Introduction

Traditional IO is blocking in nature. For example traditional socket connection is blocking, wherein the server waits for a client connection and once the client connection is obtained an independent thread handles the operation of the client. Java NIO introduces non blocking IO wherein a thread does not have to wait for read or write events. It also introduces multiplexing, a concept that can be explained as follows : imagine a waiter in a restaurant attending at a table. If the restaurant manager had to assign one waiter per table then the restaurant would quickly go out of business. Instead the waiter attends multiple tables (multiplexing) and does not wait permanently on one table (non blocking). Before we go into the code, lets look into some basic classes that make up the ‘Selector’ framework.

Important Classes

• NetworkChannel– is a channel to a network socket. The bind method binds the socket to a local address.
• SelectableChannel– Defines method using what a channel can be multiplexed (used in conjunction with other channels) with the help of a Selector. Selectable channels can be used by multiple threads A selectable channel may operate in a blocking or nonblocking mode.
• Selector – Selector handles the combination of multiple selectablechannels. A selectablechannel is registered to a selector. This registration is represented by a selectionKey. A selector mentions the registered keys, selected keys and cancelled keys. The operating system is checked for the readiness of each channel. As soon as a channel is ready its key is put into the selected keys. A selector may be used by concurrent threads by their selection keys may not.
• AbstactSelectableChannel-Represents the basic implementation of selectable channels. It contains methods for registering, deregistering and closing channels. It also has the information of whether the channel is blocked or not and keeps a list of current keys
• SocketChannel-It represents a channel for a socket, such that the channel can be selected. Socket channels support non blocking connections. Socket Channels are thread safe

We now look at an example of the above classes in action. This is a server socket that listens for incoming client connections.

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.Set;

public class ServerSocketChannelExample {
	private static String clientChannel = "clientChannel";
	private static String serverChannel = "serverChannel";
	private static String channelType = "channelType";

	/**
	 * ServerSocketChannel represents a channel for sockets that listen to
	 * incoming connections.
	 * 
	 * @throws IOException
	 */
	public static void main(String[] args) throws IOException {
		int port = 4444;
		String localhost = "localhost";

		// create a new serversocketchannel. The channel is unbound.
		ServerSocketChannel channel = ServerSocketChannel.open();

		// bind the channel to an address. The channel starts listening to
		// incoming connections.
		channel.bind(new InetSocketAddress(localhost, port));

		// mark the serversocketchannel as non blocking
		channel.configureBlocking(false);

		// create a selector that will by used for multiplexing. The selector
		// registers the socketserverchannel as
		// well as all socketchannels that are created
		Selector selector = Selector.open();

		// register the serversocketchannel with the selector. The OP_ACCEPT
		// option marks
		// a selection key as ready when the channel accepts a new connection.
		// When the
		// socket server accepts a connection this key is added to the list of
		// selected keys of the selector.
		// when asked for the selected keys, this key is returned and hence we
		// know that a new connection has been accepted.
		SelectionKey socketServerSelectionKey = channel.register(selector,
				SelectionKey.OP_ACCEPT);
		// set property in the key that identifies the channel
		Map<String, String> properties = new HashMap<String, String>();
		properties.put(channelType, serverChannel);
		socketServerSelectionKey.attach(properties);
		// wait for the selected keys
		for (;;) {

			// the select method is a blocking method which returns when atleast
			// one of the registered
			// channel is selected. In this example, when the socket accepts a
			// new connection, this method
			// will return. Once a socketclient is added to the list of
			// registered channels, then this method
			// would also return when one of the clients has data to be read or
			// written. It is also possible to perform a nonblocking select
			// using the selectNow() function.
			// We can also specify the maximum time for which a select function
			// can be blocked using the select(long timeout) function.
			if (selector.select() == 0)
				continue;
			// the select method returns with a list of selected keys
			Set<SelectionKey> selectedKeys = selector.selectedKeys();
			Iterator<SelectionKey> iterator = selectedKeys.iterator();
			while (iterator.hasNext()) {
				SelectionKey key = iterator.next();
				// the selection key could either by the socketserver informing
				// that a new connection has been made, or
				// a socket client that is ready for read/write
				// we use the properties object attached to the channel to find
				// out the type of channel.
				if (((Map) key.attachment()).get(channelType).equals(
						serverChannel)) {
					// a new connection has been obtained. This channel is
					// therefore a socket server.
					ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key
							.channel();
					// accept the new connection on the server socket. Since the
					// server socket channel is marked as non blocking
					// this channel will return null if no client is connected.
					SocketChannel clientSocketChannel = serverSocketChannel
							.accept();

					if (clientSocketChannel != null) {
						// set the client connection to be non blocking
						clientSocketChannel.configureBlocking(false);
						SelectionKey clientKey = clientSocketChannel.register(
								selector, SelectionKey.OP_READ,
								SelectionKey.OP_WRITE);
						Map<String, String> clientproperties = new HashMap<String, String>();
						clientproperties.put(channelType, clientChannel);
						clientKey.attach(clientproperties);

						// write something to the new created client
						CharBuffer buffer = CharBuffer.wrap("Hello client");
						while (buffer.hasRemaining()) {
							clientSocketChannel.write(Charset.defaultCharset()
									.encode(buffer));
						}
						buffer.clear();
					}

				} else {
					// data is available for read
					// buffer for reading
					ByteBuffer buffer = ByteBuffer.allocate(20);
					SocketChannel clientChannel = (SocketChannel) key.channel();
					int bytesRead = 0;
					if (key.isReadable()) {
						// the channel is non blocking so keep it open till the
						// count is >=0
						if ((bytesRead = clientChannel.read(buffer)) > 0) {
							buffer.flip();
							System.out.println(Charset.defaultCharset().decode(
									buffer));
							buffer.clear();
						}
						if (bytesRead < 0) {
							// the key is automatically invalidated once the
							// channel is closed
							clientChannel.close();
						}
					}

				}

				// once a key is handled, it needs to be removed
				iterator.remove();

			}
		}

	}
}

The client that connects to the server.

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.CharBuffer;
import java.nio.channels.SocketChannel;
import java.nio.charset.Charset;

public class SocketChannelExample {
	
	public static void main(String[] args) throws IOException,
			InterruptedException {
		int port = 4444;
		SocketChannel channel = SocketChannel.open();

		// we open this channel in non blocking mode
		channel.configureBlocking(false);
		channel.connect(new InetSocketAddress("localhost", port));

		while (!channel.finishConnect()) {
			// System.out.println("still connecting");
		}
		while (true) {
			// see if any message has been received
			ByteBuffer bufferA = ByteBuffer.allocate(20);
			int count = 0;
			String message = "";
			while ((count = channel.read(bufferA)) > 0) {
				// flip the buffer to start reading
				bufferA.flip();
				message += Charset.defaultCharset().decode(bufferA);

			}

			if (message.length() > 0) {
				System.out.println(message);
				// write some data into the channel
				CharBuffer buffer = CharBuffer.wrap("Hello Server");
				while (buffer.hasRemaining()) {
					channel.write(Charset.defaultCharset().encode(buffer));
				}
				message = "";
			}

		}
	}
}