Android static object lifecycle (Application act crazy)

Question

I am creating event search application, we set search criteria from one screen populate in another screen then user can edit search criteria from 3rd screen and goes to 4th screen.

To achieve above task i am using static object which remember the values around the application and i don't need to do any thing extra.

But i am afraid if about static object life cycle in android if low memory found android delete static objects ???

As android supports multi tasking, if user switches to another application and when user comes back application start acting crazy, does static object get removed when it multi task ??? any idea ?? and also suggest holding static object via singleton method is better approach ???

Answer 1

Lets start with a bit of background: What happens when you start an application?
The OS starts a process and assigns it a unique process id and allocates a process table.A process start an instance of DVM(Dalvik VM); Each application runs inside a DVM.
A DVM manages class loading unloading, instance lifecycle, GC etc.

Lifetime of a static variable: A static variable comes into existence when a class is loaded by the JVM and dies when the class is unloaded.

So if you create an android application and initialize a static variable, it will remain in the JVM until one of the following happens:
1. the class is unloaded
2. the JVM shuts down
3. the process dies

Note that the value of the static variable will persist when you switch to a different activity of another application and none of the above three happens. Should any of the above three happen the static will lose its value.

You can test this with a few lines of code:

1. print the uninitialized static in onCreate of your activity -> should print null
2. initialize the static. print it -> value would be non null
3. Hit the back button and go to home screen. Note: Home screen is another activity.
4. Launch your activity again -> the static variable will be non-null
5. Kill your application process from DDMS(stop button in the devices window).
6. Restart your activity -> the static will have null value.

Hope that helps.

Answer 2

Well, the Singleton pattern is also based on using static variables so actually you would be in the same position. While the static approach may work most of the times, it may happen that in some cases when memory is full and another activity takes the foreground before your application moves to its next screen, your activity's process could be killed and you lose the static values. However Android offers a few options of persisting values between states or transmitting them such as:

• using an Intent, you could pass along your search criteria from activity to activity (similar to a web http request)
• using application preferences, you could save the values and retrieve them in the activity that needs them
• using the sqlite database you can persist them in a table and retrieve them later
• if you need to just save activity state so that on restart, the fields get filled with their previously selected values, you can implement the onSaveInstanceState() activity method - note that this is not recommended for between activities persistance of states.

You can get some code examples of the usage of preferences, intents and the sqlite database by looking at the aegis-shield source code tree in google code or in other open source Android applications.