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()

1    @Test  //利用通道完成文件复制
2    public void test4() throws FileNotFoundException{
3        FileInputStream fis = new FileInputStream("1.mp4");
4        FileOutputStream fos=new FileOutputStream("2.mp4");
5        //1、获取通道
6        FileChannel inChannel = fis.getChannel();
7        FileChannel outChannel = fos.getChannel();
8        try {
9            //2、分配一个指定大小的缓冲区
10            ByteBuffer buf=ByteBuffer.allocate(1024);
11            //3、将通道中的数据存入缓冲区
12            while(inChannel.read(buf)!=-1){
13                //4、将缓冲区中的数据写入通道中
14                buf.flip();  //切换读取数据模式
15                outChannel.write(buf);
16                buf.clear();
17            }
18        } catch (FileNotFoundException e) {
19            e.printStackTrace();
20        } catch (IOException e) {
21            e.printStackTrace();
22        }finally{
23            try {
24                outChannel.close();
25            } catch (Exception e) {
26                e.printStackTrace();
27            }
28            try {
29                inChannel.close();
30            } catch (IOException e) {
31                e.printStackTrace();
32            }
33            try {
34                fos.close();
35            } catch (IOException e) {
36                e.printStackTrace();
37            }
38            try {
39                fis.close();
40            } catch (IOException e) {
41                e.printStackTrace();
42            }   
43        }
44    }
45
46
47    @Test  //使用直接缓冲区完成文件的复制
48    public void test5() throws IOException{
49        FileChannel inChannel=FileChannel.open(Paths.get("1.mp4"),StandardOpenOption.READ);
50        FileChannel outChannel=FileChannel.open(Paths.get("2.mp4"),StandardOpenOption.WRITE,StandardOpenOption.READ,StandardOpenOption.CREATE_NEW);
51        //内存映射文件
52        MappedByteBuffer inMappedBuf=inChannel.map(MapMode.READ_ONLY,0,inChannel.size());
53        MappedByteBuffer outMappedBuf=outChannel.map(MapMode.READ_WRITE,0,inChannel.size());
54        //直接对缓冲区进行数据的读写操作
55        byte[] dst=new byte[inMappedBuf.limit()];
56        inMappedBuf.get(dst);
57        outMappedBuf.put(dst);
58        outChannel.close();
59        inChannel.close();
60    }

四、通道之间的数据传输

1    @Test  // 通道之间的数据传输(直接缓冲区)
2    public void test6() throws IOException{
3        FileChannel inChannel=FileChannel.open(Paths.get("1.mp4"),StandardOpenOption.READ);
4        FileChannel outChannel=FileChannel.open(Paths.get("2.mp4"),StandardOpenOption.WRITE,StandardOpenOption.READ,StandardOpenOption.CREATE);
5        //inChannel.transferTo(0,inChannel.size(),outChannel); //二选一即可
6        outChannel.transferFrom(inChannel,0,inChannel.size());
7        outChannel.close();
8        inChannel.close();
9    }