Static Variable becomes null after class completed

Question

I Have three classes

1. StaticHolder.java - Which holds a static variable.
2. StaticInitializer.java -Responsible only for initializing the variable through a static method.
3. Application.java - Retrieves the static variables value through getter method.

I thought initializing a static variable once in JVM will not go until we stop the JVM. So I called ran StaticInitializer once which will do the initialization. And tired to access its value from another class which is not working and returning null. Can anyone explain why. Thanks In Advance.

public class StaticHolder {
    private static String hello;

    public static void ini() {
        hello = "Hello World";
    }

    public static String getHello() {
        return hello;
    }

    public static void setHello(String hello) {
        StaticHolder.hello = hello;
    }
}

class StaticInitializer {
    public static void main(String[] args) {
        StaticHolder.ini();
        while (true) {
            Thread.sleep(1000);
        }
    }
}

public class Application {
    public static void main(String[] args) {
        System.out.println(StaticHolder.getHello());
    }
}

Answer 1

static does not mean that this value is there forever!

It is only theree for the current java session.

Invocing the java command at the command line starts a new java session where the value needs to be initialized again.

---

Actually I have a daemon thread which does the initialization and stays alive.And I have another stand alone java program which tries to get the value.

Without knowing that other code involved my gueass is that you did not establish inter process communication.

The easiest way it that you "deamon" opens a server socket and your "stand alone java program" connects to it an queries the desired data through it.

Answer 2

So there is only one main method that can be executed as entry point for the entire application for each JVM run.

When the JVM is executed you can specify which class has to be loaded at start. The Classloader take care to load that class and then the JVM can execute the only one public static void main(String[] args) method.

In Java you need to have at least one class with a public static method named main. I suggest to read this post to understand why it is public static.

The Java Classloader is a part of the Java Runtime Environment that dynamically loads Java classes into the Java Virtual Machine. Usually classes are only loaded on demand.

So returning to your question, given that when Application.main is running there is no way to execute StaticHolder.init(), I suggest to change your main in this way:

public class Application {
    public static void main(String[] args) {
        StaticHolder.init();
        System.out.println(StaticHolder.getHello());
    }
}

or change StaticHolder in this way and remove the init:

public class StaticHolder {
    private static String hello;

    static {
        hello = "Hello World";
    }

    public static String getHello() {
        return hello;
    }

    public static void setHello(String hello) {
        StaticHolder.hello = hello;
    }
}

On the other hand, just to be clear if you run the StaticInitializer.main this has no effect on Application.main execution.

Answer 3

In your program , when main method of StaticInitializer is first executed, a String named hello is initalized. and as ini() method is called, the value 'Hello world' is assigned to hello. Then jvm exists main method, and then stops working. Again when we compile application class,instead of the previous hello variable , a new hello string variable is created with no value assigned(null valued) . That's why you're getting null as output. Thankyou.