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Client/Server Distributed SystemsClient/Server Distributed Systems

ObjectivesObjectives– describe basic networking in Javadescribe basic networking in Java– Web page retrieval and sockets programmingWeb page retrieval and sockets programming

240-322, Semester 1, 2005-2006

11. Java Networking

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ContentsContents

1.1. Networking Restrictions in JavaNetworking Restrictions in Java

2.2. Basic Networking ClassesBasic Networking Classes

3.3. Sockets (Java Style)Sockets (Java Style)

4.4. Retrieving a Web PageRetrieving a Web Page– 4 different approaches 4 different approaches

5.5. A Stream Socket Client/ServerA Stream Socket Client/Server

continued

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6.6. A Datagram Socket A Datagram Socket Client/ServerClient/Server

7.7. Networked Tic-Tac-ToeNetworked Tic-Tac-Toe

8.8. More InformationMore Information

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1. Networking Restrictions in Java1. Networking Restrictions in Java

Many browsers place networking Many browsers place networking restrictions restrictions on appletson applets– usually communication is limited to being usually communication is limited to being

between your machine and the one where the between your machine and the one where the applet came fromapplet came from

There are There are no restrictions on Java applicationsno restrictions on Java applications or when using or when using appletviewerappletviewer

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Relaxing SecurityRelaxing Security

Applets can be Applets can be signedsigned with with trusted trusted certificatescertificates– a browser can be configured to relax security a browser can be configured to relax security

depending on an applet’s signaturedepending on an applet’s signature– an advanced topican advanced topic

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2. Basic Networking Classes2. Basic Networking Classes

The J2SE documentation for The J2SE documentation for java.netjava.net lists lists over 30 classes. A few of the key ones:over 30 classes. A few of the key ones:

InetAddressInetAddress

– the class that represents IP addresses and contaithe class that represents IP addresses and contains operations for manipulating them ns operations for manipulating them

see section 2.1see section 2.1

continued

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URLURL

– used to retrieve the Web page at the given URLused to retrieve the Web page at the given URL

URLConnectionURLConnection

– also used to retrieve a Web pagealso used to retrieve a Web page– allows extra parameters to be sent to the URLallows extra parameters to be sent to the URL

e.g HTTP request headerse.g HTTP request headers

continued

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SocketSocket– the client-side socket class for TCPthe client-side socket class for TCP

ServerSocketServerSocket– the server-side socket class for TCPthe server-side socket class for TCP

DatagramSocketDatagramSocket– allows a client or server to send/receive UDP paallows a client or server to send/receive UDP pa

cketsckets

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2.1. Finding an IP Address2.1. Finding an IP Address

Java’s Java’s InetAddressInetAddress class makes the mappin class makes the mapping between hostnames and IP addresses mucg between hostnames and IP addresses much easier than in UNIX.h easier than in UNIX.

For details, look at the the documentation foFor details, look at the the documentation for r java.net.InetAddressjava.net.InetAddress

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WhatIP.javaWhatIP.java

import java.io.*;import java.net.*;

public class WhatIP { public static void main(String args[])

throws IOException { InetAddress addr =

InetAddress.getByName(args[0]); System.out.println("Inet address is "+ addr); } }

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UseUseCarried out from a DOS

prompt on my Windows machine.

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3. Sockets (Java Style)3. Sockets (Java Style) 1. Stream Sockets1. Stream Sockets

– the client/server connection exists for the entire the client/server connection exists for the entire duration of the request and answerduration of the request and answer

similar to a telephone callsimilar to a telephone call

– a a connection-oriented serviceconnection-oriented service corresponds to TCPcorresponds to TCP

– Java has separate classes for client and server Java has separate classes for client and server stream sockets (see section 5)stream sockets (see section 5)

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2. Datagram Sockets2. Datagram Sockets– the client/server send messages (the client/server send messages (packetspackets, ,

datagramsdatagrams) to each other) to each other similar to the postal servicesimilar to the postal service

– a a connectionless serviceconnectionless service corresponds to UDPcorresponds to UDP

– Java has classes for datagram sockets and Java has classes for datagram sockets and packets (see section 6).packets (see section 6).

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3.1. Pinging3.1. Pinging

pingping uses the ICMP protocol's uses the ICMP protocol's ECHO_REQUESTECHO_REQUEST datagram. Th datagram. The host’s answer is an ICMP e host’s answer is an ICMP ECHO_RESPONSEECHO_RESPONSE..

ping is a DOS command, not Java.

