Introduction to Java

1
Introduction to Java

• April 26, 2007
• David Goldschmidt, Ph.D.
• goldschd_at_strose.edu

2
Applications vs. Applets (i)

• Because JFrame and JApplet are both subclasses of
  Container, all user interface components, layout
  managers, and event-handling features are the
  same
• Applets require HTML for deployment
• Both applications and applets require JVM (often
  via JRE or Web browser plug-in)

3
Applications vs. Applets (ii)

• Applications are invoked from static main()
  method by Java interpreter
• Applets are run by Web browser
• Applications require a frame (e.g. JFrame)
• Applets require a Web browser to provide
  graphical environment

4
Applications vs. Applets (iii)

• Applets have security restrictions
• An applet is not allowed to read from or write to
  the file system of the Web browsers computer
• An applet is not allowed to run any programs on
  the Web browsers computer
• An applet is not allowed to establish connections
  between the users computer and another computer
• Except with the server on which the applet is
  stored

5
Applications vs. Applets (iv)

• Converting an applet into an application is
  always possible (and usually mechanical)
• Converting an application into an applet is
  sometimes possible, depending on whether security
  restrictions on applets are not violated

6
Tic Tac Toe (i)
7
Tic Tac Toe (ii)
8
Overcoming Applet Restrictions

• Security restrictions disallow applets from
  accessing local or network files
• How can applets load image, audio, video, and
  other local or network files?
• Use the java.net.URL class to identify local
  files or files available via the Internet

9
Meta-Objects (i)

• When a class is loaded, the JVM creates a
  meta-object for the class
• java.lang.Class metaObject this.getClass()
• Meta-object instances provide information about
  the class, including
• Data fields
• Constructors
• Methods

10
Meta-Objects (ii)

• Obtain URL of a file in the class directory
• String filename ca.gif
• Class metaObject this.getClass()
• URL url metaObject.getResource(filename)
• If the class directory was C\Java\rpi-week12\
  then the URL would be
• C\Java\rpi-week12\ca.gif

11
Deploying Java Applets (i)

• A Java Archive ( JAR) file groups all project
  files into a single compressed file for
  deployment
• Includes Java class files, images, audio files,
  etc.
• JAR files are based on the ZIP file format
• Can usually open JAR files using WinZip
• JAR files enables efficient deployment of both
  Java applets and applications

12
Deploying Java Applets (ii)

• Use jar command from the Java SDK to create a
  Java archive file
• jar -cvf TicTacToe.jar TicTacToe.class
  TicTacToeCell.class
• Command-line arguments very similar to tar
• Use -c to create a new JAR file
• Use -v to specify verbose mode
• Use -f to specify the name of the JAR file

Files to be added to the JAR file
Name of the JAR file
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Deploying Java Applets (iii)

• Use the following HTML to deploy the JAR file
• ltapplet
• codeTicTacToe.class
• archiveTicTacToe.jar
• width350
• height200gt
• lt/appletgt

14
JDBC

• JDBC is a platform-independent Java API for
  executing SQL statements
• Using JDBC, you can
• Connect to databases
• Send SQL statements
• Receive results (i.e. rows of data)

15
JDBC Architecture
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JDBC Drivers (i)

• Before connecting to a database, a driver class
  must first be loaded into the JVM
• A driver is simply a class in Java
• Access sun.jdbc.odbc.JdbcOdbcDriver
• MySQL com.mysql.jdbc.Driver
• Oracle oracle.jdbc.driver.OracleDriver
• Oracle driver located in a JAR file of the Oracle
  distribution

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JDBC Drivers (ii)

• To load a driver, use static forName() method of
  the java.lang.Class class
• Class.forName(sun.jdbc.odbc.JdbcOdbcDriver)
• For Oracle
• Class.forName("oracle.jdbc.driver.OracleDriver")
  

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JDBC Interfaces
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JDBC Connection Strings

• Connect to a database using its connect string
  (i.e. its connection URL)
• Access jdbcodbcdataSource
• MySQL jdbcmysql//hostname/dbname
• Oracle jdbcoraclethin_at_hostnameportSID
• Use a Connection object
• Connection connection
• DriverManager.getConnection(dbConnectURL)
  

20
Sample Access Database
21
Using JDBC to Process Data
22
Client/Server Architecture

• Web-based and other applications are often
  designed using a client/server architecture
• An application server is always running, waiting
  for connections from clients
• A client connects to a server as
  necessary on behalf of its
  user
• A socket serves as
  the communications
  channel between
  
  client and server

Server
socket
socket
socket
Client
Client
Client
23
Creating a Server in Java (i)

