從AsyncTask中的上下文讀取R.string

Question

我是Java的新手，但不是編程的新手，但是有人可以幫助解釋為什么從AsyncTask讀取R.string時遇到以下崩潰嗎？

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference

我有一個與AsyncTask的類：

public class getBitmapFromURL extends AsyncTask<String, Void, Bitmap>{

    ImageView bmImage;
    private Context mContext;

    public getBitmapFromURL( Context context, ImageView bmImage) {
        this.bmImage = bmImage;
        mContext = context;
    }

在doInBackground方法中：

protected Bitmap doInBackground(String... urls) {

try {
            savebitmap( mIcon11, target_file[1]);
        } catch (IOException e) {
            e.printStackTrace();
        }
}

以及文件保存方法本身：

private File savebitmap(Bitmap bmp, String file_name) throws IOException {

//**** --- >>> This line causes the crash:
String app_name = mContext.getApplicationContext().getString(R.string.app_name);



    Log.e( "APP_NAME INFO:", app_name);
                File app_dest_folder = new File(Environment.getExternalStorageDirectory () + "/" + app_name );
                if(!app_dest_folder.exists()){
                    app_dest_folder.mkdir();
...

調用AsyncTask是從GetContent類完成的：

public class GetContent extends AppCompatActivity {

private Context context;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_get_content);
    loadimages();
}

    public void loadimages() {
new getBitmapFromURL(context, (ImageView)findViewById(R.id.wc_latest_iv)).execute(url);
}

...

Answer 1

使用它來執行asynctask類

   new getBitmapFromURL(GetContent.this, (ImageView)findViewById(R.id.wc_latest_iv)).execute(url);

這是因為您尚未初始化context 。