上下文context为类Application无法创建数据库

原来的DatabaseOption和DatabaseManager总是报空指针异常，是因为上下文为Application时无法通过SQLiteOpenHelper的方法getWritableDatabase获取可读写的数据库。

在menifest文件中初始化后可以解决该问题。即在application标签下添加name属性，并在name属性对应的包名下新建一个类继承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;
    }
}