Why Am I Getting 'java.net.ConnectException: Connection refused'?

"java.net.ConnectException: Connection refused" Explained

When executing a TCP client program, encountering the error "java.net.ConnectException: Connection refused" indicates that the client is unable to establish a connection with the target IP and port.

Causes of the Error:

• Incorrect IP or Port: Ensure that the client is targeting the correct IP address and port number of the server.
• Server Unavailability: Verify that the server is running and listening for connections on the specified port.
• Firewall or Network Restrictions: Check if a firewall or network configuration is blocking the connection attempt.
• Server Listen Queue Full (Windows Only): Windows servers may have a limited listen backlog queue. If the queue is full, new connections may be rejected.

Resolving the Issue:

• Validate IP and Port: Double-check the IP address and port number used in the client code.
• Start the Server: Ensure that the TCP server is up and running before running the client.
• Configure Network Access: If applicable, configure firewall rules to allow connections to the server's IP and port.
• Adjust Listen Queue (Windows Only): If using Windows, increase the listen backlog queue by adjusting the "ListenLimit" registry value.

Additional Troubleshooting:

• Check Server Logs: Examine server logs for any errors or warnings related to connection attempts.
• Use a Network Sniffer: Utilize a tool like Wireshark to monitor network traffic and identify any dropped packets or connection issues.
• Contact Network Administrator: If all else fails, consult with the network administrator to rule out infrastructure-related problems.

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