How to prevent auto start of tomcat/jetty in Spring Boot when I only want to use RestTemplate

Spring Boot is not going to start a web container if it's not present. spring-web does not provide any embedded container. You may want to analyse the dependencies of your project (try mvn dependency:tree).

If you want to make sure a web server is not started in your spring boot application, you can set the following configuration key

spring.main.web-application-type=none

Or you can use the SpringApplicationBuilder

new SpringApplicationBuilder(YourApp.class)
        .web(WebApplicationType.NONE).run(args);

Since Spring Boot 2.0.0 this property is deprecated and following is the new way:

spring.main.web-application-type=none

This change is because Spring Boot the support for reactive server.