Lecture 8. Java Distributed Computing

1
Lecture 8. Java Distributed Computing

• Cheng-Chia Chen

2
Distributed Computing

• Network programming
• JDBC
• Servlets JSPs
• Remote Method Invocation (RMI)
• Others not covered
• CORBA
• Enterprise JavaBeans (EJB)
• JINI
• JavaSpaces

3
Network programming

• Historically error-prone, difficult, complex
• but network programming in java is rather easy
• IO stream library works quite well for TCP/IP.
• Threading is also very useful and relatively easy
  here
• References
• The java tutorial, Trail Custom Networking
• http//java.sun.com/docs/books/tutorial/index.htm
  l
• Thinking in java, ch 15 http//www.eckelobjects.co
  m/TIJ2/index.html.
• Java Network Programming by Elliotte Rusty
  Harold, OReilly Books, 2nd Edition 2000. visit
  http//www.ibooks.com for a free online reference.

4
Networking Basics

• TCP/IP Network Protocol

5
TCP and UDP

• TCP (Transmission Control Protocol) is a
  connection-based protocol that provides a
  reliable flow of data between two computers.
• UDP (User Datagram Protocol) is a protocol that
  sends independent packets of data, called
  datagrams, from one computer to another with no
  guarantees about arrival. UDP is not
  connection-based like TCP.

6
Identifying a Machine

• Uniquely identify a machine from all the others
  in the world
• IP (Internet Protocol) address that can exist in
  two forms
• DNS (Domain Name System) form xml.cs.nccu.edu.tw
• Dotted quad (IP) form 140.119.162.230
• Represented internally by 32-bit number (4 bytes)
  
• 4.3 billion possibilities.
• IPV6 128-bit coming
• static InetAddress.getByName(String) produces
  Java object containing address

7
Servers Clients

• Two machines must connect
• Server waits around for connection
• Client initiates connection
• Once the connection is made, server client look
  identical
• Both ends are turned into InputStream and
  OutputStream objects, which can then be converted
  to Reader and Writer objects

8
Testing w/o a Network

• Here, we have no network
• Also, you may not trust your program enough yet.
• localhost the local loopback IP address for
  testing without a network
• InetAddress addr InetAddress.getByName(nul
  l)
• Equivalently
• InetAddress.getByName("localhost")
• Or using the reserved IP number for the loopback
• InetAddress.getByName("127.0.0.1")

9
WhoAmI

• // Finds out your network address when
• // you're connected to the Internet.
• import java.net.
• public class WhoAmI
• public static void main(String args) throws
  Exception
• if(args.length ! 1)
• System.err.println( "Usage WhoAmI
  MachineName")
• System.exit(1)
• InetAddress a InetAddress.getByName(args0
  )
• System.out.println(a)

10
Port Unique Place in a Machine

• IP address isnt enough to identify a unique
  server
• Many servers can exist on one machine
• IP machines also contain port numbers
• When you set up client and server, you must
  specify IP address and port, so they can find
  each other
• Not a physical location, but a software
  abstraction to represent a service
• There are 65536 ports
• in a machine
• 0-1023 reserved, others usable
• 80 ? HTTP 110 ? POP3
• 20,21 ? ftp 25 ? smtp
• 23 ? telnet, 119 ? nntp

11
Sockets

• Software abstraction used to represent the
  terminals of a connection between two machines
• Java provides a class abstraction
  java.io.Socket
• Socket is the actual 2-way connector. Can
  initiate a connection with a server
• ServerSocket isnt really a socket but more of a
  ServerConnector that produces a Socket as the
  return value of accept( ), which waits for a
  connection.
• In the end, you get a Socket on each machine

12
Just Like Files...

• Once you have a Socket, you call
• getInputStream( ) and getOutputStream( ) to
  produce the corresponding InputStream and
  OutputStream objects
• You convert these to readers and writers, wrap
  them in a BufferedReader or BufferedWriter and
  PrintWriter
• From then on, its like reading and writing any
  other IO stream!

