URLConnection getContentLength()返回一个负值

Here is my code:

url = paths[0];
HttpURLConnection connection = (HttpURLConnection) url.openConnection();
int length = connection.getContentLength(); // i get negetive length
InputStream is = (InputStream) url.getContent();
byte[] imageData = new byte[length]; 
int buffersize = (int) Math.ceil(length / (double) 100);
int downloaded = 0;
int read;
while (downloaded < length) {
    if (length < buffersize) {
        read = is.read(imageData, downloaded, length);
    } else if ((length - downloaded) <= buffersize) {
        read = is.read(imageData, downloaded, length - downloaded);
    } else {
        read = is.read(imageData, downloaded, buffersize);
    downloaded += read;
    publishProgress((downloaded * 100) / length);
Bitmap bitmap = BitmapFactory.decodeByteArray(imageData, 0,
        length);
if (bitmap != null) {
    Log.i(TAG, "Bitmap created");
} else {
    Log.i(TAG, "Bitmap not created");
is.close();
return bitmap;
我在Java文档中看了一下，长度是负的，因为有以下原因。
  
"the number of bytes of the content, or a negative number if unknown. If the content >length is known but exceeds Long.MAX_VALUE, a negative number is returned."  
这可能是什么原因？我正试图下载一张图片。我想指出的是，这是我试图下载图像的第四种方法。其他三种方法都提到了here.
Edit: 
按照要求，这里是我使用的全部方法。
protected Bitmap getImage(String imgurl) {
    try {
        URL url = new URL(imgurl);
        HttpURLConnection connection = (HttpURLConnection) url.openConnection();
        int length = connection.getContentLength();
        InputStream is = (InputStream) url.getContent();
        byte[] imageData = new byte[length];
        int buffersize = (int) Math.ceil(length / (double) 100);
        int downloaded = 0;
        int read;
        while (downloaded < length) {
            if (length < buffersize) {
                read = is.read(imageData, downloaded, length);
            } else if ((length - downloaded) <= buffersize) {
                read = is.read(imageData, downloaded, length
                        - downloaded);
            } else {
                read = is.read(imageData, downloaded, buffersize);
            downloaded += read;
        //  publishProgress((downloaded * 100) / length);
        Bitmap bitmap = BitmapFactory.decodeByteArray(imageData, 0,length);
        if (bitmap != null) {
             System.out.println("Bitmap created");
        } else {
            System.out.println("Bitmap not created");
        is.close();
        return bitmap;
    } catch (MalformedURLException e) {
        System.out.println(e);
    } catch (IOException e) {
        System.out.println(e);
    } catch (Exception e) {
        System.out.println(e);
    return null;