Appendix F: Network Programming in Java ©SoftMoore ConsultingSlide 1.

Appendix F:Network Programming in Java

©SoftMoore Consulting Slide 1

Basic Network Concepts

• Packet (or datagram)– little bundle of information– sent from one node to another– each packet is delivered separately (possibly by different routes)

• Protocol– roles, vocabulary, rules for communication– network protocols (e.g., IP) versus application protocol (e.g.,

HTTP)

• IP– Internet Protocol– enables different local area networks to communicate– basis for connecting computers around the world

Slide 2©SoftMoore Consulting

Two Types of Information

• Application data– the information one computer wants to send to another

• Network protocol data– describes how to reach the intended computer– describes how to check for errors in the transmission


TCP/IP: The Internet Protocol


Physical Network

Transport Layer (TCP, UDP)

Internet Layer (IP)

Application Layer (HTTP, FTP, SMTP)

IP Addresses

• IP Address– identifies a host– serves as a destination address for a packet– IPv4 (most widely deployed)

• 4 bytes/32 bits for an IP address• Expressed using “dot” notation: 192.168.0.13

– IPv6 (the next generation)• 16 bytes/128 bits for an IP address

• Domain name– easy-to-remember name for a host– Example: www.google.com

• Domain Naming Service (DNS)– translates from domain names to IP addresses


Socket

• Two-way connection

• Provides inter-process communication using IP

• Derived from BSD (UNIX) sockets

• Read/write streams between hosts– use either UDP and TCP protocols (layered on top of IP) – uses IP addresses and port numbers to locate servers


Port

• Meeting place on a host– one service per port

• Port numbers identify services on a server

• Range from 1 to 65535 (use type int)

• Port numbers 1 through 255 are reserved for well-known services– HTTP is 80– TELNET is 23– etc.

• Port numbers less than 1024 are privileged


Sockets and Ports (Diagram)


Client

port 13

port 80

Time Service

Web Service

Socket

Server

Socket

Well-Known Ports

• 13: time

• 20,21: FTP

• 23: telnet

• 25: SMTP

• 43: whois

• 80: HTTP

• 119: NNTP

• 1099: RMI


Time of Day Service

• From the command prompt, enter the following command:telnet time-A.timefreq.bldrdoc.gov 13

• Connects to the time-of-day service on a computer operated by the National Institute of Standards and Technology in Boulder, Colorado

• Gives time based on a Cesium atomic clock– result not completely accurate due to network delays


By default, the telnet client is not installed on WindowsVista. To install telnet Start → Control Panel → Programs and Features

→ Turn Windows features on or off→ Telnet Client option

The Socket Class

• Hides the complexities of establishing a network connection and sending information across it

• Provides the same programming interface used for working with files

• Primary constructor/methodsSocket(String host, int port)InputStream getInputStream()OutputStream getOutputStream()void close()


Streams

• A stream is a sequence of bytes

• I/O from disk, network, memory, etc. is handled in exactly the same way.

• Class InputStreamabstract int read()abstract void close()

• Class OutputStreamabstract void write(int b)abstract void close()


Class PrintStream

• Method print() outputs an object, primitive, or string.

• Method println() does the same thing, but it adds a newline at the end.

• Object System.out is a PrintStream.

• ExamplePrintStream ps = new PrintStream(outputstream);


Class BufferedReader

• Provides methodString readLine()

• ExamplesInputStreamReader isReader = new InputStreamReader(System.in);BufferedReader reader = new BufferedReader(isReader)


Example: Using Sockets

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

...

Socket s = new Socket("time-A.timefreq.bldrdoc.gov", 13);InputStream is = s.getInputStream();InputStreamReader isReader = new InputStreamReader(is);BufferedReader in = new BufferedReader(isReader);String line;

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

in.close();

...