13
Simple Server Client

• // Very simple server that just echoes whatever
  the client sends.
• import java.io.
• import java.net.
• public class JabberServer
• // Choose a port outside of the range 0-1023
• public static final int PORT 8080
• public static void main(String args) throws
  IOException
• ServerSocket s new ServerSocket(PORT)
• System.out.println("Started " s)

14

• try // Blocks until a connection occurs
• Socket socket s.accept()
• try
• System.out.println( "Connection accepted "
  socket)
• BufferedReader in new BufferedReader(
• new InputStreamReader(
  socket.getInputStream()))
• // Output is automatically flushed by
  PrintWriter
• PrintWriter out new PrintWriter(
• new BufferedWriter(
• new OutputStreamWriter
  ( socket.getOutputStream())),true)

15

• while (true) // main server response
• String str in.readLine()
• if (str.equals("END")) break
• System.out.println("Echoing " str)
• out.println(str)
• // Always close the two sockets...
• finally
• System.out.println("closing...")
• socket.close()
• finally s.close()

16
The client program

• // Thinking in Java c15, JabberClient.java
• // Very simple client that just sends lines to
  the server and reads lines
• // that the server sends.
• import java.net.
• import java.io.
• public class JabberClient
• public static void main(String args)
  throws IOException
• // "Local Loopback" IP address
• InetAddress addr InetAddress.getByName(null)
• System.out.println("addr " addr)
• // Unlike ServerSocket, client neednt specify
  its local port, which is allocated dynamically by
  machine.
• Socket socket new Socket(addr,
  JabberServer.PORT)
• // Guard everything in a try-finally to make
  sure that the socket is closed

17

• try System.out.println("socket " socket)
• BufferedReader in new
  BufferedReader( new InputStreamReader(
• socket.getInputStream()))
• // Output is automatically flushed by
  PrintWriter
• PrintWriter out new PrintWriter(
  new BufferedWriter(
• new
  OutputStreamWriter (socket.getOutputStream() ) ),
  true)
• for(int i 0 i lt 10 i )
• out.println(sending " i)
  String str in.readLine()
• System.out.println(str)
• out.println("END")
• finally System.out.println("closing...
  ") socket.close()

18
Serving Multiple Clients via Threads

• // A server that uses multithreading to
  handle any number of clients.
• import java.io. import java.net.
• class ServeOneJabber extends Thread
• private Socket socket private
  BufferedReader in
• private PrintWriter out
• public ServeOneJabber(Socket s) throws
  IOException
• socket s in new
  BufferedReader( new InputStreamReader
• (socket.getInputStream()
  ))
• // Enable auto-flush
• out new PrintWriter(new
  BufferedWriter( new OutputStreamWriter
• 
  ( socket.getOutputStream())), true)
• // If any of the above calls throw an
  exception, the caller is responsible
• // for closing the socket. Otherwise the
  thread will close it.

19

• public void run()
• try
• while (true)
• String str in.readLine()
• if (str.equals("END")) break
• System.out.println("Echoing "
  str)
• out.println(str)
• System.out.println("closing...")
• catch (IOException e)
• finally try socket.close()
  catch(IOException e)

20
MultiJabberServer

• public class MultiJabberServer static final
  int PORT 8080
• public static void main(String args)
  throws IOException
• ServerSocket s new ServerSocket(PORT)
• System.out.println("Server Started")
• try
• while(true)
• // wait for a connection established
• Socket socket s.accept()
• try new ServeOneJabber(socket).start
  ()
• catch(IOException e)
• // If it fails, close the socket,
  otherwise the thread will close it
• socket.close()
• finally s.close()

21
JabberClientThread

• // Client that tests the MultiJabberServer by
  starting up multiple clients.
• import java.net. import java.io.
• class JabberClientThread extends Thread
• private Socket socket private
  BufferedReader in
• private PrintWriter out private
  static int counter 0
• private int id counter private static
  int threadcount 0
• public static int threadCount() return
  threadcount
• public JabberClientThread(InetAddress addr)
  
• System.out.println("Making client "
  id)
• threadcount
• try socket new Socket(addr,
  MultiJabberServer.PORT)
• catch(IOException e)
• // If the creation of the socket fails,
  nothing needs to be cleaned up.

22

• try in new BufferedReader( new
  InputStreamReader
• ( socket.getInputStream()))
• // Enable auto-flush
• out new PrintWriter( new
  BufferedWriter(
• new OutputStreamWriter(
  socket.getOutputStream())), true)
• start()
• catch(IOException e)
• // The socket should be closed on any
  failures other than the socket
• // constructor
• try socket.close()
  catch(IOException e2)
• // Otherwise the socket will be closed by
• // the run() method of the thread.

