Validating URL in Java

Question

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.

Answer 1

Just important to point that the URL object handle both validation and connection. Then, only protocols for which a handler has been provided in sun.net.www.protocol are authorized (file, ftp, gopher, http, https, jar, mailto, netdoc) are valid ones. For instance, try to make a new URL with the ldap protocol:

new URL("ldap://myhost:389")

You will get a java.net.MalformedURLException: unknown protocol: ldap.

You need to implement your own handler and register it through URL.setURLStreamHandlerFactory(). Quite overkill if you just want to validate the URL syntax, a regexp seems to be a simpler solution.

Answer 2

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
}

Answer 3

Using only standard API, pass the string to a URL object then convert it to a URI object. This will accurately determine the validity of the URL according to the RFC2396 standard.

Example:

public boolean isValidURL(String url) {  

    URL u = null;

    try {  
        u = new URL(url);  
    } catch (MalformedURLException e) {  
        return false;  
    }

    try {  
        u.toURI();  
    } catch (URISyntaxException e) {  
        return false;  
    }  

    return true;  
} 

Answer 4

There is a way to perform URL validation in strict accordance to standards in Java without resorting to third-party libraries:

boolean isValidURL(String url) {
  try {
    new URI(url).parseServerAuthority();
    return true;
  } catch (URISyntaxException e) {
    return false;
  }
}

The constructor of URI checks that url is a valid URI, and the call to parseServerAuthority ensures that it is a URL (absolute or relative) and not a URN.

Answer 5

Thanks. Opening the URL connection by passing the Proxy as suggested by NickDK works fine.

//Proxy instance, proxy ip = 10.0.0.1 with port 8080
Proxy proxy = new Proxy(Proxy.Type.HTTP, new InetSocketAddress("10.0.0.1", 8080));
conn = new URL(urlString).openConnection(proxy);

System properties however doesn't work as I had mentioned earlier.

Thanks again.

Regards, Keya

Answer 6

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");
}