475d901131
Motivation: Javadoc reports errors about invalid docs. Modifications: Fix some errors reported by javadoc. Result: A lot of javadoc errors are fixed by this patch.
741 lines
28 KiB
Java
741 lines
28 KiB
Java
/*
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* Copyright 2012 The Netty Project
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*
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* The Netty Project licenses this file to you under the Apache License,
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* version 2.0 (the "License"); you may not use this file except in compliance
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* with the License. You may obtain a copy of the License at:
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
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* WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
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* License for the specific language governing permissions and limitations
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* under the License.
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*/
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package io.netty.util;
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import io.netty.util.internal.MpscLinkedQueueNode;
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import io.netty.util.internal.PlatformDependent;
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import io.netty.util.internal.StringUtil;
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import io.netty.util.internal.logging.InternalLogger;
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import io.netty.util.internal.logging.InternalLoggerFactory;
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import java.util.Collections;
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import java.util.HashSet;
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import java.util.Queue;
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import java.util.Set;
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import java.util.concurrent.CountDownLatch;
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import java.util.concurrent.Executors;
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import java.util.concurrent.ThreadFactory;
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import java.util.concurrent.TimeUnit;
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import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
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/**
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* A {@link Timer} optimized for approximated I/O timeout scheduling.
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*
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* <h3>Tick Duration</h3>
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*
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* As described with 'approximated', this timer does not execute the scheduled
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* {@link TimerTask} on time. {@link HashedWheelTimer}, on every tick, will
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* check if there are any {@link TimerTask}s behind the schedule and execute
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* them.
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* <p>
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* You can increase or decrease the accuracy of the execution timing by
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* specifying smaller or larger tick duration in the constructor. In most
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* network applications, I/O timeout does not need to be accurate. Therefore,
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* the default tick duration is 100 milliseconds and you will not need to try
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* different configurations in most cases.
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*
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* <h3>Ticks per Wheel (Wheel Size)</h3>
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*
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* {@link HashedWheelTimer} maintains a data structure called 'wheel'.
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* To put simply, a wheel is a hash table of {@link TimerTask}s whose hash
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* function is 'dead line of the task'. The default number of ticks per wheel
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* (i.e. the size of the wheel) is 512. You could specify a larger value
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* if you are going to schedule a lot of timeouts.
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*
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* <h3>Do not create many instances.</h3>
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*
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* {@link HashedWheelTimer} creates a new thread whenever it is instantiated and
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* started. Therefore, you should make sure to create only one instance and
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* share it across your application. One of the common mistakes, that makes
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* your application unresponsive, is to create a new instance for every connection.
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*
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* <h3>Implementation Details</h3>
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*
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* {@link HashedWheelTimer} is based on
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* <a href="http://cseweb.ucsd.edu/users/varghese/">George Varghese</a> and
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* Tony Lauck's paper,
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* <a href="http://cseweb.ucsd.edu/users/varghese/PAPERS/twheel.ps.Z">'Hashed
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* and Hierarchical Timing Wheels: data structures to efficiently implement a
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* timer facility'</a>. More comprehensive slides are located
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* <a href="http://www.cse.wustl.edu/~cdgill/courses/cs6874/TimingWheels.ppt">here</a>.
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*/
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public class HashedWheelTimer implements Timer {
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static final InternalLogger logger =
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InternalLoggerFactory.getInstance(HashedWheelTimer.class);
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private static final ResourceLeakDetector<HashedWheelTimer> leakDetector =
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new ResourceLeakDetector<HashedWheelTimer>(
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HashedWheelTimer.class, 1, Runtime.getRuntime().availableProcessors() * 4);
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private static final AtomicIntegerFieldUpdater<HashedWheelTimer> WORKER_STATE_UPDATER;
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static {
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AtomicIntegerFieldUpdater<HashedWheelTimer> workerStateUpdater =
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PlatformDependent.newAtomicIntegerFieldUpdater(HashedWheelTimer.class, "workerState");
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if (workerStateUpdater == null) {
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workerStateUpdater = AtomicIntegerFieldUpdater.newUpdater(HashedWheelTimer.class, "workerState");
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}
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WORKER_STATE_UPDATER = workerStateUpdater;
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}
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private final ResourceLeak leak;
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private final Worker worker = new Worker();
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private final Thread workerThread;
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public static final int WORKER_STATE_INIT = 0;
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public static final int WORKER_STATE_STARTED = 1;
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public static final int WORKER_STATE_SHUTDOWN = 2;
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@SuppressWarnings({ "unused", "FieldMayBeFinal", "RedundantFieldInitialization" })
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private volatile int workerState = WORKER_STATE_INIT; // 0 - init, 1 - started, 2 - shut down
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private final long tickDuration;
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private final HashedWheelBucket[] wheel;
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private final int mask;
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private final CountDownLatch startTimeInitialized = new CountDownLatch(1);
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private final Queue<HashedWheelTimeout> timeouts = PlatformDependent.newMpscQueue();
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private final Queue<Runnable> cancelledTimeouts = PlatformDependent.newMpscQueue();
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private volatile long startTime;
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/**
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* Creates a new timer with the default thread factory
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* ({@link Executors#defaultThreadFactory()}), default tick duration, and
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* default number of ticks per wheel.
