threaddemo.java

These are the examples from the book Java Examples in a Nutshell, 2nd Edition, by David Flanagan.

/*
 * Copyright (c) 2000 David Flanagan.  All rights reserved.
 * This code is from the book Java Examples in a Nutshell, 2nd Edition.
 * It is provided AS-IS, WITHOUT ANY WARRANTY either expressed or implied.
 * You may study, use, and modify it for any non-commercial purpose.
 * You may distribute it non-commercially as long as you retain this notice.
 * For a commercial use license, or to purchase the book (recommended),
 * visit http://www.davidflanagan.com/javaexamples2.
 */
package com.davidflanagan.examples.thread;

/**
 * This class demonstrates the use of threads.  The main() method is the 
 * initial method invoked by the interpreter.  It defines and starts two
 * more threads and the three threads run at the same time.  Note that this
 * class extends Thread and overrides its run() method.  That method provides
 * the body of one of the threads started by the main() method
 **/
public class ThreadDemo extends Thread {
    /**
     * This method overrides the run() method of Thread.  It provides
     * the body for this thread.
     **/
    public void run() {	for(int i = 0; i < 5; i++) compute(); }
    
    /** 
     * This main method creates and starts two threads in addition to the 
     * initial thread that the interpreter creates to invoke the main() method.
     **/
    public static void main(String[] args) {
	// Create the first thread: an instance of this class.  Its body is
	// the run() method above
	ThreadDemo thread1 = new ThreadDemo();
	
	// Create the second thread by passing a Runnable object to the 
	// Thread() construtor.  The body of this thread is the run() method
	// of the anonymous Runnable object below.
	Thread thread2 = new Thread(new Runnable() {
		public void run() { for(int i = 0; i < 5; i++) compute(); }
	    });
	
	// Set the priorities of these two threads, if any are specified
	if (args.length >= 1) thread1.setPriority(Integer.parseInt(args[0]));
	if (args.length >= 2) thread2.setPriority(Integer.parseInt(args[1]));
	
	// Start the two threads running
	thread1.start();
	thread2.start();

	// This main() method is run by the initial thread created by the
	// Java interpreter.  Now that thread does some stuff, too.
	for(int i = 0; i < 5; i++) compute();

	// We could wait for the threads to stop running with these lines
	// But they aren't necessary here, so we don't bother.
	// try {
	//     thread1.join();
	//     thread2.join();
	// } catch (InterruptedException e) {}

	// The Java VM exits only when the main() method returns, and when all
	// threads stop running (except for daemon threads--see setDaemon()).
    }

    // ThreadLocal objects respresent a value accessed with get() and set().
    // But they maintain a different value for each thread.  This object keeps
    // track of how many times each thread has called compute().
    static ThreadLocal numcalls = new ThreadLocal();

    /** This is the dummy method our threads all call */
    static synchronized void compute() {
	// Figure out how many times we've been called by the current thread
	Integer n = (Integer) numcalls.get();
	if (n == null) n = new Integer(1);
	else n = new Integer(n.intValue() + 1);  
	numcalls.set(n);

	// Display the name of the thread, and the number of times called
	System.out.println(Thread.currentThread().getName() + ": " + n);

	// Do a long computation, simulating a "compute-bound" thread
	for(int i = 0, j=0; i < 1000000; i++) j += i;

	// Alternatively, we can simulate a thread subject to network or I/O
	// delays by causing it to sleep for a random amount of time:
	try {
	    // Stop running for a random number of milliseconds
	    Thread.sleep((int)(Math.random()*100+1));  
	}
	catch (InterruptedException e) {}

	// Each thread politely offers the other threads a chance to run.
	// This is important so that a compute-bound thread does not "starve"
	// other threads of equal priority.
	Thread.yield();
    }
}