android sqlite 空指针错误

Question

我正在开发一个需要数据库的android项目。我遇到的问题是当我启动它时，出现空指针异常错误。通过 logcat，我已经将范围缩小到数据库更新并尝试打开的时间。这是正常现象还是我看得太少了？
logcat：

02-12 15:41:33.810: ERROR/AndroidRuntime(354): java.lang.RuntimeException: Unable to     start activity ComponentInfo{arch.field/arch.field.ChooseSite}: java.lang.NullPointerException
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.app.ActivityThread.access$1500(ActivityThread.java:117)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.os.Handler.dispatchMessage(Handler.java:99)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.os.Looper.loop(Looper.java:123)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.app.ActivityThread.main(ActivityThread.java:3683)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at java.lang.reflect.Method.invokeNative(Native Method)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at java.lang.reflect.Method.invoke(Method.java:507)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at dalvik.system.NativeStart.main(Native Method)
02-12 15:41:33.810: ERROR/AndroidRuntime(354): Caused by: java.lang.NullPointerException
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at arch.field.BmDb$DatabaseHelper.onUpgrade(BmDb.java:96)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.database.sqlite.SQLiteOpenHelper.getWritableDatabase(SQLiteOpenHelper.java:132)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at arch.field.BmDb.open(BmDb.java:110)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at arch.field.ChooseSite.onCreate(ChooseSite.java:30)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
02-12 15:41:33.810: ERROR/AndroidRuntime(354):     ... 11 more

OnUpgrade 代码：

public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) 
       {
        Log.d("MonkeyDatabase","DbUpdater");
        db.execSQL("Drop table if exists "+TABLE_SITE_NAME);
        db.execSQL("Drop table if exists "+TABLE_UNIT_NAME);
        db.execSQL("Drop table if exists "+TABLE_BONESHELL_NAME);
        db.execSQL("Drop table if exists "+TABLE_CERAMIC_NAME);
        db.execSQL("Drop table if exists "+TABLE_LITHIC_NAME);
        onCreate(db);
        }

开放代码：

public BmDb open() throws SQLException
{
    Log.d("Database","open");
    db= BmDb.getWritableDatabase();
    return this;
}

更多代码：

public BmDb(Context context)
{
    this.context= context;
    BmDb= new DatabaseHelper(context);
}
public static class DatabaseHelper extends SQLiteOpenHelper
{
    DatabaseHelper(Context context){
        super(context, DATABASE_NAME, null, VERSION);

}

这是 ChooseSite 文件中的代码（带行号）：

 final BmDb db=new BmDb(this);
    db.open(); **this is the line mentioned in my logcat**
    final Cursor c= db.GetSite();
    if (c.moveToFirst()){
        do{
            sites(c);
        }while (c.moveToNext());
    }

对我正在查看的内容有什么建议吗？

Answer 1

从此代码中

 final BmDb db=new BmDb(this);
db.open(); **this is the line mentioned in my logcat**
final Cursor c= db.GetSite();
if (c.moveToFirst()){
    do{
        sites(c);
    }while (c.moveToNext());
}

调用此方法两次 db.open(); 将其置于 try{}catch{} 黑色中。

如下 try{db.open();db.open();}catch(){}

Answer 2

如果应用程序代码中的数据库版本增加，则在 onUpgrade 中

public void onUpgrade(SQLiteDatabase arg0, int arg1, int arg2) 
       {
        Log.d("MonkeyDatabase","DbUpdater");
        db.execSQL("Drop table if exists "+TABLE_SITE_NAME);
        db.execSQL("Drop table if exists "+TABLE_UNIT_NAME);
        db.execSQL("Drop table if exists "+TABLE_BONESHELL_NAME);
        db.execSQL("Drop table if exists "+TABLE_CERAMIC_NAME);
        db.execSQL("Drop table if exists "+TABLE_LITHIC_NAME);
         //onCreate(db);
         onCreate(arg0);
        }

调用 onUpgrade()。检查你的代码..