(November 18, 2002) - Most of the Java developers I have come across create multi-threaded servers without knowing a jack about deadlocks, I/O Performance, Thread starvation, scalability and other OS dependent issues. While forking new thread for every socket connection and managing predefined thread pools are the most-taught techniques, robust servers can be written for enabling multiple simultaneous connections with a single thread configured with Non-blocking sockets using Java's NIO socket channels. Coding Non-blocking I/O for sockets and files was introduced with JDK1.4 NIO APIs. Let's create a simple Non-blocking server socket and a client socket.

Code:

1. import java.nio.*;
2. import java.nio.channels.*;
3. import java.io.*;
4. import java.net.*;
5.
6. public class nbsock{
7.
8. /* Creating a Non-blocking Server
9. socket */
10. public static void mySsc(int port){
11. try{
12. /* Create a non-blocking server
13. socket channel on port 8678 */
14. ServerSocketChannel ssc =
ServerSocketChannel.open();
15. ssc.configureBlocking(false);
16.
17. /* Binding Address and port */
18. ssc.socket().bind
(new InetSocketAddress(port));
19. } catch (IOException e) {
20. System.out.println(""+e);
21. }
22. }
23.
24. /* Creating a Non-blocking
25. Client Socket */
26. public static SocketChannel mysc
(String host, int port){
27. SocketChannel sc=null;
28. try{
29. /* Create a non-blocking
30. socket channel */
31. sc = SocketChannel.open();
32. sc.configureBlocking(false);
33.
34. /* A Non-blocking connection
35. request to the server */
36. sc.connect(new InetSocketAddress
(host, port));
37. }catch (IOException e) {
38. System.out.println(""+e);
39. }
40. return sc;
41. }
42.
43. public static void main(String ar[]){
44. try {
45. /* Create a non-blocking
46. socket channel */
47. mySsc(8678);
48. SocketChannel sc = mysc("afj", 8678);
49.
50. /* Completing the connection by
51. calling finishConnect().
52. If this channel is in non-blocking
53. mode then this method will return
54. false if the connection process
55. is not yet complete. If this channel
56. is in blocking mode then this
57. method will block until the
58. connection either completes or fails. */
59. while (!sc.finishConnect()) {}
60.
61. /* Socket channel is ready */
62. System.out.println("Channel Ready!");
63. } catch (IOException e) {
64. System.out.println(""+e);
65. }
66.
67. }
68. }

Socket channels are not an absolute abstraction of connecting network sockets. Binding, shutdown, and the manipulation of socket options must be done through an associated socket object which can obtained by invoking the socket() method. Socket channels are safer for use by multiple concurrent threads. The connect() and finishConnect() methods are mutually synchronized , and an attempt to initiate a read or write operation while an invocation of one of these methods is in progress will block until that invocation is complete.

Assignment
Try using a selector to manage Non-blocking sockets and accept connection requests. Hint: Use the java.nio.channels.Selector class.