RocketMQ作为一款优秀的开源消息中间件，实现了文件的高性能存储和读取，在众多消息中间件中脱颖而出，其文件模块设计思想很值得我们学习和借鉴。因此很多开发者在使用的时候，也开始研究其文件存储的实现原理，但是在学习过程中，由于自身知识储备不足，往往只能了解其基本原理和整体架构，对于具体是怎么实现是，用到了什么技术，往往是一知半解。目前网上有很多介绍RocketMQ原理和源码的文章，但是很多都是讲解整体架构，对源代码的分析也仅仅是停留在代码注释层面，导致对整体和细节的把握不能统一， 给人一种"不识庐山真面目，只缘身在此山中"的感觉。
笔者针对开发者在研究RocketMQ的过程中遇到的困惑，基于对RocketMQ的文件存储原理和源码研究，结合JAVA NIO的文件读写，自己动手实现了一个简化版本的RocketMQ文件系统，分享出来，希望能抽丝剥茧，帮助开发者从本质上理解RocketMQ文件存储的原理，起到抛砖引玉，举一反三的作用。
本文不是一篇介绍RocketMQ存储基本原理的文章，本文假设读者对RocketMQ的CommitLog,ConsumeQueue,IndexFile已经有一定的了解，熟悉java NIO文件读写。本文适合对RocketMQ的文件存储原理有一定的了解，并且希望进一步了解RocketMQ是如何通过java NIO实现的读者。
在向commitLog文件写入消息的时候，需要记录该条消息在commitLog文件的偏移量offset（消息在commitLog的起始字节数），读取的时候根据offset读取。RocketMQ保存offset的文件为consumeQueue 和indexFile。
RockeetMQ文件读写流程
RocketMQ文件存储示意图
RocketMQ文件逻辑存储结构
RocketMQ文件offset查找示意图
CommitLog读写
commitLog文件写入的是完整的消息，长度不固定，因此读取的时候只能根据文件存储偏移量offset读取。实际上offset保存在consumeQueue，indexFile文件中。
consumeQueue读写
consumeQueue在消费方拉取消息的时候读取，读取原理比较简单。
consumeQueue每条数据固定长度是20（8:offset+4:msgLen+8:tagCode），顺序写入，每写入一条消息，写入位置postition+20。读取的时候按消息序号index（第几条消息）读取。
假设消费方要消费消息序号index=2的消息（第2条消息），过程如下：
1.定位consumeQueue文件，然后读位置postition定位到40(2*20)，读取数据。
2.根据1读取 的数据取到offset值(存储在consumeQueue的偏移量)。
3.根据2得到的offset值，定位commitLog文件，然后读取commitLog上的整条消息。
参见RocketMQ文件offset查找示意图
indexFile读写
indexFile由indexHead(长度40),500W个hash槽位（每个槽位长度固定4），2000W个indexData组成。
indexFile是为了方便通过messageId读取消息而设计的，因此需要将messageId和消息序号index做一层映射，将messageId取模后得到槽位下标（第几个槽位），然后将当前messageId对应的消息index（消息序号）放到对应的槽位，并将数据顺序保存到indexFile的indexData部分。
写入过程：
1.hash(messageId)%500W得到槽位(slot)的下标slot_index（第几个槽位，槽位长度固定4），
然后将消息序号index存放到对应的槽位(为简化设计，暂不考虑hash冲突的情况)。
2.存储indexData数据，起始存储位置postition 为
indexDataOffset = 40（文件头长度） + 500W * 4+(index-1)*20
读取过程:
1.hash(messageId) % 500W定位到槽位的下标slot_index（第几个槽位）。
2.然后根据槽位下标计算槽位的偏移量slot_offset（每个槽位的固定长度 是4）。
slot_offset = 40（文件头长度） + slot_index * 4。
3.然后根据slot_offset获取到槽位上存储的消息的序号index。
4.根据消息的index计算该条消息存储在indexFile的indexData部分的偏移量indexDataOffset，
indexDataOffset = 40（文件头长度） + 500W * 4+( index - 1 ) * 20
5.根据indexDataOffset读取indexFile的IndexData部分，然后获取commitLog的offset，即可读取到实际的消息。
参见RocketMQ文件offset查找示意图
1.手动生成10个消息，并创建commitLog文件，consumeQueue,indexFile文件
public class CommitLogWriteTest {
private static Long commitLogOffset = 0L;//8byte(commitlog offset)
private static List<ConsumerQueueData> consumerQueueDatas = new ArrayList<>();
private static List<IndexFileItemData> indexFileItemDatas = new ArrayList<>();
private static int MESSAGE_COUNT = 10;
public static void mAIn(String[] args) throws IOException {
createCommitLog();
createConsumerQueue();
createIndexFile();
}
private static void createCommitLog() throws IOException {
System.out.println("");
System.out.println("commitLog file create!" );
FileChannel fileChannel = FileChannel.open(Paths.get(URI.create("file:/c:/123/commitLog.txt")),
StandardOpenOption.WRITE, StandardOpenOption.READ);
MAppedByteBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 409600);
fileChannel.close();
Random random = new Random();
int count = 0;
for (int i = 0; i < MESSAGE_COUNT; i++) {
String topic = "Topic-test";
String msgId = UUID.randomUUID().toString();
