JVM Question

[Nidhin Kannan]

Hi,
A question that an interviewer put forward.

1. Consider there are 2 JVMs on ONE machine. Consider also that you have a Class that has a static variable.
2. Now consider that you are initiate the static variable via the main method of that class on one JVM.
3. If I display the values of the class' static variable on the second JVM, what value will I get? Will it be the same value that I had initiated using the first JVM or will it be the value specific to the JVM in question where Im running the programs/classes.

Its a pure application. SOmething as simple as a HelloWorld program.

Any comments/answers would be welcome.

Thanx,
Nidhin.

[Anandh Ramesh]

it returns the first value of the static variable.when you
have not initialised it, the default value 0 will be displayed.

-Anandh

[Joel McNary]

I'm not sue I understand the question, but separate JVMs do not share the same memory space. Thus, the values of the variables are independent. Try the following:

public class HelloWorld{
    public static String HELLO_WORLD = "Hello World";
 
    public static void main(String argv[]) throws Exception{
        try{
           if(argv.length == 0){
               HELLO_WORLD = "Bonjour, Monde!";
               //Set the variable, then sleep for an hour.
               Thread.sleep(3600000);
           }
           else{
               System.out.println(HELLO_WORLD);
           }
        }
        catch(Exception e){
           e.printStackTrace();
        }
    }
}

Run this class as : java HelloWorld
Then, while it's sleeping (it's sleep for an hour -- you'll have the time), launch another: java HelloWorld Print

You will get "Hello, World" printed, even though the first program set it to "Bonjour, Monde" -- the variables are separate.

[Nidhin Kannan]

Originally posted by Joel McNary:
I'm not sue I understand the question, but separate JVMs do not share the same memory space. Thus, the values of the variables are independent. Try the following:

public class HelloWorld{
    public static String HELLO_WORLD = "Hello World";
 
    public static void main(String argv[]) throws Exception{
        try{
           if(argv.length == 0){
               HELLO_WORLD = "Bonjour, Monde!";
               //Set the variable, then sleep for an hour.
               Thread.sleep(3600000);
           }
           else{
               System.out.println(HELLO_WORLD);
           }
        }
        catch(Exception e){
           e.printStackTrace();
        }
    }
}

Run this class as : java HelloWorld
Then, while it's sleeping (it's sleep for an hour -- you'll have the time), launch another: java HelloWorld Print

You will get "Hello, World" printed, even though the first program set it to "Bonjour, Monde" -- the variables are separate.

Hi Joel,
thanx for the reply. this programe has this argument and no argument methods to call the HelloWorld. thats bound to give different results even on the same JVM.
However, i think the results should be same because u cant have 2 jvms sharing the same memory on the same CPU based machine, meaning your copy of the program would have initiated to the same value in case of the static variables.
Whatsay??
Nidhin. your copy of the program

[Joel McNary]

I'm still not sure I understand the situation that you are describing -- perhaps the best way to answer the question is to set it up yourself and give it a quick test.

Anyway, based on you statements, I'll propose this code:

public class HelloWorld{
    public static String HELLO_WORLD = "Hello World";
 
    public static void main(String argv[]) throws Exception{
        try{
           HELLO_WORLD = "Bonjour, Monde!";
           //Set the variable, then sleep for an hour.
           System.out.println(HelloWorld.HELLO_WORLD);
           Thread.sleep(3600000);
        }
        catch(Exception e){
           e.printStackTrace();
        }
    }
}
 

public class Test{
    public static void main(String argv[]){
        System.out.println(HelloWorld.HELLO_WORLD);
    }
}

Now, run "java HelloWorld" and get "Bonjour, Monde!" as the result.
While that JVM is running, start another with:
"java Test" and get "Hello, World!" as the result.

That's 2 JVMs with two separaet memory spaces. Thus, changing the value of the static variable in one does not affect the other.

[Mike Gershman]

The answer lies in how you do this test.

If you launch the first program in a command window, you won't get another command prompt until the first program finishes. So you have to open another command window to launch the second program. That exists in a separate address space, with no access to application data, including static variables, in the first address space.

I'm sure there is system software around that can tie the two jvm's together, but the standard jvm's don't share their variables between address spaces.

[madhup narain]

These are my views, the JVM runs within an environment and the address space is unique , thus what ever would be declared under the context of one JVM would not be valid on the others context.
Thus this should clearify your thoughts as to what answer you should have given.


Thanks

[David Harkness]

Let's see how many interesting ways we can all say the same thing!

Two JVMs running on the same machine are no different than two JVMs running on different machines as far as static variables are concerned.

Of course, this holds for all objects and memory spaces. You can get a similar effect with regard to separate static "pools" in a single JVM using different class loaders. This is what J2EE containers do with their WAR and EAR class loader hierarchies. That gets truly messy.