Java socket connect exception issue (Connection timed out)

Question

Some background:

I have an Android app that uses a 3G or 4G data connection. But it also connects to a wifi hotspot to transfer some data between the hotspot device and the app.

What I want to do: create a socket connection to this wifi hotspot and send/receive data over this socket.

I added the following code to ensure that we use the wifi hotspot wifi while creating a socket (else it sometimes ends up using the data connection):

android.net.wifi.WifiManager wm = (android.net.wifi.WifiManager) getSystemService(WIFI_SERVICE);
String ip = Formatter.formatIpAddress(wm.getConnectionInfo().getIpAddress());

This seems to be fine. However, I see issues while trying to connect to the remote server address. Here's the code I am using:

InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
InetAddress localAddress = InetAddress.getByName(ip);
socket = new Socket(serverAddr,SERVERPORT,localAddress,0);

I am using a socket call that connects to a remote server address from a local address (the wifi connection wifi address obtained as above). I am setting the local port to 0 since that will allow the socket to pick any local available port. However, when I do this, I get a connection timed out error all the time.

java.net.ConnectException: failed to connect to /A.B.C.D (port 3003) from /A.B.C.E (port 48492): connect failed: ETIMEDOUT (Connection timed out)

I'm sure there's no issue with the server socket code, since I am able to connect to the server if I do not use a local address. The below code always works:

InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
                socket = new Socket(serverAddr, SERVERPORT);

However, I cannot use it always because I want data and wifi to be ON simultaneously and the above code does not guarantee that it will always use the wifi connection to connect. (in fact it rarely works)

I also tried manually doing a bind to the local wifi ip address and some random port. and then doing a socket connect. But that did not work either.

I initially thought that the port number does not get closed properly if I use it once. But then I noticed that this issue often occurs even when I open the app for the first time. I even tried setting port to 0 (ephemeral port) to ensure it picks up any available port.

Any suggestions. If there's a better way to solve this problem, pl let me know.

Thanks in advance.

This is a network connectivity problem, not a coding problem. Can you ping that server from that client? — user207421
As mentioned in the post, I can connect to the server from that client if I just connect from client to server without specifying that the socket should use the local wifi address. — Omi
In addition, my team member has a test app that jst tests if you can connect to a particular server and if aconnection is alive. That also showed that the server is alive. For all these reasons, I am sure tht the server is good. — Omi
Just figured out the problem. Android does not support running both wifi and 3G (or 4G) on the same phone. By default, it chooses the wifi network if it has internet conectivity. Else, it chooses the data connection.In this case, it tries to connect over the 3G data connection instead of the wifi - even if I explicitly choose the wifi network ip address. — Omi

dhahn dhahn · Accepted Answer · 2019-07-16T04:36:04

Make sure to run the smartphone and computer are connected to the same internet connection! (i.e.: smartphone may be using 4G or its own network whereas computer can be on wifi)

Java socket connect exception issue (Connection timed out)

1 Answers