Using the Application Class
Depending on what you're doing in your initialization you could consider creating a new class that extends Application
and moving your initialization code into an overridden onCreate
method within that class.
public class MyApplicationClass extends Application {
@Override
public void onCreate() {
super.onCreate();
// TODO Put your application initialization code here.
}
}
The onCreate
in the application class is only called when the entire application is created, so the Activity restarts on orientation or keyboard visibility changes won't trigger it.
It's good practice to expose the instance of this class as a singleton and exposing the application variables you're initializing using getters and setters.
NOTE: You'll need to specify the name of your new Application class in the manifest for it to be registered and used:
<application
android:name="com.you.yourapp.MyApplicationClass"
Reacting to Configuration Changes [UPDATE: this is deprecated since API 13; see the recommended alternative]
As a further alternative, you can have your application listen for events that would cause a restart – like orientation and keyboard visibility changes – and handle them within your Activity.
Start by adding the android:configChanges
node to your Activity's manifest node
<activity android:name=".MyActivity"
android:configChanges="orientation|keyboardHidden"
android:label="@string/app_name">
or for Android 3.2 (API level 13) and newer:
<activity android:name=".MyActivity"
android:configChanges="keyboardHidden|orientation|screenSize"
android:label="@string/app_name">
Then within the Activity override the onConfigurationChanged
method and call setContentView
to force the GUI layout to be re-done in the new orientation.
@Override
public void onConfigurationChanged(Configuration newConfig) {
super.onConfigurationChanged(newConfig);
setContentView(R.layout.myLayout);
}
After tracking down this problem as well, I found this note in the Android documentation:
http://source.android.com/source/using-eclipse.html
*Note: Eclipse sometimes likes to add an "import android.R" statement at the
top of your files that use resources,
especially when you ask Eclipse to
sort or otherwise manage imports. This
will cause your make to break. Look
out for these erroneous import
statements and delete them.*
While going through the Android sample tutorials, I would often use the Ctrl + Shift + O command to "Organize Imports" and generate any missing import statements. Sometimes this would generate the incorrect import statement which would hide the R.java
class that is automatically generated when you build.
Best Answer
Every Android application runs in its own process, with its own instance of the Dalvik virtual machine. Dalvik has been written so that a device can run multiple VMs efficiently.
The Dalvik VM executes files in the Dalvik Executable (
.dex
) format which is optimised for minimal memory footprint.The VM is register-based, and runs classes compiled by a Java language compiler that have been transformed into the
.dex
format by the includeddx
tool.Also have a look at What is... The Dalvik Virtual Machine for detailed description about DVM.