零拷贝指的是没有CPU拷贝，并不是不拷贝；减少上下文切换
一、概念说明
1、传统IO
需要4次拷贝，3次上下文切换
2、mmap
mmap 通过内存映射，将文件映射到内存缓冲区，同时用户空间可以共享内存缓冲区的数据，减少内核空间到用户空间的拷贝
需要3次拷贝，3次上下文切换
3、sendfile
Linux 2.4 避免了从内核缓冲区到Socket Buffer的拷贝，直接拷贝到协议栈，从而减少一次数据拷贝
需要2次拷贝，3次上下文切换
4、mmap与sendfile
mmap适合小数据量读写，sendfile适合大文件传输
mmap需要4次上下文切换，3次数据拷贝；sendfile需要3次上下文切换，最少2次数据拷贝
send可用利用DMA方式，减少CPU拷贝，mmap则不能（必须从内核拷贝到Socket缓冲区）
二、传统IO传输文件代码示例
1、服务端代码
import java.io.DataInputStream;import java.io.FileOutputStream;import java.io.IOException;import java.net.ServerSocket;import java.net.Socket;public class BIOServer { public static void main(String[] args) throws IOException { ServerSocket serverSocket = new ServerSocket(7000); while (true) { Socket socket = serverSocket.accept(); DataInputStream dataInputStream = new DataInputStream(socket.getInputStream()); try { long total = 0; byte[] bytes = new byte[4096]; FileOutputStream fileOutputStream = new FileOutputStream("d:\\temp\\04.zip"); while (true) { int read = dataInputStream.read(bytes, 0, bytes.length); if (read == -1) { //文件读取结束，退出循环 break; } total += read; fileOutputStream.write(bytes, 0, read); } System.out.println("收到客户端发送文件，总字节数：" + total); fileOutputStream.close(); } catch (Exception e) { e.printStackTrace(); } } }}2、客户端代码
import java.io.DataOutputStream;import java.io.FileInputStream;import java.io.IOException;import java.net.Socket;public class BIOClient { public static void main(String[] args) throws IOException { Socket socket = new Socket("127.0.0.1", 7000); FileInputStream fileInputStream = new FileInputStream("d:\\temp\\03.zip"); DataOutputStream dataOutputStream = new DataOutputStream(socket.getOutputStream()); byte[] bytes = new byte[4096]; long readCount; long total = 0; long start = System.currentTimeMillis(); while ((readCount = fileInputStream.read(bytes)) >= 0) { total += readCount; dataOutputStream.write(bytes); } System.out.println("发送总字节数：" + total + ", 总耗时：" + (System.currentTimeMillis() - start)); dataOutputStream.close(); socket.close(); fileInputStream.close(); }}3、控制台出输出
测试发送9M的压缩文件，耗时在26ms左右
发送总字节数：9428963, 总耗时：26三、NIO传输文件代码示例
1、服务端代码
package com.hj.io.nio.zero;import java.io.FileOutputStream;import java.io.IOException;import java.net.InetSocketAddress;import java.nio.ByteBuffer;import java.nio.channels.ServerSocketChannel;import java.nio.channels.SocketChannel;public class NIOServerFile { public static void main(String[] args) throws IOException { InetSocketAddress address = new InetSocketAddress(7000); ServerSocketChannel serverSocketChannel = ServerSocketChannel.open(); serverSocketChannel.socket().bind(address); //创建buffer ByteBuffer byteBuffer = ByteBuffer.allocate(4096); while (true) { //等待客户端链接 SocketChannel socketChannel = serverSocketChannel.accept(); System.out.println("收到客户端链接"); int total = 0; FileOutputStream fileOutputStream = new FileOutputStream("d:\\temp\\05.zip"); //循环读取数据，并存储到硬盘 while (true) { try { int readCount = socketChannel.read(byteBuffer); if (readCount == -1) { //文件读取结束 break; } total += readCount; fileOutputStream.write(byteBuffer.array(),0,readCount); //将buffer倒带 byteBuffer.rewind(); } catch (IOException e) { break; } } System.out.println("收到客户端发送文件，总字节数：" + total); fileOutputStream.close(); } }}2、客户端代码
import java.io.FileInputStream;import java.io.IOException;import java.net.InetSocketAddress;import java.nio.channels.FileChannel;import java.nio.channels.SocketChannel;public class NIOClientFile { public static void main(String[] args) throws IOException { //打开一个SocketChannel并链接到服务器端 SocketChannel socketChannel = SocketChannel.open(new InetSocketAddress("127.0.0.1", 7000)); //打开一个文件 FileChannel fileChannel = new FileInputStream("d:\\temp\\03.zip").getChannel(); //发送文件到服务器 long start = System.currentTimeMillis(); //在linux下，一次transferTo调用就可以完成传输 //在windows下，一次transferTo调用最多只能传8M，大文件需要分段传输，需要注意传输位置 //transferTo底层使用零拷贝 long total = fileChannel.size(); long sended = 0; while (sended < total) { //从上一次传输位置继续发送 long lenth = fileChannel.transferTo(sended, fileChannel.size(), socketChannel); sended += lenth; } System.out.println("发送总字节数：" + sended + "，总耗时：" + (System.currentTimeMillis() - start)); //关闭channel fileChannel.close(); socketChannel.close(); }}3、控制台出输出
测试发送9M的压缩文件，耗时在16ms左右
发送总字节数：9428963，总耗时：16四、总结
使用零拷贝传输，性能明显高于传统IO传输
原文地址：https://blog.csdn.net/wlddhj/article/details/123778139