百度uid-generator学习
学习自https://www.cnblogs.com/yeyang/p/10226284.html
百度uid-generator源码
https://github.com/baidu/uid-generator
解析uid-generator
uid-generator是基于Twitter开源的snowflake算法实现的。snowflake雪花算法上文有介绍 传送门
snowflake将long的64位分为了3部分，时间戳、工作机器id和序列号、其中，时间戳部分的时间单位一般为毫秒。也就是说1台工作机器1毫秒可产生4096个id（2的12次方）。
源码实现分析
与原始的snowflake算法不同，uid-generator支持自定义时间戳、工作机器id和序列号等各部分的位数，以应用于不同场景。默认分配方式如下。
-
sign(1bit)
固定1bit符号标识，即生成的UID为正数。 -
delta seconds (28 bits)
当前时间，相对于时间基点"2016-05-20"的增量值，单位：秒，最多可支持约8.7年（注意：1. 这里的单位是秒，而不是毫秒！ 2.注意这里的用词，是“最多”可支持8.7年，为什么是“最多”，后面会讲） -
worker id (22 bits)
机器id，最多可支持约420w次机器启动。内置实现为在启动时由数据库分配，默认分配策略为用后即弃，后续可提供复用策略。 -
sequence (13 bits)
每秒下的并发序列，13 bits可支持每秒8192个并发。（注意下这个地方，默认支持qps最大为8192个）
DefaultUidGenerator
DefaultUidGenerator的产生id的方法与基本上就是常见的snowflake算法实现，仅有一些不同，如以秒为为单位而不是毫秒。
DefaultUidGenerator的产生id的方法如下。
/*
* Copyright (c) 2017 Baidu, Inc. All Rights Reserve.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.baidu.fsg.uid.impl;
import java.util.Date;
import java.util.concurrent.TimeUnit;
import org.apache.commons.lang.StringUtils;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.InitializingBean;
import com.baidu.fsg.uid.BitsAllocator;
import com.baidu.fsg.uid.UidGenerator;
import com.baidu.fsg.uid.exception.UidGenerateException;
import com.baidu.fsg.uid.utils.DateUtils;
import com.baidu.fsg.uid.worker.WorkerIdAssigner;
/**
* Represents an implementation of {@link UidGenerator}
*
* The unique id has 64bits (long), default allocated as blow:
* sign: The highest bit is 0
* delta seconds: The next 28 bits, represents delta seconds since a customer epoch(2016-05-20 00:00:00.000).
* Supports about 8.7 years until to 2024-11-20 21:24:16
* worker id: The next 22 bits, represents the worker's id which assigns based on database, max id is about 420W
* sequence: The next 13 bits, represents a sequence within the same second, max for 8192/s
*
* The {@link DefaultUidGenerator#parseUID(long)} is a tool method to parse the bits
*
* {@code
* +------+----------------------+----------------+-----------+
* | sign | delta seconds | worker node id | sequence |
* +------+----------------------+----------------+-----------+
* 1bit 28bits 22bits 13bits
* }
*
* You can also specified the bits by Spring property setting.
