将图像文件转换为Base 64以进行上载

Question

我是android的新手，想要将选中的图像从缩略图网格转换为string64，然后传递给服务器。请帮忙。！！！！ 我不知道如何将图像文件转换为字符串变量并将它们传递给String64进行转换。此外，我希望图片是.JPEG类型。

public class UploadFiles extends Activity {
private int count;
private Bitmap[] thumbnails;
private boolean[] thumbnailsselection;
private String[] arrPath;
private ImageAdapter imageAdapter;

/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.main);

    final String[] columns = { MediaStore.Images.Media.DATA,MediaStore.Images.Media._ID };
    final String orderBy = MediaStore.Images.Media._ID;
    Cursor imagecursor = managedQuery(
            MediaStore.Images.Media.EXTERNAL_CONTENT_URI, columns, null,
            null, orderBy);
    int image_column_index = imagecursor.getColumnIndex(MediaStore.Images.Media._ID);
    this.count = imagecursor.getCount();
    this.thumbnails = new Bitmap[this.count];
    this.arrPath = new String[this.count];
    this.thumbnailsselection = new boolean[this.count];

    for (int i = 0; i < this.count; i++) {
        imagecursor.moveToPosition(i);
        int id = imagecursor.getInt(image_column_index);
        int dataColumnIndex = imagecursor.getColumnIndex(MediaStore.Images.Media.DATA);
        thumbnails[i] = MediaStore.Images.Thumbnails.getThumbnail(
                getApplicationContext().getContentResolver(), id,
                MediaStore.Images.Thumbnails.MICRO_KIND, null);
        arrPath[i]= imagecursor.getString(dataColumnIndex);
    }
    GridView imagegrid = (GridView) findViewById(R.id.PhoneImageGrid);
    imageAdapter = new ImageAdapter();
    imagegrid.setAdapter(imageAdapter);

    final Button selectBtn = (Button) findViewById(R.id.selectBtn);
    selectBtn.setOnClickListener(new OnClickListener() {

        public void onClick(View v) {
            // TODO Auto-generated method stub
            final int len = thumbnailsselection.length;
            int cnt = 0;
            String selectImages = "";
            for (int i =0; i<len; i++)
            {
                if (thumbnailsselection[i]){
                    cnt++;
                    selectImages = selectImages + arrPath[i] + "|";
                }
            }
            if (cnt == 0){
                Toast.makeText(getApplicationContext(),
                        "Please select at least one image",
                        Toast.LENGTH_LONG).show();
            } else {

                Toast.makeText(getApplicationContext(),
                        "You've selected Total " + cnt + " image(s).",
                        Toast.LENGTH_LONG).show();
                Log.d("SelectedImages", selectImages);
            }
        }
    });
}

public class ImageAdapter extends BaseAdapter {
    private LayoutInflater mInflater;

    public ImageAdapter() {
        mInflater = (LayoutInflater) getSystemService(Context.LAYOUT_INFLATER_SERVICE);
    }

    public int getCount() {
        return count;
    }

    public Object getItem(int position) {
        return position;
    }

    public long getItemId(int position) {
        return position;
    }

    public View getView(int position, View convertView, ViewGroup parent) {
        ViewHolder holder;
        if (convertView == null) {
            holder = new ViewHolder();
            convertView = mInflater.inflate(
                    R.layout.galleryitem, null);
            holder.imageview = (ImageView) convertView.findViewById(R.id.thumbImage);
            holder.checkbox = (CheckBox) convertView.findViewById(R.id.itemCheckBox);

            convertView.setTag(holder);
        }
        else {
            holder = (ViewHolder) convertView.getTag();
        }
        holder.checkbox.setId(position);
        holder.imageview.setId(position);
        holder.checkbox.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                // TODO Auto-generated method stub
                CheckBox cb = (CheckBox) v;
                int id = cb.getId();
                if (thumbnailsselection[id]){
                    cb.setChecked(false);
                    thumbnailsselection[id] = false;
                } else {
                    cb.setChecked(true);
                    thumbnailsselection[id] = true;
                }
            }
        });
        holder.imageview.setOnClickListener(new OnClickListener() {

            public void onClick(View v) {
                // TODO Auto-generated method stub
                int id = v.getId();
                Intent intent = new Intent();
                intent.setAction(Intent.ACTION_VIEW);
                intent.setDataAndType(Uri.parse("file://" + arrPath[id]), "image/*");
                startActivity(intent);
            }
        });
        holder.imageview.setImageBitmap(thumbnails[position]);
        holder.checkbox.setChecked(thumbnailsselection[position]);
        holder.id = position;
        return convertView;
    }
}
class ViewHolder {
    ImageView imageview;
    CheckBox checkbox;
    int id;
}