05-netty基础-ByteBuf数据结构

1 基本概念

在网络编程中，字节数据的处理是核心环节之一。无论是客户端与服务器之间的通信，还是数据的编解码操作，都离不开对字节缓冲区的高效管理。Java 原生的 ByteBuffer 虽然提供了基础功能，但在灵活性、性能和易用性上存在诸多局限。而 Netty 框架提供的 ByteBuf 则彻底解决了这些问题，成为处理网络字节数据的首选工具。

2 为什么需要 Bytebuf

Java 原生的 ByteBuffer 存在以下明显短板：

单指针设计：仅有一个 position 指针，读写操作需频繁调用 flip() 切换模式，容易出错；
固定容量：创建后容量不可动态调整，超出容量时需手动扩容，操作繁琐；
内存管理复杂：堆外内存（直接内存）的释放依赖 GC，可能导致内存泄漏；
API 设计不友好：缺乏直接读写基本类型、字符串的便捷方法。

相比之下，ByteBuf 针对网络编程场景进行了全面优化：

采用双指针设计，读写无需切换模式；
支持自动扩容，无需手动处理容量不足问题；
内置引用计数机制，精准控制内存生命周期；
提供丰富的 API，简化字节数据操作。

3 Bytebuf数据结构

3.1 创建一个空Bytebuf

 public static void main(String[] args) {// 空ByteBufemptyByteBuf();}private static void emptyByteBuf() {// byteBuf 分配一块内存，自动判断是否分配堆内存或者堆外内存ByteBuf byteBuf = ByteBufAllocator.DEFAULT.buffer();printMsg(byteBuf);}private static void printMsg(ByteBuf byteBuf) {StringBuilder stringBuilder = new StringBuilder();stringBuilder.append("read index:").append(byteBuf.readerIndex()).append("\n");stringBuilder.append("write index:").append(byteBuf.writerIndex()).append("\n");stringBuilder.append("capacity:").append(byteBuf.capacity()).append("\n");stringBuilder.append("maxCapacity:").append(byteBuf.maxCapacity()).append("\n");ByteBufUtil.appendPrettyHexDump(stringBuilder, byteBuf);System.out.println(stringBuilder.toString());}

输出
read index:0
write index:0
capacity:256
maxCapacity:2147483647

数据为空时bytebuf的内存结构

3.2 往Bytebuf中写入数据

 private static void writeByte() {// byteBuf 分配一块内存，自动判断是否分配堆内存或者堆外内存ByteBuf byteBuf = ByteBufAllocator.DEFAULT.buffer();// 写入1个字节byteBuf.writeBytes(new byte[]{(byte)1});printMsg(byteBuf);}private static void printMsg(ByteBuf byteBuf) {StringBuilder stringBuilder = new StringBuilder();stringBuilder.append("read index:").append(byteBuf.readerIndex()).append("\n");stringBuilder.append("write index:").append(byteBuf.writerIndex()).append("\n");stringBuilder.append("capacity:").append(byteBuf.capacity()).append("\n");stringBuilder.append("maxCapacity:").append(byteBuf.maxCapacity()).append("\n");ByteBufUtil.appendPrettyHexDump(stringBuilder, byteBuf);System.out.println(stringBuilder.toString());}

read index:0
write index:1
capacity:256
maxCapacity:2147483647
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 01 |. |
+--------+-------------------------------------------------+----------------+

写入1个字节内存结构

3.3 Bytebuf中读取数据

private static void readByte() {// byteBuf 分配一块内存，自动判断是否分配堆内存或者堆外内存ByteBuf byteBuf = ByteBufAllocator.DEFAULT.buffer();// 写入1个字节byteBuf.writeBytes(new byte[]{(byte)1, (byte)2});printMsg(byteBuf);// 读取1个字节byte b = byteBuf.readByte();printMsg(byteBuf);}private static void printMsg(ByteBuf byteBuf) {StringBuilder stringBuilder = new StringBuilder();stringBuilder.append("read index:").append(byteBuf.readerIndex()).append("\n");stringBuilder.append("write index:").append(byteBuf.writerIndex()).append("\n");stringBuilder.append("capacity:").append(byteBuf.capacity()).append("\n");stringBuilder.append("maxCapacity:").append(byteBuf.maxCapacity()).append("\n");ByteBufUtil.appendPrettyHexDump(stringBuilder, byteBuf);System.out.println(stringBuilder.toString());}

read index:0
write index:2
capacity:256
maxCapacity:2147483647
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 01 02 |.. |
+--------+-------------------------------------------------+----------------+
read index:1
write index:2
capacity:256
maxCapacity:2147483647
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 02 |. |
+--------+-------------------------------------------------+----------------+

