Spring Boot4 min read2026-03-01

Spring Boot Port 8080 Already in Use

Fix WebServerException: Port 8080 was already in use by killing the conflicting process or assigning a custom port.

Error Code / Stack Trace

org.springframework.boot.web.server.PortInUseException: Port 8080 was already in use

Problem Overview

When launching a Spring Boot application, Embedded Tomcat fails to start because port 8080 is already bound by another background application, a previously unclosed JVM instance, or Docker container.

Why Does This Happen?

A previous instance of the Spring Boot application is still running in the background.
Another local server (Jenkins, Tomcat, Oracle XE, or Docker) is bound to port 8080.
The IDE crashed without terminating the spawned Java child process.

Step-by-Step Solution

Step 1: Identify the process listening on port 8080

Run the terminal command to find the Process Identifier (PID) using port 8080.

bash

lsof -i :8080
# On Windows: netstat -ano | findstr :8080

Step 2: Terminate the blocking process

Kill the process using its PID (replace 12345 with the actual PID from step 1).

bash

kill -9 12345
# On Windows: taskkill /PID 12345 /F

Step 3: Change the server port in application properties

If port 8080 is reserved by a permanent system service, configure Spring Boot to use another port.

properties

# application.properties
server.port=8081

# Or application.yml
server:
  port: 8081

Alternative Workarounds

Assign a random available port (ideal for automated tests)

Set server.port to 0 to let the OS allocate any free ephemeral port.

text

server.port=0

Override port via command line argument

Pass the port override flag at runtime without modifying code.

text

java -jar target/app.jar --server.port=9090

Common Mistakes to Avoid

•Restarting the IDE without checking if the orphaned background Java process is still active.
•Changing the port in application.properties but forgetting that environment variable SERVER_PORT overrides it.
•Forgetting that Docker host port mapping (-p 8080:8080) reserves the host port.

Prevention & Best Practices

Always configure graceful shutdown hooks in Spring Boot.
Use docker-compose port variables rather than hardcoding port 8080 across multiple microservices.

Frequently Asked Questions

Why does Spring Boot default to port 8080?

Port 8080 is the standard alternate HTTP port established by Apache Tomcat and the Java Servlet Specification.

How do I check what port Spring Boot picked when server.port=0?

Inject ServletWebServerApplicationContext and call getWebServer().getPort(), or watch the console logs for 'Tomcat started on port(s): XXXXX'.