comp201 java programming part iii: advanced features
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COMP201 Java Programming Part III: Advanced Features. Topic 14: Networking Volume II,Chapter 3. Outline. Networking basics IP addresses, ports, protocols, client-server interaction Socket-level programming Writing a client Writing a server URL connections and web servers - PowerPoint PPT PresentationTRANSCRIPT
COMP201 Java Programming
Part III: Advanced Features
Topic 14: Networking
Volume II,Chapter 3
COMP201 Topic 14 / Slide 2
Outline
Networking basics IP addresses, ports, protocols, client-server interaction
Socket-level programming Writing a client Writing a server
URL connections and web servers Retrieving information Sending information
COMP201 Topic 14 / Slide 3
Networking Basics Internet protocol (IP) address
Every host on Internet has a unique IP address
143.89.40.46, 203.184.197.198
203.184.197.196, 203.184.197.197, 127.0.0.1
More convenient to refer to using hostname string
cs.ust.hk, tom.com, localhost
Domain Naming Service (DNS) maps names to numbers
One hostname can correspond to multiple internet addresses:
tom.com: 203.184.197.196, 203.184.197.196, 203.184.197.197 (down to one now)
COMP201 Topic 14 / Slide 4
java.net.InetAddress class converts between hostnames and internet addresses InetAddress tm = InetAddress.getByName(“tom.com");InetAddress tm= InetAddress.getByName(“localhost"); //127.0.0.1InetAddress tm = InetAddress.getLocalHost();
Can get array of addresses (if more than one)
InetAddress[] addrs;
addrs=InetAddress.getAllByName(“tom.com");
for (int i = 0; i < addr.length; i++)
System.out.println(addrs[i].getHostAddress());InetAddressTest.java
Networking Basics
COMP201 Topic 14 / Slide 5
Ports
A port identifies a service within a host
Many different services can be running on the host
Many standard port numbers are pre-assigned
time of day 13, ftp 21, telnet 23, smtp 25, finger 79, http 80
see /etc/services on workstation for list of all assigned ports
IP address + port number = "phone number“ for service
Networking Basics
COMP201 Topic 14 / Slide 6
protocols : rules that facilitate communications between machinesTime of day just reports time, ftp has put/get commands, etc.
Protocols must be standardized and documentedSo machines can reliably work with one another
TCP (Transmission Control Protocol) vs UDP (User Datagram Protocol, good for, e.g., video delivery)
Networking Basics
COMP201 Topic 14 / Slide 7
Client-Server interaction
Communication between hosts is two-way, but usually the two hosts take different roles
Server waits for client to make request
Server registered on a known port with the host ("public phone number")
Usually running in endless loop
Listens for incoming client connections
Networking Basics
COMP201 Topic 14 / Slide 8
Client "calls" server to start a conversation
Client making calls uses hostname/IP address and port number
Sends request and waits for response
Standard services always running
ftp, http, smtp, etc. server running on host using expected port
Server offers shared resource (information,database, files, printer, compute power) to clients
Networking Basics
COMP201 Topic 14 / Slide 9
Using telnet to try out some services of servers:
Telnet assumes you want to connect to port 23 on the receiving host (port 23 is where the telnet server is listening)
However there is an optional argument after the hostname that allows you to connect to a different port
Try the following
Get time: telnet time-A.timefreq.bldrdoc.gov 13
Finger some one: telnet cssu100.cs.ust.hk 79 & type a username
Get HTML page: telnet tom.com 80 and enter a GET command
Networking Basics
COMP201 Topic 14 / Slide 10
Socket-Level Programming
A socket is a bi-directional communication channel between hosts
Send and receive data using streams
Client Server
OutputStream
InputStream
InputStream
OutputStream
Next: How to write a client How to write a server
COMP201 Topic 14 / Slide 11
To write a client Create a new Socket with hostname and port number of the connection
Socket s = New Socket(String hostName,int portNumber);
Call s.getOutputStream() and s.getInputStream() to get streams for sending and receiving infomation
Need to learn protocol used to communicate
– Know how to properly form requests to send to server
– Know how to interpret the server’s responses
Socket-Level Programming/Client
COMP201 Topic 14 / Slide 12
Socket-Level Programming/Client Example: A client using http to request page
Socket s = new Socket(“tom.com", 80);
PrintWriter send = new PrintWriter(s.getOutputStream());
send.print("GET \n");
send.flush();
BufferedReader recv = new BufferedReader(
new InputStreamReader(s.getInputStream()));
String line;
while ((line = recv.readLine()) != null)
System.out.println(line);
s.close()//HttpTest.java. This is basically what web browsers do.