* timeBits: default as 28
* workerBits: default as 22
* seqBits: default as 13
* epochStr: Epoch date string format 'yyyy-MM-dd'. Default as '2016-05-20'
*
* Note that: The total bits must be 64 -1
*
* @author yutianbao
*/
public class DefaultUidGenerator implements UidGenerator, InitializingBean {
private static final Logger LOGGER = LoggerFactory.getLogger(DefaultUidGenerator.class);
/** Bits allocate */
protected int timeBits = 28;
protected int workerBits = 22;
protected int seqBits = 13;
/** Customer epoch, unit as second. For example 2016-05-20 (ms: 1463673600000)*/
protected String epochStr = "2016-05-20";
protected long epochSeconds = TimeUnit.MILLISECONDS.toSeconds(1463673600000L);
/** Stable fields after spring bean initializing */
protected BitsAllocator bitsAllocator;
protected long workerId;
/** Volatile fields caused by nextId() */
protected long sequence = 0L;
protected long lastSecond = -1L;
/** Spring property */
protected WorkerIdAssigner workerIdAssigner;
@Override
public void afterPropertiesSet() throws Exception {
// initialize bits allocator
bitsAllocator = new BitsAllocator(timeBits, workerBits, seqBits);
// initialize worker id
workerId = workerIdAssigner.assignWorkerId();
if (workerId > bitsAllocator.getMaxWorkerId()) {
throw new RuntimeException("Worker id " + workerId + " exceeds the max " + bitsAllocator.getMaxWorkerId());
}
LOGGER.info("Initialized bits(1, {}, {}, {}) for workerID:{}", timeBits, workerBits, seqBits, workerId);
}
@Override
public long getUID() throws UidGenerateException {
try {
return nextId();
} catch (Exception e) {
LOGGER.error("Generate unique id exception. ", e);
throw new UidGenerateException(e);
}
}
@Override
public String parseUID(long uid) {
long totalBits = BitsAllocator.TOTAL_BITS;
long signBits = bitsAllocator.getSignBits();
long timestampBits = bitsAllocator.getTimestampBits();
long workerIdBits = bitsAllocator.getWorkerIdBits();
long sequenceBits = bitsAllocator.getSequenceBits();
// parse UID
long sequence = (uid << (totalBits - sequenceBits)) >>> (totalBits - sequenceBits);
long workerId = (uid << (timestampBits + signBits)) >>> (totalBits - workerIdBits);
long deltaSeconds = uid >>> (workerIdBits + sequenceBits);
Date thatTime = new Date(TimeUnit.SECONDS.toMillis(epochSeconds + deltaSeconds));
String thatTimeStr = DateUtils.formatByDateTimePattern(thatTime);
// format as string
return String.format("{\"UID\":\"%d\",\"timestamp\":\"%s\",\"workerId\":\"%d\",\"sequence\":\"%d\"}",
uid, thatTimeStr, workerId, sequence);
}
/**
* Get UID
*
* @return UID
* @throws UidGenerateException in the case: Clock moved backwards; Exceeds the max timestamp
*/
protected synchronized long nextId() {
long currentSecond = getCurrentSecond();
// Clock moved backwards, refuse to generate uid
if (currentSecond < lastSecond) {
long refusedSeconds = lastSecond - currentSecond;
throw new UidGenerateException("Clock moved backwards. Refusing for %d seconds", refusedSeconds);
}
// At the same second, increase sequence
if (currentSecond == lastSecond) {
sequence = (sequence + 1) & bitsAllocator.getMaxSequence();
// Exceed the max sequence, we wait the next second to generate uid
if (sequence == 0) {
currentSecond = getNextSecond(lastSecond);
}
// At the different second, sequence restart from zero
} else {
sequence = 0L;
}
lastSecond = currentSecond;
// Allocate bits for UID
return bitsAllocator.allocate(currentSecond - epochSeconds, workerId, sequence);
}
/**
* Get next millisecond
*/
private long getNextSecond(long lastTimestamp) {
