nio~view buffer

此文介绍nio中ByteBuffer的特性之一,视图.

*view buffer概念

---把ByteBuffer转换为其他数据类型的buffer,比如char,long,float等,这样就方便处理连续的多字节情况

---相关的api,形如as*Buffer,如LongBuffer asLongBuffer()之类

*功能特性(ByteBuffer注释中"views"条目有详细的注解)

---视图和原ByteBuffer共享数据,但各自独立位置信息.

"Changes to the byte buffer's content
will be visible in the view buffer, and vice versa; the two buffers'
position, limit, and mark values are independent."

---view buffer的字节次序在创建时被确定

"The byte order of a view buffer is fixed to be that of its byte buffer
at the time that the view is created. "

---优点

方便处理大块数据

*api分析

以子类HeapByteBuffer中的实现方式为例,其他类型的实现方式相同

public CharBuffer asCharBuffer() {
　　int size = this.remaining() >> 1;  //char比byte多一个字节,容量／２
　　int off = offset + position();  //offset从原buffer的position开始
　　return (bigEndian
	? (CharBuffer)(new ByteBufferAsCharBufferB(this, //与原buffer共享数据
							       -1,　　//mark =-1
							       0,    //position =0
							       size, //limit　＝size
							       size, //compacity =size
							       off)) //offset =off
	: (CharBuffer)(new ByteBufferAsCharBufferL(this,
							       -1,
							       0,
							       size,
							       size,
							       off)));
}
/*
*1.view buffer的数据内容是原buffer的remaining部分
*2.view buffer和原buffer保持相同的字节顺序
*/

*测试示例

/**
 * Feb 16, 2011 by dzh
 */
package buffer.view;

import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.nio.CharBuffer;

/**
 * @author dzh
 *
 */
public class ViewBufferTest {
	
	public static void main(String[] args) {
		ByteBuffer byteBuf =ByteBuffer.allocate(6).order(ByteOrder.BIG_ENDIAN);
		byteBuf.put((byte) 0).put((byte) 'A').mark();
		byteBuf.put(new byte[]{0,'B',0,'C'}).reset();
		
		CharBuffer charBuf =byteBuf.asCharBuffer();
		
		System.out.println(byteBuf.toString());  //java.nio.HeapByteBuffer[pos=2 lim=6 cap=6]
		System.out.println(charBuf.toString());  //BC
	}
	
}