Threads

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Threads

• OOP in Java

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Threads in Java

• Definition
• unit of program control that represents an
  execution sequence
• Java supports multi-threaded execution
• Programs we have written up to this point have
  been on a single thread of control

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The Thread class

• Thread creation
• instantiate a class that extends Thread
• instantiate the class Thread but provide a run()
  method
• Thread execution
• let t refer to a thread object
• t.start() causes the thread to execute its run()
  method in the background

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Example 1 PrintThread

• PrintThread class extends Thread
• Attributes name timedelay
• Constructor sets name and timedelay
• run method prints name twice but with a time
  delay in between the print statements
• use Thread method sleep(timedelay)

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run() Methodfor PrintThread

• public class PrintThread extends Thread
• ...
• public void run()
• System.out.println(name)
• try sleep(timedelay)
• catch(Exception e)
• System.out.println(name)

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Using PrintThread

• PrintThread t1,t2,t3
• t1 new PrintThread("tic",5000)
• t2 new PrintThread("tac",1000)
• t3 new PrintThread("toe",3000)
• t1.start()
• t2.start()
• t3.start()
• // expected output ? last output line should be
  tic

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Example 2 MovingCircle

• MovingCircle class extends Applet
• Attributes xpos size (of circle)
• provide initial values
• paint() method draws the circle
• Thread created inside init()
• run() method has a loop that continuously updates
  xpos size and then repaints
• A button executes the thread (t.start())

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MovingCircle class (attributes paint())

• // sample animation using Threads
• public class MovingCircle extends Applet
• int xpos 5 // these variables will be
  updated
• int size 10 // on a different thread
• ...
• public void paint(Graphics g)
• g.drawOval(xpos,50,size,size)

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MovingCircle class(thread code)

• t new Thread()
• public void run()
• while (xpos lt 80)
• try sleep(1000) catch(Exception e)
• xpos 5
• size 3
• repaint() // forces paint() to be re-executed
• // this statement placed inside the init()
  method

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The Runnable Interface

• Runnable is an interface that contains the run()
  method
• One of the constructors for Thread
• public Thread(Runnable r)
• Can create a thread by giving it an argument
  (object) that implements run()
• alternative to extending thread and then
  overriding run()

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Thread Issues

• Deadlock
• some methods have been deprecated because of this
  possibility
• Synchronization
• several threads running the same method and/or
  updating the same data

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Networking

• OOP in Java

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Client/Server Computing

• Communication over the network often occurs
  between a client and a server
• A server listens for connection requests and then
  responds to messages
• A client establishes a connection to a server and
  then sends messages
• TCP/IP abstract layer that simplifies the above
  activities

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Hosts, Ports, and Sockets

• Computers on the network are (uniquely) specified
  by a host name or an IP address
• Communication between hosts occurs through their
  ports
• each port allows several connections
• Socket
• connection handle that facilitates communication
  over the network

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Networking in Java

• java.net package
• import java.net.
• Most important classes
• Socket
• ServerSocket
• URL (discussed earlier)

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The Socket class

• Constructor requires a host and port that the
  client intends to connect to
• Useful Socket methods
• InputStream getInputStream()
• OutputStream getOutputStream()
• Use file/stream interfaces to carry out the
  communication

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The ServerSocket class

• Constructor requires a port number that the
  server wishes to listen from
• accept() method returns a Socket object once a
  connection from a client has been established
• blocks (hangs) until the connection occurs

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On the Server Program ...

• Create a ServerSocket object
• specify port
• Invoke accept() on that object
• Obtain Socket object
• returned by accept()
• Obtain I/O streams from the socket
• carry out communication using these streams

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On the Client Program ...

• Create Socket object
• specify host and port of server
• Obtain I/O streams from the socket
• carry out communication using these streams
• execute the server before the client

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Allowing Multiple Connections to a Server

• Use Threads
• Have a loop that continuously calls accept()
• main thread
• Create and start a thread whenever accept()
  returns a socket
• facilitate communication on the socket using a
  separate thread