Advanced Java Unit 3

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Advanced JavaUnit 3

• CMSC 291
• Shon Vick

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Agenda

• Final Socket Notes
• Look at URLs
• Summarize I/O
• Present some ideas about distributed computing in
  Java

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Avoiding Infinite Block

• If not properly constructed the Socket
  constructor may block indefinitely
• JDK 1.4 offers a solution

Socket (String host, int port) Socket s new
Socket() s.connect(new InetSocketAddress(host,por
t) , timeout)
Blocks waiting for host
Only waits timeout milisecs for host
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Thread Method
public final void join(long millis) throws
InterruptedException Waits at most millis
milliseconds for this thread to die. A timeout
of 0 means to wait forever. Parameters
millis - the time to wait in
milliseconds. Throws
InterruptedException - if another thread has
interrupted the current thread. The
interrupted status of the current
thread is cleared when this exception is thrown
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Older Version Workaround
Socket s null Thread t new Thread()
public void run( ) try s new
Socket(host, port) catch (IOException e)
t.start() try t.join(timeout) catch
(InterruptedException e)

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Half Close
Socket s new Socket(host,port) BufferedReade
r r new BufferedReader( new
InputStreamReader(s.getInputStream())) PrintWrite
r w new PrintWriter(s.getOutputStream(),
true) // send request data w.print(
.) s.shutdownOutput() // now socket is half
closed I.e open for input only String
line while ((line r.readLine()) ! null)
. . s.close()
Tells server that you are done
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URLs

• Now lets look at URLs and which provide a
  relatively high-level mechanism for accessing
  resources on the Internet
• Client uses some service
• Server - provides some service

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Working with URLs

• URL is the acronym for Uniform Resource Locator
• It is a reference (an address) to a resource on
  the Internet.
• You provide URLs to your favorite Web browser so
  that it can locate resources on the Internet

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Properties and Uses

• Java programs use a class called URL in the
  java.net package to represent a URL address.
• Applications
• You can use a URL to read in application data
• Have an applet load specific web pages by having
  users specify a URL

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Constructing a URL

• Think of a URL as the name of a file on the Web
  because most URLs refer to a file on some machine
  on the network.
• However, remember that URLs also can point to
  other resources on the network, such as database
  queries and other data

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The anatomy of a URL

• A URL has two main components
• Protocol identifier
• Resource name

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The anatomy of a URL

• The protocol identifier and the resource name are
  separated by a colon and two forward slashes.
• The protocol identifier indicates the name of the
  protocol to be used to fetch the resource

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Sample Protocols

• Hypertext Transfer Protocol (HTTP)
• Usually used to serve up hypertext documents
• Other protocols include
• File Transfer Protocol FTP
• Gopher, File, and News

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The anatomy of a Resource name

• The format of the resource name depends entirely
  on the protocol used, but for many protocols
  including HTTP, the resource name contains these
  components
• Host Name The name of the machine on which the
  resource lives.
• Filename The pathname to the file on the machine

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More Components of the Anatomy

• Host Name The name of the machine on which the
  resource lives.
• Filename The pathname to the file on the machine
  (sometimes implied I.e with index.html)
• Port Number The port number to which to connect
  (typically optional).
• Reference A named anchor within a resource that
  usually identifies a specific location within a
  file (typically optional).

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Reading from and Writing to a URLConnection

• Some URLs, such as many that are connected to
  cgi-bin scripts, allow you to (or even require
  you to) write information to the URL
• Example
• A search script may require detailed query data
  to be written to the URL before the search can be
  performed. Lets see how to write to a URL and
  how to get results back.

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Reading

• You can read and write to a URL
• First we will look at simple examples using the
  URL class then we will look at examples involving
  the http centric URLConnection class

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Creating a URL

• Within your Java programs, you can create a URL
  object that represents a URL address
• The URL object always refers to an absolute URL
  but can be constructed from an absolute URL, a
  relative URL, or from URL components.

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URL Example

• The easiest way to create a URL object is from a
  String that represents the human-readable form of
  the URL address.
• Example
• http//heaven.org/apply.html

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Creating a URL Relative to Another

• A relative URL contains only enough information
  to reach the resource relative to (or in the
  context of) another URL
• Consider a html file named JoesHomePage.html
  (http//www.JoesPlace.com/JoesHomePage.html )
• Within this page there arelinks to other pages
• lta href"PicturesOfMe.html"gtPictures of Melt/agt

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Relative URLs

• General Form
• new URL(URL baseURL, String relativeURL)
• URL Joes
• new URL (http//www.JoesPlace.com/
  )
• URLPix
• new URL (Joes, PicturesOfMe.html)

Base
Relative
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Other Constructs

• public URL(String protocol, String host,
• int port, String file)
• throws MalformedURLException
• public URL(String url)
• throws MalformedURLException
• public URL(String host, String file)
• throws MalformedURLException

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MalformedURLException

• Each URL constructor throws a MalformedURLExceptio
  n
• Java does not verify to throw this exception it
  only checks URL specification form
• try
• URL myURL new URL(. . .)
• catch (MalformedURLException e)
• . . .
• // exception handler code here
• . . .

