Are private methods really safe?

Question

In Java the private access modifier consider as safe since it is not visible outside of the class. Then outside world doesn\'t know about that method either.

Answer 1

By saying 'safe', you are protecting you or other developers, which are using your API to not harm the object by calling your private method. But if you or they really need to call this method, they can do it with Reflection.

Answer 2

The question is who are you trying to save it from. In my opinion, such a client of your code is the one at a loss here.

Any piece of code (written by you or others) which tries to access a private member of the above class is essentially digging its own grave. private members don't make a part of the public API and are subject to change without notice. If a client happens to consume one of such private members in the manner given above, it's going to break if it upgrades to a newer version of the API in which the private member got modified.

Answer 3

Access modifiers have nothing to do with security. In fact you can and should look at access modifiers as the reverse of security -- it is not to protect your data or algorithims, it is to protect people from the requirement to know about your data and algorithims. This is why the default modifier is package -- if they are working on the package they probably already need to know.

Along with the knowledge of the data and the methods of your code, comes the responibility to know when and how to use it. You don't put private on your inIt method to keep someone from finding out about it, you do so because (a) they aren't going to know that you only call that after foo and only if bar = 3.1415 and (b) because it does them no good to know about it.

Access modifers can be summed up in a simple phrase "TMI, dude, I so didn't need to know that".

Answer 4

Assuming you trust the client programmer of your API, another way of looking at is how 'safe' it is for them to use those particular functions.

Your publicly available functions should provide a clear, well-documented, rarely-changing interface into your code. Your private functions can be considered an implementation detail and may change over time, so are not safe to use directly.

If a client programmer goes out of their way to circumvent these abstractions, they are in a way declaring that they know what they are doing. More importantly, they understand that it is unsupported and may stop working with future versions of your code.

Answer 5

With facility, there comes responsibility. There are thing's you can't do, & things you can do but you shouldn't do.

Private modifier is provided/used as/in the most restricted manner. Members which should not be visible outside the class shall be defined as private. But this can be broken with Reflection as we see. But this does not mean that you should not use private - or they are unsafe. It is about you shall use things judiciously or in constructive manner (like reflection).

Answer 6

It depends on what you mean by "safe". If you're running with a security manager that allows this sort of thing, then yes, you can do all kinds of nasty things with reflection. But then in that kind of environment the library can probably just be modified to make the method public anyway.

Access control is effectively "advisory" in an environment like that - you're effectively trusting the code to play nicely. If you don't trust the code you're running, you should use a more restrictive security manager.