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*/
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public HashedWheelTimer() {
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this(Executors.defaultThreadFactory());
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}
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/**
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* Creates a new timer with the default thread factory
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* ({@link Executors#defaultThreadFactory()}) and default number of ticks
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* per wheel.
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*
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* @param tickDuration the duration between tick
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* @param unit the time unit of the {@code tickDuration}
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* @throws NullPointerException if {@code unit} is {@code null}
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* @throws IllegalArgumentException if {@code tickDuration} is <= 0
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*/
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public HashedWheelTimer(long tickDuration, TimeUnit unit) {
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this(Executors.defaultThreadFactory(), tickDuration, unit);
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}
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/**
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* Creates a new timer with the default thread factory
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* ({@link Executors#defaultThreadFactory()}).
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*
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* @param tickDuration the duration between tick
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* @param unit the time unit of the {@code tickDuration}
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* @param ticksPerWheel the size of the wheel
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* @throws NullPointerException if {@code unit} is {@code null}
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* @throws IllegalArgumentException if either of {@code tickDuration} and {@code ticksPerWheel} is <= 0
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*/
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public HashedWheelTimer(long tickDuration, TimeUnit unit, int ticksPerWheel) {
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this(Executors.defaultThreadFactory(), tickDuration, unit, ticksPerWheel);
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}
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/**
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* Creates a new timer with the default tick duration and default number of
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* ticks per wheel.
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*
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* @param threadFactory a {@link ThreadFactory} that creates a
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* background {@link Thread} which is dedicated to
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* {@link TimerTask} execution.
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* @throws NullPointerException if {@code threadFactory} is {@code null}
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*/
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public HashedWheelTimer(ThreadFactory threadFactory) {
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this(threadFactory, 100, TimeUnit.MILLISECONDS);
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}
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/**
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* Creates a new timer with the default number of ticks per wheel.
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*
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* @param threadFactory a {@link ThreadFactory} that creates a
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* background {@link Thread} which is dedicated to
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* {@link TimerTask} execution.
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* @param tickDuration the duration between tick
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* @param unit the time unit of the {@code tickDuration}
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* @throws NullPointerException if either of {@code threadFactory} and {@code unit} is {@code null}
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* @throws IllegalArgumentException if {@code tickDuration} is <= 0
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*/
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public HashedWheelTimer(
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ThreadFactory threadFactory, long tickDuration, TimeUnit unit) {
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this(threadFactory, tickDuration, unit, 512);
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}
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/**
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* Creates a new timer.
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*
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* @param threadFactory a {@link ThreadFactory} that creates a
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* background {@link Thread} which is dedicated to
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* {@link TimerTask} execution.
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* @param tickDuration the duration between tick
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* @param unit the time unit of the {@code tickDuration}
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* @param ticksPerWheel the size of the wheel
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* @throws NullPointerException if either of {@code threadFactory} and {@code unit} is {@code null}
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* @throws IllegalArgumentException if either of {@code tickDuration} and {@code ticksPerWheel} is <= 0
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*/
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public HashedWheelTimer(
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ThreadFactory threadFactory,
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long tickDuration, TimeUnit unit, int ticksPerWheel) {
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if (threadFactory == null) {
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throw new NullPointerException("threadFactory");
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}
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if (unit == null) {
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throw new NullPointerException("unit");
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}
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if (tickDuration <= 0) {
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throw new IllegalArgumentException("tickDuration must be greater than 0: " + tickDuration);
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}
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if (ticksPerWheel <= 0) {
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throw new IllegalArgumentException("ticksPerWheel must be greater than 0: " + ticksPerWheel);
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}
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// Normalize ticksPerWheel to power of two and initialize the wheel.
