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Changing the port number of a Spring Boot application is a common requirement, especially when you need to run multiple applications on the same machine or avoid conflicts with other services. Spring Boot provides several ways to customize the port number. Here are the most common methods you can use:

1. Using application.properties or application.yml

The simplest way to change the port number is by setting it in the application.properties or application.yml file. Depending on the format of your configuration file, you can specify the port as follows:

• In application.properties:

properties

server.port=8081

• In application.yml:

yaml

server:
  port: 8081

This method is useful for applications where you want a persistent configuration and is easily editable.

2. Using Command-Line Arguments

Another way to change the port is by passing the server.port property as a command-line argument when starting your Spring Boot application:

bash

java -jar myapp.jar --server.port=8081

This method is convenient when you want to quickly change the port number without modifying configuration files.

3. Using System Properties

You can also set the port number by passing it as a system property when running the application. This is done using the -D flag:

bash

java -jar -Dserver.port=8081 myapp.jar

Setting the port number this way is particularly useful if you're running your application in an environment where you want to programmatically set the port before starting.

4. Using Environment Variables

Spring Boot allows you to configure the port number using environment variables. This method is especially useful in cloud environments, Docker containers, or other environments where you prefer to set configuration outside of the application code.

bash

export SERVER_PORT=8081
java -jar myapp.jar

Alternatively, you can also set the environment variable directly when running the application in a one-liner:

bash

SERVER_PORT=8081 java -jar myapp.jar

5. Embedded Servlet Container Customization

If you're using an embedded servlet container like Tomcat, Jetty, or Undertow, you can customize the port number programmatically in your Spring Boot application. Here’s how you can do this in your main application class:

java

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.servlet.support.SpringBootServletInitializer;
import org.springframework.boot.web.embedded.tomcat.TomcatServletWebServerFactory;
import org.springframework.boot.web.servlet.server.ConfigurableServletWebServerFactory;
import org.springframework.context.annotation.Bean;

@SpringBootApplication
public class MyApplication extends SpringBootServletInitializer {

    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }

    @Bean
    public ConfigurableServletWebServerFactory webServerFactory() {
        TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
        factory.setPort(8081); // Set custom port
        return factory;
    }
}

This allows you to programmatically configure the port and other properties of the embedded servlet container.

Conclusion

Spring Boot provides multiple ways to change the default port number for your application. Depending on your needs—whether it's for local development, testing, or production—any of the methods outlined above can be used. Choose the method that fits your workflow best!