An program As a simple example of socket programming we can implement a program that sends to a remote site As a simple example of socket programming we can implement a program that sends to a remote site  This is taken from Core Java, vol 2, chapter 3 servers use port number 25 which is the SMTP port servers use port number 25 which is the SMTP port  Simple mail transport protocol

A client socket to the mail server can be opened on port 25 A client socket to the mail server can be opened on port 25 SMPT uses the following protocol SMPT uses the following protocol Socket s=new Socket(“engmail.bham.ac.uk”,25); HELO sending host MAIL FROM:sender address RCPT TO: recipient address DATA mail message …. QUIT

The program uses a simple GUI to input the required information The program uses a simple GUI to input the required information The send button on the GUI calls the sendMail() method which outputs the correct protocol to the mail server The send button on the GUI calls the sendMail() method which outputs the correct protocol to the mail server Socket s=new Socket(“engmail.bham.ac.uk”,25); out=new PrintWriter(s.getOutputStream()); String hostname=InetAddress.getLocalHost().getHostName(); out.println(“HELO “ + hostname); out.println(“MAIL FROM: “ + from.getText()); etc

URL connections We have seen that to send or retrieve information to a web server, we use the HTTP protocol through a client socket attached to port 80 We have seen that to send or retrieve information to a web server, we use the HTTP protocol through a client socket attached to port 80 We can do this by using normal socket-based programming and sending the correct HTTP commands We can do this by using normal socket-based programming and sending the correct HTTP commands However, Java has specific support for the HTTP protocol through its URLConnection class However, Java has specific support for the HTTP protocol through its URLConnection class  Its very easy to retrieve a file from a web server by simply providing the file’s url as a string

This sets up an input stream from a url connection This sets up an input stream from a url connection  Can turn this into a Scanner object for text processing The URLConnection class also has additional functionality related to the HTTP protocol The URLConnection class also has additional functionality related to the HTTP protocol  Querying the server for header information  Setting request properties URL u=new URL(“ URLConnection c=u.openConnection(); InputStream in=c.getInputStream();

The following simple example program opens a web page and displays the HTML The following simple example program opens a web page and displays the HTML It also checks the server response code It also checks the server response code  404 if the page is not found  200 if the connection succeeded Uses a Scanner object to output the lines of HTML Uses a Scanner object to output the lines of HTML

public class URLGet { public static void main(String[] args) throws IOException { String urlName; if (args.length > 0) urlName = args[0]; else urlName = " URL url = new URL(urlName); URLConnection c = url.openConnection(); // Check response code HttpURLConnection httpConnection = (HttpURLConnection) c; int code =httpConnection.getResponseCode(); String message=httpConnection.getResponseMessage(); System.out.println(code + " " + message); if (code!=HttpURLConnection.HTTP_OK) return; // Read server response InputStream instream=connection.getInputStream(); Scanner in=new Scanner(instream); while (in.hasNextLine()) { String input=in.nextLine(); System.out.println(input); }