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wheel = createWheel(ticksPerWheel);
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mask = wheel.length - 1;
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// Convert tickDuration to nanos.
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this.tickDuration = unit.toNanos(tickDuration);
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// Prevent overflow.
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if (this.tickDuration >= Long.MAX_VALUE / wheel.length) {
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throw new IllegalArgumentException(String.format(
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"tickDuration: %d (expected: 0 < tickDuration in nanos < %d",
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tickDuration, Long.MAX_VALUE / wheel.length));
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}
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workerThread = threadFactory.newThread(worker);
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leak = leakDetector.open(this);
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}
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private static HashedWheelBucket[] createWheel(int ticksPerWheel) {
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if (ticksPerWheel <= 0) {
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throw new IllegalArgumentException(
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"ticksPerWheel must be greater than 0: " + ticksPerWheel);
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}
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if (ticksPerWheel > 1073741824) {
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throw new IllegalArgumentException(
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"ticksPerWheel may not be greater than 2^30: " + ticksPerWheel);
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}
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ticksPerWheel = normalizeTicksPerWheel(ticksPerWheel);
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HashedWheelBucket[] wheel = new HashedWheelBucket[ticksPerWheel];
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for (int i = 0; i < wheel.length; i ++) {
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wheel[i] = new HashedWheelBucket();
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}
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return wheel;
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}
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private static int normalizeTicksPerWheel(int ticksPerWheel) {
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int normalizedTicksPerWheel = 1;
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while (normalizedTicksPerWheel < ticksPerWheel) {
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normalizedTicksPerWheel <<= 1;
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}
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return normalizedTicksPerWheel;
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}
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/**
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* Starts the background thread explicitly. The background thread will
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* start automatically on demand even if you did not call this method.
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*
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* @throws IllegalStateException if this timer has been
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* {@linkplain #stop() stopped} already
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*/
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public void start() {
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switch (WORKER_STATE_UPDATER.get(this)) {
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case WORKER_STATE_INIT:
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if (WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_INIT, WORKER_STATE_STARTED)) {
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workerThread.start();
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}
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break;
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case WORKER_STATE_STARTED:
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break;
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case WORKER_STATE_SHUTDOWN:
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throw new IllegalStateException("cannot be started once stopped");
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default:
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throw new Error("Invalid WorkerState");
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}
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// Wait until the startTime is initialized by the worker.
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while (startTime == 0) {
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try {
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startTimeInitialized.await();
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} catch (InterruptedException ignore) {
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// Ignore - it will be ready very soon.
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}
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}
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}
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@Override
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public Set<Timeout> stop() {
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if (Thread.currentThread() == workerThread) {
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throw new IllegalStateException(
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HashedWheelTimer.class.getSimpleName() +
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".stop() cannot be called from " +
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TimerTask.class.getSimpleName());
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}
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if (!WORKER_STATE_UPDATER.compareAndSet(this, WORKER_STATE_STARTED, WORKER_STATE_SHUTDOWN)) {
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// workerState can be 0 or 2 at this moment - let it always be 2.
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WORKER_STATE_UPDATER.set(this, WORKER_STATE_SHUTDOWN);
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if (leak != null) {
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leak.close();
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}
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return Collections.emptySet();
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}
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boolean interrupted = false;
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while (workerThread.isAlive()) {
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workerThread.interrupt();
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try {
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workerThread.join(100);
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} catch (InterruptedException ignored) {
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interrupted = true;
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}
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}
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if (interrupted) {
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Thread.currentThread().interrupt();
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}
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if (leak != null) {
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leak.close();
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}
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return worker.unprocessedTimeouts();
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}
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@Override
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public Timeout newTimeout(TimerTask task, long delay, TimeUnit unit) {
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if (task == null) {
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throw new NullPointerException("task");
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}
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if (unit == null) {
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throw new NullPointerException("unit");
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}
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start();
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// Add the timeout to the timeout queue which will be processed on the next tick.
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// During processing all the queued HashedWheelTimeouts will be added to the correct HashedWheelBucket.
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long deadline = System.nanoTime() + unit.toNanos(delay) - startTime;
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HashedWheelTimeout timeout = new HashedWheelTimeout(this, task, deadline);
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timeouts.add(timeout);
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return timeout;
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}
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private final class Worker implements Runnable {
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private final Set<Timeout> unprocessedTimeouts = new HashSet<Timeout>();
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private long tick;
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@Override
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public void run() {
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// Initialize the startTime.
