Android: have Singleton class that will never been killed

Question

In my Android application I have a Singleton class. It works like a charm when the application task is alive. But after the application task is being killed, the Singleton class gets destroyed, which is obviously. I need to keep this class always alive. Is it possible? And what is the best approach to have this class never been killed by Android system, do I need to use a background Service in this situation?

Answer 1

Using a Service for the purpose of trying to ensure a Singleton class always stays in memory for subsequent invocations of the application would be terribly bad design. It's simply not what Services are for. See: http://www.androidguys.com/2009/09/09/diamonds-are-forever-services-are-not/ .

If you want to ensure that a bunch of data is kept around ready for the next time the application is created then you should choose an appropriate means to persist that data. This could be, for example, by using SharedPreferences or SQL , or a cache, etc. depending on what kind of data you're trying to keep around. Then, design all Activity classes to properly cope with the scenario where they're resumed when all Singleton data is not in an initialised state.

I personally maintain a complex scientific application that relies on having quite a lot of singleton data held in RAM. One significant problem I encountered during early stages of development is that it's possible for the following situation to happen:

• You have three Activity classes (for example); call them A, B and C.
• You have Singleton data attached to the Application class.
• User makes use of the application and then navigates to Application C. By this time, lots of Singleton data has been initialised. Then, the user navigates back to home (the launcher).
• In the meantime, your app's Application class is destroyed and created (perhaps several times over) along with any Service s it uses while the user interacts with other apps.
• Later, the user goes back to your app. The Activity stack is resumed, meaning that the user goes straight to Activity C. However, the Singleton data is now all gone. Having Activity C start up with an "empty" non-initialised set of data might have resulted in a crash or the user being presented with a blank page.
• Key point: It's possible for the Activity stack to be resumed so that the app is resumed back on Activity C even though the Application class has been restarted and all global data has been nuked from orbit.

To cope with the above scenario, the key thing I did was to ensure all Activities don't just assume that global (Singleton) data is present and intact when the Activity is started. Each Activity in my application that depends on the global Singleton data checks that data first and, if it is not initialised, calls a method in my Application class to trigger regeneration of that data (which in my case involves retrieving that data from a file from the SD card).

So in short, handling global / Singleton data in Android takes some care and thought. You cannot force the system to indefinitely avoid killing your Application and therefore all of your Singleton data. The key things I learned therefore are to persist and retrieve your data, and design your Activity classes to properly handle the situation where no Singleton data is found to be already initialised when that Activity is created.

Answer 2

是的，您需要使用后台服务。后台服务是与其他所有组件相比在被杀死方面具有最高优先级的组件，但是请确保或服务应连续运行。

Answer 3

Try using moveTaskToBack() . That should do the trick.