Threads

1
Threads

• How do we mimic two computers, communicating?
• How do many programs run at once on the same
  computer?
• What is meant by client / server?

2
what are threads?

• independently running programs within one Unix
  process
• can communicate through (static) variables that
  they share
• define the Client / Server relationship
• used when a task needs undivided attention
• playing mp3s
• running a task while waiting for user input

3
how threads operate
Server
main
.start
.run()
.start
communicate?
Client
.run()
4
main typically exists in the server
Server
main
.start
.run()
.start
Communicate usually through a separate class
Client
.run()
5
timeline
Server thread
?
main
end (not exit)
?
Client thread
6
writing a client thread

• public class ClientClass extends Thread
• public void run()
• while (true)
• // stuff

7
writing a Server

• public class ServerClass extends Thread
• public void run()
• while
• // does stuff here
• ( true )
• // end run()
• public static void main (String args)
• ClientClass clientThread new ClientClass()
• clientThread.start()
• ServerClass serverThread new ServerClass()
• serverThread.start()
• // end main
• // end class

8
your Server Class responsibility

• main
• start own thread
• start all client threads
• run forever until commanded to stop
• System.exit(0) from this classs .run() ends all
  threads
• actually any System.exit(0) ends all threads.
• many ways to command - shared variables are often
  used files etc.

9
talk to each other

• public class commClass
• public static String ClientToServer "idle"
• public static String ServerToClient "idle"
• // end commClass
• static means everyone can use the Strings
• commClass.ClientToServer exit
• defines a protocol

10
typical uses of thread communications
Server
e.g. web server
registration Ive started
display this page
mouse clicks new pages
Client
e.g. web page
11
some useful methods

• Thread.sleep( milliseconds ) - suspends this
  threads activity

12
stopping a thread

• a clean end to a threads .run( ) method ends
  that thread.
• A .run( ) method that is in a while (true)
  loop will not end.
• System.exit(0) from ANYWHERE ends all threads

13
Assignment 1

• Due one week from today
• Write a client/server program.
• Only the server takes text input from the user
• The server builds the entire user text input in a
  text buffer that is shared by both
• Upon command from the user, the client prints it
  to the screen
• Upon command from the user, the client halts
• Upon separate command from the user, the server
  halts
• The user communicates only with the server
• Use submit_cse453 .java to submit your files