Spring Boot is an opinionated convention-over-configuration framework for creating stand-alone Spring-based Java applications.

Its objective is to provide you with a preselected best practice choice of libraries (both from the Spring platform and by third parties) for a given task, e.g.

• building a web application that connects to a database and provides REST resources
• creating a scheduled batch task that exports and aggregates data from various data sources
• providing an API that connects to SOAP Web Services

It both helps you with getting started quickly and allows you to go about application development in a pattern-like manner that makes application behaviour more predictable and easier to understand.

In keeping with that stand-alone, cookie cutter approach the usual way Spring Boot applications are deployed is as a JAR file (including an embedded web container such as Tomcat if your application is a web app). This allows you to run your application right away without any configuration hassle. In addition to that, this self-contained approach is expedient to the microservices design pattern, which has made Spring Boot a prime choice for modern microservice architectures.

Sometimes however for instance due to existing infrastructure that can’t be changed quickly it might be necessary to deploy applications – web applications in particular – the old-fashioned way as WAR (Web application ARchive) files in existing servlet containers or application servers.

In order to do so you only have to make a few changes to your application configuration (as described in the Spring Boot documentation part on traditional deployment):

• Provide a SpringBootServletInitializer and configure() method:
  
  @SpringBootApplication public class Application extends SpringBootServletInitializer { @Override protected SpringApplicationBuilder configure(SpringApplicationBuilder application) { return application.sources(Application.class); } public static void main(String[] args) throws Exception { SpringApplication.run(Application.class, args); } }

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  @SpringBootApplication public class Application extends SpringBootServletInitializer {       @Override     protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {         return application.sources(Application.class);     }       public static void main(String[] args) throws Exception {         SpringApplication.run(Application.class, args);     }   }

• Change the packaging option in the application’s pom.xml to war:
  <packaging>war</packaging>

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  <packaging>war</packaging>

• Finally, tell Maven to not use the provided embedded Tomcat servlet container in production in order to not interfere with the existing servlet container you deploy the WAR file to:
  
  <dependencies> <dependency> <groupid>org.springframework.boot</groupid> <artifactid>spring-boot-starter-tomcat</artifactid> <scope>provided</scope> </dependency> </dependencies>

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  <dependencies>     <dependency>         <groupid>org.springframework.boot</groupid>         <artifactid>spring-boot-starter-tomcat</artifactid>         <scope>provided</scope>     </dependency> </dependencies>

If you’d like to know more about Spring Boot application deployment in general and converting Spring Boot applications from JAR to WAR deployments in particular please have a look at these posts on the Spring website:

• Deploying Spring Boot Applications
• Converting a Spring Boot JAR Application to a WAR (including directions for using Gradle instead of Maven, which I omitted in this article for the sake of brevity)