Understanding URLs, HTTP, and Sockets for Client-Server Communication, Slides of Computer Science

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Clients and Servers

URL review

• A URL has the syntax:
  • protocol :// hostname : port / path # anchor
• import java.net.*;
  • This is the package that defines sockets, URLs, etc.
• URL url = new URL( String );
  • Constructs a URL object from a text string
• MalformedURLException
  • This exception is thrown if the given String cannot be parsed by newURL( String )
• We have used URLs to display a page in an applet:
  • appletContext .showUrl( URL )

Using a URL

• URLConnection c = url .openConnection();
  • The URLConnection is the basic way to access the resource information
• c .getHeaderField( name )
  • Returns the value of the named header field (as a String)
  • Frequently used fields have shorthand methods, for example, c .getLastModified() = c .getHeaderField("last-modified")
• getHeaderField( int )
  • Returns the value of the int -th header field (as a String)
  • The 0-th header field is the status line
• c .getInputStream()
  • Returns an InputStream containing the “content” of the resource
  • url .openStream() is shorthand for url .openConnection().getInputStream()

Socket review

• A socket is a low-level software device for connecting

two programs (possibly on different computers)

together

• new Socket(String host , int port )

• Creates a client socket and makes the connection
• Methods include getInputStream(), getOutputStream(),

and close()

• new ServerSocket(int port )

• Creates a server socket that listens on the specified port
• accept() returns a Socket that can be used for I/O
  • accept() is a blocking method, so multithreading is highly desirable

How to write a client

• Socket server = new Socket( ip_address , port )

– The ip_address can be the String "localhost"

• This method makes the actual connection
• InputStream inStream = server .getInputStream();
• As on the previous slide
• OutputStream outStream = server .getOutputStream();
• As on the previous slide
• input .close(), output .close(), server .close()
• As on the previous slide

How to write an HTTP server

• An HTTP server is just a server that follows the

HTTP protocol (request/status line, header,

blank line, body)

– Since HTTP is a text-based protocol, compliance is

easy

• There are two versions of HTTP: 1.0 and 1.

– HTTP 1.0 is simpler and should be used if the

special features of 1.1 are not required

– The most important change in HTTP 1.1 is that it

can accomodate proxy servers

Multithreading

• server .accept() is a blocking call--Java stops and waits for a

response before it continues

• This is only acceptable if the server never has more than one client
• A server needs to have a separate thread for each client
• There are two ways to create a Thread:
• Write a class that extends Thread
• Override the public void run() method
• Create an instance of your class and call its (inherited) start() method
• Write a class that implements Runnable
• Implement the public void run() method
• Create an instance of your class
• Create a Thread object with this instance as a parameter to the constructor
• Call the Thread object’s start() method

Synchronization

• While an object is being modified by one thread, no other

thread should try to access it

• This leads to unpredictable (and difficult to debug) results
• You can synchronize an object:
• synchronized ( obj ) { code that uses/modifies obj }
• synchronized is a statement type, like if or while
• No other code can use or modify this object at the same time
• You can synchronize a method:
• synchronized void addOne( arg1, arg2, ...) { code }
• synchronized is a method modifier, like public or abstract
• Only one synchronized method in a class can be used at a time (but this doesn’t restrict other, non- synchronized methods)
• Synchronization can really hurt efficiency (and response time)
• It can be very difficult to make a program both safe and efficient