String msgBody = "消息内容" + "msgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsg".substring(0, random.nextInt(48) + 1);//
long queueOffset = i;//索引偏移量
String transactionId = UUID.randomUUID().toString();
/* 数据格式，位置固定
int totalSize;//消息长度
String msgId;
String topic;
long queueOffset;//索引偏移量
long bodySize;//消息长度
byte[] body;//消息内容
String transactionId;
long commitLogOffset;//从第一个文件开始算的偏移量
*/
int msgTotalLen = 8 //msgTotalLen field
+ 64 //msgId field长度
+ 64 //topic field长度
+ 8 //索引偏移量field长度
+ 8 //消息长度field长度
+ msgBody.getBytes(StandardCharsets.UTF_8).length //field
+ 64 //transactionId field长度
+ 64 //commitLogOffset field长度;
;
// 定位写入文件的起始位置
//如果3个消息长度分别是100，200，350，则偏移量分别是0,100,300
mappedByteBuffer.position(Integer.valueOf(commitLogOffset + ""));
mappedByteBuffer.putLong(msgTotalLen);//msgTotalLen
mappedByteBuffer.put(getBytes(msgId, 64));//msgId
mappedByteBuffer.put(getBytes(topic, 64));//topic,定长64
mappedByteBuffer.putLong(queueOffset);//索引偏移量
mappedByteBuffer.putLong(msgBody.getBytes(StandardCharsets.UTF_8).length);//bodySize
mappedByteBuffer.put(msgBody.getBytes(StandardCharsets.UTF_8));//body
mappedByteBuffer.put(getBytes(transactionId, 64));
mappedByteBuffer.putLong(commitLogOffset);//commitLogOffset
System.out.println("写入消息，第:" + i + "次");
System.out.println("msgTotalLen:" + msgTotalLen);
System.out.println("msgId:" + msgId);
System.out.println("topic:" + topic);
System.out.println("msgBody:" + msgBody);
System.out.println("transactionId:" + transactionId);
System.out.println("commitLogOffset:" + commitLogOffset);
ConsumerQueueData consumerQueueData = new ConsumerQueueData();
consumerQueueData.setOffset(commitLogOffset);
consumerQueueData.setMsgLength(msgTotalLen);
consumerQueueData.setTagCode(100L);
//准备生成consumeQueue文件
consumerQueueDatas.add(consumerQueueData);
IndexFileItemData indexFileItemData = new IndexFileItemData();
indexFileItemData.setKeyHash(msgId.hashCode());
indexFileItemData.setMessageId(msgId);
indexFileItemData.setPhyOffset(commitLogOffset);
//准备生成indexFile文件
indexFileItemDatas.add(indexFileItemData);
mappedByteBuffer.force();
commitLogOffset = msgTotalLen + commitLogOffset;
count++;
}
System.out.println("commitLog数据保存完成,totalSize:" + count);
}
public static void createConsumerQueue() throws IOException {
System.out.println("");
System.out.println("ConsumerQueue file create!" );
FileChannel fileChannel = FileChannel.open(Paths.get(URI.create("file:/c:/123/consumerQueue.txt")),
StandardOpenOption.WRITE, StandardOpenOption.READ);
MappedByteBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 4096);
fileChannel.close();
int count = 0;
for (int i = 0; i < consumerQueueDatas.size(); i++) {
ConsumerQueueData consumerQueueData = consumerQueueDatas.get(i);
//指定写入位置
mappedByteBuffer.position(i * 20);
mappedByteBuffer.putLong(consumerQueueData.getOffset());//8byte(commitlog offset)
mappedByteBuffer.putInt(consumerQueueData.getMsgLength());//4byte （msgLength)
mappedByteBuffer.putLong(consumerQueueData.getTagCode());//8byte (tagCode)
count++;
System.out.println("consumerQueue数据写入完成:" + JSON.toJSONString(consumerQueueData));
