Java URLConnection : how could I find out a file's size ?

Question

I'm working on a project for school , and I'm implementing a tool which can be used to download files from the web ( with a throttling option ) . The thing is , I'm gonna have a GUI for it , and I will use a JProgressBar widget , which I would like to reflect how much the tool downloaded so far , and for that I would need to know the size of the file. How could I do this ?

Answer 1

Any HTTP response is supposed to contain a Content-Length header, so you could query the URLConnection object for this value.

//once the connection has been opened
List values = urlConnection.getHeaderFields().get("content-Length")
if (values != null && !values.isEmpty()) {

    // getHeaderFields() returns a Map with key=(String) header 
    // name, value = List of String values for that header field. 
    // just use the first value here.
    String sLength = (String) values.get(0);

    if (sLength != null) {
       //parse the length into an integer...
       ...
    }

It might not always be possible for a server to return an accurate Content-Length, so the value could be inaccurate, but at least you would get some usable value most of the time.

update: Or, now that I look at the URLConnection javadoc more completely, you could just use the getContentLength() method.

Answer 2

You'll want to use the content length (URLConnection.getContentLength()). Unfortunately, this won't always be accurate, or may not always be provided, so it's not always safe to rely on it.