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startTime = System.nanoTime();
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if (startTime == 0) {
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// We use 0 as an indicator for the uninitialized value here, so make sure it's not 0 when initialized.
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startTime = 1;
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}
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// Notify the other threads waiting for the initialization at start().
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startTimeInitialized.countDown();
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do {
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final long deadline = waitForNextTick();
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if (deadline > 0) {
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int idx = (int) (tick & mask);
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processCancelledTasks();
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HashedWheelBucket bucket =
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wheel[idx];
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transferTimeoutsToBuckets();
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bucket.expireTimeouts(deadline);
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tick++;
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}
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} while (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_STARTED);
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// Fill the unprocessedTimeouts so we can return them from stop() method.
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for (HashedWheelBucket bucket: wheel) {
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bucket.clearTimeouts(unprocessedTimeouts);
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}
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for (;;) {
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HashedWheelTimeout timeout = timeouts.poll();
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if (timeout == null) {
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break;
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}
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if (!timeout.isCancelled()) {
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unprocessedTimeouts.add(timeout);
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}
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}
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processCancelledTasks();
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}
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private void transferTimeoutsToBuckets() {
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// transfer only max. 100000 timeouts per tick to prevent a thread to stale the workerThread when it just
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// adds new timeouts in a loop.
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for (int i = 0; i < 100000; i++) {
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HashedWheelTimeout timeout = timeouts.poll();
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if (timeout == null) {
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// all processed
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break;
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}
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if (timeout.state() == HashedWheelTimeout.ST_CANCELLED) {
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// Was cancelled in the meantime.
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continue;
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}
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long calculated = timeout.deadline / tickDuration;
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timeout.remainingRounds = (calculated - tick) / wheel.length;
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final long ticks = Math.max(calculated, tick); // Ensure we don't schedule for past.
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int stopIndex = (int) (ticks & mask);
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HashedWheelBucket bucket = wheel[stopIndex];
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bucket.addTimeout(timeout);
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}
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}
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private void processCancelledTasks() {
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for (;;) {
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Runnable task = cancelledTimeouts.poll();
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if (task == null) {
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// all processed
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break;
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}
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try {
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task.run();
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} catch (Throwable t) {
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if (logger.isWarnEnabled()) {
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logger.warn("An exception was thrown while process a cancellation task", t);
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}
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}
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}
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}
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/**
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* calculate goal nanoTime from startTime and current tick number,
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* then wait until that goal has been reached.
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* @return Long.MIN_VALUE if received a shutdown request,
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* current time otherwise (with Long.MIN_VALUE changed by +1)
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*/
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private long waitForNextTick() {
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long deadline = tickDuration * (tick + 1);
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for (;;) {
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final long currentTime = System.nanoTime() - startTime;
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long sleepTimeMs = (deadline - currentTime + 999999) / 1000000;
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if (sleepTimeMs <= 0) {
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if (currentTime == Long.MIN_VALUE) {
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return -Long.MAX_VALUE;
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} else {
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return currentTime;
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}
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}
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// Check if we run on windows, as if thats the case we will need
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// to round the sleepTime as workaround for a bug that only affect
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// the JVM if it runs on windows.
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//
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// See https://github.com/netty/netty/issues/356
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if (PlatformDependent.isWindows()) {
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sleepTimeMs = sleepTimeMs / 10 * 10;
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}
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try {
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Thread.sleep(sleepTimeMs);
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} catch (InterruptedException ignored) {
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if (WORKER_STATE_UPDATER.get(HashedWheelTimer.this) == WORKER_STATE_SHUTDOWN) {
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return Long.MIN_VALUE;
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}
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}
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}
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}
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public Set<Timeout> unprocessedTimeouts() {
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return Collections.unmodifiableSet(unprocessedTimeouts);
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}
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}
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private static final class HashedWheelTimeout extends MpscLinkedQueueNode<Timeout>
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implements Timeout {
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private static final int ST_INIT = 0;
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private static final int ST_CANCELLED = 1;
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private static final int ST_EXPIRED = 2;
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private static final AtomicIntegerFieldUpdater<HashedWheelTimeout> STATE_UPDATER;
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static {
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AtomicIntegerFieldUpdater<HashedWheelTimeout> updater =
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PlatformDependent.newAtomicIntegerFieldUpdater(HashedWheelTimeout.class, "state");
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if (updater == null) {
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updater = AtomicIntegerFieldUpdater.newUpdater(HashedWheelTimeout.class, "state");
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}
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STATE_UPDATER = updater;
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}
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private final HashedWheelTimer timer;
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private final TimerTask task;
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private final long deadline;
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@SuppressWarnings({"unused", "FieldMayBeFinal", "RedundantFieldInitialization" })
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private volatile int state = ST_INIT;
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// remainingRounds will be calculated and set by Worker.transferTimeoutsToBuckets() before the
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// HashedWheelTimeout will be added to the correct HashedWheelBucket.