23

• public void run()
• try
• for(int i 0 i lt 25 i)
• out.println("Client " id " "
  i)
• String str in.readLine()
• System.out.println(str)
• out.println("END")
• catch(IOException e)
• finally // Always close it
• try socket.close()
• catch(IOException e)
• threadcount-- // Ending this thread

24
MultiJabberClient

• public class MultiJabberClient
• static final int MAX_THREADS 40
• public static void main(String args)
• throws IOException, InterruptedException
  
• InetAddress addr
• InetAddress.getByName(null)
• while(true)
• if(JabberClientThread.threadCount() lt
  MAX_THREADS)
• new JabberClientThread(addr)
• Thread.currentThread().sleep(100) //
  Why not just sleep(100)?

25
Port Warning

• Problem program works fine, except when client
  is behind a firewall
• Firewall doesnt like to see anything but
  ordinary network connections to WWW server via
  its standard http port 80
• Servlets solve the problem

26
User Datagram Protocol (UDP)

• Previous examples used Transmission Control
  Protocol (TCP).
• Very high reliability, message will always get
  there.
• Also high overhead
• User Datagram Protocol (UDP) is an unreliable
  protocol which is much faster, but the messages
  wont always get there
• c.f.
• TCP ? Telephone system
• UDP ? postal service

27
Datagrams

• You make your own packets and write the address
  on the outside, send it
• Based on packet contents, recipients decide
  whether they got everything, sends a message if
  they didnt, and you retransmit in that case.
• Reliability must be enforced by your program, not
  by the UDP protocol
• Java APIs provided in two classes
• java.net.DatagramPacket
• java.net.DatagramSocket

28
DatagramPacket

• Encapsulation of data(packet) to be
  received/transmitted and related information.
• Constructors
• // for receiving packets
• DatagramPacket(byte data , int offset , int
  length)
• // for sending packets
• DatagramPacket(byte data , int offset, int
  length,
• 
  InetAddress ia ,int port )

29
DatagramPacket Methods

• InetAddress getAddress()
• Returns the IP address of the peer
  machine.
•  byte getData()      Returns the data buffer.
•  int getLength()        Returns the length of the
  data.
•  int getOffset()          Returns the offset of
  the data to be sent/received.
•  int getPort()           Returns the port number
  on the remote host.
•  SocketAddress getSocketAddress()           Gets
  the SocketAddress (usually IP address port
  number) of the remote host.
•  void setAddress(InetAddress iaddr)          
• void setData(byte buf , int offset,
  int length)       
•  void setLength(int length)      
•  void setPort(int iport)
• void setSocketAddress(SocketAddress )

30
DatagramSocket

• represents a socket for sending and receiving
  datagram packets
• Constructor
• // bind to local IP laddr and port port or local
  SocketAddr bindaddr
• // info not given gt allocated by machine
• DatagramSocket( int port ,InetAddress laddr
• DatagramSocket(SocketAddress bindaddr )
• Methods
• // for local info manipulation
• void bind(SocketAddress),
• isBound(),
• getLocalAddress(), getLocalPort(),
  getLocalSocketAddress()

31

• // for remote info manipulation
• // dedicated to this remote socket address only
• connect(InetAddress, int), connect(SocketAddress),
  
• disconnect(),
• isConnected() // return null/-1 if not
  connected.
• getInetAddress(), getPort(), getRemoteSocketAddres
  s()
• //send/receive data
• send(DatagramPacket)
• receive(DatagramPacket)
• // others
• isClosed()
• getBroadcast(), setBroadcast(boolean),
• getSoTimeout(), setSoTimeout(int),

