Object/variable synchronization

Question

i need to synchronize a primitive type (or object), am i doing it correctly?
private static boolean threadStarted = false;
public void init(){
    sunchronized (threadStarted){
        if (!threadStarted){
            schedulerThread = new Scheduler();
            schedulerThread.start();
            threadStarted = true;
}this is basically a servlet that does some database maintainance (I'm forced to make it a Servlet - so we can do different DB maintainance task from anywhere, by using the HTTP GET).
what i wanted to happen is that on init(), the servlet will create an instance of the Scheduler(which extends Thread to create and starts severals TimeTask objects using a Timer). i wanted to make sure only the scheduler thread is ever created get created once and started once. is the above code guareentee to start only one instance of the scheduler.
my previous code does not have synchronization, and i noticed that the scheduler is created twice 9therby - doing the same DB maintainance twice - which is extremely costly)
I thought of making the Scheduler a Singleton, and return null if the instance has been created...like the code below
public class Scheduler extends Thread{
    private static Scheduler scheduler = null;
    private Scheduler(){
        // .. initialize some local variables in here
    public synchronized static Scheduler getInstance(){
        if (scheduler == null)
            return new Scheduler();
        return null; 
    public void run(){
        // do some thread stuff here
public void init(){
    Scheduler temp = Scheduler.getInstance();
    if (temp != null){
        schedulerThread = temp;
        schedulerThread.start();
} any comment or better design? (yes, i have to use a servlet)

Answer 1

First, synchronized has a 'y' in it, not a 'u'. :^)yeah, i have a bad habit of typo. for me, most compile error (90%) aare typo
i really need to take a typing class or two.
Second, your methodology of checking null before creating the object involves
a race condition. Two requests, theoretically, could come in at the same time,
and you might get unexpected results. actually..the method is synchronized, so there wont (shouldn't) be any race condition. the first time the method is invoke..it will lock the method from all other..until the method is complete. I think this is pretty much a standard way of doing a Singleton (cept return null should be returning the scheduler object.)
I aslo knows about double locks and its flaw..that's why the method is synchronized instead of using lazy creation method.
You can only synchronize an object, so you would have to wrap boolean in an
instance of java.lang.Boolean. cool, this i didn't know about. guess i can make the primitive a Boolean instead of a boolean.
Be VERY wary of spinning off your own threads in a J2EE container (e.g.,
Servlet). The J2EE specification specifically requests you avoid doing this. The
primary reason is that the container should be responsible for multi-threading
and thread management. That having been said, you CAN spin off your own
threads, just be very careful.yes, i heard about this so many time..but there's really no choice but to use thread in my application. that's why i come here to ask..hoping someone works with this type of problem before..and know a better design.