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long remainingRounds;
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|
// This will be used to chain timeouts in HashedWheelTimerBucket via a double-linked-list.
|
|
// As only the workerThread will act on it there is no need for synchronization / volatile.
|
|
HashedWheelTimeout next;
|
|
HashedWheelTimeout prev;
|
|
|
|
// The bucket to which the timeout was added
|
|
HashedWheelBucket bucket;
|
|
|
|
HashedWheelTimeout(HashedWheelTimer timer, TimerTask task, long deadline) {
|
|
this.timer = timer;
|
|
this.task = task;
|
|
this.deadline = deadline;
|
|
}
|
|
|
|
@Override
|
|
public Timer timer() {
|
|
return timer;
|
|
}
|
|
|
|
@Override
|
|
public TimerTask task() {
|
|
return task;
|
|
}
|
|
|
|
@Override
|
|
public boolean cancel() {
|
|
// only update the state it will be removed from HashedWheelBucket on next tick.
|
|
if (!compareAndSetState(ST_INIT, ST_CANCELLED)) {
|
|
return false;
|
|
}
|
|
// If a task should be canceled we create a new Runnable for this to another queue which will
|
|
// be processed on each tick. So this means that we will have a GC latency of max. 1 tick duration
|
|
// which is good enough. This way we can make again use of our MpscLinkedQueue and so minimize the
|
|
// locking / overhead as much as possible.
|
|
//
|
|
// It is important that we not just add the HashedWheelTimeout itself again as it extends
|
|
// MpscLinkedQueueNode and so may still be used as tombstone.
|
|
timer.cancelledTimeouts.add(new Runnable() {
|
|
@Override
|
|
public void run() {
|
|
HashedWheelBucket bucket = HashedWheelTimeout.this.bucket;
|
|
if (bucket != null) {
|
|
bucket.remove(HashedWheelTimeout.this);
|
|
}
|
|
}
|
|
});
|
|
return true;
|
|
}
|
|
|
|
public boolean compareAndSetState(int expected, int state) {
|
|
return STATE_UPDATER.compareAndSet(this, expected, state);
|
|
}
|
|
|
|
public int state() {
|
|
return state;
|
|
}
|
|
|
|
@Override
|
|
public boolean isCancelled() {
|
|
return state() == ST_CANCELLED;
|
|
}
|
|
|
|
@Override
|
|
public boolean isExpired() {
|
|
return state() == ST_EXPIRED;
|
|
}
|
|
|
|
@Override
|
|
public HashedWheelTimeout value() {
|
|
return this;
|
|
}
|
|
|
|
public void expire() {
|
|
if (!compareAndSetState(ST_INIT, ST_EXPIRED)) {
|
|
return;
|
|
}
|
|
|
|
try {
|
|
task.run(this);
|
|
} catch (Throwable t) {
|
|
if (logger.isWarnEnabled()) {
|
|
logger.warn("An exception was thrown by " + TimerTask.class.getSimpleName() + '.', t);
|
|
}
|
|
}
|
|
}
|
|
|
|
@Override
|
|
public String toString() {
|
|
final long currentTime = System.nanoTime();
|
|
long remaining = deadline - currentTime + timer.startTime;
|
|
|
|
StringBuilder buf = new StringBuilder(192)
|
|
.append(StringUtil.simpleClassName(this))
|
|
.append('(')
|
|
.append("deadline: ");
|
|
if (remaining > 0) {
|
|
buf.append(remaining)
|
|
.append(" ns later");
|
|
} else if (remaining < 0) {
|
|
buf.append(-remaining)
|
|
.append(" ns ago");
|
|
} else {
|
|
buf.append("now");
|
|
}
|
|
|
|
if (isCancelled()) {
|
|
buf.append(", cancelled");
|
|
}
|
|
|
|
return buf.append(", task: ")
|
|
.append(task())
|
|
.append(')')
|
|
.toString();
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Bucket that stores HashedWheelTimeouts. These are stored in a linked-list like datastructure to allow easy
|
|
* removal of HashedWheelTimeouts in the middle. Also the HashedWheelTimeout act as nodes themself and so no
|
|
* extra object creation is needed.
