How to check if internet connection is present in java?

Question

How do you check if you can connect to the internet via java? One way would be:

final URL url = new URL("http://www.google.com");
final URLConnection conn = url.openConnection();
... if we got here, we should have net ...

But is there something more appropriate to perform that task, especially if you need to do consecutive checks very often and a loss of internet connection is highly probable?

Answer 1

You should connect to the place that your actual application needs. Otherwise you're testing whether you have a connection to somewhere irrelevant (Google in this case).

In particular, if you're trying to talk to a web service, and if you're in control of the web service, it would be a good idea to have some sort of cheap "get the status" web method. That way you have a much better idea of whether your "real" call is likely to work.

In other cases, just opening a connection to a port that should be open may be enough - or sending a ping. InetAddress.isReachable may well be an appropriate API for your needs here.

Answer 2

This seems to be a duplicate of this question: Detect internet Connection using Java

Answer 3

1) Figure out where your application needs to be connecting to.

2) Set up a worker process to check InetAddress.isReachable to monitor the connection to that address.

Answer 4

People have suggested using INetAddress.isReachable. The problem is that some sites configure their firewalls to block ICMP Ping messages. So a "ping" might fail even though the web service is accessible.

And of course, the reverse is true as well. A host may respond to a ping even though the webserver is down.

And of course, a machine may be unable to connect directly to certain (or all) web servers due to local firewall restrictions.

The fundamental problem is that "can connect to the internet" is an ill-defined question, and this kind of thing is difficult to test without:

1. information on the user's machine and "local" networking environment, and
2. information on what the app needs to access.

So generally, the simplest solution is for an app to just try to access whatever it needs to access, and fall back on human intelligence to do the diagnosis.

Answer 5

If you're on java 6 can use NetworkInterface to check for available network interfaces. I.e. something like this:

Enumeration<NetworkInterface> interfaces = NetworkInterface.getNetworkInterfaces();
while (interfaces.hasMoreElements()) {
  NetworkInterface interf = interfaces.nextElement();
  if (interf.isUp() && !interf.isLoopback())
    return true;
}

Haven't tried it myself, yet.

Answer 6

URL url=new URL("http://[any domain]"); URLConnection con=url.openConnection();

/*now errors WILL arise here, i hav tried myself and it always shows "connected" so we'll open an InputStream on the connection, this way we know for sure that we're connected to d internet */

url.getInputStream();

//put the above statements in try catch blocks and if an exception in caught means that //there's no internet connection established. :-)

Answer 7

I usually break it down into three steps.

1. I first see if I can resolve the domain name to an IP address.
2. I then try to connect via TCP (port 80 and/or 443) and close gracefully.
3. Finally, I'll issue an HTTP request and check for a 200 response back.

If it fails at any point, I provide the appropriate error message to the user.