Network Programming Clients

1 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
Network Programming:
Clients

Network Programming: Clients2 www.corewebprogramming.com
Agenda
• Creating sockets
• Implementing a generic network client
• Parsing data using StringTokenizer
• Retrieving files from an HTTP server
• Retrieving Web documents by using the
URL class

Client vs. Server
• Traditional definition
– Client: User of network services
– Server: Supplier of network services
• Problem with traditional definition
– If there are 2 programs exchanging data, it seems unclear
– Some situations (e.g., X Windows) seem reversed
• Easier way to remember distinction
– Server starts first. Server doesn't specify host (just port).
– Client starts second. Client specifies host (and port).
• Analogy: Company phone line
– Installing phone is like starting server
– Extension is like port
– Person who calls is the client: he specifies both host
(general company number) and port (extension)

Client vs. Server (Continued)
• If server has to start first, why are we
covering clients before we cover servers?
– Clients are slightly easier.
– We can test clients by connecting to existing servers that
are already on the internet.
• Point: clients created in Java need not
communicate with servers written in Java.
– They can communicate with any server that accepts
socket connections (as long as they know the proper
communication protocol).
– Exception: ObjectInputStream and ObjectOutputStream
allow Java programs to send complicated data structures
back and forth. Only works in Java, though.

Steps for Implementing a Client
1. Create a Socket object
Socket client = new Socket("hostname", portNumber);
2. Create an output stream that can be used to send
info to the Socket
// Last arg of true means autoflush -- flush stream
// when println is called
PrintWriter out =
new PrintWriter(client.getOutputStream(), true);
3. Create an input stream to read the response from
the server
BufferedReader in =
new BufferedReader
(new InputStreamReader(client.getInputStream()));

Steps for Implementing a Client
(Continued)
4. Do I/O with the input and output Streams
– For the output stream, PrintWriter, use print and println, similar to
System.out.println
• The main difference is that you can create
PrintWriters for different Unicode
characters sets, and you can’t with
PrintStream (the class of System.out).
– For the input stream, BufferedReader, you can call read to get a
single character or an array of characters, or call readLine to get a
whole line
• Note that readLine returns null if the
connection was terminated (i.e. on EOF),
but waits otherwise
5. Close the socket when done
client.close();
• Also closes the associated input and

A Generic Network Client
import java.net.*;
import java.io.*;
/** A starting point for network clients. */
public class NetworkClient {
protected String host;
protected int port;
public NetworkClient(String host, int port) {
this.host = host;
this.port = port;
}
public String getHost() {
return(host);
}
public int getPort() {
return(port);
}
...

(Continued)
...
/** Establishes the connection, then passes the socket
* to handleConnection. */
public void connect() {
try {
Socket client = new Socket(host, port);
handleConnection(client);
} catch(UnknownHostException uhe) {
System.out.println("Unknown host: " + host);
uhe.printStackTrace();
} catch(IOException ioe) {
System.out.println("IOException: " + ioe);
ioe.printStackTrace();
}
}
...

(Continued)
/** This is the method you will override when
* making a network client for your task.
* This default version sends a single line
* ("Generic Network Client") to the server,
* reads one line of response, prints it, then exits.
*/
protected void handleConnection(Socket client)
throws IOException {
PrintWriter out =
SocketUtil.getPrintWriter(client);
BufferedReader in =
SocketUtil.getBufferedReader(client);
out.println("Generic Network Client");
System.out.println
("Generic Network Client:n" +
"Made connection to " + host +
" and got '" + in.readLine() + "' in response");
client.close();
}
}

SocketUtil – Simplifying
Creation of Reader and Writer
import java.net.*;
import java.io.*;
public class SocketUtil {
/** Make a BufferedReader to get incoming data. */
public static BufferedReader getBufferedReader
(Socket s) throws IOException {
return(new BufferedReader(
new InputStreamReader(s.getInputStream())));
}
/** Make a PrintWriter to send outgoing data.
* This PrintWriter will automatically flush stream
* when println is called.
*/
public static PrintWriter getPrintWriter(Socket s)
// 2nd argument of true means autoflush
return(new PrintWriter(s.getOutputStream(), true));
}
}

Example Client
public class NetworkClientTest {
public static void main(String[] args) {
String host = "localhost";
if (args.length > 0)
host = args[0];
int port = 8088;
if (args.length > 1)
port = Integer.parseInt(args[1]);
NetworkClient nwClient
= new NetworkClient(host, port);
nwClient.connect();
}
}