mappedByteBuffer.force();
}
System.out.println("ConsumerQueue数据保存完成count:" + count);
}
public static void createIndexFile() throws IOException {
System.out.println("");
System.out.println("IndexFile file create!" );
//文件场创建时间，在写第一条消息的时候创建
FileChannel fileChannel = FileChannel.open(Paths.get(URI.create("file:/c:/123/index.txt")),
StandardOpenOption.WRITE, StandardOpenOption.READ);
MappedByteBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 409600);
ByteBuffer headerByteBuffer = mappedByteBuffer.slice();
long firstDataTime = System.currentTimeMillis();
fileChannel.close();
//开始写hash槽，从头部后写入
/* 已经填充有index的slot数量
（并不是每个slot槽下都挂载有index索引单元，这 里统计的是所有挂载了index索引单元的slot槽的数量，hash冲突）*/
int hashSlotCount = 0;
/* 已该indexFile中包含的索引单元个数（统计出当前indexFile中所有slot槽下挂载的所有index索引单元的数量之和），
如果没有hash冲突,hashSlotCount = indexCount*/
int indexCount = 0;
//假设建立100个槽位（总长度400）
int soltNum = 100;
for (int i = 0; i < MESSAGE_COUNT; i++) {
IndexFileItemData indexFileItemData = indexFileItemDatas.get(i);
int keyHash = indexFileItemData.getKeyHash();
//取模，计算第几个槽位
int slotPos = keyHash % 100 > 0 ? keyHash % 100 : -1 * (keyHash % 100);
// slot存放的文件偏移量（字节长度）
int absSlotPos = 40 + slotPos * 4;
// 存储实际数据的文件偏移量（字节长度）
int absIndexPos =
40 + soltNum * 4
+ indexCount * 20;
//将indexCount存到对应的hash槽
mappedByteBuffer.putInt(absSlotPos, indexCount);
//写入数据(IndecFile的实际数据部分)
mappedByteBuffer.putInt(absIndexPos, indexFileItemData.getKeyHash());//8byte msg hashcode
mappedByteBuffer.putLong(absIndexPos + 4, indexFileItemData.getPhyOffset());//8byte msg hashcode
mappedByteBuffer.putInt(absIndexPos + 4 + 8, Integer.valueOf((System.currentTimeMillis() - firstDataTime) + ""));//8byte (timeDiff)
mappedByteBuffer.putInt(absIndexPos + 4 + 8 + 4, 0);//8byte (preIndex)暂不考虑hash冲突的情况
//模拟最后一个文件，写入header
if (i == 0) {
//该indexFile中第一条消息的存储时间
headerByteBuffer.putLong(0, firstDataTime);
//该indexFile种第一条消息在commitlog种的偏移量commitlog offset
mappedByteBuffer.putLong(16, indexFileItemData.getPhyOffset());
}
//模拟第一个文件，写入header
if (i == MESSAGE_COUNT - 1) {
//该indexFile种最后一条消息存储时间
headerByteBuffer.putLong(8, System.currentTimeMillis());
//该indexFile中最后一条消息在commitlog中的偏移量commitlog offset
headerByteBuffer.putLong(24, indexFileItemData.getPhyOffset());
}
//已经填充有index的slot数量
headerByteBuffer.putInt(32, hashSlotCount + 1);
//该indexFile中包含的索引单元个数
headerByteBuffer.putInt(36, indexCount + 1);
mappedByteBuffer.force();
System.out.println("msgId:" + indexFileItemData.getMessageId() + ",keyHash:" + keyHash + ",保存槽位为" + slotPos + "的数据,absSlotPos=" + absSlotPos + ",值index=" + indexCount + ",绝对位置:" + absIndexPos + ",commit-phyOffset:" + indexFileItemData.getPhyOffset());
indexCount++;
hashSlotCount++;
}
}
//将变长字符串定长byte[]，方便读取
private static byte[] getBytes(String s, int length) {
int fixLength = length - s.getBytes().length;
if (s.getBytes().length < length) {
byte[] S_bytes = new byte[length];
System.arraycopy(s.getBytes(), 0, S_bytes, 0, s.getBytes().length);
for (int x = length - fixLength; x < length; x++) {
S_bytes[x] = 0x00;
}
return S_bytes;
}
return s.getBytes(StandardCharsets.UTF_8);
}
}
运行结果：
commitLog file create!
