URLs, InetAddresses, and URLConnections

1. URLs, InetAddresses, and URLConnections Chapter 9 Prepared By E. Musa Alyaman

2. We will learn how Java handles • URLs • CGI • URLConnection • Content and Protocol handlers Prepared By E. Musa Alyaman

3. URLs • A URL, short for "Uniform Resource Locator", is a way to identify the location of a resource on the Internet. • Examples http://java.sun.com/ ftp://ftp.info.apple.com/pub/ mailto:elharo@metalab.unc.edu telnet://utopia.poly.edu

4. The Pieces of a URL • the protocol. • the authority • :// • host name or address • port • the path Prepared By E. Musa Alyaman

5. The java.net.URL class • A URL object represents a URL. • The URL class contains methods to • create new URLs • parse the different parts of a URL • get an input stream from a URL so you can read data from a server • get content from the server as a Java object Prepared By E. Musa Alyaman

6. Content and Protocol Handlers • Content and protocol handlers separate the data being downloaded from the the protocol used to download it. • The protocol handler negotiates with the server and parses any headers. It gives the content handler only the actual data of the requested resource. • The content handler translates those bytes into a Java object like an InputStream. Prepared By E. Musa Alyaman

7. Finding Protocol Handlers • When the virtual machine creates a URL object, it looks for a protocol handler that understands the protocol part of the URL such as "http". • If no such handler is found, the constructor throws a MalformedURLException. Prepared By E. Musa Alyaman

8. URL Constructors • There are four constructors in the java.net.URL class. public URL(String u) throws MalformedURLException public URL(String protocol, String host, String file) throws MalformedURLException public URL(String protocol, String host, int port, String file) throws MalformedURLException public URL(URL context, String url) throws MalformedURLException Prepared By E. Musa Alyaman

9. Constructing URL Objects • An absolute URL like http://www.poly.edu/fall97/grad.html try { URL u = new URL("http://www.poly.edu/fall97/grad.html"); } catch (MalformedURLException e) {} Prepared By E. Musa Alyaman

10. Constructing URL Objects in Pieces You can also construct the URL by passing its pieces to the constructor, like this: URL u = null; try { u = new URL("http", "www.poly.edu", "/schedule/fall97/bgrad.html"); } catch (MalformedURLException e) {} Prepared By E. Musa Alyaman

11. Including the Port URL u = null; try { u = new URL("http", "www.poly.edu", 8000, "/fall97/grad.html"); } catch (MalformedURLException e) {} Prepared By E. Musa Alyaman

12. Relative URLs • Many HTML files contain relative URLs. • Consider the page http://metalab.unc.edu/javafaq/index.html • On this page a link to “books.html" refers to http://metalab.unc.edu/javafaq/books.html. Prepared By E. Musa Alyaman

13. Constructing Relative URLs • The fourth constructor creates URLs relative to a given URL. For example, try { URL u1 = new URL("http://metalab.unc.edu/index.html"); URL u2 = new URL(u1, ”books.html"); } catch (MalformedURLException e) {} Prepared By E. Musa Alyaman

14. Parsing URLs • The java.net.URL class has five methods to split a URL into its component parts. These are: public String getProtocol() public String getHost() public int getPort() public String getFile() Prepared By E. Musa Alyaman

15. For example, try { URL u = new URL("http://www.poly.edu/fall97/grad.html "); System.out.println("The protocol is " + u.getProtocol()); System.out.println("The host is " + u.getHost()); System.out.println("The port is " + u.getPort()); System.out.println("The file is " + u.getFile()); } catch (MalformedURLException e) { } Prepared By E. Musa Alyaman

16. Missing Pieces • If a port is not explicitly specified in the URL it's set to -1. This means the default port is to be used. • If the file is left off completely, e.g. http://java.sun.com, then it's set to "/". Prepared By E. Musa Alyaman

17. Reading Data from a URL • The openStream() method connects to the server specified in the URL and returns an InputStream object fed by the data from that connection. • public final InputStream openStream() throws IOException • Any headers that precede the actual data are stripped off before the stream is opened. • Network connections are less reliable and slower than files. Buffer with a BufferedReader or a BufferedInputStream. Prepared By E. Musa Alyaman

