How to declare global variables in Android

Question

I am creating an application which requires login  I created the main and the login activity   In the main activity onCreate method I added the following condition   public void onCreate Bundle savedInstanceState        super onCreate savedInstanceState       setContentView R layout main                 loadSettings        if strSessionString    null                login                      The onActivityResult method which is executed when the login form terminates looks like this    Override public void onActivityResult int requestCode                               int resultCode                               Intent data        super onActivityResult requestCode  resultCode  data       switch requestCode                case SHOW SUBACTICITY LOGIN                         if resultCode    Activity RESULT OK                                 strSessionString   data getStringExtra Login SESSIONSTRING                   connectionAvailable   true                  strUsername   data getStringExtra Login USERNAME                                   The problem is the login form sometimes appears twice  the login   method is called twice  and also when the phone keyboard slides the login form appears again and I guess the problem is the variable strSessionString   Does anyone know how to set the variable global in order to avoid login form appearing after the user already successfully authenticates

User · Answer

The approach of subclassing has also been used by the BARACUS framework. From my point of view subclassing Application was intended to work with the lifecycles of Android; this is what any Application Container does. Instead of having globals then, I register beans to this context an let them beeing injected into any class manageable by the context. Every injected bean instance actually is a singleton.

See this example for details

Why do manual work if you can have so much more?

User · Answer

you can use Intents   Sqlite   or Shared Preferences   When it comes to the media storage  like documents   photos   and videos  you may create the new files instead

User · Answer

Just you need to define an application name like below which will work    lt application   android name  ApplicationName  android icon   drawable icon  gt   lt  application gt

User · Answer

class GlobaleVariableDemo extends Application        private String myGlobalState       public String getGlobalState         return myGlobalState            public void setGlobalState String s        myGlobalState   s           class Demo extends Activity     Override public void onCreate Bundle b               GlobaleVariableDemo appState     GlobaleVariableDemo getApplicationContext         String state   appState getGlobalState

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What about ensuring the collection of native memory with such global structures   Activities have an onPause onDestroy   method that s called upon destruction  but the Application class has no equivalents   What mechanism are recommended to ensure that global structures  especially those containing references to native memory  are garbage collected appropriately when the application is either killed or the task stack is put in the background

User · Answer

On activity result is called before on resume  So move you login check to on resume and your second login can be blocked once the secomd activity has returned a positive result  On resume is called every time so there is not worries of it not being called the first time

User · Answer

Like there was discussed above OS could kill the APPLICATION without any notification  there is no onDestroy event  so there is no way to save these global variables   SharedPreferences could be a solution EXCEPT you have COMPLEX STRUCTURED variables  in my case I had integer array to store the IDs that the user has already handled   The problem with the SharedPreferences is that it is hard to store and retrieve these structures each time the values needed   In my case I had a background SERVICE so I could move this variables to there and because the service has onDestroy event  I could save those values easily

User · Answer

If some variables are stored in sqlite and you must use them in most activities in your app  then Application maybe the best way to achieve it  Query the variables from database when application started and store them in a field  Then you can use these variables in your activities   So find the right way  and there is no best way

User · Answer

Create this subclass   public class MyApp extends Application     String foo      In the AndroidManifest xml add android name   Example   lt application android name   MyApp          android icon   drawable icon          android label   string app name  gt

User · Answer

You can have a static field to store this kind of state  Or put it to the resource Bundle and restore from there on onCreate Bundle savedInstanceState   Just make sure you entirely understand Android app managed lifecycle  e g  why login   gets called on keyboard orientation change

