I have a class in my Android application that sub-classes the AndroidApplication object. The documents say:
public void onCreate ()
Since: API Level 1 Called when the
application is starting, before any
other application objects have been
created. Implementations should be as
quick as possible (for example using
lazy initialization of state) since
the time spent in this function
directly impacts the performance of
starting the first activity, service,
or receiver in a process. If you
override this method, be sure to
call super.onCreate().
I placed a breakpoint on my sub-class's constructor and when I run my application, it is never reached. Naturally, when I call the sub-class's getInstance() method from other code it returns NULL since the instance variable is (supposed to be) initialized when the constructor is called.
Can anyone tell me what is wrong? I would assume from the docs that I don't have to create an instance of the AndroidApplication sub-class myself, or do I? Am I supposed to modify my manifest file somehow to add the AndroidApplication sub-class and if so, how?
-- roschler
I'm posting the answer here for others. Yes you need to add the name of your Application object sub-class's name to the Android manifest. For Eclipse users, the easiest way to do this is to open the AndroidManifest.xml file, select the Application tab in the manifest editor, and use the Browse button next to the Name field to find your Android Application object sub-class name and select it. The manifest file will be updated properly to register it. I just did that and it worked.
I had a problem of not having a . before my application class name.
Should be:
android:name=".MyApp"
since the MyApp class is in the package defined in the manifest.
Related
In some cases I can see that Activity.onCreate is called before the Appication object gets created (before Application.onCreate is called). Is that ever possible?
May be you forgot to add your application class in manifest file.
Place your application class in AndroidManifest.xml class under <application> tag.
i.e.,
<application
android:name=".{YourApplicationClassName}"
...
...
In some cases I can see that Activity.onCreate is called before the
Appication object gets created (before Application.onCreate is
called).
This is not what Android document says about the Application class. As per the official android documents,
The Application class, or your subclass of the Application class, is
instantiated before any other class when the process for your
application/package is created.
Also below is specific explanation of onCreate() of an Application class
Called when the application is starting, before any activity, service,
or receiver objects (excluding content providers) have been created.
Hence the onCreate() of Application has to be invoked 1st and then onCreate() of Activity class
So the scenario you have mentioned is not possible as per the flow of instantiation of Application class and Activity class
In the case you are using logging to determine when application is created, check if you are using external logging system, like Timber which is usually instantiated in the end of application onCreate(). So it may appear that nothing before this system is instantiated when it is called.
Try to instantiate logging tool before super.onCreate() in application onCreate().
I have a project (in Eclipse) which I've turned into an Android Project Library so as to re-use some of the code in another similar project. I think I've shot myself in the foot however as I'm getting the error:
Unable to start activity ComponentInfo{com.test.scroller1/com.lib.scrolltest.ScrollTestActivity}: java.lang.ClassCastException: android.app.Application cannot be cast to com.lib.scrolltest.resAppVars
com.lib.scrolltest is my Project Library which instantiates a class extending Application (resAppVars). In the onCreate() method I call:
mRav = (resAppVars) getApplicationContext ();
This way, I can use methods in the mRav object which would otherwise be a lot of duplicated code in other classes (such as passing a query to a generic select statement which returns an ArrayList of results).
What's the problem here? It seems I've hit a limitation in the way I've implemented the Application class.
Calling getApplicationContext() returns the Application object for the current application (i.e. the application that owns the activity that onCreate() is running inside of).
Unless you're doing something strange, you don't get to pick which Application class is used. There's even a note in the documentation for Application saying not to do this:
There is normally no need to subclass Application. In most situation,
static singletons can provide the same functionality in a more modular
way. If your singleton needs a global context (for example to register
broadcast receivers), the function to retrieve it can be given a
Context which internally uses Context.getApplicationContext() when
first constructing the singleton.
You should just create a regular shared class inside of your library project. Or if you don't have a need for the special functionality library projects offer, you can also just use a regular .jar file.
And if you need shared state, just make it a singleton. ;)
Although this is a very old post. I encountered the same problem and solved it. So I thought I'll post the solution for everyone.
It turns out that I forgot to declare the subclassed application name in the manifest file.
The android:name should point to the extended app class, even if it is defined in the referenced library.
<application
android:allowBackup="true"
android:icon="#drawable/ic_launcher"
android:label="#string/app_name"
android:theme="#style/AppTheme"
android:name="com.example.lib.MyApp">
After I added that I could use the extended app with (<cast>) getApplication() anywhere in my project.
