package Scheduler;import java.util.*;/** * TestScheduler.java * * This program demonstrates how the scheduler operates. * This creates the scheduler and then the three example threads. * * @author Greg Gagne, Peter Galvin, Avi Silberschatz * @version 1.0 - July 15, 1999. * Copyright 2000 by Greg Gagne, Peter Galvin, Avi Silberschatz * Applied Operating Systems Concepts - John Wiley and Sons, Inc. */class TestScheduler{   public static void main(String args[]) {	/**	* This must run at the highest priority to ensure that	* it can create the scheduler and the example threads.	* If it did not run at the highest priority, it is possible	* that the scheduler could preempt this and not allow it to	* create the example threads.	*/      Thread.currentThread().setPriority(Thread.MAX_PRIORITY);      Scheduler CPUScheduler = new Scheduler();      CPUScheduler.start();      TestThread t1 = new TestThread("Thread 1");      t1.start();      CPUScheduler.addThread(t1);      TestThread t2 = new TestThread("Thread 2");      t2.start();      CPUScheduler.addThread(t2);      TestThread t3 = new TestThread("Thread 3");      t3.start();      CPUScheduler.addThread(t3);   }}/** * CircularList.java * * This class implements a circular list using the Vector class * note that elements in a vector with n elements are numbered 0 .. (n-1) * * @author Greg Gagne, Peter Galvin, Avi Silberschatz * @version 1.0 - July 15, 1999. * Copyright 2000 by Greg Gagne, Peter Galvin, Avi Silberschatz * Applied Operating Systems Concepts - John Wiley and Sons, Inc. */public class CircularList{   private Vector List;   private int index;   public CircularList() {      List = new Vector(10);      index = 0;   }   /**    * this method returns the next element in the list.    * @return Object    */   public Object getNext() {      Object nextElement = null;      int lastElement;      if (!List.isEmpty() ) {         if (index == List.size() )            index = 0;         nextElement = List.elementAt(index);         ++index;      }      return nextElement;   }   /**    * this method adds an item to the list    * @return void    */   public void addItem(Object t) {      List.addElement(t);   }}/** * TestThread.java * * This thread is used to demonstrate how the scheduler operates. * This thread runs forever, periodically displaying its name. * * @author Greg Gagne, Peter Galvin, Avi Silberschatz * @version 1.0 - July 15, 1999. * Copyright 2000 by Greg Gagne, Peter Galvin, Avi Silberschatz * Applied Operating Systems Concepts - John Wiley and Sons, Inc. */class TestThread extends Thread{private String name;   public TestThread(String id) {      name = id;   }   public void run() {	/*	 * The thread does something  	 **/  	 //while(true){     for(int count=0;count<5;count++){	for (int i = 0; i < 50000; i++)		;	System.out.println("I am thread " + name);      }   }}/** * Scheduler.java * * This class is a simple round-robin scheduler. * The idea for this scheduler came from "Java Threads" * by Oaks and Wong (Oreilly, 1999). * * @author Greg Gagne, Peter Galvin, Avi Silberschatz * @version 1.0 - July 15, 1999. * Copyright 2000 by Greg Gagne, Peter Galvin, Avi Silberschatz * Applied Operating Systems Concepts - John Wiley and Sons, Inc. */public class Scheduler extends Thread{   private CircularList queue;   private int timeSlice;   private static final int DEFAULT_TIME_SLICE = 1000; // 1 second   public Scheduler() {      timeSlice = DEFAULT_TIME_SLICE;      queue = new CircularList();   }   public Scheduler(int quantum) {      timeSlice = quantum;      queue = new CircularList();   }   /**    * adds a thread to the queue    * @return void    */   public void addThread(Thread t) {      t.setPriority(2);      queue.addItem(t);   }   /**    * this method puts the scheduler to sleep for a time quantum    * @return void    */   private void schedulerSleep() {      try {         Thread.sleep(timeSlice);      } catch (InterruptedException e) { };   }   public void run() {      Thread current;      // set the priority of the scheduler to the highest priority      this.setPriority(6);   for(int a=0;a<5;a++){      //while(true) {            try {               current = (Thread)queue.getNext();               if ( (current != null) && (current.isAlive()) ) {                 current.setPriority(4);                 schedulerSleep();                System.out.println("* * * Context Switch * * * ");                 current.setPriority(2);               }            } catch (NullPointerException e3) { } ;      }   }}