Netty學習12-緩衝區【2】ByteBuf

1 概述

《Netty學習11-緩衝區之JDK ByteBuffer》一文中介紹了JDK的緩衝區ByteBuffer的用法和不足。爲了彌補這些不足，Netty提供了自己的緩衝區實現ByteBuf（netty3.X是ChannelBuffer）。

2 實現策略

JDK的ByteBuffer已經提供了基礎的能力，Netty的ByteBuf的實現可以有兩種策略。

[1] 參考JDK ByteBuffer的實現，增加額外功能，解決原ByteBuffer的問題。
[2] 句話JDK ByteBuffer，通過Facade模式對其進行包裝。

3 代碼

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;

public class NettyByteBuf {
	public static void main(String[] args) {

		// 實例初始化
		ByteBuf buffer = Unpooled.buffer(20);
		System.out.println(String.format(
				"#####初始化 capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));

		// 寫數據
		String msg = "xy";
		buffer.writeBytes(msg.getBytes());
		System.out.println(String.format(
				"#####寫數據 capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));

		// 讀數據（只讀取一個字節）
		// byte[] vArray = new Byte[ buffer.writerIndex()];
		byte[] vArray = new byte[1];
		buffer.readBytes(vArray);
		System.out.println("result is " + new String(vArray));
		System.out.println(String.format(
				"#####讀數據 capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));

		// discardReadBytes
		buffer.discardReadBytes();
		System.out.println(String.format(
				"#####discard capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));

		// clear
		buffer.clear();
		System.out.println(String.format(
				"#####clear capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));
		
	}
}

#####初始化 capacity:20 readerIndex:0 writeIndex:0 readeableBytes:0 writableBytes:20
#####寫數據 capacity:20 readerIndex:0 writeIndex:2 readeableBytes:2 writableBytes:18
result is x
#####讀數據 capacity:20 readerIndex:1 writeIndex:2 readeableBytes:1 writableBytes:18
#####discard capacity:20 readerIndex:0 writeIndex:1 readeableBytes:1 writableBytes:19
#####clear capacity:20 readerIndex:0 writeIndex:0 readeableBytes:0 writableBytes:20

4 圖解

上述過程的圖解說明，圖片摘自李林峯老師《Netty權威指南》一書。

  

  

 

5 markReaderIndex和resetReaderIndex

import io.netty.buffer.ByteBuf;
import io.netty.buffer.Unpooled;

public class NettyByteBuf2 {
	public static void main(String[] args) {

		// 實例初始化
		ByteBuf buffer = Unpooled.buffer(20);
		System.out.println(String.format(
				"#####初始化 capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));

		// 寫數據
		String msg = "xy";
		buffer.writeBytes(msg.getBytes());
		System.out.println(String.format(
				"#####寫數據 capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));

		// 讀數據（只讀取一個字節）
		byte[] vArray = new byte[1];
		// 標記讀索引位置
		buffer.markReaderIndex();
		buffer.readBytes(vArray);
		System.out.println("result is " + new String(vArray));
		System.out.println(String.format(
				"#####讀數據 capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));		
		
		// 還原讀索引位置
		buffer.resetReaderIndex();
		System.out.println(String.format(
				"#####resetReaderIndex capacity:%s readerIndex:%s writeIndex:%s readeableBytes:%s writableBytes:%s",
				buffer.capacity(), buffer.readerIndex(), buffer.writerIndex(),buffer.readableBytes(),buffer.writableBytes()));		
		
	}
}

#####初始化 capacity:20 readerIndex:0 writeIndex:0 readeableBytes:0 writableBytes:20
#####寫數據 capacity:20 readerIndex:0 writeIndex:2 readeableBytes:2 writableBytes:18
result is x
#####讀數據 capacity:20 readerIndex:1 writeIndex:2 readeableBytes:1 writableBytes:18
#####resetReaderIndex capacity:20 readerIndex:0 writeIndex:2 readeableBytes:2 writableBytes:18