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所以，我正在开发一个应该播放“\sdcard\movies”中的视频的应用程序，为此它必须查阅我数据库中的一个表。但是，选择查询在该行返回一个空指针异常：

Cursor query = myDataBase.rawQuery("SELECT Arquivo FROM Comercial ORDER BY UltimaExecucao ASC , Random DESC LIMIT 1 ", null);

这是我的代码：

DBHandler.java

public class DBHandler extends SQLiteOpenHelper {

    private static String DB_PATH = "/sdcard/movies/";
    private static String DB_NAME = "audiostore.db";
    private SQLiteDatabase myDataBase;
    private final Context myContext;

    public DBHandler(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_READWRITE);
        } 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 {
        String myPath = DB_PATH + DB_NAME;
        myDataBase = SQLiteDatabase.openDatabase(myPath, null, SQLiteDatabase.OPEN_READWRITE);
    }

    @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 String getData() {
        String str = new String();
        Cursor query = myDataBase.rawQuery("SELECT Arquivo FROM Comercial ORDER BY UltimaExecucao ASC , Random DESC LIMIT 1 ", null);
        str = query.getString(query.getColumnIndex(str));

        return str;

    }
}

有人可以告诉我为什么它得到一个空值？

=====编辑=====

这是日志猫：

10-02 21:19:50.004: E/AndroidRuntime(507): FATAL EXCEPTION: main
10-02 21:19:50.004: E/AndroidRuntime(507): java.lang.RuntimeException: Unable to start     activity     ComponentInfo{com.sample.VideoViewExample/com.sample.VideoViewExample.VideoViewExample}: java.lang.NullPointerException
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1736)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1752)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.app.ActivityThread.access$1500(ActivityThread.java:123)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:993)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.os.Handler.dispatchMessage(Handler.java:99)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.os.Looper.loop(Looper.java:126)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.app.ActivityThread.main(ActivityThread.java:3997)
10-02 21:19:50.004: E/AndroidRuntime(507):  at java.lang.reflect.Method.invokeNative(Native Method)
10-02 21:19:50.004: E/AndroidRuntime(507):  at java.lang.reflect.Method.invoke(Method.java:491)
10-02 21:19:50.004: E/AndroidRuntime(507):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:841)
10-02 21:19:50.004: E/AndroidRuntime(507):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:599)
10-02 21:19:50.004: E/AndroidRuntime(507):  at dalvik.system.NativeStart.main(Native Method)
10-02 21:19:50.004: E/AndroidRuntime(507): Caused by: java.lang.NullPointerException
10-02 21:19:50.004: E/AndroidRuntime(507):  at com.sample.VideoViewExample.DBHandler.getData(DBHandler.java:158)
10-02 21:19:50.004: E/AndroidRuntime(507):  at com.sample.VideoViewExample.VideoViewExample.onCreate(VideoViewExample.java:21)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1048)
10-02 21:19:50.004: E/AndroidRuntime(507):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1700)
10-02 21:19:50.004: E/AndroidRuntime(507):  ... 11 more

在哪里：

10-02 21:19:50.004: E/AndroidRuntime(507): 在 com.sample.VideoViewExample.VideoViewExample.onCreate(VideoViewExample.java:21)

是另一个类，其中方法应该返回游标的值。

4

2 回答 2

0

您从未真正打开数据库。您的 checkDatabase 函数打开它并关闭它以检查它是否存在，但您从未真正打开它以便您可以使用它。

public void createDataBase() throws IOException {
    boolean dbExist = checkDataBase();

    if (dbExist) {
       //****This is where you need to open it*****
       myDataBase = openDataBase();
    } else {
        this.getReadableDatabase();

        try {
            copyDataBase();
        } catch (IOException e) {
            throw new Error("Error copying database");
        }
    }
}
于 2013-10-03T21:50:44.270 回答
0

你永远不会调用openDataBase方法，所以myDataBase对象是null执行到达线的时候。-

Cursor query = myDataBase.rawQuery("SELECT Arquivo FROM Comercial ORDER BY UltimaExecucao ASC , Random DESC LIMIT 1 ", null);

投掷NullPointerException.

于 2013-10-03T21:39:20.537 回答