18. Common Gateway Interface (CGI) • Normal web uses these two steps: • The browser requests a page • The server sends the page • Data flows primarily from the server to the client. Prepared By E. Musa Alyaman

19. Forms • There are times when the server needs to get data from the client rather than the other way around. The common way to do this is with a form like this one: Prepared By E. Musa Alyaman

20. CGI • The user types the requested data into the form and hits the submit button. • The client browser then sends the data to the server using the Common Gateway Interface, CGI for short. • CGI uses the HTTP protocol to transmit the data, either as part of the query string or as separate data following the MIME header. Prepared By E. Musa Alyaman

21. MIME • MIME is stands for "Multipurpose Internet Mail Extensions". • an Internet standard defined in RFCs 2045 through 2049 • originally intended for use with email messages, but has been been adopted for use in HTTP. Prepared By E. Musa Alyaman

22. GET and POST • When the data is sent as a query string included with the file request, this is called CGI GET. • When the data is sent as data attached to the request following the MIME header, this is called CGI POST Prepared By E. Musa Alyaman

23. HTTP • Web browsers communicate with web servers through a standard protocol known as HTTP, ( HyperText Transfer Protocol). • This protocol defines • how a browser requests a file from a web server • how a browser sends additional data along with the request (e.g. the data formats it can accept), • how the server sends data back to the client Prepared By E. Musa Alyaman

24. A Typical HTTP Connection • Client opens a socket to port 80 on the server. • Client sends a GET request including the name and path of the file it wants and the version of the HTTP protocol it supports. • The client sends a MIME header. • The client sends a blank line. • The server sends a MIME header • The server sends the data in the file. • The server closes the connection. Prepared By E. Musa Alyaman

25. URL Encoding • Alphanumeric ASCII characters (a-z, A-Z, and 0-9) and the $-_.!*'(), punctuation symbols are left unchanged. • The space character is converted into a plus sign (+). • Other characters (e.g. &, =, ^, #, %, ^, {, and so on) are translated into a percent sign followed by the two hexadecimal digits corresponding to their numeric value. Prepared By E. Musa Alyaman

26. The URLEncoder class • The java.net.URLEncoder class contains a single static method which encodes strings in x-www-form-url-encoded format URLEncoder.encode(String s) Prepared By E. Musa Alyaman

27. For example String qs = "Author=Sadie, Julie&Title=Women Composers"; String eqs = URLEncoder.encode(qs); System.out.println(eqs); • This prints: • Author%3dSadie%2c+Julie%26Title%3dWomen+Composers Prepared By E. Musa Alyaman

28. The URLDecoder class • In Java 1.2 the java.net.URLDecoder class contains a single static method which decodes strings in x-www-form-url-encoded format URLEncoder.decode(String s) Prepared By E. Musa Alyaman

29. URLConnections • The java.net.URLConnection class is a class that handles communication with different kinds of servers like ftp servers and web servers. • Protocol specific subclasses of URLConnection handle different kinds of servers. • By default, connections to HTTP URLs use the GET method. Prepared By E. Musa Alyaman

30. URLConnections vs. URLs • Can send output as well as read input • Can post data to CGIs • Can read headers from a connection Prepared By E. Musa Alyaman

31. URLConnection five steps: 1. The URL is constructed. 2. The URL’s openConnection() method creates the URLConnection object. 3. The parameters for the connection and the request properties that the client sends to the server are set up. 4. The connect() method makes the connection to the server. 5. The response header information is read using getHeaderField(). Prepared By E. Musa Alyaman

32. I/O Across a URLConnection • Data may be read from the connection in one of two ways • raw by using the input stream returned by getInputStream() • through a content handler with getContent(). • Data can be sent to the server using the output stream provided by getOutputStream(). Prepared By E. Musa Alyaman

33. For example, try { URL u = new URL("http://www.sd99.com/"); URLConnection uc = u.openConnection(); uc.connect(); InputStream in = uc.getInputStream(); // read the data... } catch (IOException e) { //... Prepared By E. Musa Alyaman

34. Chapter Highlights In this chapter, you have learned: • How to bind to a local port using DatagramSocket • How to create a DatagramPacket • How to read from, and write to, a DatagramPacket using ByteArrayInputStream and ByteArrayOutputStream • How to listen for UDP packets • How to send UDP packets • How to create a UDP server and a UDP client Prepared By E. Musa Alyaman