Q:

Validating URL in Java

I wanted to know if there is any standard APIs in Java to validate a given URL?
I want to check both if the URL string is right i.e. the given protocol is valid and then to check if a connection can be established.
I tried using HttpURLConnection, providing the URL and connecting to it.  The first part of my requirement seems to be fulfilled but when I try to perform HttpURLConnection.connect(), 'java.net.ConnectException: Connection refused' exception is thrown.
Can this be because of proxy settings?  I tried setting the System properties for proxy but no success.
Let me know what I am doing wrong.

A:

For the benefit of the community, since this thread is top on Google when searching for
"url validator java"

Catching exceptions is expensive, and should be avoided when possible. If you just want to verify your String is a valid URL, you can use the UrlValidator class from the Apache Commons Validator project.
For example:
String[] schemes = {"http","https"}; // DEFAULT schemes = "http", "https", "ftp"
UrlValidator urlValidator = new UrlValidator(schemes);
if (urlValidator.isValid("ftp://foo.bar.com/")) {
   System.out.println("URL is valid");
} else {
   System.out.println("URL is invalid");
}

A:

You need to create both a URL object and a URLConnection object.  The following code will test both the format of the URL and whether a connection can be established:
try {
    URL url = new URL("http://www.yoursite.com/");
    URLConnection conn = url.openConnection();
    conn.connect();
} catch (MalformedURLException e) {
    // the URL is not in a valid form
} catch (IOException e) {
    // the connection couldn't be established
}

A:

The java.net.URL class is in fact not at all a good way of validating URLs. MalformedURLException is not thrown on all malformed URLs during construction. Catching IOException on java.net.URL#openConnection().connect() does not validate URL either, only tell wether or not the connection can be established.
Consider this piece of code:
    try {
        new URL("http://.com");
        new URL("http://com.");
        new URL("http:// ");
        new URL("ftp://::::@example.com");
    } catch (MalformedURLException malformedURLException) {
        malformedURLException.printStackTrace();
    }

..which does not throw any exceptions.
I recommend using some validation API implemented using a context free grammar, or in very simplified validation just use regular expressions. However I need someone to suggest a superior or standard API for this, I only recently started searching for it myself.
Note
It has been suggested that URL#toURI() in combination with handling of the exception java.net. URISyntaxException can facilitate validation of URLs. However, this method only catches one of the very simple cases above.
The conclusion is that there is no standard java URL parser to validate URLs.

