CS401

Communication Across Internet

You can not only perform file I/O with the machine containing the java application but, also with files/systems accross the Internet.
Sometimes referred to as making your application "Net-aware".
Examples:

load a document from over the Internet,
create client-server program,
create distributed computer programs.

Java Applet Limitations:

1) cannot read or write from the disk on the machine where the browser is running.

3) If you try to load files off of the system where the applet is originally stored, you will get a security exception and other error messages.

Java Applet Possibilities:

Do so by refering to the file by its URL (Universal Resource Locator),
Note: if you do this kind of I/O communications in an applet, you can no longer use the appletviewer tool to test it -> you must test it inside of a Web browser AND have the applet on a web server along with the HTML that contains it!!!

Package = java.net

java.net Classes and important methods

URL	This is a class that represents a URL. Can create an URLConnection object with its openConnection() method.
URLConnection	This class represents a connection with a URL. Can use this to get an Inputer stream (getInputStream() method) to read/write information.
Socket	Abstraction of standard TCP socket. Provides a client-side socket interface similar to a standard UNIX socket. This provides Internet connectivity beyond URL and URLConnection classes. Socket(hostName, portNum) must know hostname and portnumber you want to make a connection with. Portnumbers often indicate the kind of protocol communications you will be performing, e.g. ftp versus telnet. Open a connnection on this socket using getInputStream() as done in URL/URLConnection example below. Once a socket is open you can use I/O streams like in URL/URLConnection example below. close() make sure you close your socket when you are done with your communications (I/O).
ServerSocket	In this case, a server-socket does not open a connection but instead has a method action() that waits and listens on the TCP port for a connection request from a client. In this way, you can create dual-applications: one a client Socket that asks for a connection and the other a ServerSocket that will accept making a connection with it.
getInputStream()	This is a method of the URLConnection class. Will retrieve an InputStream to data I/O with the corresponding Internet/URL file.

Using URL to get to remote items:

1) Create a URL object representing file

2) Create an URLConnection object that can load this URL and make a connection to the site hosting it.

3) Using getInputStream() method of the URLConnection object, create an InputStreamReader that can read from this URL.

4) If you wish create a different kind of reader, e.g. BufferedReader, as we did before with File I/O to more efficiently read the data as you wish.

Examples connecting with URLs

Example_Application

Example_Applet

Issues in Creating a Chat Program:

Communication's Model Architecture
GUI design
Data design
Business Object Design

Discussion of Client-Server Chat

Chat Related Links

Using Java- Server Chat SW

Example using Java Applet