Android's Application Object as a "Singleton"

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 참조: http://androidcookbook.com/Recipe.seam?recipeId=1218

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public class AndroidApplication extends Application 
{

     private static AndroidApplication sInstance;

     public static AndroidApplication getInstance() 
     {

         return sInstance;

     }

 

     @Override

     public void onCreate()
     {

         super.onCreate();

         sInstance = this;

         sInstance.initializeInstance();

     }

  

     protected void initializeInstance()

     {

         // do all you initialization here

     }

}

 

Also, don't forget to add the application declaration to your AndroidManifest.xml file.

<application android:icon="@drawable/app_icon" 
  android:label="@string/app_name"  
  android:name="com.company.abc.AbcApplication">

 

 

---

 

 

[Samples]

 

Step 1 - extend the Application class

import android.app.Application; 
import android.content.Context; 
/** 
 * This class is created automatically when the app launches. 
 * It is used to provide an application-level context for the SQLiteOpenHelper 
 */ 
public class ApplicationContext extends Application 
{ 
    private static ApplicationContext instance; 
    public ApplicationContext() 
    { 
        instance = this; 
    } 
    public static Context getContext() 
    { 
        return instance; 
    } 
} 

 

 

Step 2 - update the manifest so that this application class is used

<application android:name="ApplicationContext" 
             android:icon="@drawable/icon"  
             android:label="@string/app_name" 
             android:debuggable="true"> 

 

 

Step 3 - build the singleton SQLdataHelper into your app

public class SQLdataHelper 
{ 
    // for logging 
    private final String TAG = this.getClass().getSimpleName(); 
    // DATABASE 
    private static final String DATABASE_NAME = "my.db"; 
    private static final int DATABASE_VERSION = 1;//initial version 
    // TABLE NAMES 
    private static final String TABLE_NAME_A = "exampleOneTable"; 
    // MEMBER VARIABLES 
    private DatabaseHelper mDBhelper; 
    private SQLiteDatabase mDB; 
    // SINGLETON 
    private static final SQLdataHelper instance = new SQLdataHelper(); 
    private SQLdataHelper() 
    { 
        final DatabaseHelper dbHelper = new DatabaseHelper(ApplicationContext.getContext()); 
        // open the DB for read and write 
        mDB = dbHelper.getWritableDatabase(); 
    } 
    public static SQLdataHelper getInstance() 
    { 
        return instance; 
    } 
    /** 
     *  INSERT FUNCTIONS consisting of "synchronized" methods  
     */ 
    public synchronized long insertTableA(String myName, int myAge) 
    { 
        Long lValueToReturn; 
        // organize the data to store as key/value pairs 
        ContentValues kvPairs = new ContentValues(); 
        kvPairs.put("ColumnOne", myName); 
        kvPairs.put("ColumnTwo", myAge); 
        lValueToReturn = mDB.insert(TABLE_NAME_A, null, kvPairs); 
        return lValueToReturn; 
    } 
    private static class DatabaseHelper extends SQLiteOpenHelper 
    { 
        DatabaseHelper(Context context) 
        { 
            super(context, DATABASE_NAME, null, DATABASE_VERSION); 
        } 
        //this is called for first time db is created. 
        // put all CREATE TABLE here 
        @Override 
        public void onCreate(SQLiteDatabase db) 
        { 
            db.execSQL( "CREATE TABLE " 
                       + TABLE_NAME_A  
                       + " (" 
                       + "_id INTEGER PRIMARY KEY AUTOINCREMENT," 
                       + "ColumnOne TEXT," 
                       + "ColumnTwo TEXT" 
                       + ")" ); 
        } 
        // this is called when an existing user updates to a newer version of the app 
        // add CREATE TABLE and ALTER TABLE here 
        @Override 
        public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
        { 
            // update SQL DB with new columns depending on old version 
            // also add new tables 
            // NOTE: whatever is done here must also go into onCreate() so that new users 
            // get the correct db created 
            switch(oldVersion) 
            { 
            case 1: 
                  // EXAMPLE         
                  db.execSQL("ALTER TABLE " + TABLE_NAME_A + " ADD COLUMN ColumnThree 
                                       INTEGER;"); 
                  // don't use a break. for next case simply let them run together to update all the way to 
                  // latest version. This way, the case just represents a starting point to start updating. 
 
            case 2: 
                  // EXAMPLE         
                  db.execSQL("ALTER TABLE " + TABLE_NAME_A + " ADD COLUMN ColumnFour 
                                      INTEGER;"); 
            } 
            // this code drops the table and will create a fresh one. Note all data lost! 
            //          db.execSQL("DROP TABLE IF EXISTS " + TABLE_NAME_C); 
            //          onCreate(db); 
        } 
    } 
} 

 

I've only included one example insert operation.
Add more as you need them and simply make sure they are 'synchronized' methods.

 

 

Step 4 - use the SQLdataHelper in your activity

    SQLdataHelper mDataHelper = SQLdataHelper.getInstance(); 
    mDataHelper.insertTableA("Someone", 100);