81.1 Create a deployable war file

The first step in producing a deployable war file is to provide a SpringBootServletInitializer subclass and override its configure method. This makes use of Spring Framework’s Servlet 3.0 support and allows you to configure your application when it’s launched by the servlet container. Typically, you update your application’s main class to extend SpringBootServletInitializer:

_@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);
    }

}

The next step is to update your build configuration so that your project produces a war file rather than a jar file. If you’re using Maven and using spring-boot-starter-parent (which configures Maven’s war plugin for you) all you need to do is modify pom.xml to change the packaging to war:

<packaging>war</packaging>

If you’re using Gradle, you need to modify build.gradle to apply the war plugin to the project:

apply plugin: 'war'

The final step in the process is to ensure that the embedded servlet container doesn’t interfere with the servlet container to which the war file will be deployed. To do so, you need to mark the embedded servlet container dependency as provided.

If you’re using Maven:

<dependencies>
    <!-- … -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-tomcat</artifactId>
        <scope>provided</scope>
    </dependency>
    <!-- … -->
</dependencies>

And if you’re using Gradle:

dependencies {
    // …
    providedRuntime 'org.springframework.boot:spring-boot-starter-tomcat'
    // …
}

	Note
If you are using a version of Gradle that supports compile only dependencies (2.12 or later), you should continue to use providedRuntime. Among other limitations, compileOnly dependencies are not on the test classpath so any web-based integration tests will fail.

If you’re using the Spring Boot build tools, marking the embedded servlet container dependency as provided will produce an executable war file with the provided dependencies packaged in a lib-provided directory. This means that, in addition to being deployable to a servlet container, you can also run your application using java -jar on the command line.

	Tip
Take a look at Spring Boot’s sample applications for a Maven-based example of the above-described configuration.