NIO UDP 编程

   和TCP的SocketChannel类似，UDP的DatagramChannel能充分利用NIO的优势。

服务端：Selector, DatagramChannel, ByteBuffer

客户端：DatagramChannel, ByteBuffer

区别：
a. 服务端没有与TCP的ServerSocketChannel相对应的Channel，服务端和客户端之间更趋于平等，不过服务端的端口号还是固定的。

b. Selector在处理完读取操作后，会触发写操作：发送数据到客户端。

1. 服务端

public class UDPEchoServerSelector {

	private static final int TIMEOUT = 4000; // 超时 (毫秒)
	private static final int CAPACITY = 255;

	public static void main(String[] args) throws IOException {
		args = new String[1];
		args[0] = "4451";

		int servPort = Integer.parseInt(args[0]);

		Selector sel = Selector.open(); // 创建选择器，可以处理多路通道。

		DatagramChannel channel = DatagramChannel.open();
		channel.configureBlocking(false);
		channel.socket().bind(new InetSocketAddress(servPort)); // 通道关联的socket绑定地址
		channel.register(sel, SelectionKey.OP_READ, new ClientData());

		while (true) { // 持续运行，接收和返回数据
			if (sel.select(TIMEOUT) == 0) {
				System.out.println("No I/O needs to be processed");
				continue;
			}

			Iterator<SelectionKey> iter = sel.selectedKeys().iterator(); // 获取可操作的选择键集合
			while (iter.hasNext()) {
				SelectionKey key = iter.next(); // 键为位掩码

				if (key.isReadable()) { // 客户端有数据发送过来
					handleRead(key);
				}

				if (key.isValid() && key.isWritable()) { // 通道正常，且客户端需要响应
					handleWrite(key);
				}

				iter.remove(); // 从集合中移除选择键
			}

		}
	}

	private static void handleRead(SelectionKey key) throws IOException {
		DatagramChannel channel = (DatagramChannel) key.channel();
		ClientData clntDat = (ClientData) key.attachment();
		clntDat.buffer.clear();
		clntDat.clientAddress = channel.receive(clntDat.buffer); // 获取客户端的地址，用以发送响应
		if (clntDat.clientAddress != null) { // 接收到数据
			key.interestOps(SelectionKey.OP_WRITE); // 关注客户端读取响应
		}
	}

	private static void handleWrite(SelectionKey key) throws IOException {
		DatagramChannel channel = (DatagramChannel) key.channel();
		ClientData clntDat = (ClientData) key.attachment();
		clntDat.buffer.flip(); // 从起始位置开始发送
		int bytesSent = channel.send(clntDat.buffer, clntDat.clientAddress);
		if (bytesSent != 0) {
			key.interestOps(SelectionKey.OP_READ); // 关注客户端发送数据
		}
	}

	public static class ClientData {
		public SocketAddress clientAddress;
		public ByteBuffer buffer = ByteBuffer.allocate(CAPACITY);
	}
}

2. 客户端

public class UDPEchoClient {

	private static final int CAPACITY = 255;
	private static final String UTF8 = "UTF-8";

	public static void main(String[] args) throws IOException {
		args = new String[2];
		args[0] = "localhost";
		args[1] = "4451";

		String servName = args[0];
		int servPort = Integer.parseInt(args[1]);

		DatagramChannel clntChan = DatagramChannel.open();
		clntChan.configureBlocking(false);
		clntChan.connect(new InetSocketAddress(servName, servPort));

		ByteBuffer sentBuffer = ByteBuffer.wrap("Hello UDP".getBytes(UTF8));
		int bytesSent = clntChan.write(sentBuffer); // 向服务器发送数据
		System.out.println("UDP client sent " + bytesSent + " bytes");

		ByteBuffer recvBuffer = ByteBuffer.allocate(CAPACITY);
		clntChan.receive(recvBuffer); // 读取响应
		recvBuffer.flip();
		System.out.println("UDP client received: " + new String(recvBuffer.array(), UTF8));
	}
}

客户端连接服务器后，发送数据可以用write或send方法：

...
clntChan.connect(new InetSocketAddress(servName, servPort));
...
int bytesSent = clntChan.write(sentBuffer);

...
clntChan.connect(new InetSocketAddress(servName, servPort));
...
int bytesSent = clntChan.send(sentBuffer, new InetSocketAddress(servName, servPort)); // 发送地址必须和前面的连接地址相同，否则报java.lang.IllegalArgumentException: Connected address not equal to target address

客户端没有连接服务器，需要用send方法指定目标地址：

...
int bytesSent = clntChan.send(sentBuffer, new InetSocketAddress(servName, servPort)); // 使用write方法会报java.nio.channels.NotYetConnectedException