上下文context爲類Application沒法建立數據庫

原來的DatabaseOption和DatabaseManager老是報空指針異常，是由於上下文爲Application時沒法經過SQLiteOpenHelper的方法getWritableDatabase獲取可讀寫的數據庫。

在menifest文件中初始化後能夠解決該問題。即在application標籤下添加name屬性，並在name屬性對應的包名下新建一個類繼承Application。

<application
        android:name=".base.RailSetApplication"
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >

public class RailSetApplication extends Application {

    private static Application mInstance;

    @Override
    public void onCreate() {
        super.onCreate();

        mInstance = this;
    }

    public static Context getInstance()
    {
        if(null == mInstance){
            mInstance = new RailSetApplication();
        }

        Log.e("RailSetApplication", "is null "+(null == mInstance));
//        return mInstance.getApplicationContext();
        return mInstance;
    }
}