|
|
*/
|
|
private static final class HashedWheelBucket {
|
|
// Used for the linked-list datastructure
|
|
private HashedWheelTimeout head;
|
|
private HashedWheelTimeout tail;
|
|
|
|
/**
|
|
* Add {@link HashedWheelTimeout} to this bucket.
|
|
*/
|
|
public void addTimeout(HashedWheelTimeout timeout) {
|
|
assert timeout.bucket == null;
|
|
timeout.bucket = this;
|
|
if (head == null) {
|
|
head = tail = timeout;
|
|
} else {
|
|
tail.next = timeout;
|
|
timeout.prev = tail;
|
|
tail = timeout;
|
|
}
|
|
}
|
|
|
|
/**
|
|
* Expire all {@link HashedWheelTimeout}s for the given {@code deadline}.
|
|
*/
|
|
public void expireTimeouts(long deadline) {
|
|
HashedWheelTimeout timeout = head;
|
|
|
|
// process all timeouts
|
|
while (timeout != null) {
|
|
boolean remove = false;
|
|
if (timeout.remainingRounds <= 0) {
|
|
if (timeout.deadline <= deadline) {
|
|
timeout.expire();
|
|
} else {
|
|
// The timeout was placed into a wrong slot. This should never happen.
|
|
throw new IllegalStateException(String.format(
|
|
"timeout.deadline (%d) > deadline (%d)", timeout.deadline, deadline));
|
|
}
|
|
remove = true;
|
|
} else if (timeout.isCancelled()) {
|
|
remove = true;
|
|
} else {
|
|
timeout.remainingRounds --;
|
|
}
|
|
// store reference to next as we may null out timeout.next in the remove block.
|
|
HashedWheelTimeout next = timeout.next;
|
|
if (remove) {
|
|
remove(timeout);
|
|
}
|
|
timeout = next;
|
|
}
|
|
}
|
|
|
|
public void remove(HashedWheelTimeout timeout) {
|
|
HashedWheelTimeout next = timeout.next;
|
|
// remove timeout that was either processed or cancelled by updating the linked-list
|
|
if (timeout.prev != null) {
|
|
timeout.prev.next = next;
|
|
}
|
|
if (timeout.next != null) {
|
|
timeout.next.prev = timeout.prev;
|
|
}
|
|
|
|
if (timeout == head) {
|
|
// if timeout is also the tail we need to adjust the entry too
|
|
if (timeout == tail) {
|
|
tail = null;
|
|
head = null;
|
|
} else {
|
|
head = next;
|
|
}
|
|
} else if (timeout == tail) {
|
|
// if the timeout is the tail modify the tail to be the prev node.
|
|
tail = timeout.prev;
|
|
}
|
|
// null out prev, next and bucket to allow for GC.
|
|
timeout.prev = null;
|
|
timeout.next = null;
|
|
timeout.bucket = null;
|
|
}
|
|
|
|
/**
|
|
* Clear this bucket and return all not expired / cancelled {@link Timeout}s.
|
|
*/
|
|
public void clearTimeouts(Set<Timeout> set) {
|
|
for (;;) {
|
|
HashedWheelTimeout timeout = pollTimeout();
|
|
if (timeout == null) {
|
|
return;
|
|
}
|
|
if (timeout.isExpired() || timeout.isCancelled()) {
|
|
continue;
|
|
}
|
|
set.add(timeout);
|
|
}
|
|
}
|
|
|
|
private HashedWheelTimeout pollTimeout() {
|
|
HashedWheelTimeout head = this.head;
|
|
if (head == null) {
|
|
return null;
|
|
}
|
|
HashedWheelTimeout next = head.next;
|
|
if (next == null) {
|
|
tail = this.head = null;
|
|
} else {
|
|
this.head = next;
|
|
next.prev = null;
|
|
}
|
|
|
|
// null out prev and next to allow for GC.
|
|
head.next = null;
|
|
head.prev = null;
|
|
head.bucket = null;
|
|
return head;
|
|
}
|
|
}
|
|
}
|