Check if URL exists (Java)

[Kelvin Little]

I can't seem to find the right method to check if a URL exists (it's not a HTTP 404).

InputStream input = url.OpenStream();
input = new BufferedInputStream(input);

if(input.available() > 0) {
//...
}

Doesn't seem to work. URLs that don't exist still pass the if statement...?

Does anyone else know the solution to this problem?

[Kelvin Little]

I found the method:

if(url.openConnection().getContentLength() > 0) {
//...
}

How reliable is this method for any kind of URL content? For example: It will return -1 if the content-length is unknown, but does that also mean the file doesn't exist?

[Modifier]

Data (the error page) comes back on the input stream for a 404 error, and moreover, available() doesn't do quite what you think it does. All in all, your approach will identify missing pages only by accident, and will falsely identify existing pages as missing by the same accident.

[kelfro]

Does anyone else know the solution to this problem?

Assuming you are using HTTP URLs, Have you tried testing

((HttpUrlConnection) url.openConnection()).getResponseCode()

?

Neither your method nor my suggestion accounts for cases where the entire server is unavailable; chances are that you will be able to handle an exception to detect that case.

[Kelvin Little]

Ok, I should have googled before I asked that question. According to the W3C any content-length of 0 or greater is valid, unless a HTTP status code is sent and in a few other (rare?) cases.

[kelfro]

HttpURLConnection.getResponseCode() seems to be the correct way to go in all aspects (clear intention and exact fit with the HTTP specification).

If you experience that checking the contentLength also work even when the server is returning an error page, I would guess it must be because the HttpURLConnection implementation then delivers all content on the error stream instead of the output stream on responses outside 200-299.

[Kelvin Little]

Thanks for the info.

I tested the getContent-Length method and it behaves correctly when the URL is unavailable (and another page is returned).

[Nabs]

Hiiii, following code is part of my pgm,will u plz tell me why its giving malformedurlexception no protocol error... plzz


for (i =nItems; i>=0;i--) // start at end,
{
String s=queArray[i];
URL hp1=new URL(s);
URLConnection hpcon1=hp1.openConnection();
int len1=hpcon1.getContentLength();
System.out.println("Welcome"+len+" "+len1);
if (len<len1) // if new item larger,
queArray[i + 1] = queArray[i]; // shift upward
else
// if smaller,
break; // done shifting
}