I have a Java Spring Boot application that at the service level creates a Socket connection to localhost and accesses another application written in C++.

When I run a Java application on a remote host and call this line, an error occurs:

Socket socket = new Socket("localhost", port);

java.net.ConnectException: Connection refused: connect
         at java.base/sun.nio.ch.Net.connect0(Native Method)  
         at java.base/sun.nio.ch.Net.connect(Net.java:579)   
         at java.base/sun.nio.ch.Net.connect(Net.java:568)  
         at java.base/sun.nio.ch.NioSocketImpl.connect(NioSocketImpl.java:593) 
         at java.base/java.net.SocksSocketImpl.connect(SocksSocketImpl.java:327) 
         at java.base/java.net.Socket.connect(Socket.java:633)   
         at java.base/java.net.Socket.connect(Socket.java:583) 
         at java.base/java.net.Socket.(Socket.java:507)    
         at java.base/java.net.Socket.(Socket.java:287) 

But at the same time, if I change the host to external IP, everything works fine.

Socket socket = new Socket("IP-Address", port);

The port that the C++ application uses has “Inbound” and “Outbound” Rules for TCP/UDP requests and it’s open.

Any ideas what is wrong here?