写入消息，第:0次
msgTotalLen:338
msgId:8d8eb486-d94c-4da1-bdfe-f0587161ea05
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgm
transactionId:874605e6-69d2-4301-a65e-01e63de75a4d
commitLogOffset:0
写入消息，第:1次
msgTotalLen:338
msgId:57c74e53-4ea1-4a8c-9c7f-c50417d8681e
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgm
transactionId:b991a3e9-66fc-4a54-97fc-1492f7f54d3c
commitLogOffset:338
写入消息，第:2次
msgTotalLen:296
msgId:a0c7c833-9811-4f17-800b-847766aef7dd
topic:Topic-test
msgBody:消息内容msgm
transactionId:9a836d21-704f-46ae-926c-b7933efe06a5
commitLogOffset:676
写入消息，第:3次
msgTotalLen:299
msgId:050d6330-1f4a-4dff-a650-4f7eaee63356
topic:Topic-test
msgBody:消息内容msgmsgm
transactionId:19506313-c7ae-4282-8bc7-1f5ca7735c44
commitLogOffset:972
写入消息，第:4次
msgTotalLen:306
msgId:f5c5be5b-2d9d-4dd8-a9e3-1fdcacc8c2c5
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgms
transactionId:09f3b762-159e-4486-8820-0bce0ef7972d
commitLogOffset:1271
写入消息，第:5次
msgTotalLen:313
msgId:e76911ad-8d05-4d0b-b735-0b2f487f89f1
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgmsgmsgmsg
transactionId:42dce613-6aaf-466b-b185-02a3f7917579
commitLogOffset:1577
写入消息，第:6次
msgTotalLen:321
msgId:05be27f8-fb7a-4662-904f-2263e8899086
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgms
transactionId:6c7db927-911c-4d19-a240-a951fad957bd
commitLogOffset:1890
写入消息，第:7次
msgTotalLen:318
msgId:9a508d90-30f6-4a25-812f-25d750736afe
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgmsgmsgmsgmsgms
transactionId:0bbc5e92-0a78-4699-a7a4-408e7bd3b897
commitLogOffset:2211
写入消息，第:8次
msgTotalLen:305
msgId:63249e08-bd0c-4a5b-954b-aea83cb442be
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgm
transactionId:22cc0dd6-2036-4423-8e6f-d7043b953724
commitLogOffset:2529
写入消息，第:9次
msgTotalLen:329
msgId:93c46c53-b097-4dd0-90d7-06d5d877f489
topic:Topic-test
msgBody:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgm
transactionId:e9078205-15be-42b1-ad7e-55b9f5e229eb
commitLogOffset:2834
commitLog数据保存完成,totalSize:10
ConsumerQueue file create!