Good idea to catch error and deal with the error
immediately
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Specific Example From Book
public Choice setChoiceList() Choice ch new
Choice() URL furl null DataInputStream
din null String buf ch.addItem("Pick
One") try furl new URL(getDocumentBase(
) "resources.html") catch
(MalformedURLException e)
e.printStackTrace() ch.addItem("No Items
Available") return ch
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Opening the URL
String files null try din new
DataInputStream(furl.openStream())
catch( IOException e)
e.printStackTrace() ch.addItem("No Items
Available") return ch
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Adding the Choices
try while((buf din.readLine()) ! null)
ch.addItem(buf) din.close()
catch(IOException e)
e.printStackTrace() ch.addItem("No Items
Available") return ch return
ch
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Another example
import java.net. import java.io.
public class URLReader
public static void main(String args) throws
Exception URL yahoo new
URL("http//www.yahoo.com/")
BufferedReader in new BufferedReader(
new
InputStreamReader(
yahoo.openStream()))
String inputLine while
((inputLine in.readLine()) ! null)
System.out.println(inputLine)
in.close()
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Sample Explanation

• When you run the program, you should see,
  scrolling by in your command window, the HTML
  commands and textual content from the HTML file
  located at http//www.yahoo.com/
• Alternatively, the program might hang or you
  might see an exception stack trace.

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Sending your Applet to a URL
...   String name getParameter("Goto")
try gotoURL new URL(name) catch
(MalformedURLException e) // remember to
handle errors!   ... public boolean
mouseDown(Event e, int x, int y)
getAppletContext().showDocument(gotoURL)
return(true)
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Goto Example

• Enable a browser to goto Web location
• You can dynamically link providing live content
  with a very small amount of code
• Of course you will have to add an applet tag

ltapplet code GotoDemo width300 height200gt
ltparam nameGoto" valuewww.gl.umbc.edu/432 gt
lt/appletgt
Tag
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Writing to a URLConnection

• Many HTML pages contain forms-- text fields and
  other GUI objects that let you enter data to send
  to the server
• After you type in the required information and
  initiate the query by clicking a button, your Web
  browser writes the data to the URL over the
  network.

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The Other End

• At the other end, a cgi-bin script (usually) on
  the server receives the data, processes it, and
  then sends you a response, usually in the form of
  a new HTML page.
• Many cgi-bin scripts use the POST METHOD for
  reading the data from the client. Thus writing to
  a URL is often called posting to a URL.
  Server-side scripts use the POST METHOD to read
  from their standard input.

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Interacting With cgi-bin scripts

• A Java program can interact with cgi-bin scripts
  also on the server side. It simply must be able
  to write to a URL, thus providing data to the
  server.
• It can do this by following a few relatively
  steps.

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The steps

• .Create a URL.
• Open a connection to the URL.
• Set output capability on the URLConnection.
• Get an output stream from the connection. This
  output stream is connected to the standard input
  stream of the cgi-bin script on the server.
• Write to the output stream.
• Close the output stream

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Example Explanation

• Say The script at our Web site reads a string
  from its standard input, reverses the string, and
  writes the result to its standard output.
• The script requires input of the form
  stringstring_to_reverse, where string_to_reverse
  is the string whose characters you want displayed
  in reverse order.

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Some Sanity checking
import java.io. import
java.net. public class Reverse
public static void main(String args)
throws Exception if
(args.length ! 1)
System.err.println("Usage java Reverse "

"string_to_reverse")
System.exit(1) .
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Creating the URL

• Next, the program creates the URL object--the URL
  for the backwards script

URL url new URL("http//java.sun.com/cgi-bin/bac
kwards") URLConnection c
url.openConnection()
c.setDoOutput(true)
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Creating the a Writer

• The program then creates an output stream on the
  connection and opens a PrintWriter on it.
• If the URL does not support output,
  getOutputStream method throws an
  UnknownServiceException.
• If the URL does support output, then this method
  returns an output stream that is connected to the
  standard input stream of the URL on the server
  side--the client's output is the server's input

PrintWriter out new PrintWriter(c.getOutputStrea
m())
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Doing the Writing

• Next, the program writes the required information
  to the output stream and closes the stream.
• This code writes to the output stream using the
  println method - just like writing data to a
  stream.
• The data written to the output stream on the
  client side is the input for the backwards script
  on the server side.

out.println("string" stringToReverse)
out.close()
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Read back the results

• BufferReader in new BufferedReader(
• new
  InputStreamReader(c.getInputStream()))
• String inputLine
• while ((inputLine in.readLine()) !
  null)
• System.out.println(inputLine)
• in.close()

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Output from example
Reverse Me reversed is eM esreveR
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Convenience methods

• getDate
• getExpiration
• getLastModified
• getContentLength
• getContentType
• getContentEncoding

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Selected References

• Advanced Techniques for Java Developers Chapter
  4
• http//java.sun.com/docs/books/tutorial/networking
  /urls/index.html
• Exploring Java, OReilly, Niemeyer Peck
  pgs.335-363
• http//userpages.umbc.edu/vick/432/lectures/Fall9
  8/UsingJava/Applets/MoreOnApplets.html