Producer-Consumer

1
Producer-Consumer

• An example of using Threads

2
The problem

• One Thread, the producer, is producing
  information (creating objects), while another
  Thread, the consumer, is consuming (using) it
• We want the consumer to use the objects in the
  same order as the producer creates them
• We dont want either Thread to be loafingif
  there is work for it to do, the Thread should be
  doing it
• We want to absolutely avoid busy loops
• Example busy loop while (!ready) (the other
  Thread will reset the ready variable)
• Busy loops will consume all available CPU cycles
  and seriously slow down everything on the computer

3
The overall structure

• import java.util.Vectorpublic class
  CommandList static Vector list new
  Vector() public static void put(String
  s) // code on next slide
  public static String get() // code on
  slide after next

4
The put method

• public static void put(String s)
  synchronized (list) list.add(s)
  list.notify()
• The synchronized(list) block will take the list
  object as soon as it becomes available, and
  lock it so no other synchronized(list) can use
  it until this block is completed
• There is nothing special about the list object
  any object can be locked for synchronization

5
The get method

• public static String get() if
  (list.size() gt 0) synchronized
  (list) return
  (String)list.remove(0)
  else try
  synchronized (list)
  list.wait() return get()
  catch
  (InterruptedException e) return
  "InterruptedException"
  

6
The Consumer class

• public class Consumer extends Thread public
  void run() while (true)
  String s CommandList.get()
  System.out.println("Consuming " s)
  

7
The test class

• public class CommandListTester public
  static void main(String args)
  Consumer consumer new Consumer()
  consumer.start() CommandList.put("one")
  CommandList.put("two")
  sleep(2000) CommandList.put("three")
  CommandList.put("four") private
  static void sleep(int ms) try
  Thread.sleep(ms) catch
  (InterruptedException e)

• Output
• onetwo
• threefour

8
The End