User · Answer

I wrote this answer back in  09 when Android was relatively new  and there were many not well established areas in Android development  I have added a long addendum at the bottom of this post  addressing some criticism  and detailing a philosophical disagreement I have with the use of Singletons rather than subclassing Application  Read it at your own risk   ORIGINAL ANSWER    The more general problem you are encountering is how to save state across several Activities and all parts of your application  A static variable  for instance  a singleton  is a common Java way of achieving this  I have found however  that a more elegant way in Android is to associate your state with the Application context   As you know  each Activity is also a Context  which is information about its execution environment in the broadest sense  Your application also has a context  and Android guarantees that it will exist as a single instance across your application   The way to do this is to create your own subclass of android app Application  and then specify that class in the application tag in your manifest  Now Android will automatically create an instance of that class and make it available for your entire application  You can access it from any context using the Context getApplicationContext   method  Activity also provides a method getApplication   which has the exact same effect   Following is an extremely simplified example  with caveats to follow   class MyApp extends Application      private String myState     public String getState        return myState        public void setState String s       myState   s         class Blah extends Activity       Override   public void onCreate Bundle b               MyApp appState     MyApp getApplicationContext         String state   appState getState                    This has essentially the same effect as using a static variable or singleton  but integrates quite well into the existing Android framework  Note that this will not work across processes  should your app be one of the rare ones that has multiple processes    Something to note from the example above  suppose we had instead done something like   class MyApp extends Application      private String myState      complicated and slow initialization        public String getState        return myState          Now this slow initialization  such as hitting disk  hitting network  anything blocking  etc  will be performed every time Application is instantiated  You may think  well  this is only once for the process and I ll have to pay the cost anyways  right   For instance  as Dianne Hackborn mentions below  it is entirely possible for your process to be instantiated -just- to handle a background broadcast event  If your broadcast processing has no need for this state you have potentially just done a whole series of complicated and slow operations for nothing  Lazy instantiation is the name of the game here  The following is a slightly more complicated way of using Application which makes more sense for anything but the simplest of uses   class MyApp extends Application      private MyStateManager myStateManager   new MyStateManager       public MyStateManager getStateManager        return myStateManager          class MyStateManager      MyStateManager            this should be fast           String getState            if necessary  perform blocking calls here           make sure to deal with any multithreading synchronicity issues                  return state         class Blah extends Activity       Override   public void onCreate Bundle b               MyStateManager stateManager     MyApp getApplicationContext    getStateManager        String state   stateManager getState                    While I prefer Application subclassing to using singletons here as the more elegant solution  I would rather developers use singletons if really necessary over not thinking at all through the performance and multithreading implications of associating state with the Application subclass   NOTE 1  Also as anticafe commented  in order to correctly tie your Application override to your application a  tag is necessary in the manifest file  Again  see the Android docs for more info  An example    lt application      android name  my application MyApp        android icon            android label       gt   lt  application gt    NOTE 2  user608578 asks below how this works with managing native object lifecycles  I am not up to speed on using native code with Android in the slightest  and I am not qualified to answer how that would interact with my solution  If someone does have an answer to this  I am willing to credit them and put the information in this post for maximum visibility   ADDENDUM   As some people have noted  this is not a solution for persistent state  something I perhaps should have emphasized more in the original answer  I e  this is not meant to be a solution for saving user or other information that is meant to be persisted across application lifetimes  Thus  I consider most criticism below related to Applications being killed at any time  etc     moot  as anything that ever needed to be persisted to disk should not be stored through an Application subclass  It is meant to be a solution for storing temporary  easily re-creatable application state  whether a user is logged in for example  and components which are single instance  application network manager for example   NOT singleton   in nature   Dayerman has been kind enough to point out an interesting conversation with Reto Meier and Dianne Hackborn in which use of Application subclasses is discouraged in favor of Singleton patterns  Somatik also pointed out something of this nature earlier  although I didn t see it at the time  Because of Reto and Dianne s roles in maintaining the Android platform  I cannot in good faith recommend ignoring their advice  What they say  goes  I do wish to disagree with the opinions  expressed with regards to preferring Singleton over Application subclasses  In my disagreement I will be making use of concepts best explained in this StackExchange explanation of the Singleton design pattern  so that I do not have to define terms in this answer  I highly encourage skimming the link before continuing  Point by point   Dianne states   There is no reason to subclass from Application  It is no different than making a singleton     This first claim is incorrect  There are two main reasons for this  1  The Application class provides a better lifetime guarantee for an application developer  it is guaranteed to have the lifetime of the application  A singleton is not EXPLICITLY tied to the lifetime of the application  although it is effectively   This may be a non-issue for your average application developer  but I would argue this is exactly the type of contract the Android API should be offering  and it provides much more flexibility to the Android system as well  by minimizing the lifetime of associated data  2  The Application class provides the application developer with a single instance holder for state  which is very different from a Singleton holder of state  For a list of the differences  see the Singleton explanation link above   Dianne continues      just likely to be something you regret in the future as you find your Application object becoming this big tangled mess of what should be independent application logic   This is certainly not incorrect  but this is not a reason for choosing Singleton over Application subclass  None of Diane s arguments provide a reason that using a Singleton is better than an Application subclass  all she attempts to establish is that using a Singleton is no worse than an Application subclass  which I believe is false   She continues   And this leads more naturally to how you should be managing these things -- initializing them on demand   This ignores the fact that there is no reason you cannot initialize on demand using an Application subclass as well  Again there is no difference   Dianne ends with  The framework itself has tons and tons of singletons for all the little shared data it maintains for the app  such as caches of loaded resources  pools of objects  etc  It works great   I am not arguing that using Singletons cannot work fine or are not a legitimate alternative  I am arguing that Singletons do not provide as strong a contract with the Android system as using an Application subclass  and further that using Singletons generally points to inflexible design  which is not easily modified  and leads to many problems down the road  IMHO  the strong contract the Android API offers to developer applications is one of the most appealing and pleasing aspects of programming with Android  and helped lead to early developer adoption which drove the Android platform to the success it has today  Suggesting using Singletons is implicitly moving away from a strong API contract  and in my opinion  weakens the Android framework   Dianne has commented below as well  mentioning an additional downside to using Application subclasses  they may encourage or make it easier to write less performance code  This is very true  and I have edited this answer to emphasize the importance of considering perf here  and taking the correct approach if you re using Application subclassing  As Dianne states  it is important to remember that your Application class will be instantiated every time your process is loaded  could be multiple times at once if your application runs in multiple processes   even if the process is only being loaded for a background broadcast event  It is therefore important to use the Application class more as a repository for pointers to shared components of your application rather than as a place to do any processing   I leave you with the following list of downsides to Singletons  as stolen from the earlier StackExchange link    Inability to use abstract or interface classes  Inability to subclass  High coupling across the application  difficult to modify   Difficult to test  can t fake mock in unit tests   Difficult to parallelize in the case of mutable state  requires extensive locking     and add my own    Unclear and unmanageable lifetime contract unsuited for Android  or most other  development

