Java Socket Programming

Java Sockets Programming  The package java.net provides support for sockets programming (and more).  Typically you import everything defined in this package with: import java.net.*; Java Socket Programming 2

Classes InetAddress Socket ServerSocket DatagramSocket DatagramPacket Java Socket Programming 3

InetAddress class  static methods you can use to create new InetAddress objects.  getByName(String host)  getAllByName(String host)  getLocalHost() InetAddress x = InetAddress.getByName( “cse.unr.edu”);  Throws UnknownHostException Java Socket Programming 4

try { InetAddress a = InetAddress.getByName(hostname); System.out.println(hostname + ":" + a.getHostAddress()); } catch (UnknownHostException e) { System.out.println("No address found for " + hostname); } try { InetAddress a = InetAddress.getByName(hostname); System.out.println(hostname + ":" + a.getHostAddress()); } catch (UnknownHostException e) { System.out.println("No address found for " + hostname); } Java Socket Programming

Socket class  Corresponds to active TCP sockets only!  client sockets  socket returned by accept();  Passive sockets are supported by a different class:  ServerSocket  UDP sockets are supported by  DatagramSocket Java Socket Programming 6

JAVA TCP Sockets (Client Socket)  java.net.Socket  Implements client sockets (also called just “sockets”).  An endpoint for communication between two machines.  Constructor and Methods • Socket(String host, int port): Creates a stream socket and connects it to the specified port number on the named host. • InputStream getInputStream() • OutputStream getOutputStream() • close() Java Socket Programming 7

ServerSocket  java.net.ServerSocket  Implements server sockets.  Waits for requests to come in over the network.  Performs some operation based on the request.  Constructor and Methods • ServerSocket(int port) • Socket Accept(): Listens for a connection to be made to this socket and accepts it. This method blocks until a connection is made. Internet 8

Sockets Client socket, welcoming socket (passive) and connection socket (active) Java Socket Programming 9

Socket Constructors  Constructor creates a TCP connection to a named TCP server.  There are a number of constructors: Socket(InetAddress server, int port); Socket(InetAddress server, int port, InetAddress local, int localport); Socket(String hostname, int port); Java Socket Programming 10

Socket Methods void close(); InetAddress getInetAddress(); InetAddress getLocalAddress(); InputStream getInputStream(); OutputStream getOutputStream();  Lots more (setting/getting socket options, partial close, etc.) Java Socket Programming 11

Socket I/O  Socket I/O is based on the Java I/O support  in the package java.io  InputStream and OutputStream are abstract classes  common operations defined for all kinds of InputStreams, OutputStreams… Java Socket Programming 12

InputStream Basics // reads some number of bytes and // puts in buffer array b int read(byte[] b); // reads up to len bytes int read(byte[] b, int off, int len); Both methods can throw IOException. Both return –1 on EOF. Java Socket Programming 13

OutputStream Basics // writes b.length bytes void write(byte[] b); // writes len bytes starting // at offset off void write(byte[] b, int off, int len); Both methods can throw IOException. Java Socket Programming 14

ServerSocket Class (TCP Passive Socket)  Constructors: ServerSocket(int port); ServerSocket(int port, int backlog); ServerSocket(int port, int backlog, InetAddress bindAddr); Java Socket Programming 15

ServerSocket Methods Socket accept(); void close(); InetAddress getInetAddress(); int getLocalPort(); throw IOException, SecurityException Java Socket Programming 16

Socket programming with TCP Example client-server app:  client reads line from standard input (inFromUser stream) , sends to server via socket (outToServer stream)  server reads line from socket  server converts line to uppercase, sends back to client  client reads, prints modified line from socket (inFromServer stream) outToServer tonetwork fromnetwork inFromServer inFromUser keyboard monitor Process clientSocket input stream input stream output stream TCP socket Input stream: sequence of bytes into processoutput stream: sequence of bytes out of process Client process client TCP socket Java Socket Programming 17

Client/server socket interaction: TCP wait for incoming connection request connectionSocket = welcomeSocket.accept() create socket, port=x, for incoming request: welcomeSocket = ServerSocket() create socket, connect to hostid, port=x clientSocket = Socket() close connectionSocket read reply from clientSocket close clientSocket Server (running on hostid) Client send request using clientSocketread request from connectionSocket write reply to connectionSocket TCP connection setup Java Socket Programming 18

Sample Echo Server TCPEchoServer.java And TCPClient.java Save both files, compile and run on separate terminal. First TCPEchoServer and then TCPClient Based on code from: TCP/IP Sockets in Java Java Socket Programming 19

TCPEchoServer.java import java.io.*; import java.net.*; Public class TCPEchoServer { public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence; ServerSocket welcomeSocket = new ServerSocket(6789); // server running and listening while(true) { Socket connectionSocket = welcomeSocket.accept(); // new client connected BufferedReader inFromClient = new BufferedReader (new InputStreamReader (connectionSocket.getInputStream())); DataOutputStream outToClient = new DataOutputStream (connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase()+ ‘n’; outToClient.writeBytes(capitalizedSentence); } } } Java Socket Programming 21

TCPClient.java import java.io.*; import java.net.*; Public class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream (clientSocket.getOutputStream()); BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine(); outToServer.writeBytes(sentence + 'n'); modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } } Java Socket Programming 23