32
Example UDP programming The client side program
33
// Fig. 18.7 Client.java // Client
that sends and receives packets to/from a
server. import java.io. import
java.net. import java.awt.
import java.awt.event. import
javax.swing. public class Client
extends JFrame private JTextField
enterField private JTextArea displayArea
private DatagramSocket socket
// set up GUI and DatagramSocket public
Client() super( "Client" )
Container container getContentPane()
enterField new JTextField( "Type message
here" ) enterField.addActionListener(
new ActionListener()
public void actionPerformed( ActionEvent event
)
34
// create and send packet
try displayArea.append( "\nSending packet
containing " event.getActionCommand() "\n"
) // get message from textfield
and convert to byte array String
message event.getActionCommand()
byte data message.getBytes()
// create sendPacket
DatagramPacket sendPacket new DatagramPacket(
data, data.length,
InetAddress.getLocalHost(), 5000 )
socket.send( sendPacket ) // send packet
displayArea.append( "Packet sent\n" )
displayArea.setCaretPosition(
displayArea.getText().length() ) //
process problems creating or sending packet
catch ( IOException ioException )
displayMessage( ioException.toString() "\n" )
ioException.printStackTrace
()
35
// end actionPerformed
// end inner class ) //
end call to addActionListener
container.add( enterField, BorderLayout.NORTH )
displayArea new JTextArea()
container.add( new JScrollPane( displayArea
), BorderLayout.CENTER )
setSize( 400, 300 ) setVisible( true
) // create DatagramSocket for
sending and receiving packets try
socket new DatagramSocket()
// catch problems creating
DatagramSocket catch( SocketException
socketException )
socketException.printStackTrace()
System.exit( 1 )
36
// end Client constructor //
wait for packets to arrive from Server, display
packet contents private void
waitForPackets() while ( true ) //
loop forever // receive packet
and display contents try // set
up packet byte data new byte
100
DatagramPacket receivePacket new
DatagramPacket( data, data.length )
socket.receive(
receivePacket ) // wait for packet
// display packet contents
displayMessage( "\nPacket received"
"\nFrom host " receivePacket.getAddres
s() "\nHost port "
receivePacket.getPort()
"\nLength " receivePacket.getLength()
"\nContaining\n\t" new String(
receivePacket.getData(), 0,
receivePacket.getLength() ) )
37
// process problems receiving or
displaying packet catch( IOException
exception ) displayMessage(
exception.toString() "\n" )
exception.printStackTrace()
// end while // end method
waitForPackets // utility method called from
other threads to manipulate // displayArea
in the event-dispatch thread private void
displayMessage( final String messageToDisplay )
// display message from event-dispatch
thread of execution SwingUtilities.invokeL
ater( new Runnable() // inner class
to ensure GUI updates properly
public void run() // updates displayArea
displayArea.append( messageToDisplay )
displayArea.setCaretPosition(
displayArea.getText().length() )
38
// end inner class )
// end call to SwingUtilities.invokeLater
public static void main( String args )
Client application new Client()
application.setDefaultCloseOperation(
JFrame.EXIT_ON_CLOSE )
application.waitForPackets() //
end class Client
39
The server side program
40
// Fig. 18.6 Server.java // Server
that receives and sends packets from/to a
client. import java.io. import
java.net. import java.awt. import
java.awt.event. import javax.swing.
public class Server extends JFrame
private JTextArea displayArea private
DatagramSocket socket // set up GUI
and DatagramSocket public Server()
super( "Server" ) displayArea
new JTextArea() getContentPane().add(
new JScrollPane( displayArea ),
BorderLayout.CENTER ) setSize( 400,
300 ) setVisible( true ) //
create DatagramSocket for sending and receiving
packets try socket new
DatagramSocket( 5000 )
41
// process problems creating
DatagramSocket catch( SocketException
socketException )
socketException.printStackTrace()
System.exit( 1 )
// end Server constructor // wait
for packets to arrive, display data and echo
packet to client private void
waitForPackets() while ( true
) // loop forever // receive
packet, display contents, return copy to client
try // set up packet
byte data new byte 100
DatagramPacket receivePacket
new DatagramPacket( data, data.length )
socket.receive( receivePacket ) // wait
for packet
42
// display information from received
packet displayMessage( "\nPacket
received" "\nFrom host "
receivePacket.getAddress()
"\nHost port " receivePacket.getPort()
"\nLength " receivePacket.getLen
gth() "\nContaining\n\t"
new String( receivePacket.getData(),
0, receivePacket.getLength() ) )
sendPacketToClient( receivePacket )
// send packet to client //
process problems manipulating packet
catch( IOException ioException )
displayMessage( ioException.toString() "\n"
) ioException.printStackTrace()
// end while
// end method waitForPackets //
echo packet to client private void
sendPacketToClient( DatagramPacket receivePacket
) throws IOException
43
displayMessage( "\n\nEcho data to
client..." ) // create packet to
send DatagramPacket sendPacket new
DatagramPacket(
receivePacket.getData(), receivePacket.getLength()
, receivePacket.getAddress(),
receivePacket.getPort() )
socket.send( sendPacket ) // send packet
displayMessage( "Packet sent\n" )
// utility method called from other
threads to manipulate // displayArea in
the event-dispatch thread private void
displayMessage( final String messageToDisplay )
// display message from event-dispatch
thread of execution SwingUtilities.invok
eLater( new Runnable() // inner
class to ensure GUI updates properly
public void run() // updates displayArea
displayArea.append(
messageToDisplay )
displayArea.setCaretPosition( displayArea.getText
().length() )
44
// end inner class )
// end call to SwingUtilities.invokeLater
public static void main( String args )
Server application new Server()
application.setDefaultCloseOperation(
JFrame.EXIT_ON_CLOSE )
application.waitForPackets() //
end class Server
45
Access resources via URLs