• When the server first starts up, it creates a
  server socket (java.net.ServerSocket) on a given
  port
• ServerSocket serverSocket new
  ServerSocket(port)
• Port numbers typically range from 0 to 65535
• First 1024 port numbers are reserved for
  privileged services (e.g. HTTP uses port 80, FTP
  uses port 21)
• ServerSocket serverSocket new
  ServerSocket(8000)
• Using a port in use throws a BindException

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Creating a Server in Java (ii)

• Once a server socket is created, the accept()
  method is called to activate the server socket as
  a listener ready to accept new incoming client
  connections
• Socket incomingSocket serverSocket.accept()
• Java supports both TCP and UDP transmission
  protocols
• For UDP, use java.net.DatagramSocket and
  java.net.DatagramPacket classes

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Creating a Client in Java

• A client connects to a listening server via the
  Socket() constructor
• Socket clientSocket new Socket(serverName,
  port)
• Socket s1 new Socket(java.rpi.edu, 8123)
• Socket s2 new Socket(128.3.127.5, 8000)
• Socket s3 new Socket(127.0.0.1, 9000)
• Socket s4 new Socket(localhost, 9001)
• Constructor could throw UnknownHostException

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Communicating via Sockets (i)

• Once established, a socket contains both an input
  and an output stream
• Methods getInputStream() and getOutputStream()
  return InputStream and OutputStream objects (from
  the java.io library)
• Interesting and useful subclasses of InputStream
  and OutputStream include
• DataInputStream and DataOutputStream
• ObjectInputStream and ObjectOutputStream

27
Communicating via Sockets (ii)

• Use DataInputStream and DataOutputStream to
  transfer primitive data types between client and
  server
• InputStream input socket.getInputStream()
• DataInputStream in new DataInputStream(input)
  
• OutputStream output socket.getOutputStream()
  
• DataOutputStream out new DataOutputStream(outp
  ut)

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Communicating via Sockets (iii)

• Use ObjectInputStream and ObjectOutputStream to
  transfer serializable objects between client and
  server
• InputStream input socket.getInputStream()
• ObjectInputStream in new ObjectInputStream(inp
  ut)
• OutputStream output socket.getOutputStream()
  
• ObjectOutputStream out new ObjectOutputStream(
  output)

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Communicating via Sockets (iv)

• For an object to be serializable, its class must
  implement the java.io.Serializable interface
• The Serializable interface has no methods or
  fields and serves only to identify the semantics
  of being serializable
• Enables the use of readObject() and writeObject()
  methods of ObjectInputStream and
  ObjectOutputStream

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Serving Multiple Clients

• So far, weve only looked at a server that blocks
  when it receives an incoming client connection
• To serve multiple clients, a server delegates
  each incoming client connection to a dedicated
  thread
• while (true)
• Socket socket serverSocket.accept()
• Thread thread new ThreadClass(socket)
• thread.start()

31
Applets as Clients

• Applets can only connect to the host from which
  they were loaded (i.e. the Web server)
• Obtain the name of the Web server by calling the
  getHost() method of the URL object returned by
  the getCodeBase() method
• URL host getCodeBase()
• socket new Socket(host.getHost())

32
Retrieving Files from a Web Server (i)

• Java provides direct programmatic access to files
  on a Web server via the java.net.URL class
• No need to create a server application!
• URL url new URL(http//webaddress)
• Throws a MalformedURLException if the URL is
  invalid

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Retrieving Files from a Web Server (ii)

• Use the openStream() method to open an input
  stream to the given URL
• URL url new URL(http//webaddress)
• InputStream input url.openStream()
• Read text from the input stream using the
  BufferedReader and other Java API classes

34
Retrieving Files from a Web Server (iii)

• Use the javax.swing.JEditorPane class to display
  plain text, HTML, or RTF files
• JEditorPane jep1 new JEditorPane()
• JEditorPane jep2 new JEditorPane(URL)
• JEditorPane jep3 new JEditorPane(urlString)
  
• Once constructed, use the setPage() method to
  load another file
• jep1.setPage(URL)

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Retrieving Files from a Web Server (iv)

• The javax.swing.JEditorPane class captures a
  javax.swing.event.HyperlinkEvent when a hyperlink
  in the editor pane is clicked
• jep.addHyperlinkListener(new
  HyperlinkListener()
• public void hyperlinkUpdate(HyperlinkEvent
  e)
• try jep.setPage(e.getURL())
• catch (IOException ex) ...
• )