Android: NullPointerException on getWritableDatabase()

问题

Here is where I try to access the database:

UserDataAdapter dbHelper = new UserDataAdapter(this);
dbHelper.open();
dbHelper.createNetwork(network);
dbHelper.close();

UserDataAdapter.java:

public UserDataAdapter open() throws SQLException {
        dbHelper = new DatabaseHelper(context); // of class DatabaseHelper
        database = dbHelper.getWritableDatabase(); // this line!!   
        return this;
    }

DatabaseHelper.java:

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

@Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL(Defines.DATABASE_CREATE_TAXI_SERVICE_DATABASE);
    }

Crash: 09-13 18:12:33.119: E

RROR/AndroidRuntime(7488): FATAL EXCEPTION: main
09-13 18:12:33.119: ERROR/AndroidRuntime(7488): java.lang.NullPointerException
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:203)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:98)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at com.basind.testapp.database.UserDataAdapter.open(UserDataAdapter.java:29)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at com.basind.testapp.FetchNetworks.addToDatabase(FetchNetworks.java:107)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at com.basind.testapp.FetchNetworks.access$1(FetchNetworks.java:105)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at com.basind.testapp.FetchNetworks$1.onResponseReceived(FetchNetworks.java:75)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at com.basind.testapp.async.CallbackWrapper.run(CallbackWrapper.java:15)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at android.os.Handler.handleCallback(Handler.java:587)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at android.os.Handler.dispatchMessage(Handler.java:92)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at android.os.Looper.loop(Looper.java:145)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at android.app.ActivityThread.main(ActivityThread.java:4937)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at java.lang.reflect.Method.invokeNative(Native Method)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at java.lang.reflect.Method.invoke(Method.java:521)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
09-13 18:12:33.119: ERROR/AndroidRuntime(7488):     at dalvik.system.NativeStart.main(Native Method)

Other details:

• Target: android-7
• Test Device: HTC Desire

回答1:

Make sure if your Context is properly being set or not.

private Context context;
private SQLiteDatabase database;
private DatabaseHelper dbHelper;

public UserDataAdapter(Context context) {
    this.context = context;
}

public UserDataAdapter open() throws SQLException
{
    dbHelper = new DatabaseHelper(context);
    database = dbHelper.getWritableDatabase();
    return this;
}

回答2:

FYI: I've had the NullPointerException error with a standard ContentProvider implementation. I finally solved it and the key is related to context/intent in the manifest file.

In the end I was missing this line from my inside my manifest's activity's .

<data android:mimeType="vnd.android.cursor.dir/vnd.pmp.smsmessage" />

Checkout the NotePad sample for what I am talking about. Also I prefer to use the structure here http://www.vogella.com/articles/AndroidSQLite/article.html however as his tut is designed for 3.0 had to leave out all the loader stuff for 2.2 implementation ...

After putting it in my database created and I could insert rows. I tested this simply using the onCreate for my default activity.

ContentValues values = new ContentValues();
values.put("somefieldname","somevalue");
values.put("otherfieldname","othervalue");
getContentResolver().insert(MyProvider.CONTENT_URI, values);

回答3:

This happens when you try to open the database from a Service or Activity constructor. Try moving the open to onCreate().

来源：https://stackoverflow.com/questions/7398802/android-nullpointerexception-on-getwritabledatabase