java - SQLiteOpenHelper 创建时出现 NullPointerException

Question

这是我第一次尝试在 Android 中使用 SQLite 数据库。我有一个包含很多电影的数据库。我有一个加载 Activity ，在retainedFragment 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 中，并从那里启动 mainActivity。

但是，当我尝试创建 SQLiteOpenHelper 时，出现 NullPointerException。从网上阅读类似的问题来看，问题似乎可能与我的上下文有关，但我无法找到使其发挥作用的方法。我按照本教程创建了 SQLiteOpenHelper:http://www.reigndesign.com/blog/using-your-own-sqlite-database-in-android-applications/

这是我的加载 Activity :

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 AsyncTask fragment :

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;
    }
}

这是我尝试运行应用程序时的 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 开口是如何工作的，这可能就是为什么我自己似乎无法弄清楚这一点。感谢您的帮助。

Answer 1

getActivity() 在 fragment 附加到其父 Activity 之前返回 null。这就是 NPE 的原因。

提交 fragment 事务不会立即执行它。这就是该 fragment 尚未附加的原因。

通常，您不应直接调用 fragment 方法(在您的情况下为 executeTask())。只需依赖 fragment 生命周期回调，例如 onCreate()。如果您需要将数据传递到 fragment ，请使用setArguments(Bundle)。