这是我第一次尝试在Android中使用SQLite数据库。我有一个包含一堆电影的数据库。我有一个加载活动，在retainFragment中有一个AsyncTask。在AsyncTask中，我尝试从数据库中获取带有查询的光标。然后我将所有项目添加到ArrayList，然后从那里启动mainActivity。
但是，当我尝试创建SQLiteOpenHelper时，我收到NullPointerException。从在线阅读类似的问题看起来问题可能与我的背景有关，但我无法尝试使其成功。我按照本教程创建了SQLiteOpenHelper：http://www.reigndesign.com/blog/using-your-own-sqlite-database-in-android-applications/
这是我的LoadingActivity：
package com.example.pickmymovie;
import java.util.ArrayList;
import com.example.pickmymovie.LoadingFragment.LoadingCallback;
import android.app.Activity;
import android.app.FragmentManager;
import android.content.Intent;
import android.os.Bundle;
import android.widget.ProgressBar;
public class LoadingActivity extends Activity implements LoadingCallback {
private ProgressBar bar;
private LoadingFragment loadFrag;
private ArrayList<Movie> movies;
public final static String TAG_TASK_FRAGMENT = "LDFRAG";
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.loading);
bar = (ProgressBar)findViewById(R.id.progress1);
bar.setMax(10000);
onPreExecute();
}
@Override
public void onPreExecute() {
connectWithRetainedFragment().executeTask();
}
@Override
public void onCancelled() {
// nothing
}
@Override
public void onRunning(int progress) {
bar.setProgress(progress);
}
@Override
public void onPostExecute(Boolean bool) {
if (bool) {
Intent intent = new Intent(LoadingActivity.this, MainActivity.class );
intent.putParcelableArrayListExtra("movies", movies);
startActivity(intent);
}
}
/**
* find the retained fragment and connect to it. then return it so you can
* calculate stuffs
*
* @return
*/
public LoadingFragment connectWithRetainedFragment() {
FragmentManager fm = getFragmentManager();
// r1 = (RetainedFragment)fm.findFragmentByTag(TAG_TASK_FRAGMENT);
if (getFragmentManager().findFragmentByTag(TAG_TASK_FRAGMENT) == null) {
loadFrag = new LoadingFragment();
fm.beginTransaction().add(loadFrag, TAG_TASK_FRAGMENT).commit();
}
return loadFrag;
}
@Override
public void setMovieList(ArrayList<Movie> movies) {
this.movies = movies;
}
}
这是我的AsmentTask片段：
package com.example.pickmymovie;
import java.io.IOException;
import java.util.ArrayList;
import android.app.Activity;
import android.app.Fragment;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.os.AsyncTask;
public class LoadingFragment extends Fragment {
DataBaseHelper DbHelper;
/**
* interface to call back to the loading activity
*/
static interface LoadingCallback {
void onPreExecute();
void onCancelled();
void onRunning(int progress);
void onPostExecute(Boolean bool);
void setMovieList(ArrayList<Movie> movies);
}
private LoadingCallback activity;
private LoadingTask task;
private Activity context;
/**
* Hold a reference to the parent Activity so we can report the task's
* current progress and results. The Android framework will pass us a
* reference to the newly created Activity after each configuration change.
*/
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
context = activity;
this.activity = (LoadingCallback) activity;
//Create and open the database.
DbHelper = new DataBaseHelper(activity);
try {
DbHelper.createDataBase();
} catch (IOException e) {
e.printStackTrace();
}
try {
DbHelper.openDataBase();
}catch(SQLException sqle){
throw sqle;
}
}
/**
* execute the LoadingTask
* @param param
*/
public void executeTask() {
task = new LoadingTask();
task.execute();
}
/**
* Set the callback to null so we don't accidentally leak the
* Activity instance.
*/
@Override
public void onDetach() {
super.onDetach();
activity = null;
}
/**
* A dummy task that performs some (dumb) background work and proxies
* progress updates and results back to the Activity.
*
* Note that we need to check if the callbacks are null in each method in
* case they are invoked after the Activity's and Fragment's onDestroy()
* method have been called.
*/
private class LoadingTask extends AsyncTask<Cursor, Integer, Boolean> {
private Cursor cursor;
ArrayList<Movie> movieList;
/**
* nothing here
*/
@Override
protected void onPreExecute() {
//Create and open the database.
DbHelper = new DataBaseHelper(getActivity().getApplicationContext());
try {
DbHelper.createDataBase();
} catch (IOException e) {
e.printStackTrace();
}
try {
DbHelper.openDataBase();
}catch(SQLException sqle){
throw sqle;
}
cursor = DbHelper.getCursor();
}
/**
* Note that we do NOT call the callback object's methods directly from
* the background thread, as this could result in a race condition.
