Java NIO Channel to Channel Transfers

最新推荐文章于 2022-07-23 16:19:40 发布

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最新推荐文章于 2022-07-23 16:19:40 发布

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分类专栏： NIO 文章标签： transferFrom transferTo zero-copy

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本文介绍Java NIO中如何使用FileChannel的transferFrom()和transferTo()方法直接从一个通道到另一个通道进行数据传输。通过实例展示了如何将数据从一个文件复制到另一个文件，以及如何将数据从文件通道传输到其他类型的通道。

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In Java NIO you can transfer(转移,复制) data directly from one channel to another, if one of the channels is a FileChannel. The FileChannel class has a transferTo() and a transferFrom() method which does this for you.

transferFrom()

The FileChannel.transferFrom() method transfers data from a source channel into the FileChannel. Here is a simple example:

import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;

/**
 * Created by wbx on 2018/12/29.
 */
public class DemoTransfers {

    public static void main(String[] args) throws Exception {
        RandomAccessFile fromFile = new RandomAccessFile("raf.txt", "rw");
        //源通道
        FileChannel fromChannel = fromFile.getChannel();

        RandomAccessFile toFile = new RandomAccessFile("raf_transfer1.txt", "rw");
        //目标通道
        FileChannel toChannel = toFile.getChannel();
        //起始位置
        long position = 0;
        //结束位置
        long count = fromChannel.size();
        //通过transferFrom进行拷贝(转移)操作
        toChannel.transferFrom(fromChannel, position, count);
    }
}

The parameters position and count, tell where in the destination(目的地) file to start writing (position), and how many bytes to transfer maximally (count). If the source channel has fewer than count bytes, less is transfered.

Additionally(另外), some SocketChannel implementations may transfer only the data the SocketChannel has ready in its internal (内部的)buffer here and now - even if the SocketChannel may later have more data available. Thus, it may not transfer the entire(全部的) data requested (count) from the SocketChannel into FileChannel.

transferTo()

The transferTo() method transfer from a FileChannel into some other channel. Here is a simple example:

import java.io.RandomAccessFile;
import java.nio.channels.FileChannel;

/**
 * Created by wbx on 2018/12/29.
 */
public class DemoTransfer2 {

    public static void main(String[] args) throws Exception {
        RandomAccessFile fromFile = new RandomAccessFile("raf.txt", "rw");
        //源通道
        FileChannel fromChannel = fromFile.getChannel();

        RandomAccessFile toFile = new RandomAccessFile("raf_transferto.txt", "rw");
        //目标通道
        FileChannel toChannel = toFile.getChannel();
        //起始位置
        long position = 0;
        //最大读取数据
        long count = fromChannel.size();

        fromChannel.transferTo(position, count, toChannel);
    }
}

Notice how similar the example is to the previous. The only real difference is the which FileChannel object the method is called on. The rest is the same.

The issue(问题) with SocketChannel is also present(存在) with the transferTo() method. The SocketChannel implementation may only transfer bytes from the FileChannel until the send buffer is full, and then stop.