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ICMP packets can only be created via a socICMP packets can only be created via a socket of type ket of type SOCK_RAWSOCK_RAW..– but Java only supports but Java only supports SOCK_STREAMSOCK_STREAM (TCP) an (TCP) an

d d SOCK_DGRAMSOCK_DGRAM (UDP) sockets (UDP) sockets

Our solutionOur solution: use a TCP connection to the : use a TCP connection to the ddaytimeaytime service to pretend to ping service to pretend to ping– the daytime server listens at port 13the daytime server listens at port 13

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3.2. DayPing.java 3.2. DayPing.java JJJJ2, p.6772, p.677

import java.io.*;import java.net.*;public class DayPing {

public static void main(String args[]) throws Exception

{ if (args.length != 1) { System.out.println(

"Usage: java DayPing <machine name>"); System.exit(0); }

:

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String machine = args[0]; Socket so = new Socket(machine, 13); // daytime server listens at port 13

BufferedReader br = new BufferedReader(

new InputStreamReader( so.getInputStream() ) );

System.out.println( machine +

" is alive at " + br.readLine());

so.close(); } // end of main()

} // end of DayPing class

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DayPing.javaDayPing.java converts the socket connectio converts the socket connection into a n into a BufferedReaderBufferedReader

– this allows this allows readLine()readLine() to be used for socket in to be used for socket inputput

Using layered streams to add I/O functionaliUsing layered streams to add I/O functionality on top of sockets is a ty on top of sockets is a powerfulpowerful feature of feature of Java.Java.

NotesNotes

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UseUse C> javac DayPing.java

C> java DayPing calvincalvin is alive at Sat May 7 13:46:06 2005

C> java DayPing fourdotsException in thread "main" java.net.ConnectException: Connection refused: no further information at java.net.PlainSocketImpl.socketConnect

(Native Method) at java.net.PlainSocketImpl.doConnect... : at ping.main(DayPing.java:23)

All the examples were tested on my Windows machine

continued

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C> java DayPing takasillaException in thread "main" java.net.UnknownHostException: takasilla at java.net.PlainSocketImpl. connect(Unknown Source)

: at ping.main(DayPing.java:23)

C> java DayPing www.amazon.comException in thread "main" java.net.NoRouteToHostException: Operation timed out: connect at java.net.PlainSocketImpl.

socketConnect(Native Method):

at ping.main(DayPing.java:23)

After a long wait

After a wait

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3.3. Why Doesn't DayPing Work?3.3. Why Doesn't DayPing Work?

The examples that don't work show The examples that don't work show threethree di different errors:fferent errors:– ConnectExceptionConnectException– UnknownHostExceptionUnknownHostException– NoRouteToHostExceptionNoRouteToHostException

The "unknown host" exception is caused by The "unknown host" exception is caused by an incorrect IP address.an incorrect IP address.

continued

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The "connection" exception probably means that the server is uThe "connection" exception probably means that the server is unavailable (switched off)navailable (switched off)– this can be confirmed for a TCP service by trying to connect to it witthis can be confirmed for a TCP service by trying to connect to it wit

h h telnettelnet::

$ telnet fourdots 13$ telnet fourdots 13Trying 172.30.255.4...Trying 172.30.255.4...telnet: Unable to connect to remote host: Connection telnet: Unable to connect to remote host: Connection refusedrefused$ $

continued

executed from a differentmachine (fivedots)

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The "no route to host" exception usually meThe "no route to host" exception usually means that there is a ans that there is a firewallfirewall preventing the co preventing the connection from being made.nnection from being made.

telnettelnet response: response:$ telnet www.amazon.com 13$ telnet www.amazon.com 13Trying 207.171.163.90...Trying 207.171.163.90...telnet: Unable to connect to remote host: Cotelnet: Unable to connect to remote host: Connection nnection timed outtimed out$ $

A long wait,and then...

tried on (fivedots)

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3.4. Firewalls at PSU3.4. Firewalls at PSU

takasila

calvin

fivedots

Departmental firewall(no external socket creation; URLs allowed)

ratree

Univ. firewall(no external socket creation;

URLs only throughthe ‘cache’ proxy)

cache

The Internet

University

CoE Department

(simplified)

8080

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Ordinary users (i.e. students) cannot write Ordinary users (i.e. students) cannot write sockesocket-basedt-based programs that communicate outside PS programs that communicate outside PSUU– this is true in any language (Java, C, etc.)this is true in any language (Java, C, etc.)

ButBut programs can retrieve Web pages (and othe programs can retrieve Web pages (and other thingsr things) ) using the Java using the Java URLURL class class– the URL request must go through PSU's the URL request must go through PSU's cachecache mac mac

hinehine

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URLs and SocketsURLs and Sockets

From your data communications course, yoFrom your data communications course, you may recall that the Web protocol, HTTP, iu may recall that the Web protocol, HTTP, is built on top of TCP/IP.s built on top of TCP/IP.

