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/*
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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;
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import java.util.concurrent.locks.*;
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/**
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* A synchronization aid that allows a set of threads to all wait for
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* each other to reach a common barrier point. CyclicBarriers are
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* useful in programs involving a fixed sized party of threads that
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* must occasionally wait for each other. The barrier is called
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* <em>cyclic</em> because it can be re-used after the waiting threads
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* are released.
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*
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* <p>A <tt>CyclicBarrier</tt> supports an optional {@link Runnable} command
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* that is run once per barrier point, after the last thread in the party
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* arrives, but before any threads are released.
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* This <em>barrier action</em> is useful
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* for updating shared-state before any of the parties continue.
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*
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* <p><b>Sample usage:</b> Here is an example of
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* using a barrier in a parallel decomposition design:
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* <pre>
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* class Solver {
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* final int N;
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* final float[][] data;
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* final CyclicBarrier barrier;
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*
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* class Worker implements Runnable {
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* int myRow;
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* Worker(int row) { myRow = row; }
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* public void run() {
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* while (!done()) {
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* processRow(myRow);
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*
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* try {
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* barrier.await();
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* } catch (InterruptedException ex) {
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* return;
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* } catch (BrokenBarrierException ex) {
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* return;
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* }
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* }
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* }
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* }
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*
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* public Solver(float[][] matrix) {
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* data = matrix;
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* N = matrix.length;
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* barrier = new CyclicBarrier(N,
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* new Runnable() {
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* public void run() {
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* mergeRows(...);
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* }
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* });
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* for (int i = 0; i < N; ++i)
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* new Thread(new Worker(i)).start();
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*
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* waitUntilDone();
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* }
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* }
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* </pre>
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* Here, each worker thread processes a row of the matrix then waits at the
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* barrier until all rows have been processed. When all rows are processed
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* the supplied {@link Runnable} barrier action is executed and merges the
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* rows. If the merger
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* determines that a solution has been found then <tt>done()</tt> will return
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* <tt>true</tt> and each worker will terminate.
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*
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* <p>If the barrier action does not rely on the parties being suspended when
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* it is executed, then any of the threads in the party could execute that
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* action when it is released. To facilitate this, each invocation of
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* {@link #await} returns the arrival index of that thread at the barrier.
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* You can then choose which thread should execute the barrier action, for
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* example:
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* <pre> if (barrier.await() == 0) {
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* // log the completion of this iteration
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* }</pre>
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*
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* <p>The <tt>CyclicBarrier</tt> uses an all-or-none breakage model
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* for failed synchronization attempts: If a thread leaves a barrier
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* point prematurely because of interruption, failure, or timeout, all
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* other threads waiting at that barrier point will also leave
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* abnormally via {@link BrokenBarrierException} (or
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* {@link InterruptedException} if they too were interrupted at about
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* the same time).
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*
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* <p>Memory consistency effects: Actions in a thread prior to calling
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* {@code await()}
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* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
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* actions that are part of the barrier action, which in turn
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* <i>happen-before</i> actions following a successful return from the
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* corresponding {@code await()} in other threads.
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*
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* @since 1.5
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* @see CountDownLatch
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*
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* @author Doug Lea
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*/
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public class CyclicBarrier {
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/**
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* Each use of the barrier is represented as a generation instance.
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* The generation changes whenever the barrier is tripped, or
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* is reset. There can be many generations associated with threads
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* using the barrier - due to the non-deterministic way the lock
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* may be allocated to waiting threads - but only one of these
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* can be active at a time (the one to which <tt>count</tt> applies)
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* and all the rest are either broken or tripped.
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* There need not be an active generation if there has been a break
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* but no subsequent reset.
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*/
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private static class Generation {
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boolean broken = false;
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}
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/** The lock for guarding barrier entry */
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private final ReentrantLock lock = new ReentrantLock();
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/** Condition to wait on until tripped */
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private final Condition trip = lock.newCondition();
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/** The number of parties */
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private final int parties;
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/* The command to run when tripped */
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private final Runnable barrierCommand;
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/** The current generation */
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private Generation generation = new Generation();
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/**
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* Number of parties still waiting. Counts down from parties to 0
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* on each generation. It is reset to parties on each new
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* generation or when broken.