long timestamp = getCurrentSecond();
while (timestamp <= lastTimestamp) {
timestamp = getCurrentSecond();
}
return timestamp;
}
/**
* Get current second
*/
private long getCurrentSecond() {
long currentSecond = TimeUnit.MILLISECONDS.toSeconds(System.currentTimeMillis());
if (currentSecond - epochSeconds > bitsAllocator.getMaxDeltaSeconds()) {
throw new UidGenerateException("Timestamp bits is exhausted. Refusing UID generate. Now: " + currentSecond);
}
return currentSecond;
}
/**
* Setters for spring property
*/
public void setWorkerIdAssigner(WorkerIdAssigner workerIdAssigner) {
this.workerIdAssigner = workerIdAssigner;
}
public void setTimeBits(int timeBits) {
if (timeBits > 0) {
this.timeBits = timeBits;
}
}
public void setWorkerBits(int workerBits) {
if (workerBits > 0) {
this.workerBits = workerBits;
}
}
public void setSeqBits(int seqBits) {
if (seqBits > 0) {
this.seqBits = seqBits;
}
}
public void setEpochStr(String epochStr) {
if (StringUtils.isNotBlank(epochStr)) {
this.epochStr = epochStr;
this.epochSeconds = TimeUnit.MILLISECONDS.toSeconds(DateUtils.parseByDayPattern(epochStr).getTime());
}
}
}
核心代码：
/**
* Get UID
*
* @return UID
* @throws UidGenerateException in the case: Clock moved backwards; Exceeds the max timestamp
*/
protected synchronized long nextId() {
long currentSecond = getCurrentSecond();
// Clock moved backwards, refuse to generate uid
if (currentSecond < lastSecond) {
long refusedSeconds = lastSecond - currentSecond;
throw new UidGenerateException("Clock moved backwards. Refusing for %d seconds", refusedSeconds);
}
// At the same second, increase sequence
if (currentSecond == lastSecond) {
sequence = (sequence + 1) & bitsAllocator.getMaxSequence();
// Exceed the max sequence, we wait the next second to generate uid
if (sequence == 0) {
currentSecond = getNextSecond(lastSecond);
}
// At the different second, sequence restart from zero
} else {
sequence = 0L;
}
lastSecond = currentSecond;
// Allocate bits for UID
return bitsAllocator.allocate(currentSecond - epochSeconds, workerId, sequence);
}
CachedUidGenerator
CachedUidGenerator支持缓存生成的id。
基本实现原理
关于CachedUidGenerator，文档上是这样介绍的。
在实现上, UidGenerator通过借用未来时间来解决sequence天然存在的并发限制; 采用RingBuffer来缓存已生成的UID, 并行化UID的生产和消费, 同时对CacheLine补齐，避免了由RingBuffer带来的硬件级「伪共享」问题. 最终单机QPS可达600万。
【采用RingBuffer来缓存已生成的UID, 并行化UID的生产和消费】
使用RingBuffer缓存生成的id。RingBuffer是个环形数组，默认大小为8192个，里面缓存着生成的id。
获取id
会从ringbuffer中拿一个id，支持并发获取
填充id
RingBuffer填充时机
-
程序启动时，将RingBuffer填充满，缓存着8192个id
-
在调用getUID()获取id时，检测到RingBuffer中的剩余id个数小于总个数的50%，将RingBuffer填充满，使其缓存8192个id
-
定时填充（可配置是否使用以及定时任务的周期）
【UidGenerator通过借用未来时间来解决sequence天然存在的并发限制】
因为delta seconds部分是以秒为单位的，所以1个worker 1秒内最多生成的id书为8192个（2的13次方）。
从上可知，支持的最大qps为8192，所以通过缓存id来提高吞吐量。
为什么叫借助未来时间？
因为每秒最多生成8192个id，当1秒获取id数多于8192时，RingBuffer中的id很快消耗完毕，在填充RingBuffer时，生成的id的delta seconds 部分只能使用未来的时间。
（因为使用了未来的时间来生成id，所以上面说的是，【最多】可支持约8.7年）
/*
* Copyright (c) 2017 Baidu, Inc. All Rights Reserve.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.baidu.fsg.uid.impl;
import java.util.ArrayList;
import java.util.List;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.DisposableBean;
import org.springframework.util.Assert;
import com.baidu.fsg.uid.BitsAllocator;
import com.baidu.fsg.uid.UidGenerator;
import com.baidu.fsg.uid.buffer.BufferPaddingExecutor;
import com.baidu.fsg.uid.buffer.RejectedPutBufferHandler;
import com.baidu.fsg.uid.buffer.RejectedTakeBufferHandler;
import com.baidu.fsg.uid.buffer.RingBuffer;
import com.baidu.fsg.uid.exception.UidGenerateException;
/**