Java's URL class uses stream sockets (TCP/Java's URL class uses stream sockets (TCP/IP) for its implementationIP) for its implementation– so why does it get past so why does it get past cachecache??

continued

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cachecache accepts socket links to valid Web ser accepts socket links to valid Web servers, but also checks that the HTTP GET (overs, but also checks that the HTTP GET (or POST) message includes additional valid r POST) message includes additional valid HTTP headers.HTTP headers.– all of this HTTP stuff is carried out by the URL all of this HTTP stuff is carried out by the URL

class for usclass for us

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4. Retrieving a Web Page4. Retrieving a Web Page

Five ways of obtaining a Web page:Five ways of obtaining a Web page:– 1. use a socket, and send a 1. use a socket, and send a GETGET message to the s message to the s

ervererver see see GetSocketPage.javaGetSocketPage.java

– 2. use a 2. use a URLURL object, and read the page via a stre object, and read the page via a streamam

see see GetURLPage.javaGetURLPage.java

continued

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– 3. use a 3. use a URLURL object, and display the page in a browser object, and display the page in a browser see see showPage.javashowPage.java

– 4. display a URL in an 4. display a URL in an JEditorPaneJEditorPane see see ShowURLPage.javaShowURLPage.java

– 5. use a HTTP URL connection, and send a 5. use a HTTP URL connection, and send a GETGET mess messageage

see Java’s networking tutorialsee Java’s networking tutorial

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4.1. Sockets and GET4.1. Sockets and GET GetSocketPage.javaGetSocketPage.java retrieves the page: retrieves the page:

http://<host name>/index.htmlhttp://<host name>/index.html

e.g.e.g. http://fivedots.coe.psu.ac.th/index.htmlhttp://fivedots.coe.psu.ac.th/index.html

It It printsprints the text of the page to the text of the page to stdoutstdout..

continued

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It opens a socket at port 80 for the host, whiIt opens a socket at port 80 for the host, which is the usually place where the Web servech is the usually place where the Web server is listening.r is listening.

It sends the HTTP It sends the HTTP GETGET message: message:GET /index.htmlGET /index.html

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DiagramDiagram

GetSocketPageclient host

Web server

GET /index.html

Web page (as text)

80

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GetSocketPage.javaGetSocketPage.java

import java.io.*;import java.net.*;

public class GetSocketPage

{ public static void main(String args[])

throws IOException { Socket sock = new Socket(args[0],80);

:

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BufferedReader dis = new BufferedReader(

new InputStreamReader(

sock.getInputStream() )); PrintStream ps =

new PrintStream( sock.getOutputStream() );

ps.println("GET /index.html"); String line; while ((line = dis.readLine()) != null) System.out.println(line);

sock.close(); }} // end of GetSocketPage.java

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GetSocketPage.javaGetSocketPage.java converts the socket con converts the socket connection into a nection into a BufferedReaderBufferedReader for input, for input, andand a a PrintStreamPrintStream for output for output– uses uses readLine()readLine()for socket inputfor socket input– uses uses println()println() for socket output for socket output

NotesNotes

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UseUse

C> javac GetSocketPage.java

C> java GetSocketPage fivedots<html><head><title>Un title page</title></head><meta http-equiv="Content-Type" content="text/html; charset=windows-874"><style type="text/css"> :C>

text of Web page printed to stdout

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But... But...

C> java GetSocketPage www.amazon.comException in thread "main" java.net.ConnectExceException in thread "main" java.net.ConnectException: ption: Connection timed out: connect Connection timed out: connect at java.net.PlainSocketImpl.at java.net.PlainSocketImpl. socketConnect(Native Method) socketConnect(Native Method)

::at GetSocketPage.main(GetSocketPage.java:18)at GetSocketPage.main(GetSocketPage.java:18)

The firewall around PSUprevents Web serveraccess by sockets.

continued

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cache rejected the GET message to the extercache rejected the GET message to the external site since the message didn't include addnal site since the message didn't include additional HTTP headers.itional HTTP headers.

These can be supplied by us, but it's easier tThese can be supplied by us, but it's easier to use the URL class.o use the URL class.

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4.2. URL Object 4.2. URL Object

GetURLPage.javaGetURLPage.java avoids the firewall proble avoids the firewall problem with sockets by using a m with sockets by using a URLURL object. object.

A A URLURL object allows a Web page to be retrie object allows a Web page to be retrieved as a stream of textved as a stream of text– our program prints the text to our program prints the text to stdoutstdout..

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GetURLPage.javaGetURLPage.java

import java.net.*;import java.io.*;

public class GetURLPage { public static void main(String args[]) { try { URL url = new URL(args[0]); BufferedReader dis =

new BufferedReader(new InputStreamReader(

url.openStream() ));:

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String line; while ( (line = dis.readLine()) != null ) System.out.println(line);

dis.close(); } catch (Exception e) { System.out.println(e); } }

} // end of GetURLPage.java

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A stream for the A stream for the URLURL object is obtained usin object is obtained using g openStream()openStream()

– after that the same input stream layering techniafter that the same input stream layering technique is used as in que is used as in GetSocketPage.javaGetSocketPage.java

– there is no need to send a there is no need to send a GETGET message message

NotesNotes

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UseUse C> javac GetURLPage.java C> java GetURLPage http://www.amazon.co.uk

java.net.ConnectException: Connection timed out: connect

C> java -DproxySet=true -DproxyHost=cache.psu.ac.th -DproxyPort=8080

GetURLPage http://www.amazon.com/<html><head><title>Amazon Page</title></head><body bgcolor=#ffffff test=#000000><H1> :C>

typed onone line

after along wait

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Ordinary users can access outside PSU by uOrdinary users can access outside PSU by using URLs, but they must route their requestsing URLs, but they must route their requests via PSU s via PSU cachecache machine. machine.

