Port already in use

The root error Port already in use occurs when an application attempts to start a network service on a port that is already occupied by another running process. Because operating systems allow only one process to listen on a specific port and protocol combination at a time, this error is common in Linux servers, Windows systems, Java and Spring Boot applications, Docker containers, and database or web server environments.

When does this error occur?

  • When starting a server application while another process is already listening on the same port
  • When a previously running service did not shut down cleanly and kept the port open
  • When multiple applications are configured to use the same default port
  • When a Docker container is still bound to a host port
  • When system services occupy common ports such as 8080, 3306, or 5432

Root cause of Port already in use

The Port already in use error is raised by the operating system’s networking stack. When a process binds to a TCP or UDP port, the OS locks that port for exclusive use. If another process attempts to bind to the same port before it is released, the OS rejects the request and reports this error. The root cause is always an existing listener, whether active, hung, or left behind after an improper shutdown.

How to fix the error (step-by-step)

Linux / macOS

sudo lsof -i :8080
sudo kill -9 <pid>

Windows

netstat -ano | findstr :8080
taskkill /PID <pid> /F

Java / Spring Boot

server.port=8081

Docker / containers

docker ps
docker stop <container_id>

Databases or other services

sudo systemctl status service-name
sudo systemctl stop service-name

Verify the fix

Restart the application after freeing or changing the port. If the service starts successfully without reporting Port already in use, the issue is resolved. You can also re-run the port check command to confirm that no other process is listening on the original port.

Common mistakes to avoid

  • Killing random processes without confirming they own the port
  • Hardcoding the same port across multiple applications
  • Ignoring background services that auto-restart
  • Forgetting to stop Docker containers after testing

Quick tip

Use environment-specific port configurations so development, testing, and production services never compete for the same ports.

FAQ

Q: Can Port already in use happen after a crash?

A: Yes. If an application crashes, the OS may keep the port reserved temporarily until the process is fully released.

Q: Does this error affect both TCP and UDP?

A: Yes. The same restriction applies separately to TCP and UDP ports.

Q: Is changing the port always the best fix?

A: No. If the port should be free, stopping the conflicting process is usually the correct solution.

Conclusion

The Port already in use error occurs when a port is occupied by another process, and resolving it requires freeing or changing that port. Check ErrorFixHub for solutions to related network and system-level root errors.

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