UDP Sockets  DatagramSocket class  DatagramPacket class needed to specify the payload  incoming or outgoing Java Socket Programming 24

Socket Programming with UDP  UDP  Connectionless and unreliable service.  There isn’t an initial handshaking phase.  Transmitted data may be received out of order, or lost.  Socket Programming with UDP  No need for a welcoming socket.  No streams are attached to the sockets.  The sending hosts creates “packets” by attaching the IP destination address and port number to each batch of bytes.  The receiving process must unravel to received packet to obtain the packet’s information bytes. Java Socket Programming 25

JAVA UDP Sockets  In Package java.net  java.net.DatagramSocket • A socket for sending and receiving datagram packets. • Constructor and Methods – DatagramSocket(int port): Constructs a datagram socket and binds it to the specified port on the local host machine. – void receive (DatagramPacket p) – void send (DatagramPacket p) – void close() Java Socket Programming 26

DatagramSocket Constructors DatagramSocket(); DatagramSocket(int port); DatagramSocket(int port, InetAddress a); All can throw SocketException or SecurityException Java Socket Programming 27

Datagram Methods void connect(InetAddress, int port); void close(); void receive(DatagramPacket p); void send(DatagramPacket p); Lots more! Java Socket Programming 28

DatagramPacket  Contain the payload  (a byte array, length of byte array, InetAddress, port)  Can also be used to specify the destination address  when not using connected mode UDP Java Socket Programming 29

DatagramPacket Constructors For receiving: DatagramPacket( byte[] buf, int len); For sending: DatagramPacket( byte[] buf, int len InetAddress a, int port); Java Socket Programming 30

DatagramPacket methods byte[] getData(); void setData(byte[] buf); void setAddress(InetAddress a); void setPort(int port); InetAddress getAddress(); int getPort(); Java Socket Programming 31

Example: Java client (UDP) sendPacket tonetwork fromnetwork receivePacket inFromUser keyboard monitor Process clientSocket UDP packet input stream UDP packet UDP socket Output: sends packet (TCP sent “byte stream”) Input: receives packet (TCP received “byte stream”) Client process client UDP socket Java Socket Programming 32

Client/server socket interaction: UDP close clientSocket Server (running on hostid) read reply from clientSocket create socket, clientSocket = DatagramSocket() Client Create, address (hostid, port=x, send datagram request using clientSocket create socket, port=x, for incoming request: serverSocket = DatagramSocket() read request from serverSocket write reply to serverSocket specifying client host address, port umber Java Socket Programming 33

Sample UDP code UDPEchoServer.java Simple UDP Echo server. Test using nc as the client (netcat): > nc –u hostname port Java Socket Programming 34

UDPEchoServer.java import java.io.*; import java.net.*; class UDPEchoServer { public static void main(String args[]) throws Exception { int port = 9876; DatagramSocket serverSocket = new DatagramSocket(port); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket (receiveData, receiveData.length); serverSocket.receive(receivePacket); String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress();

int clientPort = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase(); sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket (sendData, sendData.length, IPAddress, clientPort); serverSocket.send(sendPacket); } } } Java Socket Programming 36

UDPClient.java import java.io.*; import java.net.*; public class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader (new InputStreamReader (System.in)); int port = 9876; DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine(); sendData = sentence.getBytes();

DatagramPacket sendPacket = new DatagramPacket (sendData, sendData.length, IPAddress, port); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket (receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); } } Java Socket Programming 38

Socket functional calls  socket (): Create a socket  bind(): bind a socket to a local IP address and port #  listen(): passively waiting for connections  connect(): initiating connection to another socket  accept(): accept a new connection  Write(): write data to a socket  Read(): read data from a socket  sendto(): send a datagram to another UDP socket  recvfrom(): read a datagram from a UDP socket  close(): close a socket (tear down the connection) Java Socket Programming 39

Java URL Class  Represents a Uniform Resource Locator  scheme (protocol)  hostname  port  path  query string Java Socket Programming 40

Parsing  You can use a URL object as a parser: URL u = new URL(“http://www.cs.unr.edu/”); System.out.println(“Proto:” + u.getProtocol()); System.out.println(“File:” + u.getFile()); Java Socket Programming 41

URL construction  You can also build a URL by setting each part individually: URL u = new URL(“http”, www.cs.unr.edu,80,”/~mgunes/”); System.out.println(“URL:” + u.toExternalForm()); System.out.println(“URL: “ + u); Java Socket Programming 42

Retrieving URL contents  URL objects can retrieve the documents they refer to!  actually this depends on the protocol part of the URL.  HTTP is supported  File is supported (“file://c:foo.html”)  You can get “Protocol Handlers” for other protocols.  There are a number of ways to do this: Object getContent(); InputStream openStream(); URLConnection openConnection(); Java Socket Programming 43

Getting Header Information  There are methods that return information extracted from response headers: String getContentType(); String getContentLength(); long getLastModified(); Java Socket Programming 44

URLConnection  Represents the connection (not the URL itself).  More control than URL  can write to the connection (send POST data).  can set request headers.  Closely tied to HTTP Java Socket Programming 45

Java Socket Programming

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