3.4 Bytebuf读取后重复读取

 public static void main(String[] args) {// jvm堆内存ByteBuf byteBuf = ByteBufAllocator.DEFAULT.buffer();// 写数据byteBuf.maxWritableBytes();byteBuf.writeBytes(new byte[] {1, 2, 3, 4});printMsg(byteBuf);// 重置，写指针复位到起始位置重新写入，覆盖写byteBuf.resetWriterIndex();byteBuf.writeInt(6);printMsg(byteBuf);// 读byteBuf.markReaderIndex();byte a = byteBuf.readByte();byte b = byteBuf.readByte();byteBuf.resetReaderIndex();printMsg(byteBuf);}private static void printMsg(ByteBuf byteBuf) {StringBuilder stringBuilder = new StringBuilder();stringBuilder.append("read index:").append(byteBuf.readerIndex()).append("\n");stringBuilder.append("write index:").append(byteBuf.writerIndex()).append("\n");stringBuilder.append("capacity:").append(byteBuf.capacity()).append("\n");stringBuilder.append("maxCapacity:").append(byteBuf.maxCapacity()).append("\n");ByteBufUtil.appendPrettyHexDump(stringBuilder, byteBuf);System.out.println(stringBuilder.toString());}

read index:0
write index:4
capacity:256
maxCapacity:2147483647
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 01 02 03 04 |.... |
+--------+-------------------------------------------------+----------------+
read index:0
write index:8
capacity:256
maxCapacity:2147483647
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 01 02 03 04 01 02 03 04 |........ |
+--------+-------------------------------------------------+----------------+
read index:0
write index:4
capacity:256
maxCapacity:2147483647
+-------------------------------------------------+
| 0 1 2 3 4 5 6 7 8 9 a b c d e f |
+--------+-------------------------------------------------+----------------+
|00000000| 00 00 00 06 |.... |
+--------+-------------------------------------------------+----------------+

备注：markWriterIndex、resetWriterIndex一般是配套使用，如果未标记写指针的位置默认从起始位置开始；markReaderIndex、resetReaderIndex一般也是配套使用，如果未标记读取的位置，则从起始位置重新读取

4 Bytebuf扩容

初始尝试：如果 minNewCapacity 小于等于 4MB（4194304 字节），新容量为大于等于 minNewCapacity 的最小 2 的幂；
超过 4MB 时：如果 minNewCapacity 大于 4MB，新容量为大于等于 minNewCapacity 且为 4MB 整数倍的值；
上限控制：新容量不能超过 ByteBuf 的最大容量。

案例：未超过4M，成倍增长

public class ByteBufCapacity {public static void main(String[] args) {// jvm堆内存ByteBuf byteBuf = ByteBufAllocator.DEFAULT.buffer();for (int i=0; i<256; i++) {byteBuf.writeBytes(new byte[] {(byte) i});}printMsg(byteBuf);for (int i=0; i<256; i++) {byteBuf.writeBytes(new byte[] {(byte) i});}printMsg(byteBuf);}private static void printMsg(ByteBuf byteBuf) {StringBuilder stringBuilder = new StringBuilder();stringBuilder.append("read index:").append(byteBuf.readerIndex()).append("\n");stringBuilder.append("write index:").append(byteBuf.writerIndex()).append("\n");stringBuilder.append("capacity:").append(byteBuf.capacity()).append("\n");stringBuilder.append("maxCapacity:").append(byteBuf.maxCapacity()).append("\n");
//        ByteBufUtil.appendPrettyHexDump(stringBuilder, byteBuf);System.out.println(stringBuilder.toString());}

read index:0
write index:256
capacity:256
maxCapacity:2147483647
read index:0
write index:512
capacity:512
maxCapacity:2147483647

大于4M，每次增加4M扩容

package com.bonnie.netty.bytebuf;import io.netty.buffer.ByteBuf;
import io.netty.buffer.ByteBufAllocator;public class ByteBufCapacity {public static void main(String[] args) {// jvm堆内存ByteBuf byteBuf = ByteBufAllocator.DEFAULT.buffer();// 4Mfor (int i=0; i<4194304; i++) {byteBuf.writeBytes(new byte[] {(byte) i});}printMsg(byteBuf);// 扩容+4Mfor (int i=0; i<4194304; i++) {byteBuf.writeBytes(new byte[] {(byte) i});}printMsg(byteBuf);// 扩容+4Mfor (int i=0; i<4194304; i++) {byteBuf.writeBytes(new byte[] {(byte) i});}printMsg(byteBuf);}private static void printMsg(ByteBuf byteBuf) {StringBuilder stringBuilder = new StringBuilder();stringBuilder.append("read index:").append(byteBuf.readerIndex()).append("\n");stringBuilder.append("write index:").append(byteBuf.writerIndex()).append("\n");stringBuilder.append("capacity:").append(byteBuf.capacity()).append("\n");stringBuilder.append("maxCapacity:").append(byteBuf.maxCapacity()).append("\n");
//        ByteBufUtil.appendPrettyHexDump(stringBuilder, byteBuf);System.out.println(stringBuilder.toString());}}

read index:0
write index:4194304
capacity:4194304
maxCapacity:2147483647
read index:0
write index:8388608
capacity:8388608
maxCapacity:2147483647
read index:0
write index:12582912
capacity:12582912
maxCapacity:2147483647

5 堆内存和堆外内存

实堆内的意思就是java虚拟机里面的堆空间，而堆外的意思是java进程中系统为它分配的堆空间，jvm堆中的数据如果想要写入磁盘，就会进行write系统调用，调用过程为：jvm堆->系统堆->PageCache->磁盘,如果数据是放在系统heap中，调用过程为：系统堆->PageCache->磁盘。我们可以看到，使用堆内内存写入数据会少一次的拷贝次数