Example Client, Result
> java NetworkClientTest ftp.netscape.com 21
Generic Network Client:
Made connection to ftp.netscape.com and got
‘220 ftp26 FTP server (UNIX(r) System V Release 4.0)
ready.’ in response
>

Aside: Parsing Strings Using
StringTokenizer
• Idea
– Build a tokenizer from an initial string
– Retrieve tokens one at a time with nextToken
– You can also see how many tokens are remaining
(countTokens) or simply test if the number of tokens
remaining is nonzero (hasMoreTokens)
StringTokenizer tok
= new StringTokenizer(input, delimiters);
while (tok.hasMoreTokens()) {
doSomethingWith(tok.nextToken());
}

StringTokenizer
• Constructors
– StringTokenizer(String input, String delimiters)
– StringTokenizer(String input, String delimiters,
boolean includeDelimiters)
– StringTokenizer(String input)
• Default delimiter set is " tnrf" (whitespace)
• Methods
– nextToken(), nextToken(String delimiters)
– countTokens()
– hasMoreTokens()
• Also see methods in String class
– substring, indexOf, startsWith, endsWith, compareTo, …
– JDK 1.4 has regular expressions in java.util.regex!

Interactive Tokenizer: Example
import java.util.StringTokenizer;
public class TokTest {
if (args.length == 2) {
String input = args[0], delimiters = args[1];
StringTokenizer tok
= new StringTokenizer(input, delimiters);
while (tok.hasMoreTokens()) {
System.out.println(tok.nextToken());
}
} else {
System.out.println
("Usage: java TokTest string delimiters");
}
}
}

Interactive Tokenizer: Result
> java TokTest http://www.microsoft.com/~gates/ :/.
http
www
microsoft
com
~gates
> java TokTest "if (tok.hasMoreTokens()) {" "(){. "
if
tok
hasMoreTokens

A Client to Verify Email
Addresses
• Talking to a mail server
– One of the best ways to get comfortable with a
network protocol is to telnet to the port a server is
on and try out commands interactively
• Example talking to apl.jhu.edu’s server
> telnet apl.jhu.edu 25
Trying 128.220.101.100 ...Connected … Escape character …
220 aplcenmp.apl.jhu.edu Sendmail SMI-8.6/SMI-SVR4 ready …
expn hall
250 Marty Hall <hall@aplcenmp.apl.jhu.edu>
expn root
250 Gary Gafke <…>
250 Tom Vellani <…>
quit
221 aplcenmp.apl.jhu.edu closing connection
Connection closed by foreign host.

Address Verifier
/** Given an email address of the form user@host,
* connect to port 25 of the host and issue an
* 'expn' request for the user. Print the results.
*/
public class AddressVerifier extends NetworkClient {
private String username;
MailAddress address = new MailAddress(args[0]);
AddressVerifier verifier
= new AddressVerifier(address.getUsername(),
address.getHostname(),
25);
verifier.connect();
}
...

Address Verifier (Continued)
protected void handleConnection(Socket client) {
try {
PrintWriter out =
SocketUtil.getPrintWriter(client);
InputStream in = client.getInputStream();
byte[] response = new byte[1000];
// Clear out mail server's welcome message.
in.read(response);
out.println("EXPN " + username);
// Read the response to the EXPN command.
// May be multiple lines!
int numBytes = in.read(response); // Can't use readLine!
// The 0 means to use normal ASCII encoding.
System.out.write(response, 0, numBytes);
out.println("QUIT");
client.close();
System.out.println("Couldn't make connection: "
+ ioe);
}
}
...}

MailAddress
// Takes a string of the form "user@host" and
// separates it into the "user" and "host" parts.
public class MailAddress {
private String username, hostname;
public MailAddress(String emailAddress) {
StringTokenizer tokenizer
= new StringTokenizer(emailAddress, "@");
this.username = getArg(tokenizer);
this.hostname = getArg(tokenizer);
}
private static String getArg(StringTokenizer tok) {
try { return(tok.nextToken()); }
catch (NoSuchElementException nsee) {
System.out.println("Illegal email address");
return(null);
}
}...
}

Address Verifier: Result
> java AddressVerifier tbl@w3.org
250 <timbl@hq.lcs.mit.edu>
> java AddressVerifier timbl@hq.lcs.mit.edu
250 Tim Berners-Lee <timbl>
> java AddressVerifier gosling@mail.javasoft.com
550 gosling... User unknown

