Problems with inheritance of static variables in Java

Question

I have three classes I can't modify. In short, I have a class Program, and other two classes, ProgramClient and ProgramServer, inheriting from Program. The class Program has a static variable.

Until now, I ran ProgramClient and ProgramServer in two different applications, without any problem. Now I need to run the two classes inside the same application. Doing this, they share the static variable of their parent class, and so bad things happen.

How can I keep the two classes in their own "context" (JVM?) in order to ensure that the static variable is used by only one of the children classes?

Answer 1

There is no inheritance or override of static method or variable because there is only one reference for it in all the program. That the aim if static. Maybe you have to create a context class which is instantiate for each program class.

Answer 2

Static variables, by definition cannot be overridden or duplicated between classes.

However the two classes can be separated by using two separate class loaders, that are not chained. This is exactly how J2EE containers provider separation between web applications.

In other words, you will load the base class Program into the JVM twice, and sandbox them apart. Thus giving each separate 'program' their own instance. More can be learnt about class loaders here.

Here is a very simple example bootstrap program. Make sure that the code for ProgramClient and ProgramServer are NOT on the system classpath when you start the java command. You will also need to change the url to the jar file which does contain the target program code.

public class Bootstrap  {
    public static void main(String[] args) throws MalformedURLException, ClassNotFoundException, NoSuchMethodException, IllegalAccessException, InvocationTargetException {
        URLClassLoader cl1 = new URLClassLoader(
                new URL[] {
                        new URL("file:///client.jar")
                },
                Bootstrap.class.getClassLoader()

        );

        URLClassLoader cl2 = new URLClassLoader(
                new URL[] {
                        new URL("file:///server.jar")
                },
                Bootstrap.class.getClassLoader()
        );

        invokeAsync(cl1, "a.b.c.ProgramClient");
        invokeAsync(cl2, "a.b.c.ProgramServer");
    }

    private static void invokeAsync(final ClassLoader cl, final String fqn, final String...progArgs) {
        new Thread() {
            public void run() {
                try {
                    Class p1 = cl.loadClass(fqn);
                    Class argType = String[].class;

                    Method m = p1.getMethod("main", argType);

                    m.invoke(null, (Object) progArgs);
                } catch ( Exception e ) {
                    e.printStackTrace();
                }
            }
        }.start();
    }

}