• What Is a URL ?
• an acronym for Uniform Resource Locator
• is a reference (an address) to a resource on the
  Internet.
• Format
• scheme // host port path
  reference
• EXs
• http//www.cs.nccu.edu.tw
• http//xml.cs.nccu.edu.tw/Courses/java2002/index.h
  tmlloc2
• ftp//java.sun.com/pub/
• http//java.sun.com/servlet/app1?userchenpasswd
  
• file///D/chencc/java2002/index.html

46
Creating a URL

• Direct from URL address
• String j2k http//xml.cs.nccu.edu.tw/Courses/ja
  va2000
• URL j2000 new URL(j2k) // or
• URL j2000 new URL
• (http, xml.cs.nccu.edu.tw,
  Courses/java2000)
• Relative to another URL address
• String coursesLoc http//xml.cs.nccu.edu.tw/Co
  urses/index.html
• URL coursesURL new URL(coursesLoc)
• URL j2000 new URL(coursesURL,
  java2002/index.html)
• Constructor Summary
• URL(String spec)
• URL(String scheme, String host , int port ,
  String file , URLStreamHandler h)
• URL(URL baseURL, String relativeURL
  ,URLStreamHandler )
• possible exception MalformedURLException

47
Parsing a URL

• getProtocol
• Returns the protocol identifier component of the
  URL.
• getHost
• Returns the host name component of the URL.
• getPort
• Returns the port number component of the URL.
• The getPort method returns an integer that is the
  port number.
• If the port is not set, getPort returns -1.
• getFile
• Returns the filename component of the URL.
• getRef
• Returns the reference component of the URL.

48
Example

• import java.net. import java.io.
• public class ParseURL
• public static void main(String args) throws
  Exception
• URL aURL new URL("http//java.sun.com80
  /docs/books/"
• 
  "tutorial/index.htmlDOWNLOADING")
• System.out.println("protocol "
  aURL.getProtocol())
• System.out.println("host "
  aURL.getHost())
• System.out.println("filename "
  aURL.getFile())
• System.out.println("port "
  aURL.getPort())
• System.out.println("ref "
  aURL.getRef())
• Here's the output displayed by the program
• protocol http host
  java.sun.com
• filename /docs/books/tutorial/index.html
  
• port 80 ref
  DOWNLOADING

49
Reading Directly from a URL

• InputStream openStream() // return a stream for
  reading
• EX
• 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()

50
Connecting to a URL

• URLConnection openConnectino() throws
  IOException
• EX
• try
• URL yahoo new URL("http//www.yahoo.com/")
• URLConnection yahooConnection
  yahoo.openConnection()
• catch (MalformedURLException e) //
  new URL() failed
• . . .
• catch (IOException e) //
  openConnection() failed
• . . .

51
Reading from and Writing to a URLConnection

• Reading from a URLConnection
• // like direct reading from URL using
  openStream()
• URL yahoo new URL("http//www.yahoo.com/")
• URLConnection yc yahoo.openConnection()
• BufferedReader in new BufferedReader(
• new
  InputStreamReader(
• 
  yc.getInputStream()))
• String inputLine
• while ((inputLine in.readLine()) ! null)
• System.out.println(inputLine)
  
• in.close()

52
Writing to a URLConnection

• procedures
• 1.Create a URL.
• 2.Open a connection to the URL.
• 3.Set output capability on the URLConnection.
• 4.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.
• 5.Write to the output stream.
• 6.Close the output stream.

