java nio transferTo方法使用场景

java nio transferTo方法最多只能发送2G的数据，超过2G需要循环发送-零拷贝
文件拷贝

try (FileChannel inputChannel = new FileInputStream(sourceFile).getChannel();
     FileChannel outputChannel = new FileOutputStream(destFile).getChannel()) {

    long position = 0;
    long size = inputChannel.size();
    while (position < size) {
        long transferred = inputChannel.transferTo(position, size - position, outputChannel);
        if (transferred <= 0) {
            break;
        }
        position += transferred;
    }
}

将文件直接发送到客户端 Socket（Web 文件服务器最常见场景）

import java.io.*;
import java.net.*;
import java.nio.channels.*;

public class FileServerZeroCopy {
    public static void main(String[] args) throws IOException {
        int port = 8080;
        ServerSocketChannel serverChannel = ServerSocketChannel.open();
        serverChannel.bind(new InetSocketAddress(port));
        System.out.println("Listening on port " + port);

        while (true) {
            SocketChannel client = serverChannel.accept();
            sendFile(client, new File("big-file.zip"));
            client.close();
        }
    }

    public static void sendFile(SocketChannel client, File file) throws IOException {
        try (FileChannel fileChannel = new FileInputStream(file).getChannel()) {
            long position = 0;
            long size = fileChannel.size();
            while (position < size) {
                long transferred = fileChannel.transferTo(position, size - position, client);
                if (transferred <= 0) break;
                position += transferred;
            }
        }
    }
}