我正在嘗試將圖像從 Web 服務器檢索到我的應用程序中。但是，當我在應用程序中查看圖像時，應用程序崩潰了。這是代碼：class DownloadFileFromURL extends AsyncTask<String, String, String> {    private String file_url=null;    /**     * Before starting background thread     * Show Progress Bar Dialog     * */    @Override    protected void onPreExecute() {        super.onPreExecute();        toggleProgress();    }    public boolean dir_exists(String dir_path)    {        boolean ret = false;        File dir = new File(dir_path);        if(dir.exists() && dir.isDirectory())            ret = true;        return ret;    }    /**     * Downloading file in background thread     * */    @Override    protected String doInBackground(String... f_url) {        int count;        try {            URL url = new URL(f_url[0]);            URLConnection conection = url.openConnection();            conection.connect();            // this will be useful so that you can show a tipical 0-100% progress bar            int lenghtOfFile = conection.getContentLength();            // download the file            InputStream input = new BufferedInputStream(url.openStream(), 8192);            String dir_path = Environment.getExternalStorageDirectory().toString() + getResources().getString(R.string.DownloadFolder);            if (!dir_exists(dir_path)){                File directory = new File(dir_path);                if(directory.mkdirs()){                    Log.v("dir","is created 1");                }else{                    Log.v("dir","not created 1");                }                if(directory.mkdir()){                    Log.v("dir","is created 2");                }else{                    Log.v("dir","not created 2");                }            }else{                Log.v("dir","is exist");            }