consumerQueue数据写入完成:{"msgLength":338,"offset":0,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":338,"offset":338,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":296,"offset":676,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":299,"offset":972,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":306,"offset":1271,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":313,"offset":1577,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":321,"offset":1890,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":318,"offset":2211,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":305,"offset":2529,"tagCode":100}
consumerQueue数据写入完成:{"msgLength":329,"offset":2834,"tagCode":100}
ConsumerQueue数据保存完成count:10
IndexFile file create!
msgId:8d8eb486-d94c-4da1-bdfe-f0587161ea05,keyHash:-358470777,保存槽位为77的数据,absSlotPos=348,值index=0,绝对位置:440,commit-phyOffset:338
msgId:57c74e53-4ea1-4a8c-9c7f-c50417d8681e,keyHash:466366793,保存槽位为93的数据,absSlotPos=412,值index=1,绝对位置:460,commit-phyOffset:676
msgId:a0c7c833-9811-4f17-800b-847766aef7dd,keyHash:1237522456,保存槽位为56的数据,absSlotPos=264,值index=2,绝对位置:480,commit-phyOffset:972
msgId:050d6330-1f4a-4dff-a650-4f7eaee63356,keyHash:-1115509881,保存槽位为81的数据,absSlotPos=364,值index=3,绝对位置:500,commit-phyOffset:1271
msgId:f5c5be5b-2d9d-4dd8-a9e3-1fdcacc8c2c5,keyHash:1219778974,保存槽位为74的数据,absSlotPos=336,值index=4,绝对位置:520,commit-phyOffset:1577
msgId:e76911ad-8d05-4d0b-b735-0b2f487f89f1,keyHash:460184183,保存槽位为83的数据,absSlotPos=372,值index=5,绝对位置:540,commit-phyOffset:1890
msgId:05be27f8-fb7a-4662-904f-2263e8899086,keyHash:-339624012,保存槽位为12的数据,absSlotPos=88,值index=6,绝对位置:560,commit-phyOffset:2211
msgId:9a508d90-30f6-4a25-812f-25d750736afe,keyHash:403329587,保存槽位为87的数据,absSlotPos=388,值index=7,绝对位置:580,commit-phyOffset:2529
msgId:63249e08-bd0c-4a5b-954b-aea83cb442be,keyHash:-1569335572,保存槽位为72的数据,absSlotPos=328,值index=8,绝对位置:600,commit-phyOffset:2834
msgId:93c46c53-b097-4dd0-90d7-06d5d877f489,keyHash:597856342,保存槽位为42的数据,absSlotPos=208,值index=9,绝对位置:620,commit-phyOffset:3163
2.读取consumeQueue文件，并根据offset从commitLog读取一条完整的消息
public class ConsumeQueueMessageReadTest {
public static MappedByteBuffer mappedByteBuffer = null;
private static int MESSAGE_COUNT = 10;
public static void main(String[] args) throws IOException {
FileChannel fileChannel = FileChannel.open(Paths.get(URI.create("file:/c:/123/consumerQueue.txt")),
StandardOpenOption.WRITE, StandardOpenOption.READ);
MappedByteBuffer mappedByteBuffer = fileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 409600);
fileChannel.close();
//根据索引下标读取索引，实际情况是用户消费的最新点位（for循环的i值），
//存在在broker的偏移量文件中
int index = 0;
for (int i = 0; i < MESSAGE_COUNT; i++) {
mappedByteBuffer.position(i * 20);
long commitlogOffset = mappedByteBuffer.getLong();
// System.out.println(commitlogOffset);
long msgLen = mappedByteBuffer.getInt();
Long tag = mappedByteBuffer.getLong();
//System.out.println("======读取到consumerQueue,commitlogOffset:"+commitlogOffset+",msgLen :"+msgLen+"===");
//根据偏移量读取CommitLog
System.out.println("=================commitlog读取第："+index+"消息，偏移量为" + commitlogOffset + "===================");
readCommitLogByOffset(Integer.valueOf(commitlogOffset + ""));
index ++;
}
}
public static MappedByteBuffer initFileChannel() throws IOException {
