Is it safe to use static class variables in an android application

Question

I am aware of the technique of extending the Application class to provide global storage. However in my case I am writing a class for a library function, so do not wish to force users of the class down this path. I have a requirement for some static class variables. I have seen passing references in StackOverflow that these might not be safe. However I've tried two different applications using the same class, and even when running both applications side by side on a Galaxy S3 in multi-window mode, the static class variables remain separate.

So, can someone with an in depth knowledge of Android internals confirm if this is safe or not.

If it is not safe, I can wrap the variables in a nested class and add them to a Serializable static HashMap, using the application package name as the key. This will force them to be safe. However if this is not necessary, then I'd rather not do it.

Answer 1

I have seen passing references in StackOverflow that these might not be safe.

They are not "safe" insofar as your process will be terminated from time to time, wiping out your static data members (and your custom Application, for that matter). Hence, static data members are good for a cache and not much else.

Within that scope, they are "safe".

You just need to make sure that this data is either stored somewhere persistent (e.g., file) or otherwise can be regenerated once the process is terminated and later is started up again. This is no different than with Application.

However I've tried two different applications using the same class, and even when running both applications side by side on a Galaxy S3 in multi-window mode, the static class variables remain separate.

Correct. Those are separate processes, with separate copies of your class and objects.

Answer 2

If your goal is to store persistant data across the twists and turns of the application lifecycle, then I would recommend not using static variables to do so. The obvious problem with this approach is that they could easily be garbage collected by the system when the operating system decides to reclaim memory (i.e. when the screen sleeps or a different application starts a memory-intensive task). I'm not sure what kind of data you are looking to "store", but I would recommend saving the state in SharedPreferences or an SQLiteDatabase instead.

Answer 3

I'm a little confused about what you're trying to do. You're trying to create a utility library in Java to be used by other applications? You're trying to create a whole Activity intended for use by other applications?

At any rate, as other posters have mentioned, applications can be killed at almost any time when resources become tight. There's simply no way to guarantee that static global values will remain resident in memory. You must provide a way to back it up on onPause() or onSaveInstanceState().

If you're writing a utility library, I presume that it returns some master object which holds all of its state. Add saveState(Bundle), restoreState(Bundle) methods to that object, and optionally saveToSharedPreferences() and restoreFromSharedPreferences() methods as well.

If it's an Activity you're writing, you're probably already familiar with the ways of saving state.

Me, I'm fond of combining the "singleton pattern" with shared preferences: https://stackoverflow.com/a/13673178/338479