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Batch Files for Long CommandsBatch Files for Long Commands

GetURLPage.batGetURLPage.bat contains: contains:@echo offecho Executing GetURLPage...java -DproxySet=true -DproxyHost=cache.psu.ac.th -DproxyPort=8080 GetURLPage %1

Use:Use:c> GetURLPage http://www.amazon.co.uk : // same output as last slide

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Proxy Username/PasswordProxy Username/Password

A further level of firewall security is to requA further level of firewall security is to require the user to enter a username and passworire the user to enter a username and passwordd– called called proxy authorizationproxy authorization

Java has network support for authorizationJava has network support for authorization– it allows a username and password to be sent by it allows a username and password to be sent by

a client program to the firewalla client program to the firewall

continued

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A good tutorial:A good tutorial:– ““Jump a Proxy/Firewall and Live to Tell About it”Jump a Proxy/Firewall and Live to Tell About it”http://www.devx.com/upload/free/features/http://www.devx.com/upload/free/features/javapro/2000/03mar00/te0003/te0003.aspjavapro/2000/03mar00/te0003/te0003.asp

Slightly modified code is in Slightly modified code is in GetThroughProxy.javaGetThroughProxy.java at:at:http://fivedots.coe.psu.ac.th/http://fivedots.coe.psu.ac.th/ Software.coe/Cliserv/ Software.coe/Cliserv/ Code%20Examples/Java%20Code/ Code%20Examples/Java%20Code/ Basic%20Networking/ Basic%20Networking/

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4.3. Passing a URL to a Browser4.3. Passing a URL to a Browser

If the Java code is in an applet, then the doIf the Java code is in an applet, then the downloaded URL can be displayed in the brownloaded URL can be displayed in the browser.wser.

showPage.htmlshowPage.html displays a user-specified We displays a user-specified Web page in the (Opera) browser, using the b page in the (Opera) browser, using the ShoSho

wPage.javawPage.java applet. applet.

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UsageUsage

continued

The dialog box appears in frontof the browser window.

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Loaded Page:

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showPage.htmlshowPage.html

<html><head><html><head><title>Web Page Loading Applet</title></head><title>Web Page Loading Applet</title></head><body><body><h1>Web Page Loading Applet</h1><h1>Web Page Loading Applet</h1><P>Applet is placed here... <P>Applet is placed here... <applet code="ShowPage.class"<applet code="ShowPage.class"

width=300 height=50>width=300 height=50></applet></applet></P></P></body></html></body></html>

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showPage.javashowPage.java

import java.net.*;import java.net.*;import javax.swing.*;import javax.swing.*;import java.applet.AppletContext;import java.applet.AppletContext;import javax.swing.JOptionPane;import javax.swing.JOptionPane;

public class ShowPage extends JApplet public class ShowPage extends JApplet {{ public void init() public void init() { { try { try {

String urlString = String urlString = JOptionPane.showInputDialog( JOptionPane.showInputDialog(

"Enter a URL:");"Enter a URL:"); URL url = new URL(urlString);URL url = new URL(urlString);

::

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AppletContext browser = getAppletContext();AppletContext browser = getAppletContext(); browser.showDocument(url); browser.showDocument(url);

} } catch (Exception e) catch (Exception e) { System.out.println(e); }{ System.out.println(e); }

} // end of init() } // end of init()

} // end of ShowPage} // end of ShowPage

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NotesNotes

The download only works because the browThe download only works because the browser is set up to work through ser is set up to work through cachecache..

There are no Java security restrictions on a There are no Java security restrictions on a applet passing a URL to a browser to be disapplet passing a URL to a browser to be displayed.played.

continued

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A URL is downloaded with A URL is downloaded with showDocument()showDocument(), , which must be called in the applet’s which must be called in the applet’s environment (i.e. the browser):environment (i.e. the browser):– AppletContext browser = AppletContext browser =

getAppletContext();getAppletContext(); browser.showDocument(url);browser.showDocument(url);

The URL is automatically loaded into the The URL is automatically loaded into the browser, replacing browser, replacing showPage.htmlshowPage.html..

continued

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There is a two-argument version of There is a two-argument version of showDocument()showDocument() that can load URLs into that can load URLs into frames.frames.

D&D contains a more complex example D&D contains a more complex example ((SiteSelector.javaSiteSelector.java, ch. 21) which allows , ch. 21) which allows the user to select a URL from a list.the user to select a URL from a list.