*/
@Override
protected Boolean doInBackground(Cursor... param) {
int total = cursor.getCount();
int margin = 10000 / total;
movieList = new ArrayList<Movie>();
// do the stuff and report back to the home activity.
for (int i = 0; i < cursor.getCount(); i++) {
if (cursor.moveToFirst()) {
do {
Movie movie = new Movie();
movie.setId(Integer.parseInt(cursor.getString(0)));
movie.setName(cursor.getString(1));
movie.setGenre(cursor.getString(2));
movie.setImage(cursor.getString(3));
movie.setRating(Integer.parseInt(cursor.getString(4)));
// Adding movie to the list
movieList.add(movie);
publishProgress(i * margin);
} while (cursor.moveToNext());
// I don't think I've ever used a Do/While in java
// they taught us this in HS C++, but I've never touched it since.
// Oh well, it was in the example code
}
}
return true;
}
/**
* cancel the thing
*/
@Override
protected void onCancelled() {
if (activity != null) {
activity.onCancelled();
}
}
/**
* update the activity
*/
protected void publishProgress(Integer progress) {
if (activity != null) {
activity.onRunning(progress);
}
}
/**
* publish the result
*/
@Override
protected void onPostExecute(Boolean result) {
if (activity != null) {
activity.onPostExecute(result);
}
}
}
}
这是我的SQLiteOpenHelper：
package com.example.pickmymovie;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.ArrayList;
import android.content.Context;
import android.database.Cursor;
import android.database.SQLException;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteException;
import android.database.sqlite.SQLiteOpenHelper;
public class DataBaseHelper extends SQLiteOpenHelper{
//The Android's default system path of your application database.
private static String DB_PATH = "/data/data/com.example.pickmymovie/databases/";
private static String DB_NAME = "movieDatabase";
private SQLiteDatabase myDataBase;
private final Context myContext;
/**
* Constructor
* Takes and keeps a reference of the passed context in order to access to the application assets and resources.
* @param context
*/
public DataBaseHelper(Context context) {
super(context, DB_NAME, null, 1);
this.myContext = context;
}
/**
* Creates a empty database on the system and rewrites it with your own database.
* */
public void createDataBase() throws IOException{
boolean dbExist = checkDataBase();
if(dbExist){
//do nothing - database already exist
}else{
//By calling this method and empty database will be created into the default system path
//of your application so we are gonna be able to overwrite that database with our database.
this.getReadableDatabase();
try {
copyDataBase();
} catch (IOException e) {
throw new Error("Error copying database");
}
}
}
/**
* Check if the database already exist to avoid re-copying the file each time you open the application.
* @return true if it exists, false if it doesn't
*/
private boolean checkDataBase(){
SQLiteDatabase checkDB = null;
try{
String myPath = DB_PATH + DB_NAME;
checkDB = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}catch(SQLiteException e){
//database does't exist yet.
}
if(checkDB != null){
checkDB.close();
}
//return checkDB != null ? true : false;
// ^ was in the example code. Seems like a goober way to do it.
return (checkDB != null);
}
/**
* Copies your database from your local assets-folder to the just created empty database in the
* system folder, from where it can be accessed and handled.
* This is done by transfering bytestream.
* */
private void copyDataBase() throws IOException{
//Open your local db as the input stream
InputStream myInput = myContext.getAssets().open(DB_NAME);
// Path to the just created empty db
String outFileName = DB_PATH + DB_NAME;
//Open the empty db as the output stream
OutputStream myOutput = new FileOutputStream(outFileName);
//transfer bytes from the inputfile to the outputfile
byte[] buffer = new byte[1024];
int length;
while ((length = myInput.read(buffer))>0){
myOutput.write(buffer, 0, length);
}
//Close the streams
myOutput.flush();
myOutput.close();
myInput.close();
}
public void openDataBase() throws SQLException{
//Open the database
String myPath = DB_PATH + DB_NAME;
myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READONLY);
}
@Override
public synchronized void close() {
if(myDataBase != null)
myDataBase.close();
super.close();
}
@Override
public void onCreate(SQLiteDatabase db) {
}
@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
}
public Cursor getCursor() {
ArrayList<Movie> movieList = new ArrayList<Movie>();
// Select All Query
String selectQuery = "SELECT * FROM movies";
SQLiteDatabase db = this.getWritableDatabase();
Cursor cursor = db.rawQuery(selectQuery, null);
return cursor;
}
}
当我尝试运行应用程序时，这是LogCat：
08-10 03:20:30.745: W/dalvikvm(18469): threadid=1: thread exiting with uncaught exception (group=0x417b2da0)
08-10 03:20:30.745: E/AndroidRuntime(18469): FATAL EXCEPTION: main
08-10 03:20:30.745: E/AndroidRuntime(18469): Process: com.example.pickmymovie, PID: 18469
08-10 03:20:30.745: E/AndroidRuntime(18469): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.pickmymovie/com.example.pickmymovie.LoadingActivity}: java.lang.NullPointerException
