问题
so I want to make an app that lists foods that I have stored in an already existing SQLite database I created. I have my foodDB.db file in the /assets/databases folder.
I know I must use a DatabaseHelper class. I have done my fair share of research, I have looked at plenty of stackoverflow posts and youtube tutorials but none satisfy my critera of populating a ListView from an existing database in the asset folder. I find them very confusing and not clear.
Could someone please clearly explain to me how I would go about this? I am really struggling and would appreciate some guidance.
My database is called foodDatabase.db.
My database contains 1 table called dataset.
The table dataset contains the colums: id, name, description, protein, fat, carbohydrates, energy, starch, sugar, cholesterol.
回答1:
First a few points to be noted.
1 - you have said that the file is FoodDB.db but then said that the database is called foodDatabase.db. The database name is the file name. As such the following example uses FoodDB.db as the file that is in the assets (you could rename the file when copying from the assets but I've not done that in this answer.)
2 - You do not need a Database Helper and the example doesn't use one.
The are two parts :-
1) Accessing the database to extract the data for the ListView. In the following example this is copied from the assets to the standard database location (/data/data/the_package/databases/the_database_name).
- The assumption has been made that once copied from the assets the database will then subsequently be used (i.e it is copied once for the lifetime of the App).
2) Displaying the extracted data (obtained as a Cursor) in a ListView.
To do 2 you will need the layout to include a ListView, as such the following layout has been used. activity_main.xml :-
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="fooddb.so49328656populatelistview.MainActivity">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Hello World!"
/>
<ListView
android:id="@+id/foodlist"
android:layout_width="match_parent"
android:layout_height="wrap_content">
</ListView>
</LinearLayout>
- Note
tools:context="fooddb.so49328656populatelistview.MainActivity"would have to reflect YOUR package
The Database Helper - Not used as warned
The invoking activity MainActivity.java (see notes) :-
public class MainActivity extends AppCompatActivity {
static final String DBNAME = "FoodDB.db";
static final String DBASSETPATH = "databases/" + DBNAME;
static final String FOODTABLE = "dataset";
static final String FOODCOLUMN = "Food";
static final String IDCOLUMN = "ID";
ListView mFoodList;
SQLiteDatabase mDB;
SimpleCursorAdapter mSCA;
Cursor mCsr;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mFoodList = (ListView) this.findViewById(R.id.foodlist);
mDB = openFoodDB();
if (mDB != null) {
mCsr = mDB.query(FOODTABLE,
new String[]{IDCOLUMN + " AS _id",
FOODCOLUMN
},
null,null,null,null,null);
mSCA = new SimpleCursorAdapter(this,android.R.layout.simple_list_item_1,mCsr,
new String[]{FOODCOLUMN},
new int[]{android.R.id.text1},0);
mFoodList.setAdapter(mSCA);
} else {
Toast.makeText(this,"Unable to open Database.",Toast.LENGTH_LONG);
}
}
private SQLiteDatabase openFoodDB() {
String dbpath = this.getDatabasePath(DBNAME).getPath();
if (this.getDatabasePath(DBNAME).exists()) {
Log.d("OPENFOODDB","Opening already existing Database");
return SQLiteDatabase.openDatabase(dbpath,null,SQLiteDatabase.OPEN_READWRITE);
}
InputStream is;
byte[] buffer;
FileOutputStream db;
try {
is = this.getAssets().open(DBASSETPATH);
buffer = new byte[is.available()];
is.read(buffer);
is.close();
} catch (Exception e) {
e.printStackTrace();
Log.d("OPENFOODDB","Unable to locate or buffer input from assets " + DBASSETPATH);
return null;
}
// Just in case the databases directory doesn't exist create it.
File dbmkdir = (this.getDatabasePath(DBNAME)).getParentFile();
dbmkdir.mkdirs();
try {
db = new FileOutputStream(this.getDatabasePath(DBNAME).getPath());
} catch (Exception e) {
e.printStackTrace();
Log.d("OPENFOODDB","Unable to create outputstream for DB at path " + dbpath);
try {
is.close();
} catch (Exception e2) {
}
return null;
}
try {
db.write(buffer);
db.flush();
db.close();
is.close();
} catch (Exception e) {
Log.d("OPENFOODDB","Failed to copy asset to DB");
e.printStackTrace();
return null;
}
return SQLiteDatabase.openDatabase(dbpath,null,SQLiteDatabase.OPEN_READWRITE);
}
}
Notes
the
openFoodDBmethod returns the SQLiteDatabase after copying if from the assets file if the database does not exist. The method will return null if there were issues.- if the database exists then the log will contain a message like
D/OPENFOODDB: Opening already existing Database - there will be no log messages if the database was copied from the assets file and opened successfully. Messages will only be logged if there is an issue.
- if for example the assets file is missing then you would get a message in the log like
D/OPENFOODDB: Unable to locate or buffer input from assets databases/FoodDB.dbthis would be preceded with a stack trace
- if the database exists then the log will contain a message like
e.g. :-
03-16 22:17:04.008 1529-1529/? W/System.err: java.io.FileNotFoundException: databases/FoodDB.db
03-16 22:17:04.008 1529-1529/? W/System.err: at android.content.res.AssetManager.openAsset(Native Method)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.content.res.AssetManager.open(AssetManager.java:315)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.content.res.AssetManager.open(AssetManager.java:289)
03-16 22:17:04.008 1529-1529/? W/System.err: at fooddb.so49328656populatelistview.MainActivity.openFoodDB(MainActivity.java:63)
03-16 22:17:04.008 1529-1529/? W/System.err: at fooddb.so49328656populatelistview.MainActivity.onCreate(MainActivity.java:37)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.app.Activity.performCreate(Activity.java:5008)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.app.ActivityThread.access$600(ActivityThread.java:130)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.os.Handler.dispatchMessage(Handler.java:99)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.os.Looper.loop(Looper.java:137)
03-16 22:17:04.008 1529-1529/? W/System.err: at android.app.ActivityThread.main(ActivityThread.java:4745)
03-16 22:17:04.008 1529-1529/? W/System.err: at java.lang.reflect.Method.invokeNative(Native Method)
03-16 22:17:04.008 1529-1529/? W/System.err: at java.lang.reflect.Method.invoke(Method.java:511)
03-16 22:17:04.008 1529-1529/? W/System.err: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
03-16 22:17:04.008 1529-1529/? W/System.err: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
03-16 22:17:04.008 1529-1529/? W/System.err: at dalvik.system.NativeStart.main(Native Method)
- A CursorAdapter requires a column specifically named _id hence the use of
IDCOLUMN + " AS _id".
The Result :-
来源：https://stackoverflow.com/questions/49328656/how-to-populate-listview-with-db-in-asset-folder