Example: Simple Browser Using Sockets

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

public class SimpleHttpBrowser { public static void main(String[] args) throws Exception { String host = args[0]; int port = Integer.parseInt(args[1]);

Socket s = new Socket(host, port);

InputStream inStream = s.getInputStream(); OutputStream outStream = s.getOutputStream();


Example: Simple Browser Using Sockets(continued)

PrintWriter out = new PrintWriter( new OutputStreamWriter(outStream), true);

BufferedReader in = new BufferedReader( new InputStreamReader(inStream));

out.println("GET / HTTP/1.1"); out.println("Host: " + host + ":" + port); out.println();

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

in.close(); out.close(); } }


Client/Server Overview

• Can communicate with remote file systems using a client/server model

• Server listens for connection requests from clients

• Clients know how to connect to the server via a port number

• Upon connection, server processes the request coming across the network from the client

• Connection can remain open or shut down after each transaction


Client-Server

• Client – initiates connection– retrieves data– displays data – responds to user input– requests more data

• Examples– web browser– chat program– PC accessing files

• Server – responds to connection– receives data request– looks up data– delivers it

• Examples– web server– database server– email server– domain name server


Difference between client and server isrelative – it’s just peers talking to each other.

URL

• Format– protocol://host[:port][/path/][file][#anchor]

• Examples– http://www.softmoore.com– http://www.softmoore.com/Bib-5.2-JavaAdvanced.html– http://java.sun.com/sfaq/index.html– http://java.sun.com/sfaq/– ftp://ftp.cs.rpi.edu/pub/stl/– http://64.233.187.99:80/

(same as http://www.google.com)


java.net.URL

• URL(String)– creates a URL object from the String representation

• URL(String, String, int, String)– creates a URL object from the specified protocol, host, port

number, and file

• URL(URL context, String spec)– creates a URL by parsing the specification spec within a

specified context.

• InputStream openStream()– opens a connection to this URL and returns an InputStream

for reading from that connection


java.net.URL(continued)

• URLConnection openConnection()– Returns a URLConnection object that represents a connection

to the remote object referred to by the URL.

• MalformedURLException– thrown if you try to create a bogus URL– usually means bad user input, so fail gracefully and informatively

• accessor methods– getProtocol() – getHost()– getPort() – getFile()– getRef()


java.net.URLConnection

• InputStream getInputStream()– returns an input stream that reads from this open connection– makes the connection if it has not already been made

• URLConnection creates the socket and handles the protocol (e.g. HTTP) for you

• There are also handlers for other protocols– ftp, gopher, file, mailto, or you can write your own– getOutputStream()

• Handles relative URLs via CodeBase/DocumentBase


java.net.URLConnection(continued)

Object getContent()

• Makes the connection if it hasn’t already been made

• Returns an object that’s appropriate for this file’s MIME type

• For text, returns an InputStream

• For images, returns an ImageObserver

• Only supports text/plain, text/Generic, image/gif, and image/jpeg

• You can add your own ContentHandler if you like


Example: Simple Browser Using URLs

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

public class SimpleBrowser { public static void main(String[] args) throws Exception { URL url = new URL(args[0]); InputStream is = url.openStream(); InputStreamReader isReader = new InputStreamReader(is); BufferedReader in = new BufferedReader(isReader); String line;

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

in.close(); } }


java.net.HttpURLConnection

• Extends URLConnection with additional support for HTTP connections.

• Usage: Cast a URLConnectionURL url = new URL("http://www.android.com/");HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();

• Selected methods in HttpURLConnection– int getResponseCode() – String getResponseMessage() – String getHeaderField(int n)– String getHeaderFieldKey(int n)


©SoftMoore Consulting Slide 27

Networking Application: Checking URLs

URLConnection conn = url.openConnection();

if (conn instanceof HttpURLConnection) { HttpURLConnection httpConn = (HttpURLConnection) conn;

// response code 400 or higher is bad if (httpConn.getResponseCode() >= 400) printBadUrl(url, httpConn.getResponseCode(), httpConn.getResponseMessage()); }else printFeedback("Skipping " + url + ": not HTTP protocol");

java.net.URLEncoder

static String encode(String)

• The ASCII characters ‘a’ through ‘z’, ‘A’ through ‘Z’, and ‘0’ through ‘9’ remain the same.

• The space character is converted into a plus sign ‘+’.

• All other characters are converted into the 3-character string “%xy”, where xy is the two-digit hexadecimal representation of the lower 8-bits of the character.


Appendix F: Network Programming in Java ©SoftMoore ConsultingSlide 1.

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