Java detect lost connection [duplicate]

Question

When I'm using e.g. PuTTY and my connection gets lost (or when I do a manual ipconfig /release on Windows), it responds directly and notifies my connection was lost.

I want to create a Java program which monitors my Internet connection (to some reliable server), to log the date/times when my internet fails.

I tried use the Socket.isConnected() method but that will just forever return "true". How can I do this in Java?

Answer 1

Well, the best way to tell if your connection is interrupted is to try to read/write from the socket. If the operation fails, then you have lost your connection sometime.

So, all you need to do is to try reading at some interval, and if the read fails try reconnecting.

The important events for you will be when a read fails - you lost connection, and when a new socket is connected - you regained connection.

That way you can keep track of up time and down time.

Answer 2

Even though TCP/IP is "connection oriented" protocol, normally no data is sent over an idle connection. You can have a socket open for a year without a single bit sent over it by the IP stack. In order to notice that a connection is lost, you have to send some data on the application level.(*) You can try this out by unplugging the phone cable from your ADSL modem. All connections in your PC should stay up, unless the applications have some kind of application level keepalive mechanism.

So the only way to notice lost connection is to open TCP connection to some server and read some data from it. Maybe the most simple way could be to connect to some FTP server and fetch a small file - or directory listing - once in a while. I have never seen a generic server which was really meant to be used for this case, and owners of the FTP server may not like clients doing this.

(*) There is also a mechanism called TCP keepalive but in many OS's you have to activate it for all applications, and it is not really practical to use if you want to notice loss of connection quickly