java - 使用 SQLiteOpenHelper 时 ContextWrapper.openOrCreateDatabase 中的 NullPointerException

Question

我正在制作一个使用数据库的新 Android 应用程序。我使用类似的代码来创建和复制数据库，但由于某种原因它不起作用。想不通，求助日志猫:

07-03 21:38:54.260: E/AndroidRuntime(12172): FATAL EXCEPTION: main
07-03 21:38:54.260: E/AndroidRuntime(12172): java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{lv.revopeng.guessthelocation/lv.revopeng.guessthelocation.LevelActivity}: java.lang.NullPointerException
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2585)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2679)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.app.ActivityThread.access$2300(ActivityThread.java:125)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2033)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.os.Handler.dispatchMessage(Handler.java:99)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.os.Looper.loop(Looper.java:123)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.app.ActivityThread.main(ActivityThread.java:4627)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at java.lang.reflect.Method.invokeNative(Native Method)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at java.lang.reflect.Method.invoke(Method.java:521)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:868)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:626)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at dalvik.system.NativeStart.main(Native Method)
07-03 21:38:54.260: E/AndroidRuntime(12172): Caused by: java.lang.NullPointerException
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.content.ContextWrapper.openOrCreateDatabase(ContextWrapper.java:203)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:112)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at lv.revopeng.guessthelocation.DataBaseHelper.open(DataBaseHelper.java:114)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at lv.revopeng.guessthelocation.DataBaseHelper.getLastId(DataBaseHelper.java:128)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at lv.revopeng.guessthelocation.LevelActivity.<init>(LevelActivity.java:31)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at java.lang.Class.newInstanceImpl(Native Method)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at java.lang.Class.newInstance(Class.java:1429)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.app.Instrumentation.newActivity(Instrumentation.java:1021)
07-03 21:38:54.260: E/AndroidRuntime(12172):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2577)
07-03 21:38:54.260: E/AndroidRuntime(12172):    ... 11 more

数据库助手.java:

public class DataBaseHelper extends SQLiteOpenHelper {

    // The Android's default system path of your application database.
    private static String DB_PATH = "/data/data/lv.revopeng.guessthelocation/databases/";
    private static String DB_NAME = "GuessTheLocationTest.db";
    public static final String TABLE_NAME1 = "gtl_levels";

    public static final String WHATTODONOW_COLUMN_ID = "_id";
    public static final String WHATTODONOW_COLUMN_INFO = "levelInfo";
    public static final String WHATTODONOW_COLUMN_ANSWER = "levelAnswer";
    public static final String WHATTODONOW_COLUMN_IMAGE = "levelImage";
    public static final String WHATTODONOW_COLUMN_CLEARED = "levelCleared";
    private DataBaseHelper openHelper;
    private SQLiteDatabase myDataBase;
    private final Context myContext;

    public DataBaseHelper(Context context) {
        super(context, DB_NAME, null, 1);
        this.myContext = context;
    }

    public void createDataBase() throws IOException {
        boolean dbExist = checkDataBase();
        if (dbExist) {
        } else {
            this.getReadableDatabase();
            try {
                copyDataBase();
            } catch (IOException e) {
                throw new Error("Error copying database");
            }
        }
    }

    private boolean checkDataBase() {

        SQLiteDatabase checkDB = null;

        try {
            String myPath = DB_PATH + DB_NAME;
            checkDB = SQLiteDatabase.openDatabase(myPath, null,
                    SQLiteDatabase.OPEN_READONLY);

        } catch (SQLiteException e) {
        }
        if (checkDB != null) {
            checkDB.close();
        }
        return checkDB != null ? true : false;
    }

    private void copyDataBase() throws IOException {

        InputStream myInput = myContext.getAssets().open(DB_NAME);

        String outFileName = DB_PATH + DB_NAME;

        OutputStream myOutput = new FileOutputStream(outFileName);

        byte[] buffer = new byte[1024];
        int length;
        while ((length = myInput.read(buffer)) > 0) {
            myOutput.write(buffer, 0, length);
        }

        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 myDataBase, int oldVersion,
            int newVersion) {
    }

    public DataBaseHelper open() throws SQLException {
        openHelper = new DataBaseHelper(myContext);
        myDataBase = openHelper.getWritableDatabase();
        return this;
    }

    public void closee() {
        openHelper.close();
    }

    String[] columns = new String[] { WHATTODONOW_COLUMN_ID,
            WHATTODONOW_COLUMN_INFO, WHATTODONOW_COLUMN_ANSWER,
            WHATTODONOW_COLUMN_IMAGE, WHATTODONOW_COLUMN_CLEARED };

    public int getLastId() {
        int id = 0;
        open();
        final String MY_QUERY = "SELECT MAX(_id) AS _id FROM " + TABLE_NAME1;
        Cursor mCursor = myDataBase.rawQuery(MY_QUERY, null);
        try {
            if (mCursor.getCount() > 0) {
                mCursor.moveToFirst();
                id = mCursor.getInt(0);
                mCursor.close();
                close();
            }
        } catch (Exception e) {
            System.out.println(e.getMessage());
        } finally {
        }
        return id;

    }

在 LevelActivity.java 上，调用了 createDataBase() 方法，并且在第一次使用 DataBaseHelper 时出现错误。

编辑:对我来说，getLastId 看起来像是 null，但它不应该是 null，因此访问数据库时出现问题。我查看了文件资源管理器，数据库在那里，其中的信息是正确的。

Answer 1

发现我的错误。在创建数据库之前，我调用了一个从数据库中获取数据的函数。