SQLite Connection leaked although everything close

问题:

I found many stuff like close the connection and close the cursor, but I do all this stuff. Still the SQLite connection leaks and I get a warning like this:

A SQLiteConnection object for database was leaked!

I have a database manager this, which I call in my activities with the following code:

DatabaseManager dbm = new DatabaseManager(this);

The code of my database manager class follows now:

public class DatabaseManager {

    private static final int DATABASE_VERSION = 9;
    private static final String DATABASE_NAME = "MyApp";
    private Context context = null;
    private DatabaseHelper dbHelper = null;
    private SQLiteDatabase db = null;


    public static class DatabaseHelper extends SQLiteOpenHelper {

         public DatabaseHelper(Context context) {
             super(context, DATABASE_NAME, null, DATABASE_VERSION);
         }

         @Override
         public void onCreate(SQLiteDatabase db) {

                   //create database tables
         }

         @Override
         public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
                      //destroy and recreate them
         }

     }

     public DatabaseManager(Context ctx) {
         this.context = ctx;
     }

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

        if (!db.isReadOnly()) {
            db.execSQL("PRAGMA foreign_keys = ON;");
        }

        return this;
    }

    private void close() {
        dbHelper.close();
    }
}

When I call a database method, I do the following thing:

public Object getData() {

    open();

            //... database operations take place ...

    close();

    return data;
}

But as I said, I still get this SQLite connection leaked warning.

What am I doing wrong?

回答1:

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);
  }
}

回答2:

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
        }
    }