentThread(), true, deadline * 1000000);
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}
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/**
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* Returns the pseudo-randomly initialized or updated secondary seed.
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* Copied from ThreadLocalRandom due to package access restrictions.
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*/
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static final int nextSecondarySeed() {
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int r;
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Thread t = Thread.currentThread();
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if ((r = t.threadLocalRandomSecondarySeed) != 0) {
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r ^= r << 13; // xorshift
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r ^= r >>> 17;
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r ^= r << 5;
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}
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else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0)
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r = 1; // avoid zero
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t.threadLocalRandomSecondarySeed = r;
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return r;
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}
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}
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