COMP201 Topic 14 / Slide 13
Socket-Level Programming/Client
Use setSoTimeout method to limit the amount of time waiting for data
Socket s = new Socket(“tom.com", 80);
s.setSoTimeout( 10000 ); // time out after 10 seconds
Can still wait indefinitely for initial connection
(construction of socket object).
COMP201 Topic 14 / Slide 14
Socket-Level Programming/Client Solution: Construct socket in a separate thread and use the join
method to time out.class SocketOpener implements Runnable
{ public SocketOpener(String aHost, int aPort)
{ socket = null; host = aHost; port = aPort;
}
public void run()
{ try
{ socket = new Socket(host, port);}
catch (IOException exception) {}
}
public Socket getSocket()
{ return socket; }
…
private String host; private int port;
private Socket socket;
COMP201 Topic 14 / Slide 15
Socket-Level Programming/Clientclass SocketOpener implements Runnable
{ // method to be called by main thread
public static Socket openSocket(String aHost, int aPort,
int timeout)
{ SocketOpener opener = new SocketOpener(aHost, aPort);
Thread t = new Thread(opener);
t.start();
try
{
t.join(timeout); // wait at most timeout milliseconds
} // for the thread to dead
catch (InterruptedException exception){}
return opener.getSocket();
} SocketOpenerTest.java
COMP201 Topic 14 / Slide 16
Socket-Level Programming/Server
To write a server Create a new ServerSocket with a port number to listen on the port
ServerSocket s = New ServerSocket( portNumber);
Use accept() to listen on the port.
accept() returns a socket incoming when a client calls
Socket incoming = s.accept();
Call incoming.getOutputStream() and incoming.getInputStream() to get streams for sending and receiving information
COMP201 Topic 14 / Slide 17
Socket-Level Programming/Server Example: Echo server
ServerSocket s = new ServerSocket(8189);
while (true)
{ Socket incoming = s.accept( );
BufferedReader in = new BufferedReader
(new InputStreamReader(incoming.getInputStream()));
PrintWriter out = new PrintWriter
(incoming.getOutputStream(), true /* autoFlush */ );
out.println( "Hello! Enter BYE to exit." );
…
}
EchoServer.java
COMP201 Topic 14 / Slide 18
Socket-Level Programming/Server Multithread server: starts a separate thread for each connection.
public class ThreadedEchoServer
{ public static void main(String[] args )
{ int i = 1;
try{ServerSocket s = new ServerSocket(8190);
while (true)
{ Socket incoming = s.accept( );
System.out.println("Spawning " + i);
new ThreadedEchoHandler(incoming, i).start();
i++;
}
} catch (Exception e) …. //ThreadedEchoServer.java
COMP201 Topic 14 / Slide 19
Socket-Level Programming/Serverclass ThreadedEchoHandler extends Thread
{ public ThreadedEchoHandler(Socket i, int c)
{ incoming = i; counter = c; }
public void run()
{ try
{ BufferedReader in = new BufferedReader
(new InputStreamReader(incoming.getInputStream()));
PrintWriter out = new PrintWriter
(incoming.getOutputStream(), true /* autoFlush */);
out.println( "Hello! Enter BYE to exit." );
…
private Socket incoming;
private int counter; }
COMP201 Topic 14 / Slide 20
ICQServer.java A simple server that listens on port 7777.
