A socket is a software endpoint that establishes bidirectional communication between a server program and one or more client programs.

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1 PART 24 Java Network Applications 24.1 Java Socket Programming A socket is a software endpoint that establishes bidirectional communication between a server program and one or more client programs. A server program typically provides resources to a network of client programs. Client programs send requests to the server program, and the server program responds to the request. Using threads, a multi-threaded server program can accept a connection from a client, start a thread for that communication, and continue listening for requests from other clients. The java.net package provides support for the two common network protocols: TCP: TCP stands for Transmission Control Protocol, which allows for reliable communication between two applications. TCP is typically used over the Internet Protocol, which is referred to as TCP/IP. UDP: UDP stands for User Datagram Protocol, a connection-less protocol that allows for packets of data to be transmitted between applications. When the connection is made, the server creates a socket object on its end of the communication. The client and server can now communicate by writing to and reading from the socket.

2 The following steps occur when establishing a TCP connection between two computers using sockets: 1. The server instantiates a ServerSocket object, denoting which port number communication is to occur on. 2. The server invokes the accept() method of the ServerSocket class. This method waits until a client connects to the server on the given port. 3. A client instantiates a Socket object, specifying the server name and port number to connect to. 4. The constructor of the Socket class attempts to connect the client to the specified server and port number. If communication is established, the client now has a Socket object capable of communicating with the server. After the connections are established, communication can occur using I/O streams. Each socket has both an OutputStream and an InputStream. The client's OutputStream is connected to the server's InputStream, and the client's InputStream is connected to the server's OutputStream. TCP is a two way communication protocol, so data can be sent across both streams at the same time.

3 SimpleServer.Java import java.net.*; import java.io.*; public class SimpleServer extends Thread private ServerSocket serversocket; public SimpleServer(int port) throws IOException serversocket = new ServerSocket(port); serversocket.setsotimeout(100000); public void run() while(true) try System.out.println("Waiting for client on port "+ serversocket.getlocalport()); Socket server = serversocket.accept(); System.out.println("Just connected to " + server.getremotesocketaddress()); DataInputStream in = new DataInputStream(server.getInputStream()); System.out.println(in.readUTF()); DataOutputStream out = new DataOutputStream(server.getOutputStream()); out.writeutf("thank you for connecting to " + server.getlocalsocketaddress() + "\ngoodbye!"); server.close(); catch(sockettimeoutexception s) System.out.println("Socket timed out!"); break; catch(ioexception e) e.printstacktrace(); break; public static void main(string [] args) try // use TCP 6666 port Thread t = new SimpleServer(6666); t.start(); catch(ioexception e) e.printstacktrace();

4 SimpleClient.Java import java.net.*; import java.io.*; public class SimpleClient public static void main(string [] args) String servername = "localhost"; int port = 6666; try System.out.println("Connecting to " + servername + " on port " + port); Socket client = new Socket(serverName, port); System.out.println("Just connected to " + client.getremotesocketaddress()); OutputStream outtoserver = client.getoutputstream(); DataOutputStream out = new DataOutputStream(outToServer); out.writeutf("hello from " + client.getlocalsocketaddress()); InputStream infromserver = client.getinputstream(); DataInputStream in = new DataInputStream(inFromServer); System.out.println("Server says " + in.readutf()); client.close(); catch(ioexception e) e.printstacktrace(); 24.2 Sending To send an using your Java Application is simple enough but to start with you should have JavaMail API installed on your machine. You can download latest version of JavaMail from Java's standard website. Download and unzip files, you will find a number of jar files for both the applications. You need to add mail.jar files in your CLASSPATH (In Eclipse drag and drop into src folder, right click mail.jar and select Build Path-> Add to Build Path) Here is an example to send a simple via GOOGLE GMAIL SMTP SERVER. Here it is assumed that your localhost is connected to the internet and capable enough to send an .

