how to I make sure that only one servlet(not instance) is call at a time?

Question

I have an application which makes lot of multiple servlet calls. How do I make sure that till a particular servlet X processing is completed no other servlet instance of Y, Z is called.

A SingleThreadModel or synchronization will make sure that only one thread per servlet is called. However that may not work on more than one servlet.

Answer 1

Use Callback mechanism for each server calls. Onsuccess of first server call you can call second.

Answer 2

You could implement manual synchronization by defining your own locks.

For Example declare a static Object to use as semaphore:

public static final Object MY_LOCK = new Object();

and where you want to synchronize you could use it like that:

public void someMethod() {
    synchronized(MY_LOCK) {
        // do stuff that may not be called at same time
    }
}

public void someOtherMethod() {
    synchronized(MY_LOCK) {
        // do other stuff that may not be called at same time
    }
}

That way you can use the same Lock in different Methods (or servlets in your case).