Embedded Tomcat in Spring Boot Web application does not start with module-info

Question

I would like to learn some new stuff with java 9+ like modules.
So I created a spring boot application with spring-boot-starter-web and module-info.java :

module com.example.demo {
    requires spring.boot;
    requires spring.boot.autoconfigure;

    exports com.example.demo;
    opens com.example.demo;
}

I used
-> Java 15.0.2
-> Spring Boot 2.4.3
-> IntelliJ IDE 2020.3
-> Ubuntu 20.04.2 LTS

---

To run the application I use a run button(green arrow) in IntelliJ - default run without any additional params. And the problem is that when I try to run the application, the embedded tomcat server does not start.
Output with module-info.java

2021-03-12 08:57:12.182  INFO 4843 --- [           main] com.example.demo.DemoApplication         : Starting DemoApplication using Java 15.0.2 on vagrant-VirtualBox with PID 4843 
2021-03-12 08:57:12.184  INFO 4843 --- [           main] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default
2021-03-12 08:57:13.081  INFO 4843 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 1.511 seconds (JVM running for 3.418)  

But, when I remove module-info.java , everything is ok, and the tomcat server started.
Output without module-info.java

2021-03-12 09:01:49.627  INFO 4928 --- [           main] com.example.demo.DemoApplication         : Starting DemoApplication using Java 15.0.2 on vagrant-VirtualBox with PID 4928 
2021-03-12 09:01:49.633  INFO 4928 --- [           main] com.example.demo.DemoApplication         : No active profile set, falling back to default profiles: default
2021-03-12 09:01:50.897  INFO 4928 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat initialized with port(s): 8080 (http)
2021-03-12 09:01:50.912  INFO 4928 --- [           main] o.apache.catalina.core.StandardService   : Starting service [Tomcat]
2021-03-12 09:01:50.912  INFO 4928 --- [           main] org.apache.catalina.core.StandardEngine  : Starting Servlet engine: [Apache Tomcat/9.0.43]
2021-03-12 09:01:50.958  INFO 4928 --- [           main] o.a.c.c.C.[Tomcat].[localhost].[/]       : Initializing Spring embedded WebApplicationContext
2021-03-12 09:01:50.958  INFO 4928 --- [           main] w.s.c.ServletWebServerApplicationContext : Root WebApplicationContext: initialization completed in 1188 ms
2021-03-12 09:01:51.179  INFO 4928 --- [           main] o.s.s.concurrent.ThreadPoolTaskExecutor  : Initializing ExecutorService 'applicationTaskExecutor'
2021-03-12 09:01:51.320  INFO 4928 --- [           main] o.s.b.w.embedded.tomcat.TomcatWebServer  : Tomcat started on port(s): 8080 (http) with context path ''
2021-03-12 09:01:51.329  INFO 4928 --- [           main] com.example.demo.DemoApplication         : Started DemoApplication in 2.271 seconds (JVM running for 3.774)

I'm wondering the difference between outputs. When I use module-info.java Tomcat doesn't run because it is beyond my module?

What can I do to run app?

How do you think, is it make sense to use java modules with Spring Boot? Why? Why not?

Answer 1

You need to add requires org.apache.tomcat.embed.core; in modules-info.java to include org.apache.catalina.startup.Tomcat.class , see this article for details on how Spring boot start the tomcat server.