问题
I have a service class having following code -
public class ReminderService extends WakeReminderIntentService{
private static final String TAG ="ReminderService";
private TasksDBAdapter dbHelper;
public ReminderService(){
super("ReminderService");
dbHelper = new TasksDBAdapter(this);
dbHelper.open(); //THIS LINE GIVES ERROR
}
code for TasksDBAdapter open() method is as -
public TasksDBAdapter(Context ctx) {
this.mCtx = ctx;
}
public TasksDBAdapter open() throws android.database.SQLException {
mDbHelper = new DatabaseHelper(mCtx);
Log.i(TAG, "mDbHelper in open method-->" + mDbHelper);
mDb = mDbHelper.getWritableDatabase(); // ACTUAL ERROR LINE
return this;
}
the log value receives address which means mDbHelper gets initialized properly.Log.i gives -
11-01 23:32:32.004: I/TasksDBAdapter(681): mDbHelper in open method-->ray.kaushik.simple.task.manager.TasksDBAdapter$DatabaseHelper@411e7a10
but
mDb = mDbHelper.getWritableDatabase();
throws the null pointer error.
Logcat gives -
11-01 23:32:32.043: E/AndroidRuntime(681): FATAL EXCEPTION: main
11-01 23:32:32.043: E/AndroidRuntime(681): java.lang.RuntimeException: Unable to instantiate service ray.kaushik.simple.task.manager.ReminderService: java.lang.NullPointerException
11-01 23:32:32.043: E/AndroidRuntime(681): at android.app.ActivityThread.handleCreateService(ActivityThread.java:2347)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.app.ActivityThread.access$1600(ActivityThread.java:130)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1277)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.os.Handler.dispatchMessage(Handler.java:99)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.os.Looper.loop(Looper.java:137)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.app.ActivityThread.main(ActivityThread.java:4745)
11-01 23:32:32.043: E/AndroidRuntime(681): at java.lang.reflect.Method.invokeNative(Native Method)
11-01 23:32:32.043: E/AndroidRuntime(681): at java.lang.reflect.Method.invoke(Method.java:511)
11-01 23:32:32.043: E/AndroidRuntime(681): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
11-01 23:32:32.043: E/AndroidRuntime(681): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
11-01 23:32:32.043: E/AndroidRuntime(681): at dalvik.system.NativeStart.main(Native Method)
11-01 23:32:32.043: E/AndroidRuntime(681): Caused by: java.lang.NullPointerException
11-01 23:32:32.043: E/AndroidRuntime(681): at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:221)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:224)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:164)
11-01 23:32:32.043: E/AndroidRuntime(681): at ray.kaushik.simple.task.manager.TasksDBAdapter.open(TasksDBAdapter.java:38)
11-01 23:32:32.043: E/AndroidRuntime(681): at ray.kaushik.simple.task.manager.ReminderService.<init>(ReminderService.java:16)
11-01 23:32:32.043: E/AndroidRuntime(681): at java.lang.Class.newInstanceImpl(Native Method)
11-01 23:32:32.043: E/AndroidRuntime(681): at java.lang.Class.newInstance(Class.java:1319)
11-01 23:32:32.043: E/AndroidRuntime(681): at android.app.ActivityThread.handleCreateService(ActivityThread.java:2344)
11-01 23:32:32.043: E/AndroidRuntime(681): ... 10 more
My databaseHelper class code -
private static class DatabaseHelper extends SQLiteOpenHelper {
DatabaseHelper(Context context) {
super(context, DATABASE_NAME, null, DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
db.execSQL(DATABASE_CREATE);
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
}
where DATABASE_NAME and all others have been defined globally -
private static final String DATABASE_NAME = "data";
private static final String DATABASE_TABLE = "reminders";
private static final int DATABASE_VERSION = 1;
public static final String KEY_TITLE = "title";
public static final String KEY_BODY = "body";
public static final String KEY_DATE_TIME = "reminder_date_time";
public static final String KEY_ROWID = "_id";
private static final String TAG = "TasksDBAdapter";
can anyone please provide some pointers.. thanks in advance..!!!
回答1:
I believe the IntentService hasn't been fully initialized when you try to create the database in the constructor. Try moving your code into onHandleIntent():
protected void onHandleIntent(Intent intent) {
dbHelper = new TasksDBAdapter(this);
dbHelper.open();
// Do something
}
回答2:
I kinda resolved the issue -
I have the class ReminderService where I was trying to call the dbHelper.open(); method.
Also this same class have one overridden method doReminderWork(intent) , where I was going to use the dbHelper initialized.
Earlier code -
public ReminderService(){
super("ReminderService");
dbHelper = new TasksDBAdapter(this);
dbHelper.open(); //THIS LINE GIVES ERROR
}
@Override
void doReminderWork(Intent intent){
/**
* retrieve data about task
*/
Cursor reminder = dbHelper.fetchReminder(rowId);
}
in which i moved the open method in doReminderWork(Intent intent) method like -
public ReminderService(){
super("ReminderService");
dbHelper = new TasksDBAdapter(this);
}
@Override
void doReminderWork(Intent intent){
/**
* retrieve data about task
*/
dbHelper.open(); //MOVED HERE
Cursor reminder = dbHelper.fetchReminder(rowId);
}
and it worked.. Can anyone give me proper detailed explanation why this worked as it was just a try that worked.
回答3:
This kind of actions must be called from a separate thread, Try to call it with a Thread object or AsyncTask.
Thread t = new Thread(new Runnable() {
public void run() {
dbHelper.open(); //MOVED HERE
Cursor reminder = dbHelper.fetchReminder(rowId);
}
});
t.start();
来源：https://stackoverflow.com/questions/13183234/getwritabledatabase-throwing-null-pointer-exception