URLs

1
URLs URIs

• Lecture 5 (chapter 7)

2
URL class

• URL class is the simplest way for Java to locate
  retrieve data from a network
• Though Java can handle only few protocols
  content types, later you will learn how to write
  install new content protocol handlers
• Java.net.URL class is an abstraction of a URL
  such as http//www.unb.ca
• It extends java.lang.Object and it is a final
  class that cannot be subclassed
• Public final class URL extends Object implements
  Serializable

3
URL class cont.

• Though storing URL as a string would be easy, it
  is helpful to think URL as object with fields
  that include the protocol, hostname, port, path,
  query string, and fragment identifier (ref), each
  of which may be set independently.
• EX http//v5o5jotqkgfu3btr91t7w5fhzedjaoaz8igl.un
  bsj.ca/jlight/

4
Creating new URLs

• We can construct instances of java.net.URL
• There are SIX constructors differing in the
  information they require

5
Constructing a URL from a string

• Public URL(String url) throws MalformedURLExceptio
  n
• Following code constructs a URL object from a
  string
• try
• URL unew URL(http//www.unb.ca)
• catch (MalformedURLException ex)
• System.err.println(ex)
• Example to find out which protocols a virtual
  machine supports. It tries to construct a URL
  object for each 14 protocols ( 8 std 3 custom
  made) for various java APIs 4 undocumented
  protocols used internally by HotJava. If the
  constructor succeeds, then the protocol is
  supported Otherwise it throws MalformedURLExcepti
  on error. Protocol Tester Example

6
Constructing a URL from its component parts

• This constructor builds a URL from THREE strings
  specifying the protocol, the hostname and the
  file
• Public URL(String protocol, String hostname,
  String file) throws MalformedURLException
• The constructor sets the port to -1, so that the
  default port for the protocol will be used
• try
• URL unew URL(http, http//www.unb.ca,
  /decker.html)
• catch (MalformedURLException ex)
• System.err.println(ex)

7
Constructing relative URL

• Public URL(URL base, String relative) throws
  MalformedURLException
• try
• URL u1 new URL(http, http//www.unb.ca,
  /prospectivestudents.html)
• URL u2 new URL(u1,studentslist.html)
• catch (MalformedURLException ex)
• System.err.println(ex)

8
Splitting a URL into pieces

• URLs are composed of five pieces
• The protocol (scheme)
• The authority
• The path
• The fragment identifier (section/ref)
• The query string
• Read only access to these parts of a URL is
  provided by FIVE public methods
• getFile() getHost() getProtocol() getPort()
  and getRef()
• Java 1.3 adds four more methods getQuery()
  getPath() getUserInfo() and getAuthority()

9
Retrieving data from a URL

• Public InputStream openStream() throws
  IOException
• Public URLConnection openConnection() throws
  IOException
• Public URLConnection openConnection(Proxy proxy)
  throws IOException //1.5
• Public Object getContent() throws IOException
• Public Object getContent(Class classes) throws
  IOException //1.3

10
URLEncoder and URLDecoder classes

• To solve difference in dealing with Oses
• Encoding Any character that is not ASCII is
  converted into byte and each byte is written as
  sign followed by two hexadecimal digits.
• Ex
• spaces are encoded as 20
• Public static String encode (String s)
• Public static String decode (String s)

11
URI Class

• Includes not only URLs but also URNs
• URI class is purely about identification of
  resources and parsing of URIs
• Most standards XML are defined interms of URIs
  //Java 1.4 after

12
Constructing a URI

• URL depends on an underlying protocol whereas URI
  does not.
• URIs are built from strings.
• Ex of some constructors for URIs
• Public URI(String uri) throws URISyntaxException
• Public URI(String scheme, String
  schemeSpecificPart, String fragment) throws
  URISyntaxException
• Public URI(String scheme, String host, String
  path, String fragment) throws URISyntaxException
• Public URI(String scheme, String host, String
  path, String query, String fragment) throws
  URISyntaxException
• Ex
• URI Lect5 http, www.unbsj.ca,
  jlight.html, v5o5jotqkgfu3btr91t7w5fhzedjaoaz8
  igl, Lect5

13
Communicating the server-side programs through Get

• URL class makes it easier for Java applets
  applications to communicate server-side programs
  such as CGIs, servlets, PHP pages, and others
  using GET method
• If you have written the server side program you
  know what form to ask
• Otherwise it is trickier!!

14
Accessing password-protected sites

• Through HTTP authentication
• Through cookies HTML forms (not a good soln)
• The authenticator class included in the Java.net
  package, use username password to authenticate
• Public PasswordAuthentication(String userName,
  char password)
• Each accessed using getter method
• Public String getUserName()
• Public char getPassword90

15
Book examples

• http//www.cafeaulait.org/books/jnp3/examples/07/