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*/
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private int count;
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/**
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* Updates state on barrier trip and wakes up everyone.
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* Called only while holding lock.
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*/
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private void nextGeneration() {
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// signal completion of last generation
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trip.signalAll();
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// set up next generation
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count = parties;
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generation = new Generation();
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}
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/**
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* Sets current barrier generation as broken and wakes up everyone.
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* Called only while holding lock.
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*/
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private void breakBarrier() {
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generation.broken = true;
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count = parties;
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trip.signalAll();
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}
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/**
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* Main barrier code, covering the various policies.
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*/
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private int dowait(boolean timed, long nanos)
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throws InterruptedException, BrokenBarrierException,
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TimeoutException {
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final ReentrantLock lock = this.lock;
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lock.lock();
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try {
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final Generation g = generation;
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if (g.broken)
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throw new BrokenBarrierException();
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if (Thread.interrupted()) {
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breakBarrier();
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throw new InterruptedException();
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}
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int index = --count;
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if (index == 0) { // tripped
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boolean ranAction = false;
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try {
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final Runnable command = barrierCommand;
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if (command != null)
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command.run();
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ranAction = true;
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nextGeneration();
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return 0;
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} finally {
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if (!ranAction)
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breakBarrier();
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}
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}
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// loop until tripped, broken, interrupted, or timed out
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for (;;) {
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try {
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if (!timed)
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trip.await();
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else if (nanos > 0L)
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nanos = trip.awaitNanos(nanos);
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} catch (InterruptedException ie) {
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if (g == generation && ! g.broken) {
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breakBarrier();
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throw ie;
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} else {
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// We're about to finish waiting even if we had not
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// been interrupted, so this interrupt is deemed to
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// "belong" to subsequent execution.
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Thread.currentThread().interrupt();
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}
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}
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if (g.broken)
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throw new BrokenBarrierException();
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if (g != generation)
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return index;
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if (timed && nanos <= 0L) {
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breakBarrier();
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throw new TimeoutException();
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}
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}
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} finally {
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lock.unlock();
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}
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}
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/**
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* Creates a new <tt>CyclicBarrier</tt> that will trip when the
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* given number of parties (threads) are waiting upon it, and which
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* will execute the given barrier action when the barrier is tripped,
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* performed by the last thread entering the barrier.
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*
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* @param parties the number of threads that must invoke {@link #await}
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* before the barrier is tripped
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* @param barrierAction the command to execute when the barrier is
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* tripped, or {@code null} if there is no action
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* @throws IllegalArgumentException if {@code parties} is less than 1
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*/
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public CyclicBarrier(int parties, Runnable barrierAction) {
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if (parties <= 0) throw new IllegalArgumentException();
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this.parties = parties;
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this.count = parties;
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this.barrierCommand = barrierAction;
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}
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/**
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* Creates a new <tt>CyclicBarrier</tt> that will trip when the
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* given number of parties (threads) are waiting upon it, and
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* does not perform a predefined action when the barrier is tripped.
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*
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* @param parties the number of threads that must invoke {@link #await}
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* before the barrier is tripped
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* @throws IllegalArgumentException if {@code parties} is less than 1
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*/
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public CyclicBarrier(int parties) {
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this(parties, null);
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}
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/**
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* Returns the number of parties required to trip this barrier.
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*
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* @return the number of parties required to trip this barrier
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*/
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public int getParties() {
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return parties;
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}
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/**
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* Waits until all {@linkplain #getParties parties} have invoked
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* <tt>await</tt> on this barrier.
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*
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* <p>If the current thread is not the last to arrive then it is
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* disabled for thread scheduling purposes and lies dormant until
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* one of the following things happens:
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* <ul>
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* <li>The last thread arrives; or
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* the current thread; or
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* <li>Some other thread {@linkplain Thread#interrupt interrupts}
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* one of the other waiting threads; or
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312 |
* <li>Some other thread times out while waiting for barrier; or
|
jaroslav@1890
|
313 |
* <li>Some other thread invokes {@link #reset} on this barrier.
