package TcpExamples;
import java.io.*;
import java.net.*;
/**
* <p>
* This program is only slightly more complex than {@link TcpExamples.TcpExample1Telnet},
* counting each connection instead of immediately exiting after the first connection.
* The only thing this program does differently is introduce a loop into the response,
* so you don't have to restart the program after one response.
* Also, it prints out the socket pair the server sees.
* </p>
* <p>
* Suggestion: connect to this program via telnet several times,
* comparing the socket-pair responses each time.
* </p>
* <p>
* <b><code>telnet localhost 2317</code></b>
* </p>
* <p>
* If you're paying attention (and sophisticated!) you can contact the
* another person's computer in class while running this program. This is
* a good way to check local-area network (LAN) connectivity.
* To learn that local computer's IPv4 Address, type
* </p>
* <p>
* <b><code>ipconfig</code></b>
* </p>
* <p>
* (On the NPS campus you might get a ipOfServerLaptop number like 172.20.148.215).
* The someone else (or even that person) can run
* </p>
* <p>
* <b><code>telnet ipOfServerLaptop 2317</code></b>
* </p>
* <p>
* If this program is running, then that machine displays the current counter value of
* socket pairs received.
* </p>
* @see <a href="../../../src/TcpExamples/TcpExample2ConnectionCountingTerminalLog.txt" target="blank">TcpExample2ConnectionCountingTerminalLog.txt</a>
* @see <a href="../../../src/TcpExamples/TcpExample2ConnectionCountingScreenshot.png" target="blank">TcpExample2ConnectionCountingScreenshot.png</a>
* @see <a href="https://savage.nps.edu/Savage/developers.html#Cygwin" target="blank">Savage Developers Guide: Cygwin</a>
* @see <a href="https://savage.nps.edu/Savage/developers.html#telnet" target="blank">Savage Developers Guide: telnet</a>
* @see <a href="https://en.wikipedia.org/wiki/Ipconfig" target="blank">Wikipedia: ipconfig</a>
*
* @author mcgredo
* @author brutzman@nps.edu
*/
public class TcpExample2ConnectionCounting
{
/** Default constructor */
public TcpExample2ConnectionCounting()
{
// default constructor
}
/**
* Program invocation, execution starts here
* @param args command-line arguments
*/
public static void main(String[] args)
{
try
{
// welcome aboard messages
System.out.println(TcpExample2ConnectionCounting.class.getName() + " has started and is waiting for a connection.");
System.out.println(" help: https://savage.nps.edu/Savage/developers.html#telnet");
System.out.println(" Windows ipconfig (or Mac ifconfig) indicates current IPv4_Address (for example local wireless IP address)");
System.out.println(" Windows enter (telnet localhost 2317) or Mac enter (nc localhost 2317) for loopback operation, or" );
System.out.println(" Windows enter (telnet IPv4_Address 2317) or Mac enter (nc IPv4_Address 2317) for LAN operation" );
System.out.println("TcpExample2ConnectionCounting server standing by..." );
// ServerSocket waits for a connection from a client.
// Notice that it is outside the loop; ServerSocket needs to be made only once.
int connectionCount = 0; // state variable
ServerSocket serverSocket = new ServerSocket(2317); // server decides here what port to listen on.
// of interest: often client doesn't care what port it uses locally when connecting to that server port.
OutputStream outputStream; // can declare these early, but...
PrintStream printStream; // note initial values are null, they still need to be intialized
// Loop, infinitely, waiting for client connections.
// If you want to stop or pause the program, go somewhere else.
while(true)
{
// note that serverSocket.accept() first blocks! then after waiting, execution proceeds once a connection is "accept"ed
// you can check this behavior by using NetBeans Debugger
try (Socket clientConnectionSocket = serverSocket.accept()) { // the accept() method blocks here until a client connects
connectionCount++; // got another one! a client has connected
outputStream = clientConnectionSocket.getOutputStream(); // clientConnectionSocket is new, every time through, so reset the streams
printStream = new PrintStream(outputStream);
// if (connectionCount == 1) // first time through, report connection
// {
// // Where are we? In other words, what is our host number? Advertise it.
// // Note that we use the serverSocket to get address, since host may have multiple network connections.
// // https://stackoverflow.com/questions/9481865/getting-the-ip-address-of-the-current-machine-using-java
// String localHostAddress = clientConnectionSocket.getInetAddress().getHostAddress();
// System.out.println("Local host address is " + localHostAddress);
// // add diagnostic for host name, note this might only show up for your system's telnet connection since hostnames aren't part of TCP
// String localHostName = clientConnectionSocket.getInetAddress().getHostName();
// System.out.println("Local host name is " + localHostName);
// if (localHostAddress.contains("/"))
// localHostAddress = localHostAddress.substring(localHostAddress.indexOf("/")+1);
// // show localhost IP number to facilitate connections over local area network (LAN, WAN)
// System.out.println(" enter (nc localhost 2317) or (telnet localhost 2317) for local operation" );
// System.out.println(" enter (nc " + localHostAddress + " 2317) or " +
// "(telnet " + localHostAddress + " 2317)..." );
// }
printStream.println("This client response was written by server " + TcpExample2ConnectionCounting.class.getName()); // to remote client
System.out.println("This server response was written by server " + TcpExample2ConnectionCounting.class.getName()); // to server console
printStream.println("You were connection #" + connectionCount + ", by my count");
// Print some information locally about the Socket connection.
// This includes the port and IP numbers on both sides (the socket pair.)
InetAddress localAddress = clientConnectionSocket.getLocalAddress();
InetAddress remoteAddress = clientConnectionSocket.getInetAddress();
int localPort = clientConnectionSocket.getLocalPort();
int remotePort = clientConnectionSocket.getPort(); // remember the prior question, why are 2 ports different?
// My socket pair connection looks like this, to localhost:
// Socket pair: (( /0:0:0:0:0:0:0:1, 2317 ), ( /0:0:0:0:0:0:0:1, 54876 )) note IPv6
// Socket pair: (( /0:0:0:0:0:0:0:1, 2317 ), ( /0:0:0:0:0:0:0:1, 54881 ))
//
// Why is first IP/port the same, while the second set has different ports?
System.out.println("Socket pair (server, client): (( " + localAddress.toString() + ", " + localPort + " ), ( " +
remoteAddress.toString() + ", " + remotePort + " ))");
System.out.println("got another connection, #" + connectionCount); // report progress
// Notice the use of flush() to ensure that messages are sent immediately,
// rather than possibly getting buffered for the program's self-perceived efficiency.
outputStream.flush();
// Notice the use of optional close() when complete. Without a close() invocation,
// the clientConnectionSocket object may stay open for a while after the client
// has stopped needing this connection. Close() explicitly ends the connection.
// Try hiding this and see if anything different occurs.
clientConnectionSocket.close();
}
}
}
catch(IOException ioe)
{
System.err.println("Problem with " + TcpExample2ConnectionCounting.class.getName() + " networking:"); // describe what is happening
System.err.println("Error: " + ioe);
// Provide more helpful information to user if exception occurs due to running twice at one time
if (ioe instanceof java.net.BindException)
System.err.println("*** Be sure to stop any other running instances of programs using this port!");
}
finally
{
// given the infinite-loop logic above, this step is "hard to reach" but a good practice nevertheless
System.out.println(TcpExample2ConnectionCounting.class.getName() + " has finished, adios!");
}
}
}