Netty ByteBuf详解
本文主要介绍Netty5 ByteBuf原理及具体使用。
基本结构：与NIO ByteBuffer类似，使用ByteBuffer往往需要在读写之间通过flip切换。ByteBuf里维护两个index,一个readerIndex,一个writerIndex;readerIndex writerIndex capactiy 的三者的关系是：
0 <=readerIndex <=writerIndex <= capacity
当往byteBuf写入一个byte时，writerIndex++,从bytebuf读一个byte时，readerIndex++;
+-------------------+------------------+------------------+ | discardable bytes | readable bytes | writable bytes | | | (CONTENT) | | +-------------------+------------------+------------------+ | | | | 0 <= readerIndex <= writerIndex <= capacity
基本操作：
通过index随机读：
ByteBuf byteBuf= Unpooled.buffer();
byteBuf.writeBytes("hello world".getBytes());
System.out.println((char)byteBuf.getByte(0));
System.out.println((char)byteBuf.getByte(1));
支持顺序读：满足readeableBytes>0 (writerIndex - readerIndex )
ByteBuf byteBuf= Unpooled.buffer();
byteBuf.writeBytes("hello world".getBytes());
while(byteBuf.readableBytes()>0){
System.out.println((char)byteBuf.readByte());
}
写操作：
支持单个byte写入，或者直接写入byte数组；可以通过writeIndex设定writerIndex,但是如果
writerIndex < readerIndex || writerIndex > capacity() 会抛出IndexOutOfBoundsException；
ByteBuf byteBuf= Unpooled.buffer(50);
while(byteBuf.writableBytes()>10){
byteBuf.writeByte('a');
byteBuf.writerIndex(byteBuf.writerIndex()+10);
}
System.out.println("writerIndex:"+byteBuf.writerIndex());
System.out.println("readerIndex:"+byteBuf.readerIndex());
System.out.println("index of 0 is:"+(char)byteBuf.getByte(0));
System.out.println("index of 11 is:"+(char)byteBuf.getByte(11));
System.out.println("index of 22 is:"+(char)byteBuf.getByte(22));
System.out.println("index of 33 is:"+(char)byteBuf.getByte(33));
System.out.println("index of 40 is:"+(char)byteBuf.getByte(40));
System.out.println(new String(byteBuf.array()));
discardReadBytes操作：
把bytebuf中读过的byte抛弃掉，writerIndex置为writerIndex-readerIndex,readerIndex置为0；
ByteBuf byteBuf= Unpooled.buffer();
byteBuf.writeBytes("hello world".getBytes());
System.out.println("readerIndex:" + byteBuf.readerIndex());
System.out.println("writerIndex:" + byteBuf.writerIndex());
System.out.println((char)byteBuf.readByte());
System.out.println(new String(byteBuf.array()));
byteBuf.discardReadBytes();
System.out.println("readerIndex:" + byteBuf.readerIndex());
System.out.println("writerIndex:" + byteBuf.writerIndex());
System.out.println(new String(byteBuf.array()));
clear mark reset 操作：
clear把bytebuf 的writerIndex=readerindex=0;
mark把当前的writerIndex或者readerIndex mark一下，记录到私有变量中
reset把当前的writerIndex或者readerIndex恢复到mark的index;
ByteBuf byteBuf= Unpooled.buffer();
byteBuf.writeBytes("hello world".getBytes());
System.out.println(new String(byteBuf.array()));
System.out.println("readerIndex:" + byteBuf.readerIndex());
System.out.println("writerIndex:" + byteBuf.writerIndex());
byteBuf.clear();
System.out.println("after clear");
System.out.println("readerIndex:" + byteBuf.readerIndex());
System.out.println("writerIndex:" + byteBuf.writerIndex());
System.out.println(new String(byteBuf.array()));
byteBuf.writerIndex(11);
byteBuf.readByte();
//在index 为1时mark ，reset之后重新回到mark的地方
byteBuf.markReaderIndex();
byteBuf.readByte();
byteBuf.readByte();
byteBuf.resetReaderIndex();
System.out.println("readerIndex:" + byteBuf.readerIndex());
Netty中分配ByteBuf有两种方式，一个是Pooled 池化的，一个是Unpooled 非池化的。
Unpooled类封装了UnpooledByteBufAllocator，用来初始化ByteBuf 包含Heap类型和Direct类型的。
PooledByteBufAllocator用来分配池化的ByteBuf,重复利用ByteBuf数组，减少分配回收开销。
同时Buffer模块还提供方便的工具类ByteBufUtil来操作ByteBuf。
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转发标注来源：http://my.oschina.net/robinyao/blog/407469