java - 数据无法从网络服务器加载到 SQLite 数据库中

Question

您好，我在从 Web 服务器获取数据并将它们直接插入 SQLite 数据库时遇到问题，但首先我需要检查数据是否已经存在于 SQLite 中，但我遇到了这个问题。

java.lang.NullPointerException:尝试调用虚方法'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.s

我的代码如下:

后台类获取数据的代码在下面我尝试指定唯一的方法来将返回的数据添加到SQLite

    @Override
protected void onPostExecute(String result) {

    try {
        DbHelper dbHelper=new DbHelper(ctx);
        boolean test;
        for (int j = 0; j < data.size(); j++) {
            Log.i("group", data.get(j));

                if(dbHelper.checkGroupId(DataId.get(j))==false)
                {
                    //now data can be added
                    test=dbHelper.saveToLocalDatabase(DataId.get(j),data.get(j),grpName.get(j),"no");
                    if(test)
                    {
                        Toast.makeText(ctx,"Data  Added "+data.get(j),Toast.LENGTH_LONG).show();
                        //notify gui
                        showNotification();
                    }
                    else
                    {
                        Toast.makeText(ctx,"Data not Added",Toast.LENGTH_LONG).show();
                    }
                }




        }
        dbHelper.close();
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}

下面是将数据添加到 SQLite 数据库的代码

    private static final int DATABASE_VERSION=1;
public static final String CREATE_TABLE="create table if not exists groups(groupId varchar(50),groupName text,targetAmount text,fetched varchar(10))";
public static final String DROP_TABLE="drop table if exists "+DbContract.TABLE_NAME;


    public DbHelper(Context context)
{
    super(context,DbContract.DATABASE_NAME,null,DATABASE_VERSION);
}
@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL(CREATE_TABLE);
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL(DROP_TABLE);
    onCreate(db);
}


    public boolean saveToLocalDatabase(String groupId,String groupName,String targetAmount,String fetched)
{
    SQLiteDatabase database=this.getWritableDatabase();
    ContentValues contentValues=new ContentValues();
    contentValues.put(DbContract.GROUP_ID,groupId);
    contentValues.put(DbContract.GROUP_NAME,groupName);
    contentValues.put(DbContract.TARGET_AMOUNT,targetAmount);
    contentValues.put(DbContract.FETCHED,fetched);
    //now add data to databse
    long result=database.insert(DbContract.TABLE_NAME,null,contentValues);
    if(result==-1)
    {
        return false;
    }
    else
    {
        return true;
    }
}

这个函数检查数据是否已经在数据库中，这里是显示错误来自 backgroundTask 类的地方

    //function to check if the given id is available
public boolean checkGroupId(String groupId)
{
    String query="select * from groups where groupId='"+groupId+"'";
    SQLiteDatabase database=this.getWritableDatabase();;
    //create cursor to return data
    Cursor cursor=database.rawQuery(query,null);
    int counter=0;
    while(cursor.moveToNext())
    {
        counter=counter+1;
    }
    //check counters returned
    if(counter>0)
    {
        return true;
    }
    else
    {
        return false;
    }
}

这是我的错误所在的输出终端日志数据

08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err: java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.sqlite.SQLiteDatabase$CursorFactory, android.database.DatabaseErrorHandler)' on a null object reference

08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err: 在 android.database.sqlite.SQLiteOpenHelper.getDatabaseLocked(SQLiteOpenHelper.java:223)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err: 在 android.database.sqlite.SQLiteOpenHelper.getWritableDatabase (SQLiteOpenHelper.java:163)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err:在 info.androidhive.uplus.DbHelper.checkGroupId(DbHelper.java:165)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err:在 info.androidhive.uplus.BackgroundTask.onPostExecute(BackgroundTask.java:136)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err:在 info.androidhive.uplus.BackgroundTask.onPostExecute(BackgroundTask.java:28)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err: 在 android.os.AsyncTask.finish (AsyncTask.java:651)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err: at android.os.AsyncTask.access$500(AsyncTask.java:180)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err: 在 android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:668)08-07 04:14:58.075 28663-28663/info.androidhive.uplus W/System.err: 在 android.os.Handler.dispatchMessage (Handler.java:102)08-07 04:14:58.076 28663-28663/info.androidhive.uplus W/System.err:在 android.os.Looper.loop(Looper.java:234)08-07 04:14:58.076 28663-28663/info.androidhive.uplus W/System.err: 在 android.app.ActivityThread.main (ActivityThread.java:5526)08-07 04:14:58.076 28663-28663/info.androidhive.uplus W/System.err:在 java.lang.reflect.Method.invoke( native 方法)08-07 04:14:58.076 28663-28663/info.androidhive.uplus W/System.err:在 com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)08-07 04:14:58.076 28663-28663/info.androidhive.uplus W/System.err: 在 com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

请任何我希望他/她可以的人谢谢

Answer 1

检查参数中的上下文，确保它不为空。

java.lang.NullPointerException: Attempt to invoke virtual method 'android.database.sqlite.SQLiteDatabase android.content.Context.openOrCreateDatabase(java.lang.String, int, android.database.s

上面的错误是因为你的context为null。

DbHelper dbHelper=new DbHelper(ctx);// ctx should not be null