NIO通道（channel）原理与获取

一、通道（Channel）: 用于源节点与目标节点的连接。在java NIO中负责缓冲区中数据的传输。Channel本身不存储数据，因此需要配合缓冲区进行传输。

二、通道的主要实现类
java.nio.channels.Channel接口：
|– FileChannel
|– SocketChannel
|– ServerSocketChannel
|– DatagramChannel

三、获取通道
1、java针对支持通道的类提供了getChannel()方法
本地IO：
FileInputStream/FileOutputStream
RandomAccessFile
网络IO：
Socket
ServerSocket
DatagramSocket
2、在jdk1.7中NIO.2针对各个通道提供了静态方法open()
3、在jdk1.7中NIO.2的Files工具类的newByteChannel()

    @Test  //利用通道完成文件复制
    public void test4() throws FileNotFoundException{
        FileInputStream fis = new FileInputStream("1.mp4");
        FileOutputStream fos=new FileOutputStream("2.mp4");
        //1、获取通道
        FileChannel inChannel = fis.getChannel();
        FileChannel outChannel = fos.getChannel();
        try {
            //2、分配一个指定大小的缓冲区
            ByteBuffer buf=ByteBuffer.allocate(1024);
            //3、将通道中的数据存入缓冲区
            while(inChannel.read(buf)!=-1){
                //4、将缓冲区中的数据写入通道中
                buf.flip();  //切换读取数据模式
                outChannel.write(buf);
                buf.clear();
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }finally{
            try {
                outChannel.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
            try {
                inChannel.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                fos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                fis.close();
            } catch (IOException e) {
                e.printStackTrace();
            }   
        }
    }


    @Test  //使用直接缓冲区完成文件的复制
    public void test5() throws IOException{
        FileChannel inChannel=FileChannel.open(Paths.get("1.mp4"),StandardOpenOption.READ);
        FileChannel outChannel=FileChannel.open(Paths.get("2.mp4"),StandardOpenOption.WRITE,StandardOpenOption.READ,StandardOpenOption.CREATE_NEW);
        //内存映射文件
        MappedByteBuffer inMappedBuf=inChannel.map(MapMode.READ_ONLY,0,inChannel.size());
        MappedByteBuffer outMappedBuf=outChannel.map(MapMode.READ_WRITE,0,inChannel.size());
        //直接对缓冲区进行数据的读写操作
        byte[] dst=new byte[inMappedBuf.limit()];
        inMappedBuf.get(dst);
        outMappedBuf.put(dst);
        outChannel.close();
        inChannel.close();
    }

四、通道之间的数据传输

    @Test  // 通道之间的数据传输(直接缓冲区)
    public void test6() throws IOException{
        FileChannel inChannel=FileChannel.open(Paths.get("1.mp4"),StandardOpenOption.READ);
        FileChannel outChannel=FileChannel.open(Paths.get("2.mp4"),StandardOpenOption.WRITE,StandardOpenOption.READ,StandardOpenOption.CREATE);
        //inChannel.transferTo(0,inChannel.size(),outChannel);   //二选一即可
        outChannel.transferFrom(inChannel,0,inChannel.size());
        outChannel.close();
        inChannel.close();
    }