Question

So, here’s an example. I have a library in the package HTTP. I define sub-sections of the library in e.g. the package HTTP.TCPProtocol. Now I want to use TCPProtocol from the HTTP package, which means I have to make the TCPProtocol functionality public. At the same time, this functionality should not be exported to users of the library.

How do I do this? I don’t want to shove my whole library into one package, as I feel the separate sub-packages really make the code more structured and navigation easier in eclipse. But browsing around, I couldn’t find a method to expose functions within my project, but not export them outside my project.

EDIT: In light of me being able to come up with a better example, I’m updating the OP.

Answer 1

Check the caller’s class to lock out all unwanted callers. The caller’s class can be obtained from the stacktrace. In the example below, only instances of Bar will trigger the system.out.println, all all other will get an exception. You can even do package-level checks this way. Make sure that all allowed caller classes methods are not public, or they can call the doSomething method indirectly. You can even do deeper checks, by inspecting the stacktrace further.

Be aware though, that a skilled develper can circumvent anything you try do do in this matter. No solution is really “secure”.

package one.two;

import one.Bar;

public class Foo {

    public void doSomething() {

        StackTraceElement[] stackTrace = Thread.currentThread().getStackTrace();
        StackTraceElement stackTraceElement = stackTrace[2];
        String className = stackTraceElement.getClassName();

        if (Bar.class.getName().equals(className)) {
            System.out.println("jay!");
        } else {
            throw new RuntimeException("not allowed");
        }

    }
}

---

package one;

import one.two.Foo;

public class Bar {

    void makeCall() {
        new Foo().doSomething();
    }

    public static void main(String[] args) {
        new Bar().makeCall();
    }

}