3 Java NIO--Channel

Channel，通道，也可以理解为连接。

在传统BIO中，流（Stream）是单向的，例如FileInputStream对向只能进行读取数据的操作，而NIO中的通道是双向的，可以读写操作。

Channel在NIO中是一个接口，常用的Channel有：
FileChannel：用于文件的数据读写
DatagramChannel：用于UDP数据读写
ServerSocketChannel和SocketChannel：用于TCP读写

通道的读写函数示例，注意读写是站在通道角度看的
read（ByteBuffer dst）从通道读取数据并放到缓冲区中
write（ByteBuffer src）把缓冲区数据写到通道中
transferFrom（ReadableByteChannel src，long position，long count）把目标通道的数据复制到当前通道中
transferTo（long position，long count，WritableByteChannel target）把当前通道的数据复制到目标通道中

package NIO;

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class ChannelCase1 {


    public static void main(String[] args) throws IOException {

        String str = "Hello, channel";

        FileOutputStream fileOutputStream = new FileOutputStream("e:\\test.txt");

        //通过fileOutputStream获取对应的filechannel
        //FileChannel真实类型是FileChannelImpl
        FileChannel channel = fileOutputStream.getChannel();
        
        //创建一个buffer
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        //对应的buffer就是写
        buffer.put(str.getBytes());

        buffer.flip();

        //把buffer的数据写入到channel中，对应的buffer就是读
        channel.write(buffer);

        //关闭资源
        fileOutputStream.close();

    }
}

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复制文件Demo

package NIO;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class ChannelCase2 {


    public static void main(String[] args) throws IOException {

        FileInputStream fileInputStream = new FileInputStream("e:\\test.txt");
        FileChannel channel1 = fileInputStream.getChannel();

        FileOutputStream fileOutputStream = new FileOutputStream("e:\\test_copy.txt");
        FileChannel channel2 = fileOutputStream.getChannel();
        
        //创建一个buffer
        ByteBuffer buffer = ByteBuffer.allocate(512);

        while (true){

            //必须对这个buffer执行clear，也就是对标志重置，否则循环将不会退出
            buffer.clear();

            int read = channel1.read(buffer);
            if(read == -1){
                break;
            }

            //将buffer数据写入到channel2,也就是读取buffer数据
            buffer.flip();
            channel2.write(buffer);

        }

        fileInputStream.close();
        fileOutputStream.close();

    }
}

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使用transferFrom简化

package NIO;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;

public class ChannelCase3 {


    //针对ChannelCase2的复制文件，这里直接实验channel的拷贝

    public static void main(String[] args) throws IOException {

        FileInputStream fileInputStream = new FileInputStream("e:\\test.txt");
        FileChannel channel1 = fileInputStream.getChannel();

        FileOutputStream fileOutputStream = new FileOutputStream("e:\\test_copy.txt");
        FileChannel channel2 = fileOutputStream.getChannel();

        channel2.transferFrom(channel1,0,channel1.size());

        fileInputStream.close();
        fileOutputStream.close();

    }
}

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