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4.4. Displaying a URL with JEditorPane4.4. Displaying a URL with JEditorPane

The Swing GUI includes The Swing GUI includes JEditorPaneJEditorPane whic which allows 'structured' text to be displayed and h allows 'structured' text to be displayed and edited.edited.– offers basic support for plain text, RTF, and offers basic support for plain text, RTF, and HTHT

MLML– see see javax.swing.JEditorPanejavax.swing.JEditorPane

continued

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A A JEditorPaneJEditorPane can be filled by calling can be filled by calling setPsetP

age()age() with a with a URL string argumentURL string argument..

JEditorPaneJEditorPane also has support for capturing also has support for capturing hyperlink clicks as events.hyperlink clicks as events.

continued

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With these features, it is possible to write a basiWith these features, it is possible to write a basic Web browser!c Web browser!

ShowURLPage.javaShowURLPage.java has a text field for the user to has a text field for the user to enter a URLenter a URL– the resulting downloaded page is displayed in a the resulting downloaded page is displayed in a JEdJEd

itorPaneitorPane – if a link is clicked inside the pane, then its page will if a link is clicked inside the pane, then its page will

be loaded automaticallybe loaded automatically

continued

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I clicked on “School of Advanced...”

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ShowURLPage’s Event ModelShowURLPage’s Event Model

enter

contents

X

hyperlinkUpdate() {...}

GUI

CodeactionPerformed() {...}

hyperlinkevents

actionevents

^

^

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ShowURLPage.javaShowURLPage.java

import java.awt.*;import java.awt.event.*;import java.net.*;import java.io.*;import javax.swing.*;import javax.swing.event.*;

public class ShowURLPage extends JFrame { private JTextField enter; private JEditorPane contents;

:

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public ShowURLPage() { super("Simple Web Browser"); Container c = getContentPane();

enter = new JTextField("Enter file URL here");

enter.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent e ) { getPage(e.getActionCommand());} });

c.add(enter, BorderLayout.NORTH);:

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contents = new JEditorPane(); contents.setEditable(false);

contents.addHyperlinkListener( new HyperlinkListener() { public void hyperlinkUpdate(

HyperlinkEvent e) { if(e.getEventType() ==

HyperlinkEvent.EventType.ACTIVATED) getPage(e.getURL().toString()); } });

:

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c.add( new JScrollPane(contents), BorderLayout.CENTER);

setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE); setSize(400, 300); setVisible(true); } // end of ShowURLPage()

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private void getPage(String location) { setCursor(Cursor.getPredefinedCursor( Cursor.WAIT_CURSOR)); try { contents.setPage(location); enter.setText(location); } catch (IOException io) { JOptionPane.showMessageDialog(this, "Error retrieving specified URL", "Bad URL", JOptionPane.ERROR_MESSAGE); } setCursor( Cursor.getPredefinedCursor( Cursor.DEFAULT_CURSOR) ); } // end of getPage()

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public static void main(String args[]) { new ShowURLPage(); }

} // end of ShowURLPage class

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UseUse

C> javac ShowURLPage.java

C> java ShowURLPage// A request for http://www.cs.ait.ac.th/~ad// causes a “Bad URL” dialog box to appear

C> java -DproxySet=true -DproxyHost=cache.psu.ac.th -DproxyPort=8080 ShowURLPage

Typed all on one line.The window shown on the left of slide 60 appears.

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NotesNotes

The The JEditorPaneJEditorPane must be set uneditable so must be set uneditable so that hyperlinks events can be captured.that hyperlinks events can be captured.

Hyperlink events include: Hyperlink events include: ENTEREDENTERED, , EXITEDEXITED, , and and ACTIVATEDACTIVATED..– see see javax.swing.event.HyperlinkEventjavax.swing.event.HyperlinkEvent

continued

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getPage()getPage() calls calls setPage()setPage() to display a URL to display a URL in the in the JEditorPaneJEditorPane – the call is surrounded by changes to the cursor ithe call is surrounded by changes to the cursor i

con so things look busycon so things look busy

getPage()getPage() is called in two possible ways: is called in two possible ways:– as a response to pressing enter in the textfieldas a response to pressing enter in the textfield– as a response to a link being clickedas a response to a link being clicked

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5. A Stream Socket Client/Server5. A Stream Socket Client/Server

AlienClient.java AlienServer1.java

socket Server socket

socketfor clientcomms.

4444

the new socket is made with accept()

Initial Internet connection

input andoutputstreams

input andoutputstreams

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The server creates a new socket when a client The server creates a new socket when a client contacts itcontacts it– client/server communication via the new socketclient/server communication via the new socket

– this frees up the main server socket to receive this frees up the main server socket to receive connections from other clientsconnections from other clients

– AlienServer1.javaAlienServer1.java is an is an iterative serveriterative server it can only process one client at a timeit can only process one client at a time

continued

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Our client and server both run on Our client and server both run on localhostlocalhost..

