I0-零拷贝

零拷贝的“零”是指用户态和内核态间copy数据的次数为零，netty采用的零拷贝

传统的数据copy（文件到文件、client到server等）涉及到四次用户态内核态切换、四次copy。四次copy中，两次在用户态和内核态间copy需要CPU参与、两次在内核态与IO设备间copy为DMA方式不需要CPU参与。零拷贝避免了用户态和内核态间的copy、减少了两次用户态内核态间的切换。

零拷贝可以提高数据传输效率，但对于需要在用户传输过程中对数据进行加工的场景（如加密）就不适合使用零拷贝。

传统IO

读取文件 文件-系统内核缓冲区-系统内存缓冲区  写文件 系统内存缓冲区-内核缓冲区-写入磁盘

 

零拷贝

文件-内核缓冲区   内核缓冲区-磁盘

 

 

Java NIO 零拷贝示例

 

 

/**
 * 零拷贝文件
 * Created by wuliangfeng on 2021/4/23.
 */
public class ZerocopyFile {
    public static void main(String[] args){
        try{
            String fromPath="D:\\json.txt";
            String toPath="E:\\json.txt";
            //copyToFile(fromPath,toPath);
           // copyFormFile(fromPath,toPath);
            copyZoreFile(fromPath,toPath);
        }catch(Exception e){
            e.printStackTrace();
        }
    }
    private static void copyZoreFile(String fromPath, String toPath) throws IOException {
        FileChannel fromChannel=FileChannel.open(Paths.get(fromPath), StandardOpenOption.READ);
        FileChannel toChannel=FileChannel.open(Paths.get(toPath),StandardOpenOption.CREATE,StandardOpenOption.READ,StandardOpenOption.WRITE);
        MappedByteBuffer formMappedByteBuffer=fromChannel.map(FileChannel.MapMode.READ_ONLY,0,fromChannel.size());
        MappedByteBuffer toMappedByteBuffer=toChannel.map(FileChannel.MapMode.READ_WRITE,0,fromChannel.size());
        byte[] bytes=new byte[formMappedByteBuffer.limit()];
        formMappedByteBuffer.get(bytes);
        toMappedByteBuffer.put(bytes);
        fromChannel.close();
        toChannel.close();;
    }
    private static void copyToFile(String fromPath, String toPath) throws IOException {
        //rw readWrite
        FileChannel fromChannel=new RandomAccessFile(fromPath,"rw").getChannel();
        FileChannel toChannel=new RandomAccessFile(toPath,"rw").getChannel();
        fromChannel.transferTo(0,fromChannel.size(),toChannel);
        fromChannel.close();
        toChannel.close();
    }
    private static  void copyFormFile(String fromPath, String toPath)throws IOException{
        FileChannel fromChannel=new FileInputStream(fromPath).getChannel();
        FileChannel toChannel=new FileOutputStream(toPath).getChannel();
        toChannel.transferFrom(fromChannel,0,fromChannel.size());
        fromChannel.close();
        toChannel.close();
    }
}