In spring boot, how do I redirect outside of the context-path?

I have a spring boot application, in the properties file I have defined:

server.servlet.context-path= /myapp

If I go to http://localhost/myapp I see everything as expected. However if I go to: http://localhost I see a standard 404 error page from Apache Tomcat.

How can I setup a redirect from / to /myapp in spring boot?

Alternatively / equivalently: Can I setup multiple context-paths in a single spring-boot application?

Your question will result in bad referenced app later because you are arrived on a webname and you are landing everytime on a redirect... Bad way to go.
If I were you, I will put the things correctly at the start : create a root virtualhost inside your Tomcat and serve your application with root "/" context, and not "/myapp", unless you (or your client, boss) want it like that...

Example config from a VirtualHost with Apache Tomcat :
From https://tecadmin.net/create-virtualhost-in-tomcat/

<Host name="example.com"  appBase="webapps" unpackWARs="true" autoDeploy="true">
 <Alias>www.example.com</Alias>
 
 <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
           prefix="example_access_log" suffix=".txt"
           pattern="%h %l %u %t %r %s %b" />
 
 <Context path="" docBase="/opt/tomcat/webapps/myapp1" debug="0" reloadable="true"/>
</Host>

<Host name="mydomain.org"  appBase="webapps" unpackWARs="true" autoDeploy="true">
 <Alias>www.mydomain.org</Alias>
 
 <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
           prefix="mydomain_access_log" suffix=".txt"
           pattern="%h %l %u %t %r %s %b" />
 
 <Context path="" docBase="/opt/tomcat/webapps/myapp2"
    debug="0" reloadable="true"/>
</Host>

If you package SpringBoot application as jar, Tomcat would be included as the default embedded container.

You can't ask the embedded Tomcat to host multi web-app like what it could do as a standalone service.

So we have two choice:

1. Run a reverse proxy in front of your SpringBoot application. Such as Nginx

2. Package your SpringBoot application as a war, and put it into some container.

   For example:

   1. Download tomcat and start it with default configuration.
   2. Build a my-app-1.0.0-SNAPSHOT.war, rename it to myapp.war and copy it to your tomcat's /webapps directory.
   3. You can visit http://localhost:8080/myapp, tomcat could host all valid folders and wars in /webapps.

PS: If you are using Spring Reactive Web(WebFlux, Netty), the second method wouldn't work.

UPDATE

Here is what I said about ServletRegistrationBean in comments.

  // use DispatcherServlet here
  private ServletRegistrationBean<? extends Servlet> createServletRegistrationBean(
      ApplicationContext context, String name, String... urlMappings) {
    final DispatcherServlet dispatcherServlet = new DispatcherServlet();
    dispatcherServlet.setApplicationContext(context);

    final ServletRegistrationBean<DispatcherServlet> servletRegistrationBean =
        new ServletRegistrationBean<>(dispatcherServlet, urlMappings);
    servletRegistrationBean.setName(name);
    return servletRegistrationBean;
  }

  @Bean
  public ServletRegistrationBean<? extends Servlet> oneContextPath(ApplicationContext context) {
    // create applicationContext or use the auto configured one
    return createServletRegistrationBean(context, "firstOne", "/*");
  }
  @Bean
  public ServletRegistrationBean<? extends Servlet> anotherContextPath(ApplicationContext context) {
    return createServletRegistrationBean(context, "secondOne", "/myapp/*");
  }

As this example, we can run http GET /foo and http GET /myapp/foo at the same time.

Notes:

1. WebFlux is NOT supported.
2. Custom applicationContext if you need.

You have so many options to do that and for external servers by WebServerFactoryCustomizer

@Bean
public WebServerFactoryCustomizer<ConfigurableServletWebServerFactory>
  webServerFactoryCustomizer() {
    return factory -> factory.setContextPath("/context");
}

With Spring Boot 1, we can create an instance of EmbeddedServletContainerCustomizer:

@Bean
public EmbeddedServletContainerCustomizer
  embeddedServletContainerCustomizer() {
    return container -> container.setContextPath("/yourContext");
}

Or with java System property

public static void main(String[] args) {
    System.setProperty("server.servlet.context-path", "/yourContext");
    SpringApplication.run(Application.class, args);
}