Brief Aside: Using the HTTP
GET Command
• For the URL http://www.apl.jhu.edu/~lmb/
Unix> telnet www.apl.jhu.edu 80
Trying 128.220.101.100 ...
Connected to aplcenmp.apl.jhu.edu.
Escape character is '^]'.
GET /~lmb/ HTTP/1.0
HTTP/1.0 200 Document follows
Date: Sat, 30 Jun 2001 14:34:58 GMT
Server: NCSA/1.5.2
Last-modified: Tue, 11 Jul 2001 15:13:56 GMT
Content-type: text/html
Content-length: 50479
<!DOCTYPE HTML PUBLIC
"-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
...
</HTML>Connection closed by foreign host.
Unix>

Talking to Web Servers
Interactively
• WebClient
– Simple graphical user interface to communicate with
HTTP servers
– User can interactively specify:
• Host
• Port
• HTTP request line
• HTTP request headers
– HTTP request is performed in a separate thread
– Response document is placed in a scrollable text area
– Download all source files for WebClient from
http://archive.corewebprogramming.com/Chapter17.html

WebClient: Example

A Class to Retrieve a Given URI
from a Given Host
import java.net.*;
import java.io.*;
public class UriRetriever extends NetworkClient {
private String uri;
UriRetriever uriClient
= new UriRetriever(args[0],
Integer.parseInt(args[1]),
args[2]);
uriClient.connect();
}
public UriRetriever(String host, int port,
String uri) {
super(host, port);
this.uri = uri;
}
...

A Class to Retrieve a Given URI
from a Given Host (Continued)
// It is safe to use blocking IO (readLine) since
// HTTP servers close connection when done,
// resulting in a null value for readLine.
protected void handleConnection(Socket uriSocket)
PrintWriter out =
SocketUtil.getPrintWriter(uriSocket);
BufferedReader in =
SocketUtil.getBufferedReader(uriSocket);
out.println("GET " + uri + " HTTP/1.0n");
String line;
while ((line = in.readLine()) != null) {
System.out.println("> " + line);
}
}
}

A Class to Retrieve a Given
URL
public class UrlRetriever {
checkUsage(args);
StringTokenizer tok = new StringTokenizer(args[0]);
String protocol = tok.nextToken(":");
checkProtocol(protocol);
String host = tok.nextToken(":/");
String uri;
int port = 80;
try {
uri = tok.nextToken("");
if (uri.charAt(0) == ':') {
tok = new StringTokenizer(uri);
port = Integer.parseInt(tok.nextToken(":/"));
uri = tok.nextToken("");
}
} catch(NoSuchElementException nsee) {
uri = "/";
}

A Class to Retrieve a Given
URL (Continued)
UriRetriever uriClient =
new UriRetriever(host, port, uri);
uriClient.connect();
}
/** Warn user if they forgot the URL. */
private static void checkUsage(String[] args) {
if (args.length != 1) {
System.out.println("Usage: UrlRetriever <URL>");
System.exit(-1);
}
}
/** Tell user that this can only handle HTTP. */
private static void checkProtocol(String protocol) {
if (!protocol.equals("http")) {
System.out.println("Don't understand protocol "
+ protocol);
System.exit(-1);
}
}}

UrlRetriever in Action
• No explicit port number
Prompt> java UrlRetriever
http://www.microsoft.com/netscape-beats-ie.html
> HTTP/1.0 404 Object Not Found
> Content-Type: text/html
>
> <body><h1>HTTP/1.0 404 Object Not Found
> </h1></body>

UrlRetriever in Action
(Continued)
• Explicit port number
Prompt> java UrlRetriever
http://home.netscape.com:80/ie-beats-netscape.html
> HTTP/1.0 404 Not found
> Server: Netscape-Enterprise/2.01
> Date: Wed, 11 Jul 2001 21:17:50 GMT
> Content-length: 207
> Content-type: text/html
>
> <TITLE>Not Found</TITLE><H1>Not Found</H1> The requested
object does not exist on this server. The link you
followed is either outdated, inaccurate, or the server
has been instructed not to let you have it.

