What is the best way of creating greenDAO DB conne

问题:

Currently I am creating the greenDAO DB connection in a class (which opens the connection in every static method) and using it wherever I need it. But I am not sure if it's the best way of doing it. Can anyone suggest a better way of doing it?

My Code:

import com.knowlarity.sr.db.dao.DaoMaster;
import com.knowlarity.sr.db.dao.DaoMaster.DevOpenHelper;
import com.knowlarity.sr.db.dao.DaoSession;
import com.knowlarity.sr.db.dao.IEntity;

public class DbUtils {

    private static Object lockCallRecord =new Object();
    private DbUtils(){};

    public static boolean saveEntity(Context context , IEntity entity){
            boolean t=false;
            DevOpenHelper helper=null;
            SQLiteDatabase db=null;
            DaoMaster daoMaster=null;
            DaoSession daoSession =null;
            try{
               helper = new DaoMaster.DevOpenHelper(context, IConstant.DB_STRING, null);
               db = helper.getReadableDatabase();
               daoMaster = new DaoMaster(db);
               daoSession = daoMaster.newSession();
               //Some business logic here for fetching and inserting the data.
            }catch (Exception e){
               Log.e("saveEntity", e.getStackTrace().toString());
            }finally{
               if(daoSession!=null)daoSession.clear();
               daoMaster=null;
               if(db.isOpen())db.close();
               helper.close();
            }
            return t;
    }

回答1:

Your approach causes the database to be loaded very often which is not necessary and may slow down your app significantly.

Open the database once and store it somewhere and request it from there if needed.

Personally I use a global DaoSession and local DaoSessions. The local DaoSessions get used where nothing should remain in the session cache (i.e. persisting a new object into the database, that is likely to be used only very infrequent or performing some queries which will load a lot of entities that are unlikely to be reused again).

Keep in mind that updating entities in a local DaoSession is a bad idea if you use the entity in your global session as well. If you do this the cached entity in your global session won't be updated and you will get wrong results unless you clear the cache of the global session!

Thus the safest way is to either just use one DaoSession or new DaoSessions all the time and to not use a global and local sessions!!!

A custom application class is a good place, but any other class will also be ok.

This is how I do it:

class DBHelper:

private SQLiteDatabase _db = null;
private DaoSession _session = null;

private DaoMaster getMaster() {
    if (_db == null) {
        _db = getDatabase(DB_NAME, false);
    }
    return new DaoMaster(_db);
}

public DaoSession getSession(boolean newSession) {
    if (newSession) {
        return getMaster().newSession();
    }
    if (_session == null) {
        _session = getMaster().newSession();
    }
    return _session;
}

private synchronized SQLiteDatabase getDatabase(String name, boolean readOnly) {
    String s = "getDB(" + name + ",readonly=" + (readOnly ? "true" : "false") + ")";
    try {
        readOnly = false;
        Log.i(TAG, s);
        SQLiteOpenHelper helper = new MyOpenHelper(context, name, null);
        if (readOnly) {
            return helper.getReadableDatabase();
        } else {
            return helper.getWritableDatabase();
        }
    } catch (Exception ex) {
        Log.e(TAG, s, ex);
        return null;
    } catch (Error err) {
        Log.e(TAG, s, err);
        return null;
    }
}

private class MyOpenHelper extends DaoMaster.OpenHelper {
    public MyOpenHelper(Context context, String name, SQLiteDatabase.CursorFactory factory) {
        super(context, name, factory);
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        Log.i(TAG, "Create DB-Schema (version "+Integer.toString(DaoMaster.SCHEMA_VERSION)+")");
        super.onCreate(db);
    }

    @Override
    public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
        Log.i(TAG, "Update DB-Schema to version: "+Integer.toString(oldVersion)+"->"+Integer.toString(newVersion));
        switch (oldVersion) {
            case 1:
                db.execSQL(SQL_UPGRADE_1To2);
            case 2:
                db.execSQL(SQL_UPGRADE_2To3);
                break;
            default:
                break;
        }
    }
}

In application class:

private static MyApplication _INSTANCE = null;

public static MyApplication getInstance() {
    return _INSTANCE;
}

@Override
public void onCreate() {
    _INSTANCE = this;
    // ...
}

private DBHelper _dbHelper = new DBHelper();

public static DaoSession getNewSession() {
    return getInstance()._dbHelper.getSession(true);
}

public static DaoSession getSession() {
    return getInstance()._dbHelper.getSession(false);
}

Of course you can also store the DaoMaster instead of the DB itself. This will reduce some small overhead.

I'm using a Singleton-like Application class and static methods to avoid casting the application (((MyApplication)getApplication())) every time I use some of the common methods (like accessing the DB).

回答2:

I would recommend to create your database in your Application class. Then you can create a method to return the DaoSession to get access to the database in other Activities.