I'm new to this and I'm sorry if this is a really dumb question. I'm just trying to clarify things. My book says I can retrieve application context for process by using the getApplicationContext() method. I just really don't know where to type this or what to do with any of it. I can go to the hierarchy but what do I do with all the script there. Also where would I write Activity Callbacks, in the main.xml? An exercise wants me to add a logging tag to my project but I'm not sure how to do this. The exact text says:
"Within the onCreate() callback method, add an informational logging message, using the Log.i() method."
and another exercise says to:
"Implement some of the Activity callback methods in addition to onCreate(), such as onStart(). Add a log message to each callback method and then run the application normally".
As these seem like basic questions, can someone please help me.
I am using the Android SDK, and Eclipse. I have made the Hello World application, but I have no idea what to do with Context or Retrieving resources. Please help!
The first rule I would give you: if you don't know why you need it, you probably don't need it. Use your activity object as the Context when you need a context.
The callbacks you talk about are on the Activity class. The Application Fundamentals describes what an Activity is: http://developer.android.com/guide/topics/fundamentals.html#Components
The only time you want to use getApplicationContext() is when you need a Context that exists outside of the lifecycle of an Activity class (or other component). You'll want to find documentation on specific cases where this is desired, there is a lot floating around. For example this one is part of the Android documentation: http://android-developers.blogspot.de/2009/01/avoiding-memory-leaks.html
For the tasks you're working with here, you'll be using the Java code that defines the behavior of the application, not the XML files that define resources and layouts or the AndroidManifest.xml file that declares basic application properties.
If you're working with Hour 3 of the Sam's Teach Yourself... book, then you need to open the src\com.androidbook.droid1\DroidActivity.java file. In general, you would need src\<package-name>\<class-name>.java. When you open that file, you'll see a class (in this case, DroidActivity) that extends Activity and already has the onCreate() callback method. Anything that you want to happen during onCreate() goes inside that method. Other callback methods can be added inside the activity class. To see an example that has all the lifecycle callbacks (but doesn't do anything in them), look here.
A logging tag is just a string. You can declare it, for example, as a private static final String inside the activity class.
If there's confusion about where methods belong, where and how to define variables or constants, how to call methods, how to use classes, and so forth, then it might be best to go through an introductory Java text before starting with Android. There are plenty of free resources available for that.
I need to start android application not with Activity^ but with some controller class that will start some activity
Is it possible?
I'm not sure if I understand your question correctly, but an Android application is built up by four "components" as mentioned in the "Android Application Fundamentals", http://developer.android.com/guide/topics/fundamentals.html (no, you don't need all four of them make your application work).
The most common way of starting an application (and actually the only one I've been in touch with) is to define an Activity in your applications AndroidManifest.xml file as described on the link above. NOTE! that an Activity doesn't have to define a UI; you are not obligated to call the "setContentView()" function. Hence your "controller class" can extend Activity and be the very Activity you define as the start-up Activity in your manifest xml. You can then call "startActivity()" function with parameters to start any other Activity, whenever you see fit, from your controller class (this is also described in the link above).
Hope this helps.
Either create a GUI-less activity without calling setContentView() or use a BroadcastReceiver that accepts launcher intents (action=MAIN, cateogry=LAUNCHER). In Activity.onCreate or receivers callback method you can place logic which will invoke the actual activity of choice.
I'm running the following line in an Activity, which is within the same application, but in a different package:
AppObject appObj = (AppObject)this.getApplication();
// FYI: AppObject is my extension class of Application.
It returns only a null pointer, while when I move it to the "main" package and run it from there it returns the application reference as expected.
I've defined the activity in my AndroidManifest.xml with the full qualified class name, since it is in another package: <activity android:name="com.foo.bar.TestActivity"></activity>
Update: As suggested in a question below android:name="AppObject" was already in the <application> tag of the AndroidManifest.xml
It's important to call getApplication() in the activity's onCreate() method, not in the constructor.
You need the update application tag to AndroidManifest.xml with your class name, which is extended from Application, with proper package name.
<application android:name=".AppObject">
As per Application tag google docs, Base class for those who need to maintain global application state. You can provide your own implementation by specifying its name in your AndroidManifest.xml's tag, which will cause that class to be instantiated for you when the process for your application/package is created.
here
Just run into the same thing, having refactored all my code still got the same issue, noticed that I was setting the local mApplication variable in the constructor, it should go in the onCreate(), I think all the objects in the manifest may be constructed first before getApplication() is setup so you need to call getApplication() in or after onCreate() has been called. Haven't refactored all my code back again to see if this works for different packages (sigh).
I think, it isn't a null pointer, but your function which you want to use next in the class AppObject maybe wrong.