SQLite Connection leaked although everything closed

The bolded font in the citation corresponds to this part in your code:

private DatabaseManager open() throws SQLException {
    dbHelper = new DatabaseHelper(context);
    db = dbHelper.getWritableDatabase();

from: http://www.androiddesignpatterns.com/2012/05/correctly-managing-your-sqlite-database.html

Approach #1: Use an Abstract Factory to Instantiate the SQLiteOpenHelper

Declare your database helper as a static instance variable and use the Abstract Factory pattern to guarantee the singleton property. The sample code below should give you a good idea on how to go about designing the DatabaseHelper class correctly.

The static factory getInstance method ensures that only one DatabaseHelper will ever exist at any given time. If the mInstance object has not been initialized, one will be created. If one has already been created then it will simply be returned.

You should not initialize your helper object using with new DatabaseHelper(context).
Instead, always use DatabaseHelper.getInstance(context), as it guarantees that only one database helper will exist across the entire application's lifecycle.

public static class DatabaseHelper extends SQLiteOpenHelper { 

  private static DatabaseHelper mInstance = null;

  private static final String DATABASE_NAME = "database_name";
  private static final String DATABASE_TABLE = "table_name";
  private static final int DATABASE_VERSION = 1;

  public static DatabaseHelper getInstance(Context ctx) {

    // Use the application context, which will ensure that you 
    // don't accidentally leak an Activity's context.
    // See this article for more information: http://bit.ly/6LRzfx
    if (mInstance == null) {
      mInstance = new DatabaseHelper(ctx.getApplicationContext());
    }
    return mInstance;
  }

  /**
   * Constructor should be private to prevent direct instantiation.
   * make call to static factory method "getInstance()" instead.
   */
  private DatabaseHelper(Context ctx) {
    super(ctx, DATABASE_NAME, null, DATABASE_VERSION);
  }
}

private void method() {
        Cursor cursor = query();
        if (flag == false) {  // WRONG: return before close()
            return;
        }
        cursor.close();
   }

Good practice should be like this:

    private void method() {
        Cursor cursor = null;
        try {
            cursor = query();
        } finally {
            if (cursor != null)
                cursor.close();  // RIGHT: ensure resource is always recovered
        }
    }

The complete example of the above-accepted answer: It may help someone.

Helper Class:

public class DatabaseHelper extends SQLiteOpenHelper {

private static final String DATABASE_NAME = "sample.db";
private static final int DATABASE_VERSION = 1;

private static DatabaseHelper mInstance;

private DatabaseHelper(@Nullable Context context) {
    super(context, DATABASE_NAME, null, DATABASE_VERSION);
}

public static synchronized DatabaseHelper getInstance(Context context) {

    if (mInstance == null) {
        mInstance = new DatabaseHelper(context.getApplicationContext());
    }
    return mInstance;
}

@Override
public void onCreate(SQLiteDatabase db) {

    // create table stuff


}

@Override
public void onUpgrade(SQLiteDatabase db, int i, int i1) {

 // drop table stuff

    onCreate(db);
 }
}

Activity:

SQLiteDatabase database = DatabaseHelper.getInstance(getApplicationContext()).getWritableDatabase();

Cursor cursor = database.query("query");

if (cursor != null) {
   while (cursor.moveToNext()) {
    // stuff
     }
   cursor.close();
   database.close();
 }