AlienServer1AlienServer1 actions: actions:– wait for a client connectionwait for a client connection– after a connection, wait for a message from the after a connection, wait for a message from the

clientclient– respond depending on the content of the messagerespond depending on the content of the message– close the client connection, and loop, waiting for close the client connection, and loop, waiting for

another clientanother client

continued

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Diagram for Client ProcessingDiagram for Client Processing

AlienClient.java AlienServer1.java1. send message

4. send answer2. analyse message3. print message locally

5. close socket

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AlienServer1.javaAlienServer1.java

import java.io.*;import java.net.*;

public class AlienServer1 { public static void main(String args[])

throws IOException { Socket sock; BufferedReader in; PrintStream out;

ServerSocket servsock = new ServerSocket(4444);

:

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while (true) { // wait for the next client connection sock = servsock.accept();

// Get I/O streams from the socket out = new PrintStream(

sock.getOutputStream() ); in = new BufferedReader(

new InputStreamReader( sock.getInputStream() ) );

:

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// get client message and respond String msg = in.readLine(); System.out.println(" Received: " + msg);

if (msg.indexOf("hello") > -1) { System.out.println(" Friendly contact made"); out.println("Welcome friend"); } else {

System.out.println(" Probably an alien"); out.println("ET go home"); } out.flush();

// Close this connection, // (not the overall server socket)

sock.close(); } } // of main()}// end of AlienServer1

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The client: AlienClient.javaThe client: AlienClient.java

AlienClient.javaAlienClient.java’s actions:’s actions:– open a connection with the serveropen a connection with the server– send a message taken from the command linesend a message taken from the command line– receive and print the server’s responsereceive and print the server’s response– closeclose

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AlienClient.javaAlienClient.java

import java.io.*;import java.net.*;

public class AlienClient { public static void main(String args[])

throws IOException { Socket sock = new Socket("localhost", 4444); // Get I/O streams from the socket BufferedReader br =

new BufferedReader( new InputStreamReader(

sock.getInputStream()) ); PrintStream ps = new PrintStream(

sock.getOutputStream() );:

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// Send a message from the command line ps.println(args[0]); ps.flush();

// Read server’s response String line = br.readLine(); System.out.println( "Got this from server:\n " + line);

sock.close(); } // end of main()

} // end of AlienClient class

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UseUse

Client: Server:

I used two separate DOS windowson the same machine.

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Telnet Client (from fivedots)Telnet Client (from fivedots)

Server:

I typed this

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A More Complex ExampleA More Complex Example

D&D gives a larger stream socket client/servD&D gives a larger stream socket client/server example (ch. 21):er example (ch. 21):– object input and output streamsobject input and output streams are used are used

– the sequence of communication is more complex the sequence of communication is more complex the client maintains a long-lived link with the server uthe client maintains a long-lived link with the server u

ntil it sends a “TERMINATE” stringntil it sends a “TERMINATE” string

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5.1. A Concurrent Server5.1. A Concurrent Server

AlienServer2.javaAlienServer2.java uses uses threadsthreads to handle to handle multiple clients concurrently.multiple clients concurrently.

Three main advantages:Three main advantages:– the server code is simpler since server processinthe server code is simpler since server processin

g is in a separate class called g is in a separate class called WorkerWorker

– clients do not have to wait clients do not have to wait – the server is more scaleablethe server is more scaleable

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AlienServer2 VisuallyAlienServer2 Visually

AlienServer2AlienClient clients 4444

Workerthreads

each Worker thread can deal with multiple messages from its client

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5.2. Java Threads (Briefly)5.2. Java Threads (Briefly)

There are There are two ways to create a threadtwo ways to create a thread::– extend the extend the ThreadThread class class– write a class to implement the write a class to implement the RunnableRunnable interfa interfa

cece

We use the first approach since We use the first approach since WorkerWorker doe does not need to extend any other classes.s not need to extend any other classes.

continued

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A class extended from A class extended from ThreadThread must define the must define the rrun()un() method: method:

class Worker extends Thread {class Worker extends Thread { public void run() public void run() { ... } { ... }

}}

A threaded object is started by calling A threaded object is started by calling start()start():: Worker w = new Worker(...):Worker w = new Worker(...):

w.start();w.start();

The thread will call The thread will call run()run() itself from within itself from within stasta

rt()rt()..

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5.3. AlienServer2.java5.3. AlienServer2.java

import java.io.*;import java.net.*;

public class Worker extends Thread { private Socket sock; private int id;

public Worker(Socket s, int c) { sock = s; id = c; }

:

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public void run() { // Get I/O streams from the socket try { PrintStream out = new PrintStream(

sock.getOutputStream() ); BufferedReader in = new BufferedReader(

new InputStreamReader(

sock.getInputStream() )); :

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// get client messages and respond String msg; while ((msg = in.readLine()) != null) { System.out.println(" Worker "+id+ " received: "+ msg); if (msg.indexOf("hello") > -1) out.println("Welcome friend") else out.println("ET go home"); out.flush(); } :

The worker contains much the same client processing code as the iterative server.

