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

User · Accepted Answer

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.

User · Answer

If you re on java 6 can use NetworkInterface to check for available network interfaces  I e  something like this   Enumeration lt NetworkInterface gt  interfaces   NetworkInterface getNetworkInterfaces    while  interfaces hasMoreElements        NetworkInterface interf   interfaces nextElement      if  interf isUp    amp  amp   interf isLoopback        return true      Haven t tried it myself  yet

User · Answer

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        Get input stream    con getInputStream      Put the above statements in try catch blocks and if an exception in caught means that there s no internet connection established   -

User · Answer

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    information on the user s machine and  local  networking environment  and 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

User · Answer

There is also a gradle option --offline which maybe results in the behavior you want

User · Answer

I usually break it down into three steps    I first see if I can resolve the domain name to an IP address  I then try to connect via TCP  port 80 and or 443  and close gracefully  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

User · Answer

public boolean checkInternetConnection          boolean status   false       Socket sock   new Socket         InetSocketAddress address   new InetSocketAddress  www google com   80         try                sock connect address  3000           if sock isConnected    status   true              catch Exception e                  status   false                     finally                try                        sock close                        catch Exception e                 return status

User · Answer

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

User · Answer

This code    127 0 0 1  equals InetAddress getLocalHost   getHostAddress   toString       Returns - to me - true if offline  and false  otherwise   well  I don t know if this true to all computers     This works much faster than the other approaches  up here     EDIT  I found this only working  if the  flip switch   on a laptop   or some other system-defined option  for the internet connection  is off  That s  the system itself knows not to look for any IP addresses

User · Answer

This code is contained within a jUnit test class I use to test if a connection is available  I always receive a connection  but if you check the content length it should be -1 if not known       try       URL url   new URL  http   www google com        URLConnection connection   url openConnection         if connection getContentLength      -1             fail  Failed to verify connection                 catch  IOException e          fail  Failed to open a connection          e printStackTrace

User · Answer

The code using NetworkInterface to wait for the network worked for me until I switched from fixed network address to DHCP  A slight enhancement makes it work also with DHCP     Enumeration lt NetworkInterface gt  interfaces   NetworkInterface getNetworkInterfaces    while  interfaces hasMoreElements          NetworkInterface interf   interfaces nextElement        if  interf isUp    amp  amp   interf isLoopback          List lt InterfaceAddress gt  adrs   interf getInterfaceAddresses        for  Iterator lt InterfaceAddress gt  iter   adrs iterator    iter hasNext               InterfaceAddress adr   iter next            InetAddress inadr   adr getAddress            if  inadr instanceof Inet4Address  return true                          This works for Java 7 in openSuse 13 1 for IPv4 network  The problem with the original code is that although the interface was up after resuming from suspend  an IPv4 network address was not yet assigned  After waiting for this assignment  the program can connect to servers  But I have no idea what to do in case of IPv6

User · Answer

You can simply write like this import java net InetAddress  import java net UnknownHostException   public class Main        private static final String HOST    quot localhost quot        public static void main String   args  throws UnknownHostException            boolean isConnected    HOST equals InetAddress getLocalHost   getHostAddress   toString              if  isConnected  System out println  quot Connected quot            else System out println  quot Not connected quot

User · Answer

The code you basically provided  plus a call to connect should be sufficient  So yeah  it could be that just Google s not available but some other site you need to contact is on but how likely is that  Also  this code should only execute when you actually fail to access your external resource  in a catch block to try and figure out what the cause of the failure was  so I d say that if both your external resource of interest and Google are not available chances are you have a net connectivity problem   private static boolean netIsAvailable         try           final URL url   new URL  http   www google com            final URLConnection conn   url openConnection            conn connect            conn getInputStream   close            return true        catch  MalformedURLException e            throw new RuntimeException e         catch  IOException e            return false

User · Answer

This code should do the job reliably   Note that when using the try-with-resources statement we don t need to close the resources   import java io IOException  import java net InetSocketAddress  import java net Socket  import java net UnknownHostException   public class InternetAvailabilityChecker       public static boolean isInternetAvailable   throws IOException               return isHostAvailable  google com      isHostAvailable  amazon com                      isHostAvailable  facebook com     isHostAvailable  apple com               private static boolean isHostAvailable String hostName  throws IOException               try Socket socket   new Socket                          int port   80              InetSocketAddress socketAddress   new InetSocketAddress hostName  port               socket connect socketAddress  3000                return true                    catch UnknownHostException unknownHost                        return false

User · Answer

InetAddress isReachable sometime return false if internet connection exist    An alternative method to check internet availability in java is   This function make a real ICMP ECHO ping   public static boolean isReachableByPing String host         try                  String cmd                       if System getProperty  os name   startsWith  Windows                                    For Windows                         cmd    ping -n 1     host                    else                              For Linux and OSX                         cmd    ping -c 1     host                                     Process myProcess   Runtime getRuntime   exec cmd                   myProcess waitFor                     if myProcess exitValue      0                             return true                    else                            return false                               catch  Exception e                      e printStackTrace                    return false

User · Answer

The following piece of code allows us to get the status of the network on our Android device     public class MainActivity extends AppCompatActivity            Override         protected void onCreate Bundle savedInstanceState                super onCreate savedInstanceState               setContentView R layout activity main               TextView mtv findViewById R id textv               ConnectivityManager connectivityManager                     ConnectivityManager  this getSystemService Context CONNECTIVITY SERVICE               if  Build VERSION SDK INT  gt   Build VERSION CODES M                    if   Network connectivityManager getActiveNetwork     null                      mtv setText  true                    else                     mtv setText  fasle

[java] How to check if internet connection is present in Java?

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