53

• String stringToReverse URLEncoder.encode(args0
  )
• URL url new URL("http//java.sun.com/cgi-bin/ba
  ckwards")
• URLConnection connection url.openConnection()
• connection.setDoOutput(true)
• PrintWriter out new PrintWriter(
• connection.getOutputStr
  eam())
• out.println("string" stringToReverse)
• out.close()
• BufferedReader in new BufferedReader( new
  InputStreamReader(
• 
  connection.getInputStream()))
• String inputLine
• while ((inputLine in.readLine()) ! null)
  System.out.println(inputLine)
• in.close()

54
JDBC

• Java DataBase Connectivity
• Allows you to write SQL without worrying about
  what brand of database youre connecting to
• Provides a simple interface for database
  programming

55
JDBC database URL

• A string used to identify data source ( and
  parameters)
• The format
• jdbc ltsubprotocolgt ltotherStuffgt
• jdbc means using JDBC
• ltsubprotocolgt the name of the driver or the name
  of a database connectivity mechanism.
• ltotherStuffgt the database identifier. This
  varies with the database driver used.
• the first subprotocol available is the jdbc-odbc
  bridge, specified by odbc
• EX String dbUrl "jdbcodbc//whitehouse.gov50
  00/CATSPWDHillary"

56

• // c15Lookup.java
• // Looks up email addresses in a local database
  using JDBC
• import java.sql.
• public class Lookup
• public static void main(String args)
• String dbUrl "jdbcodbcpeople"
• String user "" String password ""
• try // Load the driver (registers
  itself)
• Class.forName( "sun.jdbc.odbc.JdbcOdbcDriv
  er") Connection c DriverManager.getConnectio
  n
• ( dbUrl, user, password)
• Statement s
  c.createStatement()

57

• // SQL code
• ResultSet r s.executeQuery(
• "SELECT FIRST, LAST, EMAIL FROM people.csv
  people "
• "WHERE (LAST'" args0 "') " " AND
  (EMAIL Is Not Null) " "ORDER BY FIRST")
• while(r.next()) // Capitalization doesn't
  matter System.out.println( r.getString("Last")
  ", " r.getString("fIRST") " "
  r.getString("EMAIL") )
• catch(Exception e) e.printStackTrace()
  

58
Servlets

• Servlets are to servers what applets are to
  browsers
• Servlet API (a standard Java extension) is truly
  cross-platform
• Substitute for CGI scripts
• Easier to write, faster to run
• Not tied to any platform-specific API
• Better than CGI scripts
• Stay resident speed, persistent
• information
• Multithreading locks prevent collisions
• Session tracking built in

59
Running servlets locally

• Download the Tomcat implementation of Java
  Servlets and JSPs
• http//jakarta.apache.org
• Official Servlet/JSP distribution
• Free! Part of Apache project
• Will eventually be integrated into Apache to make
  it a true application server
• Execute Tomcat.bat start

60
Works with both GET POST

• import javax.servlet.
• import javax.servlet.http.
• import java.io.
• public class ServletsRule extends HttpServlet
• int i 0
• // Servlet "persistence"
• public void service(HttpServletRequest req,
  HttpServletResponse res) throws IOException
• PrintWriter out res.getWriter()
• out.print("ltHEADgtltTITLEgt")
• out.print("A server-side strategy")
• out.print("lt/TITLEgtlt/HEADgtltBODYgt")
• out.print("lth1gtServlets Rule! " i)
• out.print("lt/h1gtlt/BODYgt")
• out.close()

61
Easy to get HTML form data

• // Dumps the name-value pairs of any HTML form
• import javax.servlet. import
  javax.servlet.http.
• import java.io. import java.util.
• public class EchoForm extends HttpServlet
• public void doGet(HttpServletRequest req,
  HttpServletResponse res) throws IOException
• res.setContentType("text/html")
• PrintWriter out res.getWriter()
• out.print("lth1gtYour form containedlt/h1gt")
• Enumeration flds req.getParameterNames()
• while(flds.hasMoreElements()) String field
  (String)flds.nextElement()
• String value req.getParameter(field)
• out.print(field " " value "ltbrgt")
• out.close()

