static files in springboot project deployed to an external Tomcat with WAR - spring-boot

Question

I'm new to Spring and Springboot, I'm just trying an easy thing I can't sort out:
As per the title, I want to put in a project some static files, then package the project with maven as war and deploy it to an existing Tomcat.
I'm using Springboot 2.7.8, Java 17 and Tomcat 8.5.47
This is my pom.xml:
<?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 https://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<parent>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-parent</artifactId>
<version>2.7.8</version>
<relativePath/>
<!-- lookup parent from repository -->
</parent>
<groupId>com.example</groupId>
<artifactId>demo-01</artifactId>
<version>0.0.1-SNAPSHOT</version>
<packaging>war</packaging>
<name>demo-01</name>
<description>Demo project for Spring Boot</description>
<properties>
<java.version>17</java.version>
</properties>
<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-test</artifactId>
<scope>test</scope>
</dependency>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-tomcat</artifactId>
<scope>provided</scope>
</dependency>
</dependencies>
</project>
and this is my application class:
package com.example.demo01;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.boot.web.servlet.support.SpringBootServletInitializer;
#SpringBootApplication
public class Demo01Application extends SpringBootServletInitializer {
#Override
protected SpringApplicationBuilder configure(SpringApplicationBuilder application) {
return application.sources(Demo01Application.class);
}
public static void main(String[] args) {
SpringApplication.run(Demo01Application.class, args);
}
}
then I've put an "Hello World" index.html under src/main/resources/static
and finally I've run
mvn clean package
and I've got a war file like this:
├───META-INF
└───WEB-INF
├───classes
│ │ application.properties
│ │
│ ├───com
│ │ └───example
│ │ └───demo01
│ │ Demo01Application.class
│ │
│ └───static
│ index.html
│
└───lib
jackson-annotations-2.13.4.jar
jackson-core-2.13.4.jar
jackson-databind-2.13.4.2.jar
jackson-datatype-jdk8-2.13.4.jar
jackson-datatype-jsr310-2.13.4.jar
jackson-module-parameter-names-2.13.4.jar
jakarta.annotation-api-1.3.5.jar
jul-to-slf4j-1.7.36.jar
log4j-api-2.17.2.jar
log4j-to-slf4j-2.17.2.jar
logback-classic-1.2.11.jar
logback-core-1.2.11.jar
slf4j-api-1.7.36.jar
snakeyaml-1.30.jar
spring-aop-5.3.25.jar
spring-beans-5.3.25.jar
spring-boot-2.7.8.jar
spring-boot-autoconfigure-2.7.8.jar
spring-boot-starter-2.7.8.jar
spring-boot-starter-json-2.7.8.jar
spring-boot-starter-logging-2.7.8.jar
spring-boot-starter-web-2.7.8.jar
spring-context-5.3.25.jar
spring-core-5.3.25.jar
spring-expression-5.3.25.jar
spring-jcl-5.3.25.jar
spring-web-5.3.25.jar
spring-webmvc-5.3.25.jar
As you can see, th index.html is under WEB-INF\classes\static
Is it supposed to be served by tomcat from that location?!
Anyway: I've deployed the war file to Tomcat, and when I point my browser to http://localhost/demo-01-0.0.1-SNAPSHOT I get a sad 404
What an I missing ?
EDIT:
I've found the cause of my problem (thanks to #khmarbaise who pointed me in the right direction): since the external Tomcat instance (8.5.47) is using Java 8, having instructed Spring Boot to make the package compliant with Java 17, with these directive in my pom.xml:
<properties>
<java.version>17</java.version>
</properties>
created the problem.
It was sufficient to remove these three lines, and now everything works fine.