if (mappedByteBuffer == null) {
FileChannel commitLogfileChannel = FileChannel.open(Paths.get(URI.create("file:/c:/123/commitLog.txt")),
StandardOpenOption.WRITE, StandardOpenOption.READ);
mappedByteBuffer = commitLogfileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 409600);
commitLogfileChannel.close();
}
return mappedByteBuffer;
}
/*
*
* 根据偏移量读取commitLog
* */
public static void readCommitLogByOffset(int offset) throws IOException {
/* 存放顺序，读到时候保持顺序一致
b.putLong(totalSize);//totalSize
b.put(getBytes(msgId, 64));//msgId
b.put(getBytes(topic, 64));//topic,定长64
b.putLong(queueOffset);//索引偏移量
b.putLong(msgBody.getBytes(StandardCharsets.UTF_8).length);//bodySize
b.put(msgBody.getBytes(StandardCharsets.UTF_8));//body
b.put(getBytes(transactionId, 64));
b.putLong(commitLogOffset);//commitLogOffset
*/
MappedByteBuffer mappedByteBuffer = initFileChannel();
mappedByteBuffer.position(offset);
long totalSize = mappedByteBuffer.getLong();//消息长度
byte[] msgIdByte = new byte[64];//uuid 固定是64
mappedByteBuffer.get(msgIdByte);
byte[] topicByte = new byte[64];// 固定是64
mappedByteBuffer.get(topicByte);
long queueOffset = mappedByteBuffer.getLong();
Long bodySize = mappedByteBuffer.getLong();
int bSize = Integer.valueOf(bodySize + "");
byte[] bodyByte = new byte[bSize];//bodySize 长度不固定
mappedByteBuffer.get(bodyByte);
byte[] transactionIdByte = new byte[64];//uuid 固定是64
mappedByteBuffer.get(transactionIdByte);
long commitLogOffset = mappedByteBuffer.getLong();//偏移量
System.out.println("totalSize:" + totalSize);
System.out.println("msgId:" + new String(msgIdByte));
System.out.println("topic:" + new String(topicByte));
System.out.println("queueOffset:" + queueOffset);
System.out.println("bodySize:" + bodySize);
System.out.println("body:" + new String(bodyByte));
System.out.println("transactionId:" + new String(transactionIdByte));
System.out.println("commitLogOffset:" + commitLogOffset);
}
}
运行结果：
=================commitlog读取第：0消息，偏移量为0===================
totalSize:338
msgId:8d8eb486-d94c-4da1-bdfe-f0587161ea05
topic:Topic-test
queueOffset:0
bodySize:58
body:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgm
transactionId:874605e6-69d2-4301-a65e-01e63de75a4d
commitLogOffset:0
=================commitlog读取第：1消息，偏移量为338===================
totalSize:338
msgId:57c74e53-4ea1-4a8c-9c7f-c50417d8681e
topic:Topic-test
queueOffset:1
bodySize:58
body:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgm
transactionId:b991a3e9-66fc-4a54-97fc-1492f7f54d3c
commitLogOffset:338
=================commitlog读取第：2消息，偏移量为676===================
totalSize:296
msgId:a0c7c833-9811-4f17-800b-847766aef7dd
topic:Topic-test
queueOffset:2
bodySize:16
body:消息内容msgm
transactionId:9a836d21-704f-46ae-926c-b7933efe06a5
commitLogOffset:676
=================commitlog读取第：3消息，偏移量为972===================
totalSize:299
msgId:050d6330-1f4a-4dff-a650-4f7eaee63356
topic:Topic-test
queueOffset:3
bodySize:19
body:消息内容msgmsgm
transactionId:19506313-c7ae-4282-8bc7-1f5ca7735c44
commitLogOffset:972
=================commitlog读取第：4消息，偏移量为1271===================
totalSize:306
msgId:f5c5be5b-2d9d-4dd8-a9e3-1fdcacc8c2c5
topic:Topic-test
queueOffset:4
bodySize:26
body:消息内容msgmsgmsgmsgms
transactionId:09f3b762-159e-4486-8820-0bce0ef7972d
commitLogOffset:1271