User · Answer

I couldn t find how to specify the application tag either  but after a lot of Googling  it became obvious from the manifest file docs  use android name  in addition to the default icon and label in the application stanza   android name The fully qualified name of an Application subclass implemented for the application  When the application process is started  this class is instantiated before any of the application s components   The subclass is optional  most applications won t need one  In the absence of a subclass  Android uses an instance of the base Application class

User · Answer

You could create a class that extends Application class and then declare your variable as a field of that class and providing getter method for it   public class MyApplication extends Application       private String str    My String        synchronized public String getMyString           return str            And then to access that variable in your Activity  use this   MyApplication application    MyApplication  getApplication    String myVar   application getMyString

User · Answer

You can do this using two approaches    Using Application class Using Shared Preferences Using Application class   Example   class SessionManager extends Application     String sessionKey     setSessionKey String key       this sessionKey key         String getSessisonKey        return this sessionKey          You can use above class to implement login in your MainActivity as below  Code will look something like this    override  public void onCreate  Bundle savedInstanceState        you will this key when first time login is successful    SessionManager session   SessionManager getApplicationContext      String key getSessisonKey getKey        Use this key to identify whether session is alive or not      This method will work for temporary storage  You really do not any idea when operating system is gonna kill the application  because of low memory  When your application is in background and user is navigating through other application which demands  more memory to run  then your application will be killed since operating system given more priority to foreground processes than background  Hence your application object will be null before user logs out  Hence for this I recommend to use second method Specified above    Using shared preferences   String MYPREF  com your application session   SharedPreferences pref  context getSharedPreferences MyPREF MODE PRIVATE      Insert key as below   Editot editor  pref edit     editor putString  key   value     editor commit       Get key as below   SharedPreferences sharedPref   getActivity   getPreferences Context MODE PRIVATE    String key  getResources   getString  key

User · Answer

DO N T  Use  another  lt application gt  tag in manifest file Just do one change in existing  lt application gt  tag   add this line android name   ApplicationName  where  ApplicationName will be name of your subclass use to store global  that  you is about to create   so  finally your ONE AND ONLY  lt application gt  tag in manifest file should look like this  -   lt application         android allowBackup  true          android icon   mipmap ic launcher          android label   string app name          android theme   style Theme AppCompat NoActionBar          android name   ApplicationName           gt

User · Answer

Just a note     add   android name   Globals    or whatever you named your subclass to the existing  lt application gt  tag   I kept trying to add another  lt application gt  tag to the manifest and would get an exception

User · Answer

The suggested by Soonil way of keeping a state for the application is good  however it has one weak point - there are cases when OS kills the entire application process  Here is the documentation on this - Processes and lifecycles   Consider a case - your app goes into the background because somebody is calling you  Phone app is in the foreground now   In this case  amp  amp  under some other conditions  check the above link for what they could be  the OS may kill your application process  including the Application subclass instance  As a result the state is lost  When you later return to the application  then the OS will restore its activity stack and Application subclass instance  however the myState field will be null   AFAIK  the only way to guarantee state safety is to use any sort of persisting the state  e g  using a private for the application file or SharedPrefernces  it eventually uses a private for the application file in the internal filesystem

[android] How to declare global variables in Android?

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