Sockets: Network io HOM DOS HVD HEE. Sockets, Object Streams and Serialization. Sockets. Sockets: Network io HOM DOS HVD HEE

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1 : Network : Network ieter van den Hombergh hijs Dorssers ichard van den Ham Uwe van Heesch, bject Fontys Hogeschool voor echniek en Logistiek April 22, 2016 /FHenL : Network April 22, /19 opics : Network, bject, bject /FHenL : Network April 22, /19 Socket address: ip number + port number, server and client are applicatns which both own a socket. : Network 1 ServerSocket Socket, bject Server Listen 2 Connectn request Client 3 Accept Server Listen 4 Client Socket Figure: networking/sockets/index.html /FHenL : Network April 22, /19 basics A socket can perform 7 basic operatns: 1 Connect to a remote machine 2 Send data 3 eceive data 4 Close a connectn 5 Bind to a port 6 Listen for incoming data 7 Accept connectns from remote machines on bound port java..net.socket (used by clients and servers) has methods corresponding to operatns 1-4 java..net.serversocket (used only by servers) has methods corresponding to operatns 5-7 : Network, bject /FHenL : Network April 22, /19 1

2 ort Scanner Example public class LowortScanner { public static void main ( String [] args ) throws IExceptn { String host = " "; if ( args. length > 0 ) { host = args [ 0 ]; int [] ports = new int []{ 22, 25, 80, 631, 8083 ; for ( int port : ports ) { : Network, bject // java 7 try with resources try ( Socket sock = new Socket ( host, port ) ) { System. out. printf ( " port %d on server %s is active %n", port, host ); catch ( UnknownHostExceptn uhe ) { System. out. printf ( " host %s not found /n", host ); catch ( IExceptn e ) { System. out. printf (" port %d on server %s is not active %n", port, host ); /FHenL : Network April 22, /19 ort Scanner Example (continued) Here is the output this program produced on a certain computer: port 25 on server localhost is not active port 53 on server localhost is not active port 80 on server localhost is active port 631 on server localhostis not active port 8083 on server localhost is not active his program helps you to understand what your system is doing, so you can find (and close) possible entrance points for attackers. f course you can use this program to scan all ports in the range Don t use LowortScanner to probe a machine you don t own; most system administrators would consider that a hostile act. : Network, bject /FHenL : Network April 22, /19 eading from and writing to a socket Each socket has an input and an output stream, which can be used to sent and receive data: ublic utputstream getutputstream() throws IExceptn eturns an output stream for writing bytes to this socket ublic InputStream getinputstream() throws IExceptn eturns an input stream for reading bytes from this socket : Network, bject /FHenL : Network April 22, /19 Web browser example public class WebBrowser { public static void main ( String [] args ) { try ( Socket http = new Socket ( " www. fontysvenlo. org ", 80 ); utputstream raw = http. getutputstream (); InputStream in = http. getinputstream (); utputstream buffered = new BufferedutputStream ( raw ); utputstreamwriter out = new utputstreamwriter ( buffered, "UF -8" ); ) { out. write ( " GE / \r\n\r\n" ); out. flush (); int r; while ( ( r = in. read () )!= -1 ) { System. out. print ( ( char ) r ); catch ( IExceptn ex ) { Logger. getlogger ( WebBrowser. class. getname () ). log ( Level. SEVEE, null, ex ); : Network, bject /FHenL : Network April 22, /19 2

3 Class ServerSocket : Network, bject /FHenL : Network April 22, /19 Life cycle of a server A new ServerSocket is created on a particular port using a ServerSocket() constructor. he ServerSocket listens for incoming connectn attempts on that port using its accept() method; accept() blocks until a client attempts to make a connectn, at which point accept() returns a Socket object connecting the client and the server. he Socket s getinputstream() and/or getutputstream() method are called to get input and/or output streams. he server and the client interact according to an agreed-upon protocol until it is time to close the connectn. If that takes too long, a Java program should spawn a thread. he server and the client (or both) close the connectn. he server returns to step 2 and waits for the next connectn. : Network, bject /FHenL : Network April 22, /19 Echo client/server sequence diagram : Network, bject /FHenL : Network April 22, /19 Echo server which serves only one client public class EchoServer { public static void main ( String [] args ) throws IExceptn { if ( args. length!= 1) { System. err. println (" Usage : java EchoServer <port number >"); int portnumber = Integer. parseint ( args [0]) ; try ( ServerSocket serversocket = new ServerSocket ( Integer. parseint ( args [0]) ); Socket clientsocket = serversocket. accept (); rintwriter out = new rintwriter ( clientsocket. getutputstream (), true ); Bufferedeader in = new Bufferedeader ( new InputStreameader ( clientsocket. getinputstream ())); ) { String inputline ; while ( inputline!= null &&! inputline. isempty () ) { out. println ( inputline ); catch ( IExceptn e) { System. out. println (" Exceptn caught when trying to listen on" + " port " + portnumber + " or listening " + " for a connectn "); System. out. println (e. getmessage ()); : Network, bject /FHenL : Network April 22, /19 3

