How to configure port for a Spring Boot application

Setting up dynamic ports

During integration testing, there may be the need for random port allocation to avoid conflicts. By assigning 0 to server.port:

server.port=0


Spring Boot will automatically choose an available port at startup.

Various methods to configure the port

In diverse development and deployment environments, you have several ways of configuring the port:

Command-line overrides

The command-line method is highly prioritised. Use -D to set system properties:

java -Dserver.port=8085 -jar app.jar


Through your IDE

In your IDE's run/debug configurations, specify application arguments directly:

--server.port=8082


Using environment variables

For dockerized systems or similar environments, you can use the system environment variables, such as:

export SERVER_PORT=8090
java -jar app.jar


Spring Boot's relaxed binding rules convert SERVER_PORT into server.port.

Programmatically setting the port

In Spring Boot 2.x, to set the port programmatically, use WebServerFactoryCustomizer:

@Component
public class ServerPortCustomizer implements WebServerFactoryCustomizer<ConfigurableWebServerFactory> {
  
    @Override
    public void customize(ConfigurableWebServerFactory factory) {
        factory.setPort(8084); // "Knock, knock! Who's there? Port 8084!"
    }
}


For older Spring Boot versions, EmbeddedServletContainerCustomizer offers similar functionality.

Using Maven or Gradle

For Maven or Gradle users, you can configure your port as part of the build process:

Maven:

mvn spring-boot:run -Dserver.port=8086


Gradle:

./gradlew bootRun --args='--server.port=8087'


An external configuration directory

Place application.properties or .yml in a ./config/ directory outside your jar to override port settings:

./config/application.properties


Dynamic environment configuration

Elastic cloud environments or container orchestration systems like Kubernetes might need dynamic configuration. Fortunately, Spring profiles and service discovery tools can manage these requirements.

Mind the troubleshooting and best practices

Configuring your port in Spring Boot is indeed fun, but few things you need to look out for to avoid walking into trap doors:

Watch for port conflicts

Ensure your chosen port isn't already taken. Use server.port=0 for a random available port.

Check for firewall restrictions

The firewall might be the doorkeeper to your port, so be sure to cross-check firewall rules to avoid blocked port access.

Use secure ports for HTTPS

Choose port 443 for HTTPS connections. Just be sure to have your SSL keys and certificates ready for the formal party!

Load balancers and proxies

If your application runs behind a load balancer or reverse proxy, configure properly to ensure the right port is exposed neatly.