Using Application context everywhere

Question

In an Android app  is there anything wrong with the following approach   public class MyApp extends android app Application        private static MyApp instance       public MyApp             instance   this             public static Context getContext             return instance             and pass it everywhere  e g  SQLiteOpenHelper  where context is required  and not leaking of course

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Application Class   import android app Application  import android content Context   public class MyApplication extends Application        private static Context mContext       public void onCreate             super onCreate            mContext   getApplicationContext               public static Context getAppContext             return mContext             Declare the Application in the AndroidManifest    lt application android name   MyApplication            gt    Usage   MyApplication getAppContext

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I m using the same approach  I suggest to write the singleton a little better   public static MyApp getInstance          if  instance    null            synchronized  MyApp class                if  instance    null                    instance   new MyApp                                        return instance      but I m not using everywhere  I use getContext   and getApplicationContext   where I can do it

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It is a good approach  I use it myself as well  I would only suggest to override onCreate to set the singleton instead of using a constructor   And since you mentioned SQLiteOpenHelper  In onCreate    you can open the database as well   Personally I think the documentation got it wrong in saying that There is normally no need to subclass Application  I think the opposite is true  You should always subclass Application

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You are trying to create a wrapper to get Application Context and there is a possibility that it might return  null  pointer   As per my understanding  I guess its better approach to call- any of the 2 Context getApplicationContext    or Activity getApplication

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Some people have asked  how can the singleton return a null pointer  I m answering that question   I cannot answer in a comment because I need to post code    It may return null in between two events   1  the class is loaded  and  2  the object of this class is created  Here s an example   class X       static X xinstance      static Y yinstance   Y yinstance      X    xinstance this     class Y       static X xinstance   X xinstance      static Y yinstance      Y    yinstance this      public class A       public static void main String   p        X x   new X        Y y   new Y        System out println  x   X xinstance   y   Y yinstance       System out println  x   Y xinstance   y   X yinstance             Let s run the code     javac A java    java A x X a63599 y Y 9036e x null y null   The second line shows that Y xinstance and X yinstance are null  they are null because the variables X xinstance ans Y yinstance were read when they were null   Can this be fixed  Yes   class X       static Y y   Y getInstance        static X theinstance      static X getInstance    if theinstance  null   theinstance   new X     return theinstance     class Y       static X x   X getInstance        static Y theinstance      static Y getInstance    if theinstance  null   theinstance   new Y     return theinstance      public class A       public static void main String   p        System out println  x   X getInstance     y   Y getInstance         System out println  x   Y x   y   X y             and this code shows no anomaly     javac A java    java A x X 1c059f6 y Y 152506e x X 1c059f6 y Y 152506e   BUT this is not an option for the Android Application object  the programmer does not control the time when it is created   Once again  the difference between the first example and the second one is that the second example creates an instance if the static pointer is null  But a programmer cannot create the Android application object before the system decides to do it   UPDATE  One more puzzling example where initialized static fields happen to be null   Main java   enum MyEnum       FIRST SECOND      private static String prefix   lt    suffix   gt        String myName      MyEnum             myName   makeMyName              String makeMyName             return prefix   name     suffix            String getMyName             return myName          public class Main       public static void main String args              System out println  first    MyEnum FIRST   second    MyEnum SECOND           System out println  first    MyEnum FIRST makeMyName     second    MyEnum SECOND makeMyName             System out println  first    MyEnum FIRST getMyName     second    MyEnum SECOND getMyName               And you get     javac Main java   java Main first  FIRST second  SECOND first   lt FIRST gt  second   lt SECOND gt  first  nullFIRSTnull second  nullSECONDnull   Note that you cannot move the static variable declaration one line upper  the code will not compile

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There are a couple of potential problems with this approach  though in a lot of circumstances  such as your example  it will work well   In particular you should be careful when dealing with anything that deals with the GUI that requires a Context  For example  if you pass the application Context into the LayoutInflater you will get an Exception  Generally speaking  your approach is excellent  it s good practice to use an Activity s Context within that Activity  and the Application Context when passing a context beyond the scope of an Activity to avoid memory leaks   Also  as an alternative to your pattern you can use the shortcut of calling getApplicationContext   on a Context object  such as an Activity  to get the Application Context

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In my experience this approach shouldn t be necessary   If you need the context for anything you can usually get it via a call to View getContext   and using the Context obtained there you can call Context getApplicationContext   to get the Application context   If you are trying to get the Application context this from an Activity you can always call Activity getApplication   which should be able to be passed as the Context needed for a call to SQLiteOpenHelper     Overall there doesn t seem to be a problem with your approach for this situation  but when dealing with Context just make sure you are not leaking memory anywhere as described on the official Google Android Developers blog

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I would use Application Context to get a System Service in the constructor  This eases testing  amp  benefits from composition  public class MyActivity extends Activity        private final NotificationManager notificationManager       public MyActivity            this MyApp getContext   getSystemService NOTIFICATION SERVICE               public MyActivity NotificationManager notificationManager           this notificationManager   notificationManager                onCreate etc      Test class would then use the overloaded constructor   Android would use the default constructor

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I like it  but I would suggest a singleton instead   package com mobidrone   import android app Application  import android content Context   public class ApplicationContext extends Application       private static ApplicationContext instance   null       private ApplicationContext                 instance   this             public static Context getInstance                 if  null    instance                        instance   new ApplicationContext                       return instance

[android] Using Application context everywhere?

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