* Represents a cached implementation of {@link UidGenerator} extends
* from {@link DefaultUidGenerator}, based on a lock free {@link RingBuffer}
*
* The spring properties you can specified as below:
*
boostPower: RingBuffer size boost for a power of 2, Sample: boostPower is 3, it means the buffer size
* will be ({@link BitsAllocator#getMaxSequence()} + 1) <<
* {@link #boostPower}, Default as {@value #DEFAULT_BOOST_POWER}
* paddingFactor: Represents a percent value of (0 - 100). When the count of rest available UIDs reach the
* threshold, it will trigger padding buffer. Default as{@link RingBuffer#DEFAULT_PADDING_PERCENT}
* Sample: paddingFactor=20, bufferSize=1000 -> threshold=1000 * 20 /100, padding buffer will be triggered when tail-cursorscheduleInterval: Padding buffer in a schedule, specify padding buffer interval, Unit as second
* rejectedPutBufferHandler: Policy for rejected put buffer. Default as discard put request, just do logging
* rejectedTakeBufferHandler: Policy for rejected take buffer. Default as throwing up an exception
*
* @author yutianbao
*/
public class CachedUidGenerator extends DefaultUidGenerator implements DisposableBean {
private static final Logger LOGGER = LoggerFactory.getLogger(CachedUidGenerator.class);
private static final int DEFAULT_BOOST_POWER = 3;
/** Spring properties */
private int boostPower = DEFAULT_BOOST_POWER;
private int paddingFactor = RingBuffer.DEFAULT_PADDING_PERCENT;
private Long scheduleInterval;
private RejectedPutBufferHandler rejectedPutBufferHandler;
private RejectedTakeBufferHandler rejectedTakeBufferHandler;
/** RingBuffer */
private RingBuffer ringBuffer;
private BufferPaddingExecutor bufferPaddingExecutor;
@Override
public void afterPropertiesSet() throws Exception {
// initialize workerId & bitsAllocator
super.afterPropertiesSet();
// initialize RingBuffer & RingBufferPaddingExecutor
this.initRingBuffer();
LOGGER.info("Initialized RingBuffer successfully.");
}
@Override
public long getUID() {
try {
return ringBuffer.take();
} catch (Exception e) {
LOGGER.error("Generate unique id exception. ", e);
throw new UidGenerateException(e);
}
}
@Override
public String parseUID(long uid) {
return super.parseUID(uid);
}
@Override
public void destroy() throws Exception {
bufferPaddingExecutor.shutdown();
}
/**
* Get the UIDs in the same specified second under the max sequence
*
* @param currentSecond
* @return UID list, size of {@link BitsAllocator#getMaxSequence()} + 1
*/
protected List nextIdsForOneSecond(long currentSecond) {
// Initialize result list size of (max sequence + 1)
int listSize = (int) bitsAllocator.getMaxSequence() + 1;
List uidList = new ArrayList<>(listSize);
// Allocate the first sequence of the second, the others can be calculated with the offset
long firstSeqUid = bitsAllocator.allocate(currentSecond - epochSeconds, workerId, 0L);
for (int offset = 0; offset < listSize; offset++) {
uidList.add(firstSeqUid + offset);
}
return uidList;
}
/**
* Initialize RingBuffer & RingBufferPaddingExecutor
*/
private void initRingBuffer() {
// initialize RingBuffer
int bufferSize = ((int) bitsAllocator.getMaxSequence() + 1) << boostPower;
this.ringBuffer = new RingBuffer(bufferSize, paddingFactor);
LOGGER.info("Initialized ring buffer size:{}, paddingFactor:{}", bufferSize, paddingFactor);
// initialize RingBufferPaddingExecutor
boolean usingSchedule = (scheduleInterval != null);
this.bufferPaddingExecutor = new BufferPaddingExecutor(ringBuffer, this::nextIdsForOneSecond, usingSchedule);
if (usingSchedule) {