|
jaroslav@1890
|
314 |
* </ul>
|
jaroslav@1890
|
315 |
*
|
jaroslav@1890
|
316 |
* <p>If the current thread:
|
jaroslav@1890
|
317 |
* <ul>
|
jaroslav@1890
|
318 |
* <li>has its interrupted status set on entry to this method; or
|
jaroslav@1890
|
319 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
jaroslav@1890
|
320 |
* </ul>
|
jaroslav@1890
|
321 |
* then {@link InterruptedException} is thrown and the current thread's
|
jaroslav@1890
|
322 |
* interrupted status is cleared.
|
jaroslav@1890
|
323 |
*
|
jaroslav@1890
|
324 |
* <p>If the barrier is {@link #reset} while any thread is waiting,
|
jaroslav@1890
|
325 |
* or if the barrier {@linkplain #isBroken is broken} when
|
jaroslav@1890
|
326 |
* <tt>await</tt> is invoked, or while any thread is waiting, then
|
jaroslav@1890
|
327 |
* {@link BrokenBarrierException} is thrown.
|
jaroslav@1890
|
328 |
*
|
jaroslav@1890
|
329 |
* <p>If any thread is {@linkplain Thread#interrupt interrupted} while waiting,
|
jaroslav@1890
|
330 |
* then all other waiting threads will throw
|
jaroslav@1890
|
331 |
* {@link BrokenBarrierException} and the barrier is placed in the broken
|
jaroslav@1890
|
332 |
* state.
|
jaroslav@1890
|
333 |
*
|
jaroslav@1890
|
334 |
* <p>If the current thread is the last thread to arrive, and a
|
jaroslav@1890
|
335 |
* non-null barrier action was supplied in the constructor, then the
|
jaroslav@1890
|
336 |
* current thread runs the action before allowing the other threads to
|
jaroslav@1890
|
337 |
* continue.
|
jaroslav@1890
|
338 |
* If an exception occurs during the barrier action then that exception
|
jaroslav@1890
|
339 |
* will be propagated in the current thread and the barrier is placed in
|
jaroslav@1890
|
340 |
* the broken state.
|
jaroslav@1890
|
341 |
*
|
jaroslav@1890
|
342 |
* @return the arrival index of the current thread, where index
|
jaroslav@1890
|
343 |
* <tt>{@link #getParties()} - 1</tt> indicates the first
|
jaroslav@1890
|
344 |
* to arrive and zero indicates the last to arrive
|
jaroslav@1890
|
345 |
* @throws InterruptedException if the current thread was interrupted
|
jaroslav@1890
|
346 |
* while waiting
|
jaroslav@1890
|
347 |
* @throws BrokenBarrierException if <em>another</em> thread was
|
jaroslav@1890
|
348 |
* interrupted or timed out while the current thread was
|
jaroslav@1890
|
349 |
* waiting, or the barrier was reset, or the barrier was
|
jaroslav@1890
|
350 |
* broken when {@code await} was called, or the barrier
|
jaroslav@1890
|
351 |
* action (if present) failed due an exception.
|
jaroslav@1890
|
352 |
*/
|
jaroslav@1890
|
353 |
public int await() throws InterruptedException, BrokenBarrierException {
|
jaroslav@1890
|
354 |
try {
|
jaroslav@1890
|
355 |
return dowait(false, 0L);
|
jaroslav@1890
|
356 |
} catch (TimeoutException toe) {
|
jaroslav@1890
|
357 |
throw new Error(toe); // cannot happen;
|
jaroslav@1890
|
358 |
}
|
jaroslav@1890
|
359 |
}
|
jaroslav@1890
|
360 |
|
jaroslav@1890
|
361 |
/**
|
jaroslav@1890
|
362 |
* Waits until all {@linkplain #getParties parties} have invoked
|
jaroslav@1890
|
363 |
* <tt>await</tt> on this barrier, or the specified waiting time elapses.