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// Close this connection, // (not the overall server socket)

System.out.println(" Worker " + id + " finished"); sock.close(); } catch(IOException ioe) {System.out.println(ioe); } } // end of run()

} // end of Worker class

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public class AlienServer2 { public static void main(String args[])

throws IOException { Socket sock; ServerSocket servsock =

new ServerSocket(4444, 6);

while (true) { // wait for the next client connection sock = servsock.accept(); new Worker(sock).start(); } } // of main()} // of AlienServer2 class

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UseUseServer:

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Client on fivedotsClient on fivedots

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Two Local Clients (one closed)Two Local Clients (one closed)

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6. A Datagram Socket Client/Server6. A Datagram Socket Client/Server

UDPClient UDPServer

Datagramsocket

5000Datagramsocket

any port msg

packets

echo

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6.1. Server Event Model6.1. Server Event Model

display

X GUI

Code

^

^

packet2) echo back

to client

waitForPackets() {...}

packet

1) receive fromclient

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6.2. UDPServer.java6.2. UDPServer.java

import java.io.*;import java.net.*;import java.awt.*;import java.awt.event.*;import javax.swing.*;

public class UDPServer extends JFrame { private JTextArea display;

:

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public UDPServer() { super("UDP Server");

display = new JTextArea(); getContentPane().add(

new JScrollPane( display), BorderLayout.CENTER ); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); setSize(400, 300); show(); DatagramSocket socket = null; try { socket = new DatagramSocket( 5000 ); } catch(SocketException se) { se.printStackTrace(); System.exit(1); } waitForPackets(socket); } // end of UDPServer()

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private void waitForPackets(DatagramSocket socket) { DatagramPacket sendPacket, receivePacket;

while (true) { try { // set up packet data structure byte data[] = new byte[100]; receivePacket = new DatagramPacket(data, data.length);

// wait for packet from client socket.receive(receivePacket);

:

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// process client packet display.append("\nPacket received:" + "\nFrom host: "+receivePacket.getAddress()+ "\nHost port: " + receivePacket.getPort() + "\nLength: " + receivePacket.getLength() + "\nContaining:\n\t" + new String(receivePacket.getData(), 0, receivePacket.getLength()));

:

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// echo packet info back to client display.append(

"\n\nEcho data to client..."); sendPacket = new DatagramPacket(

receivePacket.getData(), receivePacket.getLength(), receivePacket.getAddress(), receivePacket.getPort() ); socket.send( sendPacket ); display.append("Packet sent\n"); display.setCaretPosition( display.getText().length() ); }

:

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catch(IOException io) { display.append(io.toString() + "\n"); io.printStackTrace(); } } } // end of waitForPackets{}

public static void main( String args[] ) { new UDPServer(); }

} // end of UDPServer

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Server NotesServer Notes

The server creates one datagram socket at The server creates one datagram socket at port 5000:port 5000:

socket = new DatagramSocket(5000);socket = new DatagramSocket(5000);

Datagram socket creation must be inside a Datagram socket creation must be inside a try/catch block.try/catch block.

continued

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The server loops inside The server loops inside waitForPackets()waitForPackets()

– creates an empty packet (100 bytes long)creates an empty packet (100 bytes long)– blocks inside blocks inside receive()receive()

When When receive()receive() returns, the packet returns, the packet contains data and:contains data and:– the Internet address of the senderthe Internet address of the sender– the port number of the senderthe port number of the sender– the actual length of the datathe actual length of the data

continued

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The packet information can be accessed via The packet information can be accessed via methods. e.g.:methods. e.g.:

receivePacket.getPort();receivePacket.getPort(); receivePacket.getData(); receivePacket.getData();

receive()receive() must be in a try/catch block. must be in a try/catch block.

continued

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The server echoes the datagram back to the The server echoes the datagram back to the client at the address and port obtained from the client at the address and port obtained from the client’s message:client’s message:

sendPacket = new DatagramPacket(sendPacket = new DatagramPacket(receivePacket.getData(),receivePacket.getData(),receivePacket.getLength(), receivePacket.getLength(), receivePacket.getAddress(),receivePacket.getAddress(),receivePacket.getPort() );receivePacket.getPort() );

socket.send(sendPacket); socket.send(sendPacket);

send()send() must be in a try/catch block. must be in a try/catch block.