Writing a Web Browser
• Wow! We just wrote a Web browser in 3
pages of code.
– Didn't format the HTML, but still not bad for 3 pages
– But we can do even better…

Browser in 1 Page: Using URL
public class UrlRetriever2 {
try {
URL url = new URL(args[0]);
BufferedReader in = new BufferedReader(
new InputStreamReader(
url.openStream()));
String line;
while ((line = in.readLine()) != null) {
System.out.println("> " + line);
}
in.close();
} catch(MalformedURLException mue) { // URL c'tor
System.out.println(args[0] + "is an invalid URL: "
+ mue);
} catch(IOException ioe) { // Stream constructors
System.out.println("IOException: " + ioe);
}
}
}

UrlRetriever2 in Action
Prompt> java UrlRetriever2 http://www.whitehouse.gov/
> <HTML>
> <HEAD>
> <TITLE>Welcome To The White House</TITLE>
> </HEAD>
> ... Remainder of HTML document omitted ...
> </HTML>

Useful URL Methods
• openConnection
– Yields a URLConnection which establishes a connection to
host specified by the URL
– Used to retrieve header lines and to supply data to the HTTP
server
• openInputStream
– Returns the connection’s input stream for reading
• toExernalForm
– Gives the string representation of the URL
• getRef, getFile, getHost, getProtocol, getPort
– Returns the different components of the URL

Using the URL Methods:
Example
import java.net.*;
public class UrlTest {
if (args.length == 1) {
try {
URL url = new URL(args[0]);
System.out.println
("URL: " + url.toExternalForm() + "n" +
" File: " + url.getFile() + "n" +
" Host: " + url.getHost() + "n" +
" Port: " + url.getPort() + "n" +
" Protocol: " + url.getProtocol() + "n" +
" Reference: " + url.getRef());
} catch(MalformedURLException mue) {
System.out.println("Bad URL.");
}
} else
System.out.println("Usage: UrlTest <URL>");
}
}

Using the URL Methods, Result
> java UrlTest http://www.irs.gov/mission/#squeezing-them-dry
URL: http://www.irs.gov/mission/#squeezing-them-dry
File: /mission/
Host: www.irs.gov
Port: -1
Protocol: http
Reference: squeezing-them-dry
Note: If the port is not explicitly stated in the URL, then the
standard port for the protocol is assumed and getPort returns –1

A Real Browser Using Swing
• The JEditorPane class has builtin support for
HTTP and HTML

Browser in Swing: Code
import javax.swing.*;
import javax.swing.event.*;
...
public class Browser extends JFrame implements HyperlinkListener,
ActionListener {
private JEditorPane htmlPane;
...
public Browser(String initialURL) {
...
try {
htmlPane = new JEditorPane(initialURL);
htmlPane.setEditable(false);
htmlPane.addHyperlinkListener(this);
JScrollPane scrollPane = new JScrollPane(htmlPane);
getContentPane().add(scrollPane, BorderLayout.CENTER);
warnUser("Can't build HTML pane for " + initialURL
+ ": " + ioe);
}

Browser in Swing (Continued)
...
Dimension screenSize = getToolkit().getScreenSize();
int width = screenSize.width * 8 / 10;
int height = screenSize.height * 8 / 10;
setBounds(width/8, height/8, width, height);
setVisible(true);
}
public void actionPerformed(ActionEvent event) {
String url;
if (event.getSource() == urlField)
url = urlField.getText();
else // Clicked "home" button instead of entering URL
url = initialURL;
try {
htmlPane.setPage(new URL(url));
urlField.setText(url);
warnUser("Can't follow link to " + url + ": " + ioe);
}
}

Browser in Swing (Continued)
...
public void hyperlinkUpdate(HyperlinkEvent event) {
if (event.getEventType() ==
HyperlinkEvent.EventType.ACTIVATED) {
try {
htmlPane.setPage(event.getURL());
urlField.setText(event.getURL().toExternalForm());
warnUser("Can't follow link to "
+ event.getURL().toExternalForm() +
": " + ioe);
}
}
}

Summary
• Opening a socket requires a hostname
(or IP address) and port number
• A PrintWriter lets you send string data
– Use autoflush to send the full line after each println
• A BufferedReader allows you to read the input
one line at a time (readLine)
– The readLine method blocks until a response is sent
– For a typical GET request, after the HTTP server sends
the response the connection is closed and readLine
returns null
• StringTokenizer provides simple parsing
• The URL and URLConnection classes simplify
communication with Web servers

42 © 2001-2003 Marty Hall, Larry Brown http://www.corewebprogramming.com
core
programming
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