28-Jun-15 Producer-Consumer An example of using Threads

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 don’t want either Thread to be “loafing”—if 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

The overall structure import java.util.Vector; public 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 } }

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.

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

The get method public static String get() { if (list.size() > 0) { synchronized (list) { return (String)list.remove(0); } } else { try { synchronized (list) { list.wait(); return get(); } } catch (InterruptedException e) { return "InterruptedException"; } } } }

The Consumer class public class Consumer extends Thread { public void run() { while (true) { String s = CommandList.get(); System.out.println("Consuming " + s); } } }

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

Another Consumer class import java.util.Random; public 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) { } } } }

Another test class import java.util.Random; public 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 < words.length; i++) { CommandList.put(words[i]); try { Thread.sleep(rand.nextInt(1000)); } catch (InterruptedException e) { } } } }

The End