COMP201 Java Programming Part III: Advanced Features Topic 14: Networking Volume II,Chapter 3

COMP201 Topic 14 / Slide 2 Outline l Networking basics n IP addresses, ports, protocols, client-server interaction l Socket-level programming n Writing a client n Writing a server l URL connections and web servers n Retrieving information n Sending information

COMP201 Topic 14 / Slide 3 Networking Basics l Internet protocol (IP) address n Every host on Internet has a unique IP address , , , n More convenient to refer to using hostname string cs.ust.hk, tom.com, localhost n Domain Naming Service (DNS) maps names to numbers n One hostname can correspond to multiple internet addresses: tom.com: , , (down to one now)

COMP201 Topic 14 / Slide 4 java.net.InetAddress class converts between hostnames and internet addresses InetAddress tm = InetAddress.getByName(“tom.com"); InetAddress tm= InetAddress.getByName(“localhost"); // InetAddress tm = InetAddress.getLocalHost(); l Can get array of addresses (if more than one) InetAddress[] addrs; addrs=InetAddress.getAllByName(“tom.com"); for (int i = 0; i < addr.length; i++) System.out.println(addrs[i].getHostAddress()); InetAddressTest.java Networking Basics

COMP201 Topic 14 / Slide 5 l Ports l A port identifies a service within a host Many different services can be running on the host l Many standard port numbers are pre-assigned time of day 13, ftp 21, telnet 23, smtp 25, finger 79, http 80 see /etc/services on workstation for list of all assigned ports l IP address + port number = "phone number“ for service Networking Basics

COMP201 Topic 14 / Slide 6 l protocols : rules that facilitate communications between machines Time of day just reports time, ftp has put/get commands, etc. l Protocols must be standardized and documented So machines can reliably work with one another l TCP (Transmission Control Protocol) vs UDP (User Datagram Protocol, good for, e.g., video delivery) Networking Basics

COMP201 Topic 14 / Slide 7 l Client-Server interaction l Communication between hosts is two-way, but usually the two hosts take different roles l Server waits for client to make request Server registered on a known port with the host ("public phone number") Usually running in endless loop Listens for incoming client connections Networking Basics

COMP201 Topic 14 / Slide 8 l Client "calls" server to start a conversation Client making calls uses hostname/IP address and port number Sends request and waits for response l Standard services always running ftp, http, smtp, etc. server running on host using expected port l Server offers shared resource (information,database, files, printer, compute power) to clients Networking Basics

COMP201 Topic 14 / Slide 9 Using telnet to try out some services of servers: l Telnet assumes you want to connect to port 23 on the receiving host (port 23 is where the telnet server is listening) l However there is an optional argument after the hostname that allows you to connect to a different port l Try the following Get time: telnet time-A.timefreq.bldrdoc.gov 13 Finger some one: telnet cssu100.cs.ust.hk 79 & type a username Get HTML page: telnet tom.com 80 and enter a GET command Networking Basics

COMP201 Topic 14 / Slide 10 Socket-Level Programming l A socket is a bi-directional communication channel between hosts l Send and receive data using streams ClientServer OutputStream InputStream OutputStream l Next: n How to write a client n How to write a server

COMP201 Topic 14 / Slide 11 l To write a client Create a new Socket with hostname and port number of the connection Socket s = New Socket(String hostName,int portNumber); Call s.getOutputStream() and s.getInputStream() to get streams for sending and receiving infomation n Need to learn protocol used to communicate –Know how to properly form requests to send to server –Know how to interpret the server’s responses Socket-Level Programming/Client

COMP201 Topic 14 / Slide 12 Socket-Level Programming/Client l Example: A client using http to request page Socket s = new Socket(“tom.com", 80); PrintWriter send = new PrintWriter(s.getOutputStream()); send.print("GET \n"); send.flush(); BufferedReader recv = new BufferedReader( new InputStreamReader(s.getInputStream())); String line; while ((line = recv.readLine()) != null) System.out.println(line); s.close() //HttpTest.java. This is basically what web browsers do.

COMP201 Topic 14 / Slide 13 Socket-Level Programming/Client Use setSoTimeout method to limit the amount of time waiting for data Socket s = new Socket(“tom.com", 80); s.setSoTimeout( ); // time out after 10 seconds l Can still wait indefinitely for initial connection (construction of socket object).

COMP201 Topic 14 / Slide 14 Socket-Level Programming/Client Solution: Construct socket in a separate thread and use the join method to time out. class SocketOpener implements Runnable { public SocketOpener(String aHost, int aPort) { socket = null; host = aHost; port = aPort; } public void run() { try { socket = new Socket(host, port);} catch (IOException exception) {} } public Socket getSocket() { return socket; } … private String host; private int port; private Socket socket;

COMP201 Topic 14 / Slide 15 Socket-Level Programming/Client class SocketOpener implements Runnable { // method to be called by main thread public static Socket openSocket(String aHost, int aPort, int timeout) { SocketOpener opener = new SocketOpener(aHost, aPort); Thread t = new Thread(opener); t.start(); try { t.join(timeout); // wait at most timeout milliseconds } // for the thread to dead catch (InterruptedException exception){} return opener.getSocket(); } SocketOpenerTest.java