=================commitlog读取第：5消息，偏移量为1577===================
totalSize:313
msgId:e76911ad-8d05-4d0b-b735-0b2f487f89f1
topic:Topic-test
queueOffset:5
bodySize:33
body:消息内容msgmsgmsgmsgmsgmsgmsg
transactionId:42dce613-6aaf-466b-b185-02a3f7917579
commitLogOffset:1577
=================commitlog读取第：6消息，偏移量为1890===================
totalSize:321
msgId:05be27f8-fb7a-4662-904f-2263e8899086
topic:Topic-test
queueOffset:6
bodySize:41
body:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgms
transactionId:6c7db927-911c-4d19-a240-a951fad957bd
commitLogOffset:1890
=================commitlog读取第：7消息，偏移量为2211===================
totalSize:318
msgId:9a508d90-30f6-4a25-812f-25d750736afe
topic:Topic-test
queueOffset:7
bodySize:38
body:消息内容msgmsgmsgmsgmsgmsgmsgmsgms
transactionId:0bbc5e92-0a78-4699-a7a4-408e7bd3b897
commitLogOffset:2211
=================commitlog读取第：8消息，偏移量为2529===================
totalSize:305
msgId:63249e08-bd0c-4a5b-954b-aea83cb442be
topic:Topic-test
queueOffsmsgm
transactionId:22cc0dd6-2036-4423-8e6f-d7043b953724
commitLogOffset:2529
=================commitlog读取第：9消息，偏移量为2834===================
totalSize:329
msgId:93c46c53-b097-4dd0-90d7-06d5d877f489
topic:Topic-test
queueOffset:9
bodySize:49
body:消息内容msgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgmsgm
transactionId:e9078205-15be-42b1-ad7e-55b9f5e229eb
commitLogOffset:2834
3.根据messageId读取indexFile,然后根据偏移量从CommitLog读取一条完整的消息
public class IndexFileMessageReadTest {
public static MappedByteBuffer mappedByteBuffer = null;
public static void main(String[] args) throws IOException {
String msgId = "8b78474f-b28a-4442-99a0-6f7883f0302b";
readByMessageId(msgId);
}
private static void readByMessageId(String messageId) throws IOException {
FileChannel indexFileChannel = FileChannel.open(Paths.get(URI.create("file:/c:/123/index.txt")),
StandardOpenOption.WRITE, StandardOpenOption.READ);
MappedByteBuffer indexMappedByteBuffer = indexFileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 4096);
indexFileChannel.close();
System.out.println("============get indexFile header===============");
System.out.println("beginTimestampIndex:" + indexMappedByteBuffer.getLong());
System.out.println("endTimestampIndex:" + indexMappedByteBuffer.getLong());
System.out.println("beginPhyoffsetIndex:" + indexMappedByteBuffer.getLong());
System.out.println("endPhyoffsetIndex:" + indexMappedByteBuffer.getLong());
System.out.println("hashSlotcountIndex:" + indexMappedByteBuffer.getInt());
System.out.println("indexCountIndex:" + indexMappedByteBuffer.getInt());
System.out.println("");
int keyHash = messageId.hashCode();
//取模，计算第几个槽位
int slotPos = keyHash % 100 > 0 ? keyHash % 100 : -1 * (keyHash % 100);
System.out.println("messageId:" + messageId + ",取模为:" + slotPos);
// slot的文件偏移量（字节长度）
int absSlotPos = 40 + slotPos * 4;
System.out.println("哈希槽的字节数组位置:(40+" + slotPos + "*4)=" + absSlotPos);
//获取hash槽上存取的件索引,第几个文件
int index = indexMappedByteBuffer.getInt(absSlotPos);
//计算数据需要存储的文件偏移量（字节长度）
int absIndexPos =
40 + 100 * 4
+ index * 20;
System.out.println("第几个文件index=" + index + "，实际存储数据的字节数组位置:(40 + 100 * 4+index *20)=" + absIndexPos);
long keyHash1 = indexMappedByteBuffer.getInt(absIndexPos);
long pyhOffset = indexMappedByteBuffer.getLong(absIndexPos + 4);
int timeDiff = indexMappedByteBuffer.getInt(absIndexPos + 4 + 8);