bufferPaddingExecutor.setScheduleInterval(scheduleInterval);
}
LOGGER.info("Initialized BufferPaddingExecutor. Using schdule:{}, interval:{}", usingSchedule, scheduleInterval);
// set rejected put/take handle policy
this.ringBuffer.setBufferPaddingExecutor(bufferPaddingExecutor);
if (rejectedPutBufferHandler != null) {
this.ringBuffer.setRejectedPutHandler(rejectedPutBufferHandler);
}
if (rejectedTakeBufferHandler != null) {
this.ringBuffer.setRejectedTakeHandler(rejectedTakeBufferHandler);
}
// fill in all slots of the RingBuffer
bufferPaddingExecutor.paddingBuffer();
// start buffer padding threads
bufferPaddingExecutor.start();
}
/**
* Setters for spring property
*/
public void setBoostPower(int boostPower) {
Assert.isTrue(boostPower > 0, "Boost power must be positive!");
this.boostPower = boostPower;
}
public void setRejectedPutBufferHandler(RejectedPutBufferHandler rejectedPutBufferHandler) {
Assert.notNull(rejectedPutBufferHandler, "RejectedPutBufferHandler can't be null!");
this.rejectedPutBufferHandler = rejectedPutBufferHandler;
}
public void setRejectedTakeBufferHandler(RejectedTakeBufferHandler rejectedTakeBufferHandler) {
Assert.notNull(rejectedTakeBufferHandler, "RejectedTakeBufferHandler can't be null!");
this.rejectedTakeBufferHandler = rejectedTakeBufferHandler;
}
public void setScheduleInterval(long scheduleInterval) {
Assert.isTrue(scheduleInterval > 0, "Schedule interval must positive!");
this.scheduleInterval = scheduleInterval;
}
}
获取uid
@Override
public long getUID() {
try {
return ringBuffer.take();
} catch (Exception e) {
LOGGER.error("Generate unique id exception. ", e);
throw new UidGenerateException(e);
}
}RingBuffer缓存已生成的id
（注意：这里的RingBuffer不是Disruptor框架中的RingBuffer，但是借助了很多Disruptor中RingBuffer的设计思想，比如使用缓存行填充解决伪共享问题）
RingBuffer为环形数组，默认容量为sequence可容纳的最大值（8192个），可以通过boostPower参数设置大小。
tail指针、Cursor指针用于环形数组上读写slot：
-
Tail指针
表示Producer生产的最大序号(此序号从0开始，持续递增)。Tail不能超过Cursor，即生产者不能覆盖未消费的slot。当Tail已赶上curosr，此时可通过rejectedPutBufferHandler指定PutRejectPolicy -
Cursor指针
表示Consumer消费到的最小序号(序号序列与Producer序列相同)。Cursor不能超过Tail，即不能消费未生产的slot。当Cursor已赶上tail，此时可通过rejectedTakeBufferHandler指定TakeRejectPolicy
CachedUidGenerator采用了双RingBuffer，Uid-RingBuffer用于存储Uid、Flag-RingBuffer用于存储Uid状态(是否可填充、是否可消费)
由于数组元素在内存中是连续分配的，可最大程度利用CPU cache以提升性能。但同时会带来「伪共享」FalseSharing问题，为此在Tail、Cursor指针、Flag-RingBuffer中采用了CacheLine 补齐方式。
public class RingBuffer {
private static final Logger LOGGER = LoggerFactory.getLogger(RingBuffer.class);
/** Constants */
private static final int START_POINT = -1;
private static final long CAN_PUT_FLAG = 0L; //用于标记当前slot的状态，表示可以put一个id进去
private static final long CAN_TAKE_FLAG = 1L; //用于标记当前slot的状态，表示可以take一个id
public static final int DEFAULT_PADDING_PERCENT = 50; //用于控制何时填充slots的默认阈值：当剩余的可用的slot的个数，小于bufferSize的50%时，需要生成id将slots填满
/** The size of RingBuffer's slots, each slot hold a UID */
private final int bufferSize; //slots的大小，默认为sequence可容量的最大值，即8192个
private final long indexMask;
private final long[] slots; //slots用于缓存已经生成的id
private final PaddedAtomicLong[] flags; //flags用于存储id的状态(是否可填充、是否可消费)
/** Tail: last position sequence to produce */
//Tail指针
//表示Producer生产的最大序号(此序号从0开始，持续递增)。Tail不能超过Cursor，即生产者不能覆盖未消费的slot。当Tail已赶上curosr，此时可通过rejectedPutBufferHandler指定PutRejectPolicy
private final AtomicLong tail = new PaddedAtomicLong(START_POINT); //
/** Cursor: current position sequence to consume */
//表示Consumer消费到的最小序号(序号序列与Producer序列相同)。Cursor不能超过Tail，即不能消费未生产的slot。当Cursor已赶上tail，此时可通过rejectedTakeBufferHandler指定TakeRejectPolicy
private final AtomicLong cursor = new PaddedAtomicLong(START_POINT);
