Detect internet Connection using Java [duplicate]

Answer 1

I must add that although the final code block given above is good, it has one flaw - it is possible for it to take a very long time to contact the address specified, but the address is still reachable.

In my instance when testing with one address the method would return true, but take 10 seconds or longer to get a response. In this case the server was reachable, but not for any useful purposes since the connection was so slow. This occurs because the default timeout for HttpURLConnection is 0, or infinite.

For this reason I'd recommend you do the checking off the UI thread, and add urlConnect.setConnectTimeout(1000); before calling urlConnect.getContent();

This way you know the address is reachable, and that it won't take 5 years to download a 10k file.

(You might of course want to change the the timeout time to suit your needs)

Also I'd recommend not checking a generic address (google.com etc) unless your program generally accesses more than a few domains. If you're just accessing one or two then check that domain.

Answer 2

That's a perfectly reasonable approach to solving the problem. The bad thing is that you are really testing DNS rather than testing the whole network, but in practice you can often get by with treating those as equivalent.

The other thing to remember, is that you will need to set a system property to turn off dns caching in the java runtime. Otherwise it may continue to report that the network is up based upon cached data (even though it is down).

Another approach would be to actually open an HTTP request to some network address such as this every so often.

Answer 3

A problem with the first solution is that InetAddress has a cache, so, when you lose the connection for the next few invocation the name is resolved via the java cache. With the URL connection aproach you have the problem that you use getContent that should fetch html so there you have data consumption. If the invocations are done very often that could be a problem (more so if you dont have an unlimited data plan on the device running the software).

I think the best solution would be to do a TCP connection to the 80 port an close it inmediatly after a successfull connection. That would behave as the final code but would have much less traffic.

Answer 4

Haven't tested this, but I suggest looking at java.net.NetworkInterface.getNetworkInterfaces(). This returns an Enumeration of all network interfaces on the machine, or null if there are none.

I'm not sure if it's safe to assume that a non-null response ensures a valid network connection -- depending on your needs, you may or may not need to filter out loopback addresses (which I think you could do with java.net.NetworkInterface.getInetAddresses() on each returned NetworkInterface, and then calling InetAddress.isLoopbackAddress() on each one.)

Answer 5

The question doesn't really have a meaning. There is no such thing as a 'connection to the Internet'. You have to try to create one. The Windows API referred to only tells you whether your modem is dialled in or not, and I've seen it actually cause dial-ins, which is not exactly the idea. Not that I've had a dial-in for the last 8 years or so.

Answer 6

The only way to be CERTAIN that you can reach a given service, is to do a dummy request to that service. Pings may be blocked by firewalls. Some server may be reachable, others not. If you need to talk to a webservice, have a static page to return for these requests.

Also, remember to ask the user before trying to reach out.