5 import java.util.*; import javax.mail.*; import javax.mail.internet.*; public class Send public static void main(string[] args) // Sender's ID needs to be mentioned String from = "sender@gmail.com"; String pass = "123456"; // Recipient's ID needs to be mentioned. String to = "someone@hotmail.com"; String host = "smtp.gmail.com"; // Get system properties Properties properties = System.getProperties(); // Setup mail server properties.put("mail.smtp.starttls.enable", "true"); properties.put("mail.smtp.host", host); properties.put("mail.smtp.user", from); properties.put("mail.smtp.password", pass); properties.put("mail.smtp.port", "587"); properties.put("mail.smtp.auth", "true"); // Get the default Session object. Session session = Session.getDefaultInstance(properties); try // Create a default MimeMessage object. MimeMessage message = new MimeMessage(session); // Set From: header field of the header. message.setfrom(new InternetAddress(from)); // Set To: header field of the header. message.addrecipient(message.recipienttype.to, new InternetAddress(to)); // Set Subject: header field message.setsubject("this is the Subject Line!"); // Now set the actual message message.settext("this is actual message"); // Send message Transport transport = session.gettransport("smtp"); transport.connect(host, from, pass); transport.sendmessage(message, message.getallrecipients()); transport.close(); System.out.println("Sent message successfully..."); catch (MessagingException mex) mex.printstacktrace();

6 24.3 Java Applets An applet is a Java program that runs in a Web browser. An applet can be a fully functional Java application because it has the entire Java API. There are some important differences between an applet and a standalone Java application, including the following: An applet is a Java class that extends the java.applet.applet class. A main() method is not invoked on an applet, and an applet class will not define main(). Applets are designed to be embedded within an HTML page. When a user views an HTML page that contains an applet, the code for the applet is downloaded to the user's machine. A JVM is required to view an applet. The JVM can be either a plug-in of the Web browser or a separate runtime environment. The JVM on the user's machine creates an instance of the applet class and invokes various methods during the applet's lifetime. Life Cycle of an Applet: Four methods in the Applet class give you the framework on which you build any serious applet: init: This method is intended for whatever initialization is needed for your applet. start: This method is automatically called after the browser calls the init method. It is also called whenever the user returns to the page containing the applet after having gone off to other pages. paint: Invoked immediately after the start() method, and also any time the applet needs to repaint itself in the browser. stop: This method is automatically called when the user moves off the page on which the applet sits. It can, therefore, be called repeatedly in the same applet. destroy: This method is only called when the browser shuts down normally.

7 "Hello, World" Applet: The following is a simple applet named HelloWorldApplet.java: import java.applet.*; import java.awt.*; public class HelloWorldApplet extends Applet public void paint (Graphics g) g.drawstring ("Hello World", 25, 50); Applet CLASS: Every applet is an extension of the java.applet.applet class. The base Applet class provides methods that a derived Applet class may call to obtain information and services from the browser context. Invoking an Applet: An applet may be invoked by embedding directives in an HTML file and viewing the file through an applet viewer or Java-enabled browser.

8 The <applet> tag is the basis for embedding an applet in an HTML file. Below is an example that invokes the "Hello, World" applet: <html> <title> Hello World Applet</title> <applet code="helloworldapplet.class" width="320" height="120"> </applet> </html> The code attribute of the <applet> tag is required. It specifies the Applet class to run. Width and height are also required to specify the initial size of the panel in which an applet runs. The applet directive must be closed with a </applet> tag. Specifying Applet Parameters: The following example demonstrates how to make an applet respond to setup parameters specified in the document. The Applet.getParameter() method fetches a parameter given the parameter's name (the value of a parameter is always a string). import java.applet.*; import java.awt.*; public class appletparameter extends Applet private String strdefault = "Hello! Java Applet."; public void paint(graphics g) String strparameter = getparameter("message"); if (strparameter == null) strparameter = strdefault; setbackground(color.green); g.drawstring(strparameter, 50, 25); The following is an example of an HTML file with an appletparameter.class embedded in it. The HTML file specifies both parameters to the applet by means of the <param> tag.

9 <html> <title>applet Test</title> <h1> This is the applet: </h1> <applet code="appletparameter.class" width="800" height="100"> <param name="message" value="this is HTML parameter Text."> </applet> </html> Note: Parameter names are not case sensitive.

JAVA - NETWORKING (SOCKET PROGRAMMING)

JAVA - NETWORKING (SOCKET PROGRAMMING) http://www.tutorialspoint.com/java/java_networking.htm Copyright tutorialspoint.com The term network programming refers to writing programs that execute across multiple