android下载url,Android:从URL下载

I'm trying an example download code online and I found out that it downloads to about 13KB and then exits. The file is 4 MB. Can anybody help me explain why this is? Here's the code. Thanks.

//this is the total size of the file

int totalSize = urlConnection.getContentLength();

//variable to store total downloaded bytes

int downloadedSize = 0;

//create a buffer...

byte[] buffer = new byte[1024];

int bufferLength = 0; //used to store a temporary size of the buffer

//now, read through the input buffer and write the contents to the file

while ( (bufferLength = inputStream.read(buffer)) > 0 ) {

//add the data in the buffer to the file in the file output stream (the file on the sd card

fileOutput.write(buffer, 0, bufferLength);

//add up the size so we know how much is downloaded

downloadedSize += bufferLength;

//this is where you would do something to report the prgress, like this maybe

//updateProgress(downloadedSize, totalSize);

Log.w( "DOWNLOAD" , "progress " + downloadedSize + " / " + totalSize);

I noticed that the totalSize is -1 when calling getContentLength()...probably because of the missing content length from the header.