int preIndexNo = indexMappedByteBuffer.getInt(absIndexPos + 4 + 8 + 4);
System.out.println("从index获取到的commitLog偏移量为:" + pyhOffset);
System.out.println("");
readCommitLogByOffset((int) pyhOffset);
}
public static MappedByteBuffer initFileChannel() throws IOException {
if (mappedByteBuffer == null) {
FileChannel commitLogfileChannel = FileChannel.open(Paths.get(URI.create("file:/c:/123/commitLog.txt")),
StandardOpenOption.WRITE, StandardOpenOption.READ);
mappedByteBuffer = commitLogfileChannel.map(FileChannel.MapMode.READ_WRITE, 0, 409600);
commitLogfileChannel.close();
}
return mappedByteBuffer;
}
/*
*
* 根据偏移量读取CcommitLog
* */
public static void readCommitLogByOffset(int offset) throws IOException {
/*b.putLong(totalSize);//totalSize
b.put(getBytes(msgId, 64));//msgId
b.put(getBytes(topic, 64));//topic,定长64
b.putLong(queueOffset);//索引偏移量
b.putLong(msgBody.getBytes(StandardCharsets.UTF_8).length);//bodySize
b.put(msgBody.getBytes(StandardCharsets.UTF_8));//body
b.put(getBytes(transactionId, 64));
b.putLong(commitLogOffset);//commitLogOffset
*/
System.out.println("=================commitlog读取偏移量为" + offset + "的消息===================");
MappedByteBuffer mappedByteBuffer = initFileChannel();
mappedByteBuffer.position(offset);
long totalSize = mappedByteBuffer.getLong();//消息长度
byte[] msgIdByte = new byte[64];//uuid 固定是64
mappedByteBuffer.get(msgIdByte);
byte[] topicByte = new byte[64];// 固定是64
mappedByteBuffer.get(topicByte);
long queueOffset = mappedByteBuffer.getLong();
Long bodySize = mappedByteBuffer.getLong();
int bSize = Integer.valueOf(bodySize + "");
byte[] bodyByte = new byte[bSize];//bodySize 长度不固定
mappedByteBuffer.get(bodyByte);
byte[] transactionIdByte = new byte[64];//uuid 固定是64
mappedByteBuffer.get(transactionIdByte);
long commitLogOffset = mappedByteBuffer.getLong();//偏移量
System.out.println("totalSize:" + totalSize);
System.out.println("msgId:" + new String(msgIdByte));
System.out.println("topic:" + new String(topicByte));
System.out.println("queueOffset:" + queueOffset);
System.out.println("bodySize:" + bodySize);
System.out.println("body:" + new String(bodyByte));
System.out.println("transactionId:" + new String(transactionIdByte));
System.out.println("commitLogOffset:" + commitLogOffset);
}
public static byte[] toByteArray(long number) {
byte length = Long.BYTES;
byte[] bytes = new byte[length];
for (byte i = 0; i < length; i++) {
bytes[length - 1 - i] = (byte) number;
number >>= 8;
}
return bytes;
}
}
运行结果：
============get indexFile header===============
beginTimestampIndex:1669602898169
endTimestampIndex:1669602898176
beginPhyoffsetIndex:338
endPhyoffsetIndex:3163
hashSlotcountIndex:10
indexCountIndex:10
messageId:9a508d90-30f6-4a25-812f-25d750736afe,取模为:87
哈希槽的字节数组位置:(40+87*4)=388
第几个文件index=7，实际存储数据的字节数组位置:(40 + 100 * 4+index *20)=580
从index获取到的commitLog偏移量为:2529
=================commitlog读取偏移量为2529的消息===================
totalSize:305
msgId:63249e08-bd0c-4a5b-954b-aea83cb442be
topic:Topic-test
queueOffset:8
bodySize:25
body:消息内容msgmsgmsgmsgm
transactionId:22cc0dd6-2036-4423-8e6f-d7043b953724
commitLogOffset:2529
本文基于java NIO实现了RocketMQ的文件系统的最精简的功能，希望能帮助开发人员加深对RocketMQ文件系统底层实现原理的了解，并能熟练运用Java NIO进行文件读写。欢迎一起交流讨论，不足的地方欢迎指正。
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