Connect two clients so that they can talk to each other.
Can handle more than one pairs.
See code for details.
Socket-Level Programming/Server
COMP201 Topic 14 / Slide 21
URL Connections
URL stands for Uniform Resource LocatorNeat scheme for uniquely identifying all kinds of network resources
Basic form <protocol>:<sitename><pathname>http://www.cs.ust.hk/~liao/comp201/index.html
ftp://ftp.cs.ust.hk/pub/lzhang/teach/201/codes/HttpTest/HttpTest.java
file:/MyDisk/Letters/ToMom2-11-98
Protocols include files, http, ftp, gopher, news, mailto, etc.
Connecting to a URL, the hard wayManually parse out the host, protocol, path from URL
Create a socket to host
Use protocol to send the right request & interpret response
But Java makes this much easier than that...
COMP201 Topic 14 / Slide 22
URL Connections/Retrieve Info Open connection with java.net.URL
URL url1 = new URL(“http://www.cs.ust.hk/~liao/comp201/index.html”);
URL url2 = new URL (“ftp://ftp.cs.ust.hk/pub/lzhang/teach/201/codes/net/HttpTest/HttpTest.java”);
To fetch contents of resource, use openStream method of URL, which returns a InputStream
InputStream in1 = url1.openStream();
InputStream in2 = url2.openStream();
Now we can nest the InputStreams with other Java streams to retrieve contents
Advantages
1. No need to worry about protocol details & port.
2. Connection to particular file, not easy to do at Socket level
COMP201 Topic 14 / Slide 23
URL Connections/Retrieve Info Use java.net.URLConnection for additional info about resource
1. Create URLConnection object from URL,
URLConnection cnn = url.openConnection();
2. Set properties of connections:
E.g. setDoOutPut(true) for sending information to the server
3. Make connection: cnn.connect();
4. Query header information:
getContentType, getContentLength, getContentEncoding,
getDate, getExpiration, getLastModified
5. getInputStream for reading and getOutputStream for writing
URLConnectionTest.java
COMP201 Topic 14 / Slide 24
URL Connections/Sending Info Web servers receive information from clients using either GET or
POST
http://maps.yahoo.com/py/maps.py:
<form action="/py/maps.py?Pyt=Tmap&YY=28457" method=GET> … </form>
http://www.census.gov/ipc/www/idbprint.html:
<form method=post action="/cgi-bin/ipc/idbsprd">
Appropriate CGI (common gateway interface) script is called to process info received and produce an HTML page to send back to client
CGI scripts usually written in C, Perl, shell script. (Out of the scope of this course.)
Will discuss servlets, Java alternative to CGI scripts
COMP201 Topic 14 / Slide 25
URL Connections/Sending Info Send information to server using GET
Attach parameters to the end of URL
http://host/script?parameters
Separate parameters using “&” and encode parameters as follows to avoid misinterpretation (URL encoding)
Static String encode (String s)
Replace space with “+”
Replace each non-alphanumeric character with “%” followed by the hexadecimal code of the character
“Mastering C++” “Mastering+C%2b%2b”
Disadvantage: long parameter string, might exceed limits of browsers.GetTest.java
COMP201 Topic 14 / Slide 26
URL Connections/Sending Info Sending information to server using POST:
Open URLConnection and send parameter using a stream
1. Open a URLConnection:
URL url = new URL(“http:/host/script”);
URLConnection cnn = url.openConnection();
2. Set up connection for output:
cnn.setDoOutput(true);
3. Get a stream for sending data:
PrinterWriter out = new PrintWriter(cnn.getOutputStream());
4. Send parameters
Out.print(name1 + “=“ + URLEncoder.encode(value1) + “&” );
Out.print(name2 + “=“ + URLEncoder.encode(value2) + “\n”);PostTest.java