/** Threshold for trigger padding buffer*/
private final int paddingThreshold; //用于控制何时填充slots的阈值
/** Reject put/take buffer handle policy */
//当slots满了，无法继续put时的处理策略。默认实现：无法进行put，仅记录日志
private RejectedPutBufferHandler rejectedPutHandler = this::discardPutBuffer;
//当slots空了，无法继续take时的处理策略。默认实现：仅抛出异常
private RejectedTakeBufferHandler rejectedTakeHandler = this::exceptionRejectedTakeBuffer;
/** Executor of padding buffer */
//用于运行【生成id将slots填满】任务
private BufferPaddingExecutor bufferPaddingExecutor;RingBuffer填充时机
-
程序启动时，将RingBuffer填充满，缓存着8192个id
-
在调用getUID()获取id时，检测到RingBuffer中的剩余id个数小于总个数的50%，将RingBuffer填充满，使其缓存8192个id
-
定时填充（可配置是否使用以及定时任务的周期）
填充RingBuffer
/**
* Padding buffer fill the slots until to catch the cursor
*/
public void paddingBuffer() {
LOGGER.info("Ready to padding buffer lastSecond:{}. {}", lastSecond.get(), ringBuffer);
// is still running
if (!running.compareAndSet(false, true)) {
LOGGER.info("Padding buffer is still running. {}", ringBuffer);
return;
}
// fill the rest slots until to catch the cursor
boolean isFullRingBuffer = false;
while (!isFullRingBuffer) {
//获取生成的id，放到RingBuffer中。
List uidList = uidProvider.provide(lastSecond.incrementAndGet());
for (Long uid : uidList) {
isFullRingBuffer = !ringBuffer.put(uid);
if (isFullRingBuffer) {
break;
}
}
}
// not running now
running.compareAndSet(true, false);
LOGGER.info("End to padding buffer lastSecond:{}. {}", lastSecond.get(), ringBuffer);
} 生成id（上面代码中的uidProvider.provide调用的就是这个方法）
/**
* Get the UIDs in the same specified second under the max sequence
*
* @param currentSecond
* @return UID list, size of {@link BitsAllocator#getMaxSequence()} + 1
*/
protected List nextIdsForOneSecond(long currentSecond) {
// Initialize result list size of (max sequence + 1)
int listSize = (int) bitsAllocator.getMaxSequence() + 1;
List uidList = new ArrayList<>(listSize);
// Allocate the first sequence of the second, the others can be calculated with the offset
//这里的实现很取巧
//因为1秒内生成的id是连续的，所以利用第1个id来生成后面的id，而不用频繁调用snowflake算法
long firstSeqUid = bitsAllocator.allocate(currentSecond - epochSeconds, workerId, 0L);
for (int offset = 0; offset < listSize; offset++) {
uidList.add(firstSeqUid + offset);
}
return uidList;
} 填充缓存行解决“伪共享”
关于伪共享，可以参考这篇文章《伪共享（false sharing），并发编程无声的性能杀手》
//数组在物理上是连续存储的，flags数组用来保存id的状态（是否可消费、是否可填充），在填入id和消费id时，会被频繁的修改。
//如果不进行缓存行填充，会导致频繁的缓存行失效，直接从内存中读数据。
private final PaddedAtomicLong[] flags;
//tail和cursor都使用缓存行填充，是为了避免tail和cursor落到同一个缓存行上。
/** Tail: last position sequence to produce */
private final AtomicLong tail = new PaddedAtomicLong(START_POINT);
/** Cursor: current position sequence to consume */
private final AtomicLong cursor = new PaddedAtomicLong(START_POINT)/**
* Represents a padded {@link AtomicLong} to prevent the FalseSharing problem
*
* The CPU cache line commonly be 64 bytes, here is a sample of cache line after padding:
* 64 bytes = 8 bytes (object reference) + 6 * 8 bytes (padded long) + 8 bytes (a long value)
* @author yutianbao
*/
public class PaddedAtomicLong extends AtomicLong {
private static final long serialVersionUID = -3415778863941386253L;
/** Padded 6 long (48 bytes) */
public volatile long p1, p2, p3, p4, p5, p6 = 7L;
/**
* Constructors from {@link AtomicLong}
*/
public PaddedAtomicLong() {
super();
}
public PaddedAtomicLong(long initialValue) {
super(initialValue);
}
/**
* To prevent GC optimizations for cleaning unused padded references
*/
public long sumPaddingToPreventOptimization() {
return p1 + p2 + p3 + p4 + p5 + p6;
}
}