一、问题引出

传统文件拷贝过程

它将硬盘上的文件读取到内核空间的缓冲区中，然后再将缓冲区内容拷贝到Socket缓冲区再拷贝到用户缓冲区中，调用写的方法时会从用户缓冲区到内核传冲区到Socket缓冲区再到服务器端的拷贝，总而言之会进行多次拷贝。
显然，会拥有极大的性能损耗。

简化拷贝过程

通过sendfile将用户空间切换到内核空间，从而节省了用户缓冲区的操作，直接由内核和硬件的对话，但是仍然会有拷贝的过程，这时候就要依赖操作系统的操作了。

零拷贝过程

经过操作系统的配合实现了零拷贝的过程，即通过scatter/gather的方法。

二、代码实验

经过理论分析，接下来要用代码实际查看是不是性能拥有差距。

老式拷贝文件

OldeIOServer

package com.demo.zerocopy;
import java.io.DataInputStream;
import java.io.IOException;
import java.net.ServerSocket;
import java.net.Socket;
/**
 * @Description:
 * @author: HandSomeMaker
 * @date: 2019/12/30 15:04
 */
public class OldIoServer {
    public static void main(String[] args) {
        try {
            ServerSocket serverSocket = new ServerSocket(9999);
            System.out.println("服务器已启动");
            while (true) {
                Socket socket = serverSocket.accept();
                System.out.println("客户端【"+socket.getInetAddress()+"】已连接到服务器");
                DataInputStream inputStream = new DataInputStream(socket.getInputStream());
                byte[] bytes = new byte[1024];
                while (inputStream.read(bytes) > 0) {
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

OldIOClient

package com.demo.zerocopy;
import java.io.*;
import java.net.Socket;
/**
 * @Description:
 * @author: HandSomeMaker
 * @date: 2019/12/30 15:05
 */
public class OldIoClient {
    public static void main(String[] args) {
        try {
            Socket socket = new Socket("localhost", 9999);
            File file = new File("C:" + File.separator + "Program Files" + File.separator + "CCleaner" + File.separator + "CCleaner64.exe");
            InputStream inputStream = new FileInputStream(file);
            DataOutputStream outputStream = new DataOutputStream(socket.getOutputStream());
            byte[] bytes = new byte[1024];
            long start = System.currentTimeMillis();
            while (inputStream.read(bytes, 0, bytes.length) > 0) {
                outputStream.write(bytes);
            }
            System.out.println("传送完成，耗时：" + (System.currentTimeMillis() - start));
            outputStream.close();
            inputStream.close();
            socket.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

新式拷贝文件

NewIOServer

package com.demo.zerocopy;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.nio.ByteBuffer;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
/**
 * @Description:
 * @author: HandSomeMaker
 * @date: 2019/12/30 15:05
 */
public class NewIoServer {
    public static void main(String[] args) {
        try {
            ServerSocketChannel serverSocketChannel=ServerSocketChannel.open();
            ServerSocket serverSocket=serverSocketChannel.socket();
            //开启端口复用
            serverSocket.setReuseAddress(true);
            serverSocket.bind(new InetSocketAddress(9999));
            System.out.println("服务器已启动");
            ByteBuffer byteBuffer = ByteBuffer.allocate(1024);
            while (true) {
                SocketChannel socketChannel = serverSocketChannel.accept();
                System.out.println("客户端已连接到服务器");
                //开启阻塞
                socketChannel.configureBlocking(true);
                while (socketChannel.read(byteBuffer) > 0) {
                    byteBuffer.clear();
                }
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

NewIOClient

package com.demo.zerocopy;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.channels.FileChannel;
import java.nio.channels.SocketChannel;
/**
 * @Description:
 * @author: HandSomeMaker
 * @date: 2019/12/30 15:05
 */
public class NewIoClient {
    public static void main(String[] args) {
        try {
            SocketChannel socketChannel = SocketChannel.open();
            socketChannel.connect(new InetSocketAddress("localhost", 9999));
            socketChannel.configureBlocking(true);
            File file = new File("C:" + File.separator + "Program Files" + File.separator + "CCleaner" + File.separator + "CCleaner64.exe");
            FileChannel fileChannel = new FileInputStream(file).getChannel();
            long start = System.currentTimeMillis();
            //将一个通道和另一个通道直接相连接
            fileChannel.transferTo(0, fileChannel.size(), socketChannel);
            System.out.println("传送完成，耗时：" + (System.currentTimeMillis() - start));
            socketChannel.close();
            fileChannel.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

效果演示

老式拷贝文件

新式拷贝文件

总结

显然，新式拷贝文件要比老式拷贝快上成倍的速度。
但是也不绝对，详情请查看https://www.cnblogs.com/interdrp/p/3785164.html