Netty ByteBuf 是NIO中ByteBuffer的封装,相比JDK ByteBuffer更加易用;

• 为读/写分别维护单独的指针,不需要通过flip()进行读写模式切换
• 容量自定伸缩(类似ArrayList)
• Fluent API(链式调用)
• 通过内置的CompositeBuffer来减少数据拷贝（Zero copy）
• 内存池，减少GC压力

一.内部结构

ByteBuf通过两个索引（reader index、writer index）划分为三个区域：

• read index前面的数据是已经读过的数据,这些数据是可丢弃的数据
• 从reader index开始,到write index之前的数据都是可读数据

• 从write index开始,为可写区域

  
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二.主要API

顺序读/写：- 改变reader/writer indexwriteByte()

writeLong()
writeXXX() - 增加write index
readByte()
readLong()
readXXX() - 增加read index

  public static void writeAndRead() {
         ByteBuf buffer = Unpooled.buffer(20); //get reference form somewhere
         int i = 0;
         ;
        while (buffer.writableBytes() >= 4) {
            buffer.writeInt(i++);
        }
        
        while (buffer.isReadable()) {
            System.out.println(buffer.readInt());
            //清除已读
//            buffer.discardReadBytes();
        }
    }

随机读/写： - 不改变read/write index

getXXX(index)
setXXX(index, byte)

 public static void byteBufSetGet() {
        Charset utf8 = Charset.forName("UTF-8");
        ByteBuf buf = Unpooled.copiedBuffer("Netty in Action rocks!", utf8);
        System.out.println((char)buf.getByte(0));
        int readerIndex = buf.readerIndex();
        int writerIndex = buf.writerIndex();
        System.out.println("readerIndex = " + readerIndex + "; writerIndex = " + writerIndex);
        buf.setByte(0, (byte)'B');
        System.out.println((char)buf.getByte(0));
        System.out.println("readerIndex = " + buf.readerIndex() + "; writerIndex = " + buf.writerIndex());
    }

mark/reset：

markReaderIndex()
markWriterIndex()
resetReaderIndex()
resetWriterIndex()
writerIndex(index)
readerIndex(index)

丢弃不可读数据

discardReadBytes方法 - 类似compact 方法,丢弃不可读的数据,readIndex置为0,writeIndex递减;
clear方法 初始化

image.png

查询方法:

indexOf
bytesBefore
forEachByte（ByteBufProcessor）

根据内存的位置

HeapByteBuf
基于数组- 内部为一个字节数组 (byte array)
hasArray()返回True
array()返回其内部的数组，可以对数组进行直接操作

public static void heapBuffer() {
        ByteBuf heapBuf = Unpooled.buffer(16);   
        if (heapBuf.hasArray()) {
            int i = 0;
            while (heapBuf.writableBytes() > 0) {
                heapBuf.writeByte(i++);
            }
            byte[] array = heapBuf.array();
            int offset = heapBuf.arrayOffset() + heapBuf.readerIndex();
            int length = heapBuf.readableBytes();
            handleArray(array, offset, length);
        }
    }

DirectByteBuf
jvm堆外内存
具有更好的性能
创建和释放开销更大

 public static void directBuffer() {
        ByteBuf directBuf = Unpooled.directBuffer(16);    
        if (!directBuf.hasArray()) {
            int i = 0;
            while (directBuf.writableBytes() > 0) {
                directBuf.writeByte(i++);
            }
            int length = directBuf.readableBytes();
            byte[] array = new byte[length];
            directBuf.getBytes(directBuf.readerIndex(), array);
            handleArray(array, 0, length);
        }
    }

三 ByteBuf分配

不直接通过new来创建，而是通过ByteBufAllocator来创建
UnpooledByteBufAllocator
PooledByteBufAllocator

image.png

 private static void createByteBuf() {
        ByteBuf buf1 = UnpooledByteBufAllocator.DEFAULT.buffer(10);
        ByteBuf buf2 = UnpooledByteBufAllocator.DEFAULT.heapBuffer(10);
        ByteBuf buf3 = UnpooledByteBufAllocator.DEFAULT.directBuffer(10);
        ByteBuf buf4 = UnpooledByteBufAllocator.DEFAULT.compositeBuffer(10);
        ByteBuf buf5 = UnpooledByteBufAllocator.DEFAULT.compositeHeapBuffer(10);
        ByteBuf buf6 = UnpooledByteBufAllocator.DEFAULT.compositeDirectBuffer(10);
        ByteBuf buf7 = UnpooledByteBufAllocator.DEFAULT.ioBuffer(10);
    }

Unpooled 的工具类，它提供了静态的辅助方法来创建未池化的ByteBuf实例

image.png

private static void createByteBuf2() {
        Charset utf8 = Charset.forName("UTF-8");
        ByteBuf buf1 = Unpooled.buffer(10);
        ByteBuf buf2 = Unpooled.directBuffer(10);
        String content = "Netty in Action rocks!";
        ByteBuf buf3 = Unpooled.wrappedBuffer(content.getBytes());
        ByteBuf buf4 = Unpooled.copiedBuffer(content, utf8);
    }

以上,简单介绍了下Netty中ByteBuf的使用,如果想详细了解内部实现,可自行翻阅源码.如有错误,欢迎指正;