|
jaroslav@1890
|
364 |
*
|
jaroslav@1890
|
365 |
* <p>If the current thread is not the last to arrive then it is
|
jaroslav@1890
|
366 |
* disabled for thread scheduling purposes and lies dormant until
|
jaroslav@1890
|
367 |
* one of the following things happens:
|
jaroslav@1890
|
368 |
* <ul>
|
jaroslav@1890
|
369 |
* <li>The last thread arrives; or
|
jaroslav@1890
|
370 |
* <li>The specified timeout elapses; or
|
jaroslav@1890
|
371 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
|
jaroslav@1890
|
372 |
* the current thread; or
|
jaroslav@1890
|
373 |
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
|
jaroslav@1890
|
374 |
* one of the other waiting threads; or
|
jaroslav@1890
|
375 |
* <li>Some other thread times out while waiting for barrier; or
|
jaroslav@1890
|
376 |
* <li>Some other thread invokes {@link #reset} on this barrier.
|
jaroslav@1890
|
377 |
* </ul>
|
jaroslav@1890
|
378 |
*
|
jaroslav@1890
|
379 |
* <p>If the current thread:
|
jaroslav@1890
|
380 |
* <ul>
|
jaroslav@1890
|
381 |
* <li>has its interrupted status set on entry to this method; or
|
jaroslav@1890
|
382 |
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
|
jaroslav@1890
|
383 |
* </ul>
|
jaroslav@1890
|
384 |
* then {@link InterruptedException} is thrown and the current thread's
|
jaroslav@1890
|
385 |
* interrupted status is cleared.
|
jaroslav@1890
|
386 |
*
|
jaroslav@1890
|
387 |
* <p>If the specified waiting time elapses then {@link TimeoutException}
|
jaroslav@1890
|
388 |
* is thrown. If the time is less than or equal to zero, the
|
jaroslav@1890
|
389 |
* method will not wait at all.
|
jaroslav@1890
|
390 |
*
|
jaroslav@1890
|
391 |
* <p>If the barrier is {@link #reset} while any thread is waiting,
|
jaroslav@1890
|
392 |
* or if the barrier {@linkplain #isBroken is broken} when
|
jaroslav@1890
|
393 |
* <tt>await</tt> is invoked, or while any thread is waiting, then
|
jaroslav@1890
|
394 |
* {@link BrokenBarrierException} is thrown.
|
jaroslav@1890
|
395 |
*
|
jaroslav@1890
|
396 |
* <p>If any thread is {@linkplain Thread#interrupt interrupted} while
|
jaroslav@1890
|
397 |
* waiting, then all other waiting threads will throw {@link
|
jaroslav@1890
|
398 |
* BrokenBarrierException} and the barrier is placed in the broken
|
jaroslav@1890
|
399 |
* state.
|
jaroslav@1890
|
400 |
*
|
jaroslav@1890
|
401 |
* <p>If the current thread is the last thread to arrive, and a
|
jaroslav@1890
|
402 |
* non-null barrier action was supplied in the constructor, then the
|
jaroslav@1890
|
403 |
* current thread runs the action before allowing the other threads to
|
jaroslav@1890
|
404 |
* continue.
|
jaroslav@1890
|
405 |
* If an exception occurs during the barrier action then that exception
|
jaroslav@1890
|
406 |
* will be propagated in the current thread and the barrier is placed in
|
jaroslav@1890
|
407 |
* the broken state.
|
jaroslav@1890
|
408 |
*
|
jaroslav@1890
|
409 |
* @param timeout the time to wait for the barrier
|
jaroslav@1890
|
410 |
* @param unit the time unit of the timeout parameter
|
jaroslav@1890
|
411 |
* @return the arrival index of the current thread, where index
|
jaroslav@1890
|
412 |
* <tt>{@link #getParties()} - 1</tt> indicates the first
|
jaroslav@1890
|
413 |
* to arrive and zero indicates the last to arrive
|
jaroslav@1890
|
414 |
* @throws InterruptedException if the current thread was interrupted
|
jaroslav@1890
|
415 |
* while waiting
|
jaroslav@1890
|
416 |
* @throws TimeoutException if the specified timeout elapses
|
jaroslav@1890
|
417 |
* @throws BrokenBarrierException if <em>another</em> thread was
|
jaroslav@1890
|
418 |
* interrupted or timed out while the current thread was
|
jaroslav@1890
|
419 |
* waiting, or the barrier was reset, or the barrier was broken
|
jaroslav@1890
|
420 |
* when {@code await} was called, or the barrier action (if
|
jaroslav@1890
|
421 |
* present) failed due an exception
|
jaroslav@1890
|
422 |
*/
|
jaroslav@1890
|
423 |
public int await(long timeout, TimeUnit unit)
|
jaroslav@1890
|
424 |
throws InterruptedException,
|
jaroslav@1890
|
425 |
BrokenBarrierException,
|
jaroslav@1890
|
426 |
TimeoutException {
|
jaroslav@1890
|
427 |
return dowait(true, unit.toNanos(timeout));
|
jaroslav@1890
|
428 |
}
|
jaroslav@1890
|
429 |
|
jaroslav@1890
|
430 |
/**
|
jaroslav@1890
|
431 |
* Queries if this barrier is in a broken state.
