Au sein du projet Java, rajouter les classes nécessaires :

Le starter :

@SpringBootApplication
public class SpringBootWebApplication extends SpringBootServletInitializer {

    /**
     * Configure the server.
     *
     * @param application the application.
     * @return the builder.
     */
    @Override
    protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
        return application.sources(SpringBootWebApplication.class);
    }

    /**
     * Start the server.
     *
     * @param args the arguments.
     */
    public static void main(String[] args) {
        SpringApplication.run(SpringBootWebApplication.class, args);
    }
}

La configuration Web :

@Configuration
@EnableWebMvc
public class ApplicationWebConfiguration extends WebMvcConfigurerAdapter {

    private static final String[] CLASSPATH_RESOURCE_LOCATIONS = {
            "classpath:/META-INF/resources/", "classpath:/resources/", "classpath:/static/", "classpath:/public/"};

    private static final int CACHE_PERIOD_ONE_WEEK = 604800;

    @Override
    public void configureDefaultServletHandling(DefaultServletHandlerConfigurer configurer) {
        configurer.enable();
    }

    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {
        registry.addResourceHandler("/**")
                .addResourceLocations(CLASSPATH_RESOURCE_LOCATIONS)
                .setCachePeriod(CACHE_PERIOD_ONE_WEEK);
    }

    @Override
    public void addViewControllers(ViewControllerRegistry registry) {
        registry.addRedirectViewController("/", "/index.html");
    }

// ou 

    //@Override
    //public void addViewControllers(ViewControllerRegistry registry) {
    //    registry.addViewController("/").setViewName("forward:/index.html");
    //}

}

Dans le pom, rajouter le format de packaging et le re-build du projet dans les plugins (spring-boot-maven-plugin).

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>com.itametis</groupId>
    <artifactId>itartefactId</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>war</packaging>

    <properties>
        <!-- Spring jar version -->
        <spring.version>5.0.4.RELEASE</spring.version>
        <spring-boot.version>1.5.10.RELEASE</spring-boot.version>

        <!-- Jackson version for Spring -->
        <jackson.version>2.9.3</jackson.version>
    </properties>

    <dependencies>
        <!-- Spring -->
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-beans</artifactId>
            <version>${spring.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-core</artifactId>
            <version>${spring.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-context</artifactId>
            <version>${spring.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-web</artifactId>
            <version>${spring.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework</groupId>
            <artifactId>spring-webmvc</artifactId>
            <version>${spring.version}</version>
            <scope>compile</scope>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>${spring-boot.version}</version>
            <scope>compile</scope>
        </dependency>

        <!-- Jackson -->
        <dependency>
            <groupId>com.fasterxml.jackson.core</groupId>
            <artifactId>jackson-core</artifactId>
            <version>${jackson.version}</version>
            <scope>compile</scope>
        </dependency>

    </dependencies>

    <build>
        <finalName>${project.artifactId}</finalName>
        <plugins>
            <!-- War maker -->
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-war-plugin</artifactId>
                <version>3.1.0</version>
                <configuration>
                    <archive>
                        <manifest>
                            <mainClass>${project.groupId}.webserver.SpringBootWebApplication</mainClass>
                            <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
                        </manifest>
                    </archive>
                </configuration>
            </plugin>

            <!-- Spring boot : re-build the war to carry the tomcat server -->
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <version>1.5.3.RELEASE</version>
                <executions>
                    <execution>
                        <goals>
                            <goal>repackage</goal>
                        </goals>
                    </execution>
                </executions>
                <configuration>
                    <mainClass>${project.groupId}.webserver.SpringBootWebApplication</mainClass>
                </configuration>
            </plugin>
        </plugins>
    </build>

</project>

Le war généré par le maven-war-plugin sera re-buildé pour embarquer le tomcat et les outils Spring.