Android: When do classes get unloaded by the system?

This is a very weird problem. My app that runs just fine but somehow if I leave my phone for an hour or two while my app is running, I get the following error when I come back to it later:

java.lang.NoClassDefFoundError: yoga.database.Manager
at
yoga.YogaActivity.openDatabase(YogaActivity.java:294)
at
yoga.YogaActivity.initData(YogaActivity.java:275)
at
yoga.YogaActivity.onCreate(YogaActivity.java:102)
at
android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1123)
at
android.app.ActivityThread.perform开发者_运维知识库LaunchActivity(ActivityThread.java:2364)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2417)
at android.app.ActivityThread.access$2100(ActivityThread.java:116)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1794)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:123)
at android.app.ActivityThread.main(ActivityThread.java:4203)
at java.lang.reflect.Method.invokeNative(Native Method)
at java.lang.reflect.Method.invoke(Method.java:521)
at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:791)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:549)
at dalvik.system.NativeStart.main(Native Method)

I know for a fact that my yoga.database.Manager class was loaded at the time when the app was launched, but somehow the class must have been unloaded by the system as I left the phone.

Does Android unload classes automatically after certain periods? What can I do when my class gets unloaded?

The above error causes my app to crash, but I can easily re-launch it and it runs just fine.

The problem occurs mostly on my HTC Magic phone running Android 1.6.

The Dalvik VM doesn't currently unload classes. If it did, it would only be able to do so when all classes associated with a particular class loader could be unloaded at once, which will not be the case while your app is running.

You need to check the logcat output for errors leading up to this exception. One way to get a NoClassDefFoundError is for something to fail during initialization of the class in question; if that happened there would likely be a trail in the log.

(Of course, by now the logcat output is probably long gone, but if the problem is repeatable you'll want to capture it next time.)

Are you playing games with custom classloaders?

I ask, because if yoga.YogaActivity and yoga.database.Manager are standard Java classes in the same Android APK file, you should not have one in memory and the other not. I have never seen Android unload classes on a one-off basis -- leastways, I have never run into this particular problem before.

Also:

I leave my phone for an hour or two while my app is running

You might want to clarify what you mean by this. Do you mean that you leave your activity in the foreground? Do you mean that you press HOME and then come back to it hours later? Do you mean something else?