4 Echo client public class EchoClient { public static void main ( String [] args ) throws IExceptn { if ( args. length!= 2) { System. err. println ( " Usage : java EchoClient <host name > <port number >"); String hostname = args [0]; int portnumber = Integer. parseint ( args [1]) ; try ( Socket echosocket = new Socket ( hostname, portnumber ); rintwriter out = new rintwriter ( echosocket. getutputstream (), true ); // true denotes autoflush Bufferedeader in = new Bufferedeader ( new InputStreameader ( echosocket. getinputstream ())); Bufferedeader stdin = new Bufferedeader ( new InputStreameader ( System. in )) ) { String userinput ; while (( userinput = stdin. readline ())!= null ) { out. println ( userinput ); System. out. println (" echo : " + in. readline ()); catch ( UnknownHostExceptn e) { System. err. println (" Don 't know about host " + hostname ); catch ( IExceptn e) { System. err. println (" Couldn 't get I/ for the connectn to " + hostname ); : Network, bject /FHenL : Network April 22, /19 Discussn of the example Note the use of try-with-resources. equires Java 7 or above Server can handle multiple requests in sequence. Server is blocked until a request is completely handled. ossible improvement: Single hread per Client pattern Server sits in a loop accepting forthcoming connectns As soon as they arrive it spawns a thread responsible for handling the client Downside: the model doesn t scale well with the number of clients (if more than a few tens) Even better: euse threads and connectns by using pools. Even better: Use executor framework and pass the ConnectnHandler with executor.execute(handler); : Network, bject /FHenL : Network April 22, /19 Simple Server with Single hread per Client and Callback : Network, bject serve()-method runs in a loop A ConnectnHandler object is created for every connectn ConnectnHandler implements unnable-interface ConnectnHandler is executed in a dedicated thread serve()-loop is immediately available for additnal requests /FHenL : Network April 22, /19 Using threads in a nutshell implement the unnable-interface override the run()-method instantiate a new hread and pass runnable in the constructor start thread by calling start()-method, N the run()-method : Network, bject /FHenL : Network April 22, /19 4

5 Echo server which serves multiple clients public class MultiEchoServer { public static void main ( String [] args ) throws IExceptn { if ( args. length!= 1) { System. err. println (" Usage : java EchoServer <port number >"); : Network, bject int portnumber = Integer. parseint ( args [0]) ; try ( ServerSocket serversocket = new ServerSocket ( Integer. parseint ( args [0]) );) { while ( true ) { Socket clientsocket = serversocket. accept (); ConnectnHandler clienteply = new ConnectnHandler ( clientsocket ); new hread ( clienteply ). start (); catch ( IExceptn e) { System. out. println (" Exceptn caught when trying to listen on" + " port " + portnumber + " or listening for a connectn "); System. out. println (e. getmessage ()); /FHenL : Network April 22, /19 ConnectnHandler public class ConnectnHandler implements unnable { private final Socket socket ; public ConnectnHandler ( Socket s) { socket = public void run () { try ( rintwriter out = new rintwriter ( socket. getutputstream (), true ); Bufferedeader in = new Bufferedeader ( new InputStreameader ( socket. getinputstream ()));) { String inputline ; while ( inputline!= null &&! inputline. isempty () ) { System. out. println ( inputline ); out. println ( inputline ); : Network, bject catch ( IExceptn ex ) { throw new untimeexceptn ( ex ); // stop this thread. /FHenL : Network April 22, /19 Questns and links : Network Not all understood? Study Lesson: All About Questns? Questns or remarks?, bject /FHenL : Network April 22, /19 5