62

• public void doPost(HttpServletRequest req,
  HttpServletResponse res) throws IOException
• doGet(req, res)

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One servlet, thread-per-request

• import javax.servlet. import
  javax.servlet.http.
• import java.io.
• public class ThreadServlet extends HttpServlet
• int i
• public void service(HttpServletRequest req,
  HttpServletResponse res) throws IOException
• res.setContentType("text/html")
• PrintWriter out res.getWriter()
• synchronized( this)
• try Thread.currentThread().sleep(5000)
• catch( InterruptedException e)
• out.print("lth1gtFinished " i "lt/h1gt")
• out.close()

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Java Server Pages (JSPs)

• Servlets still require web pages
• Why not combine only the essential servlet code
  with the HTML code?
• Why not minimize the Java code you have to write?
• Inspired by ASPs but much nicer
• The easiest way to experiment with servlet
  properties
• You can include special servlets, calls to Java
  libraries, etc.

65
Java Server Web Dev Kit

• From java.sun.com, exceptionally easy to install
  and run
• All you do is put .jsp files in the appropriate
  directory, point your web browser at them
• JSP is automatically built, compiled and loaded
  on the first call, just like a servlet

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Most code is built for you

• lthtmlgtltbodygt ltH1gtThe time in seconds is
• lt System.currentTimeMillis()/1000 gt
• lt/H1gt lt/bodygtlt/htmlgt
• Plenty of Java code is produced, HTML
• The lt coerces the resulting expression to a
  String
• Code automatically rebuilt recompiled when JSP
  source file changes

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Basic JSP operations

• lt-- This JSP comment will not appear in the
  generated html --gt
• lt-- This is a JSP directive --gt
• lt_at_ page import"java.util." gt
• lt-- These are declarations --gt
• lt!
• long loadTime System.currentTimeMillis()
• Date loadDate new Date()
• int hitCount 0
• gt
• lthtmlgtltbodygt
• lt-- The next several lines are the result of a
  JSP expression inserted in the generated html
  the '' indicates a JSP expression --gt
• ltH1gtThis page was loaded at lt loadDate gt lt/H1gt

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• ltH1gtHello, world! It's lt new Date() gtlt/H1gt
• ltH2gtHere's an object lt new Object() gtlt/H2gt
• ltH2gtThis page has been up lt (System.currentTimeM
  illis()-loadTime)/1000 gt secondslt/H2gt
• ltH3gtPage has been accessed lt hitCount gt
  times since lt loadDate gtlt/H3gt
• lt-- A "scriptlet" which writes to the server
  console. Note that a '' is required --gt
• lt System.out.println("Goodbye")
  out.println("Cheerio") gt
• lt/bodygtlt/htmlgt

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Extracting fields and values

• You can mix Java code HTML
• An if statement can print HTML in its body
• lt-- Fetching the data from an HTML form --gt
• lt-- Also generates the form --gt
• lt_at_ page import"java.util." gt
• lthtmlgtltbodygt ltH1gtDisplayFormDatalt/H1gtltH3gt
• lt Enumeration f request.getParameterNames()
• if(!f.hasMoreElements()) // No fields
  gt
• ltform method"POST" action"DisplayFormData.jsp"gt
  
• lt for(int i 0 i lt 10 i) gt
• Fieldltigt ltinput type"text" size"20"
  name"Fieldltigt" value"Valueltigt"gtltbrgt lt
  gt

70

• ltINPUT TYPEsubmit namesubmit Value"Submit"gt
  lt/formgt
• lt gt
• lt
• Enumeration flds request.getParameterNames()
  
• while(flds.hasMoreElements())
• String field (String)flds.nextElement()
• String value request.getParameter(field)
• gt
• ltligt lt field gt lt value gt lt/ligt
• lt gt
• lt/H3gtlt/bodygtlt/htmlgt

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Remote Method Invocation (RMI)

• Basic idea the network of computers becomes the
  computing platform
• Objects can live on other computers
• You can invoke methods on those objects, passing
  them object arguments, and they can return
  objects
• Network connections and JDBC are particular ways
  to compute across the network, RMI is the general
  solution

