new Thread(new Runnable() {

    @Override

    public void run() {

        String fileName = url.substring(url.lastIndexOf("/") + 1);

        // 如果不是，就加尾缀

        if (!fileName.endsWith(MimeUtils.guessExtensionFromMimeType(mimetype))) {

            fileName = fileName + "." + MimeUtils.guessExtensionFromMimeType(mimetype);

        }

        // 限制文件名的长度

        if (fileName.length() > 50) {

            fileName = fileName.substring(fileName.length() - 50);

        }

        HttpParams params = new BasicHttpParams();

        HttpConnectionParams.setConnectionTimeout(params, 5 * 1000);

        HttpConnectionParams.setSoTimeout(params, 5 * 1000);

        HttpGet httpGet = new HttpGet(url);

        try {

            File file = new File(Environment.getExternalStorageDirectory(), fileName);

            if (!file.exists()) {

                file.createNewFile();

            } else {

                boolean flag = file.delete();

                if (flag) {

                    file.createNewFile();

                } else {

                    // 目前设定是: 如果删除失败，就不进行下载了 
　　　　　　　　　　　　return;

                }

            }

            RandomAccessFile randomFile = new RandomAccessFile(file, "rw");

            HttpResponse response = new DefaultHttpClient(params).execute(httpGet);

            HttpEntity entity = response.getEntity();

            InputStream in = entity.getContent();

            randomFile.seek(randomFile.length());

            byte[] buffer = new byte[1024];

            int lenght = 0;

            while ((lenght = in.read(buffer)) > 0) {

                randomFile.write(buffer, 0, lenght);

                if (randomFile.length() == contentLength) {

                    // 下载完成

                }

            }

            randomFile.close();

            httpGet.abort();

        } catch (Exception e) {

            e.printStackTrace();

        }

    }

}).start();