Channel(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

通道的主要实现类

java.nio.channels.Channel 接口：

• FileChannel : 用于读取、写入、映射和操作文件的通道
• SocketChannel : 通过TCP 读写网络中的数据
• ServerSocketChannel : 可以监听新进来的TCP 连接，对每一个新进来的连接都会创建一个SocketChannel
• DatagramChannel : 通过UDP 读写网络中的数据通道

获取通道的方式

• Java 针对支持通道的类提供了 getChannel() 方法

  本地 IO：

  1. FileInputStream/FileOutputStream
  2. RandomAccessFile

  网络IO：

  1. Socket
  2. ServerSocket
  3. DatagramSocket

• 在 JDK 1.7 中的 NIO.2 针对各个通道提供了静态方法 open()

• 在 JDK 1.7 中的 NIO.2 的 Files 工具类的 newByteChannel()

public class TestChannel {
    //使用直接缓冲区完成文件的复制(内存映射文件)
    @Test
    public void test2() throws IOException{//2127-1902-1777
        long start = System.currentTimeMillis();

        FileChannel inChannel = FileChannel.open(Paths.get("d:/1.mkv"), StandardOpenOption.READ);
        FileChannel outChannel = FileChannel.open(Paths.get("d:/2.mkv"), StandardOpenOption.WRITE, StandardOpenOption.READ, StandardOpenOption.CREATE);

        //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);

        inChannel.close();
        outChannel.close();

        long end = System.currentTimeMillis();
        System.out.println("耗费时间为：" + (end - start));
    }

    //利用通道完成文件的复制（非直接缓冲区）
    @Test
    public void test1(){//10874-10953
        long start = System.currentTimeMillis();

        FileInputStream fis = null;
        FileOutputStream fos = null;
        //①获取通道
        FileChannel inChannel = null;
        FileChannel outChannel = null;
        try {
            fis = new FileInputStream("d:/1.mkv");
            fos = new FileOutputStream("d:/2.mkv");

            inChannel = fis.getChannel();
            outChannel = fos.getChannel();

            //②分配指定大小的缓冲区
            ByteBuffer buf = ByteBuffer.allocate(1024);

            //③将通道中的数据存入缓冲区中
            while(inChannel.read(buf) != -1){
                buf.flip(); //切换读取数据的模式
                //④将缓冲区中的数据写入通道中
                outChannel.write(buf);
                buf.clear(); //清空缓冲区
            }
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            if(outChannel != null){
                try {
                    outChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            if(inChannel != null){
                try {
                    inChannel.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            if(fos != null){
                try {
                    fos.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }

            if(fis != null){
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }

        long end = System.currentTimeMillis();
        System.out.println("耗费时间为：" + (end - start));

    }

}

通道之间的数据传输

• transferFrom()
• transferTo() ```java public class TestChannel{ //通道之间的数据传输(直接缓冲区) @Test public void test3() throws IOException{

    FileChannel inChannel = FileChannel.open(Paths.get("d:/1.mkv"), StandardOpenOption.READ);
    FileChannel outChannel = FileChannel.open(Paths.get("d:/2.mkv"), StandardOpenOption.WRITE, StandardOpenOption.READ, StandardOpenOption.CREATE);
  

// inChannel.transferTo(0, inChannel.size(), outChannel); outChannel.transferFrom(inChannel, 0, inChannel.size());

    inChannel.close();
    outChannel.close();
}

}

## 分散(Scatter)与聚集(Gather)
* 分散读取（Scattering Reads）：将通道中的数据分散到多个缓冲区中
* 聚集写入（Gathering Writes）：将多个缓冲区中的数据聚集到通道中
 ```java
public class TestChannel{
//分散和聚集
    @Test
    public void test4() throws IOException{
        RandomAccessFile raf1 = new RandomAccessFile("1.txt", "rw");

        //1. 获取通道
        FileChannel channel1 = raf1.getChannel();

        //2. 分配指定大小的缓冲区
        ByteBuffer buf1 = ByteBuffer.allocate(100);
        ByteBuffer buf2 = ByteBuffer.allocate(1024);

        //3. 分散读取
        ByteBuffer[] bufs = {buf1, buf2};
        channel1.read(bufs);

        for (ByteBuffer byteBuffer : bufs) {
            byteBuffer.flip();
        }

        System.out.println(new String(bufs[0].array(), 0, bufs[0].limit()));
        System.out.println("-----------------");
        System.out.println(new String(bufs[1].array(), 0, bufs[1].limit()));

        //4. 聚集写入
        RandomAccessFile raf2 = new RandomAccessFile("2.txt", "rw");
        FileChannel channel2 = raf2.getChannel();

        channel2.write(bufs);
    }
}

字符集：Charset

• 编码：字符串 -> 字节数组

• 解码：字节数组 -> 字符串

  public class TestChannel{
   //字符集
   @Test
   public void test6() throws IOException{
       Charset cs1 = Charset.forName("GBK");
  
       //获取编码器
       CharsetEncoder ce = cs1.newEncoder();
  
       //获取解码器
       CharsetDecoder cd = cs1.newDecoder();
  
       CharBuffer cBuf = CharBuffer.allocate(1024);
       cBuf.put("天王盖地虎！");
       cBuf.flip();
  
       //编码
       ByteBuffer bBuf = ce.encode(cBuf);
  
       for (int i = 0; i < 12; i++) {
           System.out.println(bBuf.get());
       }
  
       //解码
       bBuf.flip();
       CharBuffer cBuf2 = cd.decode(bBuf);
       System.out.println(cBuf2.toString());
  
       System.out.println("------------------------------------------------------");
  
       Charset cs2 = Charset.forName("utf-8");
       bBuf.flip();
       CharBuffer cBuf3 = cs2.decode(bBuf);
       System.out.println(cBuf3.toString());
   }
  
   //遍历编码
   @Test
   public void test5(){
       Map<String, Charset> map = Charset.availableCharsets();
  
       Set<Entry<String, Charset>> set = map.entrySet();
  
       for (Entry<String, Charset> entry : set) {
           System.out.println(entry.getKey() + "=" + entry.getValue());
       }
   }
  }