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2334)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2392)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.ActivityThread.access$900(ActivityThread.java:169)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1280)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.os.Handler.dispatchMessage(Handler.java:102)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.os.Looper.loop(Looper.java:146)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.ActivityThread.main(ActivityThread.java:5487)
08-10 03:20:30.745: E/AndroidRuntime(18469): at java.lang.reflect.Method.invokeNative(Native Method)
08-10 03:20:30.745: E/AndroidRuntime(18469): at java.lang.reflect.Method.invoke(Method.java:515)
08-10 03:20:30.745: E/AndroidRuntime(18469): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1283)
08-10 03:20:30.745: E/AndroidRuntime(18469): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1099)
08-10 03:20:30.745: E/AndroidRuntime(18469): at dalvik.system.NativeStart.main(Native Method)
08-10 03:20:30.745: E/AndroidRuntime(18469): Caused by: java.lang.NullPointerException
08-10 03:20:30.745: E/AndroidRuntime(18469): at com.example.pickmymovie.LoadingFragment$LoadingTask.onPreExecute(LoadingFragment.java:102)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:587)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.os.AsyncTask.execute(AsyncTask.java:535)
08-10 03:20:30.745: E/AndroidRuntime(18469): at com.example.pickmymovie.LoadingFragment.executeTask(LoadingFragment.java:70)
08-10 03:20:30.745: E/AndroidRuntime(18469): at com.example.pickmymovie.LoadingActivity.onPreExecute(LoadingActivity.java:33)
08-10 03:20:30.745: E/AndroidRuntime(18469): at com.example.pickmymovie.LoadingActivity.onCreate(LoadingActivity.java:28)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.Activity.performCreate(Activity.java:5451)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1093)
08-10 03:20:30.745: E/AndroidRuntime(18469): at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
08-10 03:20:30.745: E/AndroidRuntime(18469): ... 11 more
08-10 03:20:33.467: I/Process(18469): Sending signal. PID: 18469 SIG: 9
我不完全确定SQLite开放是如何工作的，这可能就是为什么我自己似乎无法解决这个问题。谢谢你的帮助。
答案 0 :(得分：2)
getActivity()在片段附加到其父活动之前返回null。这就是NPE的原因。
提交片段事务不会立即执行它。这就是片段尚未附加的原因。
通常，您不应直接调用片段方法（在您的情况下为executeTask()）。只需依赖片段生命周期回调，例如onCreate()。如果您需要将数据传递到片段，请使用setArguments(Bundle)。
答案 1 :(得分：0)
我设法通过将AsyncTask移动到LoadingActivity类来解决问题，而不是使用保留的片段来保存它。由于片段无头，我无法找到上下文。这很大程度上简化了本节的代码，虽然我不相信它是最好的解决方案。这是我的LoadingActivity类现在的样子：
package com.example.pickmymovie;
import java.io.IOException;
import java.util.ArrayList;
import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.database.Cursor;
import android.database.SQLException;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.ProgressBar;
public class LoadingActivity extends Activity {
private ProgressBar bar;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.loading);
bar = (ProgressBar)findViewById(R.id.progress1);
bar.setMax(10000);
LoadingTask task = new LoadingTask(this);
task.execute();
}
/**
* A dummy task that performs some (dumb) background work and proxies
* progress updates and results back to the Activity.
*
* Note that we need to check if the callbacks are null in each method in
* case they are invoked after the Activity's and Fragment's onDestroy()
* method have been called.
*/
private class LoadingTask extends AsyncTask<Cursor, Integer, Boolean> {
private Cursor cursor;
private ArrayList<Movie> movieList;
private DataBaseHelper DbHelper;
private Context context;
public LoadingTask(Context context) {
this.context = context;
}
/**
* nothing here
*/
@Override
protected void onPreExecute() {
//Create and open the database.
DbHelper = new DataBaseHelper(context);
try {
DbHelper.createDataBase();
} catch (IOException e) {
e.printStackTrace();
}
try {
DbHelper.openDataBase();
}catch(SQLException sqle){
throw sqle;
}
cursor = DbHelper.getCursor();
}
/**
* Note that we do NOT call the callback object's methods directly from
* the background thread, as this could result in a race condition.
*/
@Override
protected Boolean doInBackground(Cursor... param) {
int total = cursor.getCount();
int margin = 10000 / total;
movieList = new ArrayList<Movie>();
// do the stuff and report back to the home activity.
for (int i = 0; i < cursor.getCount(); i++) {
if (cursor.moveToFirst()) {
do {
Movie movie = new Movie();
movie.setId(Integer.parseInt(cursor.getString(0)));
movie.setName(cursor.getString(1));
movie.setGenre(cursor.getString(2));
movie.setImage(cursor.getString(3));
movie.setRating(Integer.parseInt(cursor.getString(4)));
// Adding movie to the list
movieList.add(movie);
publishProgress(i * margin);
} while (cursor.moveToNext());
// I don't think I've ever used a Do/While in java
// they taught us this in HS C++, but I've never touched it since.
// Oh well, it was in the example code
}
}
return true;
}
/**
* cancel the thing
*/
@Override
protected void onCancelled() {
// nothing
}
/**
* update the activity
*/
protected void publishProgress(Integer progress) {
bar.setProgress(progress);
}
/**
* publish the result
*/
@Override
protected void onPostExecute(Boolean result) {
if (result) {
Intent intent = new Intent(LoadingActivity.this, MainActivity.class );
intent.putParcelableArrayListExtra("movies", movieList);
Log.w("movies", movieList.get(0).title());
startActivity(intent);
}
}
}
}