Spring Boot: Bootstrapping a web server

Spring Boot auto-configures an embedded Tomcat server the moment spring-boot-starter-web is on the classpath. A single @SpringBootApplication annotation on your main class combines @Configuration, @EnableAutoConfiguration, and @ComponentScan, getting a production-capable web server running with zero XML. This article covers project setup, the application entry point, and changing the default port.

When the jar present in class path, and when the application it automatically configures and starts a Tomcat server that is embedded inside this jar.

Bootstrapping a web server
How to change default port number for web server

Bootstrapping a web server

Add spring-boot-starter-web maven dependency to your project, for the below example we are using version 2.7.6. For the latest version of the jar available, check maven repository here spring-boot-starter-web

XML

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-web</artifactId>
    <version>2.7.6</version>
</dependency>

Next create a Java class with main() method and annotate the class with @SpringBootApplication.
By declaring the class with @SpringBootApplication annotation, it automatically configures the app, and applies component scan on the current package and sub package classes. This is equivalent to annotating the class with @Configuration, @EnableAutoConfiguration, and @ComponentScan.

Java

package io.cscode.springboot.web;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class BootstrapWebServer {

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

}

Note: Note that in the above example, we did not specify any spring profile, so it uses the default profile with default values.

When you launch the class, this automatically starts the embedded tomcat and binds to port 8080, which is the default port.

Output

INFO 1012 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)

How to change default port number for web server

As you see in above example, Tomcat server started with default port number 8080, if you want to change the port number, you can suply server.port property in different ways.

You can supply server.port as JVM argument, -Dserver.port=9090
You can supply server.port as application argument, --server.port=9090

In case if you specify both, application argument --server.port value takes precedence.

You can supply it through Spring's application.properties file or application.yml file.

Below is an example specifying port number as 9090 in application.properties.

server.port=9090

Below is an example specifying port number as 9090 in application.yaml.

YAML

server:
  port: 9090

Contents

Bootstrapping a web server

How to change default port number for web server