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6.3. Client Event Model6.3. Client Event Model

enter

display

XGUI

CodeactionPerformed() {...}

actionevents

^^

packet

1) send to server

waitForPackets() {...}

packet2) receive echo

from sender

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6.4. UDPClient.java6.4. UDPClient.java

import java.io.*;import java.net.*;import java.awt.*;import java.awt.event.*;import javax.swing.*;

public class UDPClient extends JFrame implements ActionListener

{ private JTextField enter; private JTextArea display;

private DatagramSocket socket;:

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public UDPClient() { super("UDP Client");

enter = new JTextField("Type message here"); enter.addActionListener(this); getContentPane().add(enter,BorderLayout.NORTH);

display = new JTextArea(); getContentPane().add(new JScrollPane(display), BorderLayout.CENTER); setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); setSize(400, 300); setVisible(true);

:

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try { socket = new DatagramSocket(); } catch(SocketException se) { se.printStackTrace(); System.exit(1); }

waitForPackets(); } // end of UDPClient()

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private void waitForPackets() { DatagramPacket receivePacket;

while (true) { try { // set up packet data structure byte data[] = new byte[100]; receivePacket = new DatagramPacket(data, data.length);

// wait for packet from server socket.receive(receivePacket);

:

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// process packet from server display.append("\nPacket received:" + "\nFrom host: "+receivePacket.getAddress()+ "\nHost port: " + receivePacket.getPort() + "\nLength: " + receivePacket.getLength() + "\nContaining:\n\t" + new String( receivePacket.getData(), 0, receivePacket.getLength())); display.setCaretPosition( display.getText().length() ); } catch(IOException ex) { display.append(ex.toString()+"\n" ); ex.printStackTrace(); } } } // end of waitForPackets()

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public void actionPerformed(ActionEvent e) { DatagramPacket sendPacket; try { display.append(

"\nSending packet containing: " + e.getActionCommand() + "\n" );

String s = e.getActionCommand(); byte data[] = s.getBytes();

sendPacket = new DatagramPacket( data, data.length,

InetAddress.getLocalHost(), 5000); socket.send(sendPacket);

:

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display.append("Packet sent\n"); display.setCaretPosition( display.getText().length() ); } catch (IOException ex) { display.append(ex.toString()+"\n"); ex.printStackTrace(); } } // end of actionPerformed()

public static void main( String args[] ) { new UDPClient(); }

} // end of UDPClient

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Client NotesClient Notes

The client gets the data for each packet The client gets the data for each packet from the user entering a string into a from the user entering a string into a TextFieldTextField..

The client creates a datagram socket with The client creates a datagram socket with no specified port number:no specified port number:

socket = new DatagramSocket();

continued

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In In actionPerformed()actionPerformed(), the user’s input is , the user’s input is converted into a packet, and sent to the server:converted into a packet, and sent to the server:

sendPacket = new DatagramPacket(data, data.length(),InetAddress.getLocalHost(), 5000);

socket.send(sendPacket);

The code assumes that the server is on the same The code assumes that the server is on the same machine as the client, and attached to port 5000.machine as the client, and attached to port 5000.

continued

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The client receives the echoed reply in The client receives the echoed reply in waitForPackets()waitForPackets()::– the loop blocks inside the loop blocks inside receive()receive() until a packet until a packet

arrives from the serverarrives from the server– the packet details are displayed in the the packet details are displayed in the JTextAreaJTextArea

The blocked The blocked waitForPackets()waitForPackets() does not stop does not stop the user from entering more strings for the the user from entering more strings for the client to send outclient to send out– the GUI always runs in a separate threadthe GUI always runs in a separate thread

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6.5. Use6.5. Use

continued

1. message sent to server

3. Echoed messagereceived from the server

2. Client messagereceived and echoed

Tested by starting the client and server intwo separate DOS windows.

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Another message sent and echoed back:

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7. Networked Tic-Tac-Toe7. Networked Tic-Tac-ToeD&D Section 21.8, p.1031

thread

thread

5000

synchronizedvalidMove()

thread

Client: Player ‘X’

Server

data

thread

Client: Player ‘O’

data

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OutputOutput p.1041-1042 p.1041-1042

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8. More Information8. More Information

Chapter 21, Deitel & Deitel (D&D)Chapter 21, Deitel & Deitel (D&D)http://java.coe.psu.ac.th/ForMember/http://java.coe.psu.ac.th/ForMember/ Books.html#Network Books.html#Network

Killer Game Programming in JavaKiller Game Programming in JavaChapter 29, Network BasicsChapter 29, Network Basicshttp://fivedots.coe.psu.ac.th/~ad/jg/ch18/

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Core Java 2, Vol II - Advanced FeaturesCore Java 2, Vol II - Advanced FeaturesCay S. Horstmann & Gary CornellCay S. Horstmann & Gary CornellSun Microsystems Press, 2001,Sun Microsystems Press, 2001,chapter 3chapter 3– multithreaded server, sending e-mail, URLConmultithreaded server, sending e-mail, URLCon

nection, HTTP POSTnection, HTTP POST– http://java.coe.psu.ac.th/ForMember/http://java.coe.psu.ac.th/ForMember/ Books.html#Network Books.html#Network

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Aj. Somchai's Java SiteAj. Somchai's Java Site– Network and Distributed Programming BooksNetwork and Distributed Programming Bookshttp://java.coe.psu.ac.th/ForMember/http://java.coe.psu.ac.th/ForMember/ Books.html#Network Books.html#Network