public final Buffer flip() {
    limit = position; // 将 limit 设置为实际写入的数据数量
    position = 0; // 重置 position 为 0
    mark = -1; // mark 之后再说
    return this;
}

ByteBuffer byteBuf = ByteBuffer.allocate(1024);
IntBuffer intBuf = IntBuffer.allocate(1024);
LongBuffer longBuf = LongBuffer.allocate(1024);
public static ByteBuffer wrap(byte[] array) {
    ...
}

    public final Buffer clear() {
        position = 0;
        limit = capacity;
        mark = -1;
        return this;
    }

    public ByteBuffer compact() {

        int pos = position();
        int lim = limit();
        assert (pos <= lim);
        int rem = (pos <= lim ? lim - pos : 0);  //position 到 limit 之间的数据

        unsafe.copyMemory(ix(pos), ix(0), (long)rem << 0); //移到左边
        position(rem);
        limit(capacity());
        discardMark();
        return this;

    }

// 读取
FileInputStream inputStream = new FileInputStream(new File("/data.txt"));
FileChannel fileChannel = inputStream.getChannel();

ByteBuffer buffer = ByteBuffer.allocate(1024);
int num = fileChannel.read(buffer);

// 写入
ByteBuffer buffer = ByteBuffer.allocate(1024);
buffer.put("随机写入一些内容到 Buffer 中".getBytes());
// Buffer 切换为读模式
buffer.flip();
while(buffer.hasRemaining()) {
    // 将 Buffer 中的内容写入文件
    fileChannel.write(buffer);
}

//打开一个 TCP 连接：
// 打开一个通道
SocketChannel socketChannel = SocketChannel.open();
// 发起连接
socketChannel.connect(new InetSocketAddress("https://www.baidu.com", 80));

// 读取数据
socketChannel.read(buffer);

// 写入数据到网络连接中
while(buffer.hasRemaining()) {
    socketChannel.write(buffer);   
}

// 实例化
ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();
// 监听 8080 端口
serverSocketChannel.socket().bind(new InetSocketAddress(8080));

while (true) {
    // 一旦有一个 TCP 连接进来，就对应创建一个 SocketChannel 进行处理
    SocketChannel socketChannel = serverSocketChannel.accept();
}

//监听端口
DatagramChannel channel = DatagramChannel.open();
channel.socket().bind(new InetSocketAddress(9090));

ByteBuffer buf = ByteBuffer.allocate(48);
buf.clear();

channel.receive(buf);

//发送数据
String newData = "New String to write to file..."
                    + System.currentTimeMillis();

ByteBuffer buf = ByteBuffer.allocate(48);
buf.clear();
buf.put(newData.getBytes());
buf.flip();

int bytesSent = channel.send(buf, new InetSocketAddress("localhost.com", 80));

//开启一个 Selector  多路复用器
Selector selector = Selector.open();

//将 Channel 注册到 Selector 上, 注册到 Selector 的 Channel 必须要支持非阻塞模式.FileChannel 不支持非阻塞，最常见的 SocketChannel 和 ServerSocketChannel。
// 将通道设置为非阻塞模式，因为默认都是阻塞模式的
channel.configureBlocking(false);
// 注册
SelectionKey key = channel.register(selector, SelectionKey.OP_READ);
//注册方法返回值是 SelectionKey 实例，它包含了 Channel 和 Selector 信息，也包括了一个叫做 Interest Set 的信息，即我们设置的我们感兴趣的正在监听的事件集合。
//监听感兴趣的事件,可以同时监听一个 Channel 中的发生的多个事件,共以下四种事件：
//        SelectionKey.OP_READ
//        SelectionKey.OP_WRITE
//        SelectionKey.OP_CONNECT
//        SelectionKey.OP_ACCEPT

//调用 select() 方法获取通道信息。用于判断是否有我们感兴趣的事件已经发生了。
while(true) {
  // 判断是否有事件准备好
  int readyChannels = selector.select();
  if(readyChannels == 0) continue;

  // 遍历
  Set<SelectionKey> selectedKeys = selector.selectedKeys();
  Iterator<SelectionKey> keyIterator = selectedKeys.iterator();
  while(keyIterator.hasNext()) {
    SelectionKey key = keyIterator.next();

    if(key.isAcceptable()) {
        // a connection was accepted by a ServerSocketChannel.

    } else if (key.isConnectable()) {
        // a connection was established with a remote server.

    } else if (key.isReadable()) {
        // a channel is ready for reading

    } else if (key.isWritable()) {
        // a channel is ready for writing
    }

    keyIterator.remove();
  }
}