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1) Make a Remote Interface

• // Thinking in Java, ch15rmi
  PerfectTimeIF.java
• // The PerfectTime remote interface.
• package c15.rmi
• import java.rmi.
• interface PerfectTimeIF extends Remote
• long getPerfectTime() throws RemoteException

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2) Create the Remote class

• Create and install
• RMISecurityManager
• Create one or more instances of a remote object
• Register at least one of the remote objects with
  the RMI remote object registry, for bootstrapping
  purposes

74

• // c15rmiPerfectTime.java
• // The implementation of the PerfectTimeIF remote
  object.
• package c15.rmi
• import java.rmi. import java.rmi.server.
• import java.rmi.registry. import java.net.
• public class PerfectTime extends
  UnicastRemoteObject implements PerfectTimeIF
• // Implementation of the interface
• public long getPerfectTime() throws
  RemoteException
• return System.currentTimeMillis()
• // Must implement constructor to throw
  RemoteException
• public PerfectTime() throws RemoteException
• // super() Called automatically

75

• // Registration for RMI serving
• public static void main(String args)
• System.setSecurityManager( new
  RMISecurityManager())
• try
• PerfectTime pt new PerfectTime()
  Naming.bind( "//peppy2005/PerfectTime", pt)
  System.out.println("Ready to do time")
• catch(Exception e) e.printStackTrace()

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Starting The Registry

• At default port 1099
• start rmiregistry (Win95)
• rmiregistry (Unix)
• At specified port
• start rmiregistry 2005 (Win95)
• rmiregistry 2005 (Unix)

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Creating Stubs Skeletons

• Must specify location starting from the class
  path
• rmic c15.rmi.PerfectTime
• This produces
• PerfectTime_Stub.class
• PerfectTime_Skel.class
• Now your RMI program can work

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3) Use the Remote Object

• // c15rmiDisplayPerfectTime.java
• package c15.rmi
• import java.rmi. import java.rmi.registry.
• public class DisplayPerfectTime
• public static void main(String args)
• System.setSecurityManager( new
  RMISecurityManager())
• try
• PerfectTimeIF t (PerfectTimeIF)
  Naming.lookup
• ( "//peppy2005/PerfectTime")
• for(int i 0 i lt 10 i)
• System.out.println("Perfect time "
  t.getPerfectTime())
• catch(Exception e) e.printStackTrace()
  

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Problem 4

• Use Socket to implement a simple file transfer
  client/server
• server-side SftpServer
• has a userdata.cfg file, each line of which
  records a user information including user-name,
  pass-word and users home directory
• Each user can get/put files on his home directory
  (and subdirectory)
• there is a global uploading directory ( say
  incoming ) to which every user can put files
• there is a global directory ( say pub) from which
  every user can get files.
• to handle simultaneous request of multi users,
  your server must be multithreaded.
• Client-side SftpClient

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• pubic class SftpClient
• // constructor
• public SftpClient(String ip, String port, String
  user, String passw)
• get(String remoteFilePathName, String
  localFileName) throws IOException,
  remoteFileNotFound
• put(String remoteFilePathName, String
  localFileName) throws IOException,
  RemoteFileNotWritable
• pubic static void main(String args )

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usage of SftpClient

• java SftpClient Option get put
  remotefileName localName
• Where Option is any of
• -server ipport -user usernamepasswd
• -terminate
• Exs
• gt java SftpClient server 140.119.162.2302021
  user chencc123456
• get pub/java2000.html
  D/temp/java2000.html
• gt java SftpClient server xml.cs.nccu.edu.tw2021
  user test1111
• put incoming/test1.txt
  test1.txt
• Run the batch to test your client

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Problem4Test.bat

• Rem launch the server at port 2021
• java SftpServer port 2021
• Rem get the file pub/test1.txt from the server
• java SftpClient -server localhost2021 user
  test1111 get pub/test1.txt c\temp\test1.txt
• Rem put the file c\temp\test2.txt to server
• java SftpClient server 127.0.0.12021 user
  test1111 put incoming/test2.txt
  c\temp\test2.txt
• Rem terminate the server
• java SftpClient server 127.0.0.12021 user
  root1111 -terminate