URL connection, Content handler

1
URL connection, Content handler Protocol handler

• Lecture 10
• Chapters 15, 16, 17

2
URL connection class

• Active connection to a resource specified by a
  URL
• Provides more control over the interaction with a
  server esp HTTP server than URL class
• You can inspect the header sent by server and
  respond accordingly
• You can set the header fields used in the client
  request
• You can use URL connection to download binary
  files.
• Allows you to sent data back to server as PUT or
  POST methods

3
URL connection class.

• It is part of Javas protocol handler mechanism
  which also includes the URLStreamHandler class

4
Opening URL connections

• Construct a URL object
• Invoke the URL objects openConnection() method
  to retrieve a URLConnection object for that URL
• Configure the URLConnection
• Read the header fields
• Get an input stream read data
• Get an output stream and write data
• Close the connection

5
To download a webpage with URL connection

• import java.net.
• import java.io.
• public class SourceViewer2 public static void
  main (String args)
• if (args.length gt 0) try //Open the
  URLConnection for reading
• URL u new URL(args0)
• URLConnection uc u.openConnection()
• InputStream raw uc.getInputStream()
• InputStream buffer new BufferedInputStream(raw)
  // chain the InputStream to a Reader
• Reader r new InputStreamReader(buffer)
• int c
• while ((c r.read()) ! -1)
• System.out.print((char) c)
• catch (MalformedURLException ex)
• System.err.println(args0 " is not a
  parseable URL")
• catch (IOException ex) System.err.println(ex)
  
• // end if
• // end main
• // end SourceViewer2

6
Book examples

• Look here
• http//www.cafeaulait.org/books/jnp3/examples/15/

7
What URL connection do that URL class cannot do

• URLConnection provides access to the HTTP header
• URLConnection can configure the request
  parameters sent to the server
• Can read as well as write data to the server

8
Reading the Header info

• Content-type
• Content-length
• Content-encoding
• Date
• Last-modified
• Expires
• Example http//www.cafeaulait.org/books/jnp3/exam
  ples/15/

9
Writing data to the server

• Configure the URLConnection
• Post the data
• Example of posting the authors name Email ID
  to the website
• http//www.cafeaulait.org/books/jnp3/examples/15/F
  ormPoster.java

10
Protocol Handlers

• Designing an architecture requires
• Handling Protocol
• Handling content
• Handling the protocol involves interaction
  between client/server
• Generating request in the correct format
• Interpreting the headers that come back with the
  data
• Acknowledging that data is received etc
• Content Handlers convert the data into the
  correct format such as InputStream or AudioClip

11
Protocol Handler Implementation

• FOUR different classes in java.net package work
  together to implement Protocol Handler
• URL (Concrete class)
• URLStreamHandler (abstract class)
• URLConnection (abstract class)
• URLStreamHandlerFactory (interface)

12
Protocol Handler Implementation

• The program constructs a URL object
• The constructor uses the arguments its passed to
  find the protocol part of the URL, such as http
• The URL() constructor tries to find a
  URLStreamHandler for the given protocol
• Retrieves from Cache if used before
• If a URLStreamHandlerFactory has been set, the
  protocol string is passed to the factorys
  createURLStreamHandler() method
• If not, the constructor attempts to instantiate
  an URLStreamHandler object named protocol.Handler
  in the java or sun protocol packages
• If all these fails, then throws an exception
• The program calls the URL objects
  openConnection()
• URL object asks the URLStreamHandler to return
  the URLConnection object
• Program uses the methods of the URLConnection
  class to interact with the remote resource

13
Writing a protocol handler

• Finger protocol URLConnection example in book
• http//www.cafeaulait.org/books/jnp3/examples/16/

14
Content Handlers

• Ability to handle various kinds of files such as
  html, gif, jpeg,
• Browsers come with plug-ins that add the ability
  to view new content types
• Java team found a way around to only download the
  parts of the program to be updated instead of the
  entire browser!!
• Unfortunately, java content handlers never really
  made it out of Suns white papers into the
  shipping software!
• Book Examples