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
Methods in java.lang.Object

• void wait()
• Causes the current thread to wait until another
  thread invokes the notify() method or the
  notifyAll() method for this object.
• void wait(long timeout)
• Causes the current thread to wait until either
  another thread invokes the notify() method or the
  notifyAll() method for this object, or a
  specified amount of time has elapsed.
• void notify()
• Wakes up a single thread that is waiting on this
  object's monitor.
• void notifyAll()
• Wakes up all threads that are waiting on this
  object's monitor.

5
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

6
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"
  

7
The Consumer class

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

8
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
• Consuming one
• Consuming two
• Consuming three
• Consuming four

9
Another Consumer class

• import java.util.Randompublic class Consumer2
  extends Thread static Random rand new
  Random() public void run()
  while (true) String s
  CommandList.get() System.out.println(
  "Consuming " s) try
  Thread.sleep(rand.nextInt(1000))
  catch (InterruptedException e)
  

10
Another test class

• import java.util.Randompublic class
  CommandListTester2 static Random rand new
  Random(100) public static void
  main(String args) Consumer consumer
  new Consumer() consumer.start()
  String words "one", "two", "three", "four",
  "five", "six" for (int i 0 i lt
  words.length i)
  CommandList.put(wordsi) try
  Thread.sleep(rand.nextInt(1000))
  catch (InterruptedException
  e)

11
The End