Stockholm == cyclist city

Last week I have been there at Stockholm. Nice and green city. Since i am living in Frankfurt for the past 2 years, it was a different experience for me. In Germany (I have lived in 3 to 4 cities in Germany), you cannot see that many cyclists in Stockholm. They really have a broader lane, even in busy routes, just for cyclists.

There were plenty of cyclists in Stockholm. They help to keep the air fresh and themselves active. Just for comparison in Frankfurt the car:cycle ratio would be 500:1 if not 1000:1.

Only after coming back and after doing a little research i came to know that these days European countries are taking effort to cut down emission by encouraging people to use cycles.

BBC News tells you more.

Java nio non blocking server & client

Java supports nonblocking io since java 1.4.

But just now i am really using it in one of my projects.

The difference is that the server socket's accept() or a normal socket's read() or write() method need not be blocking (in a single thread) any more.
E.g. with the normal io, if a server is executing the accept() method then it cannot do anything with the previously accepted sockets.

It has changed with nio in the sense that a server can handle (within a single thread)
1. several existing connections and
2. new incoming connections

Let us see an see an example.

The server (Server.java) listens on port 9999 for any incoming connections.
$>javac Server.java
$>java Server

The client (Client.java) sends the text "I am Client : clientXXX" to the server. XXX->is the command line argument; it is just an identifier to distinguish (on the server's console) different clients.
$>javac Client.java
$>java Client 354
$>java Client dfdsfsd

Sample Output:
Sample Output:
serverSocketChannel's registered key is : sun.nio.ch.ServerSocketChannelImpl[/127.0.0.1:9999]

Server is listening on: 127.0.0.1:9999
Key ready to perform accept() : sun.nio.ch.ServerSocketChannelImpl[/127.0.0.1:9999]
Key ready to perform read() : java.nio.channels.SocketChannel[connected local=/127.0.0.1:9999 remote=/127.0.0.1:2633]
I am Client : 354
Key ready to perform read() : java.nio.channels.SocketChannel[connected local=/127.0.0.1:9999 remote=/127.0.0.1:2633]
I am Client : 354
Key ready to perform read() : java.nio.channels.SocketChannel[connected local=/127.0.0.1:9999 remote=/127.0.0.1:2633]
I am Client : 354
Key ready to perform accept() : sun.nio.ch.ServerSocketChannelImpl[/127.0.0.1:9999]
Key ready to perform read() : java.nio.channels.SocketChannel[connected local=/127.0.0.1:9999 remote=/127.0.0.1:2636]
I am Client : dfdsfsd
Key ready to perform read() : java.nio.channels.SocketChannel[connected local=/127.0.0.1:9999 remote=/127.0.0.1:2633]
I am Client : 354
Key ready to perform read() : java.nio.channels.SocketChannel[connected local=/127.0.0.1:9999 remote=/127.0.0.1:2636]
I am Client : dfdsfsd
Key ready to perform read() : java.nio.channels.SocketChannel[connected local=/127.0.0.1:9999 remote=/127.0.0.1:2633]
I am Client : 354


Server.java

import java.io.IOException; import java.net.InetAddress; import java.net.InetSocketAddress; import java.nio.ByteBuffer; import java.nio.channels.SelectionKey; import java.nio.channels.Selector; import java.nio.channels.ServerSocketChannel; import java.nio.channels.SocketChannel; import java.util.Iterator; import java.util.Set; public class Server { // this is equivalent to the server socket in the non nio world ServerSocketChannel serverSocketChannel; // this is the multiplexer which multiplexes the messages received from different clients Selector selector; public Server() { try { // get a selector selector = Selector.open(); // get a server socket channel serverSocketChannel = ServerSocketChannel.open(); // we force the socket to be Non-blocking. // if it is set to "true" then this socket acts as a normal (blocking) server socket serverSocketChannel.configureBlocking(false); // port and ip address where the server listens for connections InetSocketAddress add = new InetSocketAddress(InetAddress.getLocalHost(), 9999); // bind the server socket to the ip/port serverSocketChannel.socket().bind(add); // register the serverSocketChannel (for incoming connection events) to the selector. // The "SelectionKey.OP_ACCEPT" parameter tells the selector that this serverSocketChannel registers // itself for incoming (acceptable) connections SelectionKey key = serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT); System.out.println("serverSocketChannel's registered key is : " + key.channel().toString()); System.out.println(); } catch (IOException e) { e.printStackTrace(); } } public static void main(String[] args) { Server server = new Server(); server.startListening(); } private void startListening() { System.out.println("Server is listening on: " + serverSocketChannel.socket().getInetAddress().getHostAddress() + ":" + serverSocketChannel.socket().getLocalPort()); while (true) { try { // this line blocks until some events has occurred in the underlying socket selector.select(); // get the selected keys set Set selectedKeys = selector.selectedKeys(); Iterator iterator = selectedKeys.iterator(); while (iterator.hasNext()) { SelectionKey key = (SelectionKey) iterator.next(); iterator.remove(); // a client has asked for a new connection if (key.isAcceptable()) { // only ServerSocketsChannels registered for OP_ACCEPT are excepted to receive an // "acceptable" key System.out.println("Key ready to perform accept() : " + key.channel().toString()); // as usual the accept returns the plain socket towards the client SocketChannel client = serverSocketChannel.accept(); // set the client socket to be non blocking client.configureBlocking(false); // register the client socket with the same selector to which we have registered the // serverSocketChannel client.register(selector, SelectionKey.OP_READ); continue; } // the client has sent something to be read by this server if (key.isReadable()) { System.out.println("Key ready to perform read() : " + key.channel().toString()); // get the underlying socket SocketChannel client = (SocketChannel) key.channel(); ByteBuffer bb = ByteBuffer.allocate(1024); // read the msg sent by the client client.read(bb); // display the message bb.flip(); byte[] array = new byte[bb.limit()]; bb.get(array); System.out.println(new String(array)); continue; } } } catch (IOException e) { e.printStackTrace(); } } } }

Client.java

import java.io.IOException; import java.net.InetAddress; import java.net.InetSocketAddress; import java.nio.ByteBuffer; import java.nio.channels.SelectionKey; import java.nio.channels.Selector; import java.nio.channels.SocketChannel; import java.util.Iterator; import java.util.Set; public class Client { String myIdentity; public Client(String pIdentity) { myIdentity = pIdentity; } void talkToServer() { try { SocketChannel mySocket = SocketChannel.open(); // non blocking mySocket.configureBlocking(false); // connect to a running server mySocket.connect(new InetSocketAddress(InetAddress.getLocalHost(), 9999)); // get a selector Selector selector = Selector.open(); // register the client socket with "connect operation" to the selector mySocket.register(selector, SelectionKey.OP_CONNECT); // select() blocks until something happens on the underlying socket while (selector.select() > 0) { Set keys = selector.selectedKeys(); Iterator it = keys.iterator(); while (it.hasNext()) { SelectionKey key = it.next(); SocketChannel myChannel = (SocketChannel) key.channel(); it.remove(); if (key.isConnectable()) { if (myChannel.isConnectionPending()) { myChannel.finishConnect(); System.out.println("Connection was pending but now is finiehed connecting."); } ByteBuffer bb = null; while (true) { bb = ByteBuffer.wrap(new String("I am Client : " + myIdentity).getBytes()); myChannel.write(bb); bb.clear(); synchronized (this) { wait(3000); } } } } } } catch (IOException e) { e.printStackTrace(); } } public static void main(String[] args) { Client client = new Client(args[0]); client.talkToServer(); } }

SCTPing in Java

blog from Chris Hegarty - the lead for SCTP integration in JDK7