|
jaroslav@1890
|
432 |
*
|
jaroslav@1890
|
433 |
* @return {@code true} if one or more parties broke out of this
|
jaroslav@1890
|
434 |
* barrier due to interruption or timeout since
|
jaroslav@1890
|
435 |
* construction or the last reset, or a barrier action
|
jaroslav@1890
|
436 |
* failed due to an exception; {@code false} otherwise.
|
jaroslav@1890
|
437 |
*/
|
jaroslav@1890
|
438 |
public boolean isBroken() {
|
jaroslav@1890
|
439 |
final ReentrantLock lock = this.lock;
|
jaroslav@1890
|
440 |
lock.lock();
|
jaroslav@1890
|
441 |
try {
|
jaroslav@1890
|
442 |
return generation.broken;
|
jaroslav@1890
|
443 |
} finally {
|
jaroslav@1890
|
444 |
lock.unlock();
|
jaroslav@1890
|
445 |
}
|
jaroslav@1890
|
446 |
}
|
jaroslav@1890
|
447 |
|
jaroslav@1890
|
448 |
/**
|
jaroslav@1890
|
449 |
* Resets the barrier to its initial state. If any parties are
|
jaroslav@1890
|
450 |
* currently waiting at the barrier, they will return with a
|
jaroslav@1890
|
451 |
* {@link BrokenBarrierException}. Note that resets <em>after</em>
|
jaroslav@1890
|
452 |
* a breakage has occurred for other reasons can be complicated to
|
jaroslav@1890
|
453 |
* carry out; threads need to re-synchronize in some other way,
|
jaroslav@1890
|
454 |
* and choose one to perform the reset. It may be preferable to
|
jaroslav@1890
|
455 |
* instead create a new barrier for subsequent use.
|
jaroslav@1890
|
456 |
*/
|
jaroslav@1890
|
457 |
public void reset() {
|
jaroslav@1890
|
458 |
final ReentrantLock lock = this.lock;
|
jaroslav@1890
|
459 |
lock.lock();
|
jaroslav@1890
|
460 |
try {
|
jaroslav@1890
|
461 |
breakBarrier(); // break the current generation
|
jaroslav@1890
|
462 |
nextGeneration(); // start a new generation
|
jaroslav@1890
|
463 |
} finally {
|
jaroslav@1890
|
464 |
lock.unlock();
|
jaroslav@1890
|
465 |
}
|
jaroslav@1890
|
466 |
}
|
jaroslav@1890
|
467 |
|
jaroslav@1890
|
468 |
/**
|
jaroslav@1890
|
469 |
* Returns the number of parties currently waiting at the barrier.
|
jaroslav@1890
|
470 |
* This method is primarily useful for debugging and assertions.
|
jaroslav@1890
|
471 |
*
|
jaroslav@1890
|
472 |
* @return the number of parties currently blocked in {@link #await}
|
jaroslav@1890
|
473 |
*/
|
jaroslav@1890
|
474 |
public int getNumberWaiting() {
|
jaroslav@1890
|
475 |
final ReentrantLock lock = this.lock;
|
jaroslav@1890
|
476 |
lock.lock();
|
jaroslav@1890
|
477 |
try {
|
jaroslav@1890
|
478 |
return parties - count;
|
jaroslav@1890
|
479 |
} finally {
|
jaroslav@1890
|
480 |
lock.unlock();
|
jaroslav@1890
|
481 |
}
|
jaroslav@1890
|
482 |
}
|
jaroslav@1890
|
483 |
}
|