传递上下文并在服务中使用SQL

Question

I want to write some SQL to a database, however when executing it I get a nullpointer error. I think I do something wrong with passing the Context from WhatsApi.java to MessageService.java . (See the Context.MODE_PRIVATE )

This part from MessageService.java

db = openOrCreateDatabase("msgstore", Context.MODE_PRIVATE, null);
            db.execSQL("CREATE TABLE IF NOT EXISTS messages(`from` TEXT, `to` TEXT, message TEXT, id TEXT, t TEXT);");

This in WhatsApi.java

MessageService msg = new MessageService();
            msg.saveMessage(mContext, "mynumber", to, message, id, time());

I suggest you guys to see this link too:

https://github.com/gi097/WhatsApi-Android/commit/3875d97458095f792c73f275648629aaaf726751

Any help is appreciated. Sorry for the maybe unclear look, but I hope you guys understand.

Answer 1

Well I use this in one of my Projects DatabaseHandler dbHandler=null;//Database handler class//global var SQLiteDatabase contactDatabase=null;// globalvar . . //some function{ if(dbHandler==null) dbHandler = new DatabaseHandler(this);//service context if(contactDatabase==null) contactDatabase=dbHandler.getWritableDatabase(); ArrayList<information>list= new ArrayList<information>(); String selectQuery = "SELECT * FROM " + DatabaseHandler.TABLE_CONTACTS; Cursor cursor = contactDatabase.rawQuery(selectQuery, null); DatabaseHandler dbHandler=null;//Database handler class//global var SQLiteDatabase contactDatabase=null;// globalvar . . //some function{ if(dbHandler==null) dbHandler = new DatabaseHandler(this);//service context if(contactDatabase==null) contactDatabase=dbHandler.getWritableDatabase(); ArrayList<information>list= new ArrayList<information>(); String selectQuery = "SELECT * FROM " + DatabaseHandler.TABLE_CONTACTS; Cursor cursor = contactDatabase.rawQuery(selectQuery, null);

Answer 2

You should never instantiate an Activity or Service yourself with the new keyword. It is Android's responsibility to start these components because Android needs to also do some additional setup, including provide a base Context for these components. That is why you get a NullPointerException in your service class. You have to actually use startService() to make the service run properly.

In my opinion, since WhatsApi already has a Context, I don't see any reason you need to bother having the service save the data. Just open the database and save the data in WhatsApi .

By the way, since you're using SQLite, you really should be using SQLiteOpenHelper instead of calling methods like openOrCreateDatabase() directly. See this guide for more information.