COMP201 Topic 14 / Slide 16 Socket-Level Programming/Server l To write a server Create a new ServerSocket with a port number to listen on the port ServerSocket s = New ServerSocket( portNumber); Use accept() to listen on the port. accept () returns a socket incoming when a client calls Socket incoming = s.accept(); Call incoming.getOutputStream() and incoming.getInputStream() to get streams for sending and receiving information

COMP201 Topic 14 / Slide 17 Socket-Level Programming/Server l Example: Echo server ServerSocket s = new ServerSocket(8189); while (true) { Socket incoming = s.accept( ); BufferedReader in = new BufferedReader (new InputStreamReader(incoming.getInputStream())); PrintWriter out = new PrintWriter (incoming.getOutputStream(), true /* autoFlush */ ); out.println( "Hello! Enter BYE to exit." ); … } EchoServer.java

COMP201 Topic 14 / Slide 18 Socket-Level Programming/Server l Multithread server: starts a separate thread for each connection. public class ThreadedEchoServer { public static void main(String[] args ) { int i = 1; try{ServerSocket s = new ServerSocket(8190); while (true) { Socket incoming = s.accept( ); System.out.println("Spawning " + i); new ThreadedEchoHandler(incoming, i).start(); i++; } } catch (Exception e) …. // ThreadedEchoServer.java

COMP201 Topic 14 / Slide 19 Socket-Level Programming/Server class ThreadedEchoHandler extends Thread { public ThreadedEchoHandler(Socket i, int c) { incoming = i; counter = c; } public void run() { try { BufferedReader in = new BufferedReader (new InputStreamReader(incoming.getInputStream())); PrintWriter out = new PrintWriter (incoming.getOutputStream(), true /* autoFlush */); out.println( "Hello! Enter BYE to exit." ); … private Socket incoming; private int counter; }

COMP201 Topic 14 / Slide 20 l ICQServer.java n A simple server that listens on port n Connect two clients so that they can talk to each other. n Can handle more than one pairs. n See code for details. Socket-Level Programming/Server

COMP201 Topic 14 / Slide 21 URL Connections l URL stands for Uniform Resource Locator Neat scheme for uniquely identifying all kinds of network resources l Basic form : ftp://ftp.cs.ust.hk/pub/lzhang/teach/201/codes/HttpTest/HttpTest.java file:/MyDisk/Letters/ToMom Protocols include files, http, ftp, gopher, news, mailto, etc. l Connecting to a URL, the hard way Manually parse out the host, protocol, path from URL Create a socket to host Use protocol to send the right request & interpret response But Java makes this much easier than that...

COMP201 Topic 14 / Slide 22 URL Connections/Retrieve Info Open connection with java.net.URL URL url1 = new URL(“ URL url2 = new URL (“ftp://ftp.cs.ust.hk/pub/lzhang/teach/201/codes/net/HttpT est/HttpTest.java”); To fetch contents of resource, use openStream method of URL, which returns a InputStream InputStream in1 = url1.openStream(); InputStream in2 = url2.openStream(); Now we can nest the InputStreams with other Java streams to retrieve contents l Advantages 1. No need to worry about protocol details & port. 2. Connection to particular file, not easy to do at Socket level

COMP201 Topic 14 / Slide 23 URL Connections/Retrieve Info Use java.net.URLConnection for additional info about resource 1. Create URLConnection object from URL, URLConnection cnn = url.openConnection(); 2. Set properties of connections: E.g. setDoOutPut(true) for sending information to the server 3. Make connection: cnn.connect(); 4. Query header information: getContentType, getContentLength, getContentEncoding, getDate, getExpiration, getLastModified 5. getInputStream for reading and getOutputStream for writing URLConnectionTest.java

COMP201 Topic 14 / Slide 24 URL Connections/Sending Info l Web servers receive information from clients using either GET or POST l … l l Appropriate CGI (common gateway interface) script is called to process info received and produce an HTML page to send back to client l CGI scripts usually written in C, Perl, shell script. (Out of the scope of this course.) l Will discuss servlets, Java alternative to CGI scripts

COMP201 Topic 14 / Slide 25 URL Connections/Sending Info l Send information to server using GET l Attach parameters to the end of URL l Separate parameters using “&” and encode parameters as follows to avoid misinterpretation (URL encoding) l Static String encode (String s) l Replace space with “+” l Replace each non-alphanumeric character with “%” followed by the hexadecimal code of the character “Mastering C++”  “Mastering+C%2b%2b” Disadvantage: long parameter string, might exceed limits of browsers. GetTest.java

COMP201 Topic 14 / Slide 26 URL Connections/Sending Info l Sending information to server using POST: Open URLConnection and send parameter using a stream 1. Open a URLConnection: URL url = new URL(“ URLConnection cnn = url.openConnection(); 2. Set up connection for output: cnn.setDoOutput(true); 3. Get a stream for sending data: PrinterWriter out = new PrintWriter(cnn.getOutputStream()); 4. Send parameters Out.print(name1 + “=“ + URLEncoder.encode(value1) + “&” ); Out.print(name2 + “=“ + URLEncoder.encode(value2) + “\n”); PostTest.java