Spring Boot 'Web server failed to start. Port 8080 was already in use.' / 'Embedded servlet container failed to start. Port 8080 was already in use': how do I fix it?

Cause: Another process (often a previous run of the same application, an IDE-launched copy, or another service) is already listening on the port Spring Boot wants. The embedded server defaults to port 8080. Spring Boot's failure analyzer prints an 'APPLICATION FAILED TO START' block whose action says to identify and stop the process listening on that port, or configure the application to use another port. Fix (best first): 1. Find what holds the port and stop it if it is a leftover: on Linux or macOS use ss -ltnp or lsof -i :8080, on Windows use netstat -ano and then check the process id in Task Manager. Stop only a process you recognize. 2. Or choose another port with the server.port property, for example server.port=9090 in application.properties (server: port: 9090 in YAML), or the SERVER_PORT environment variable, which works through relaxed binding. 3. For tests and parallel runs, use server.port=0 so the operating system picks a free port; in @SpringBootTest use webEnvironment = RANDOM_PORT and inject the chosen port with @LocalServerPort (tests only). 4. If you want no HTTP endpoints at all but still a web application context, server.port=-1 disables them. 5. In containers or Kubernetes, check that two processes in the same pod or host-network container are not both binding the port. Verify: The application starts and logs the embedded server started on the port you configured, and a request to that port is answered by your application. Sources: https://docs.spring.io/spring-boot/how-to/webserver.html https://docs.spring.io/spring-boot/reference/features/spring-application.html