Here you can find simple demo project for Spring hosting thymeleaf templates and react app under /admin/

For simple react app i used create-react-app and react router to test routes

The goal was to make site urls look like this

/admin/static/** => react static folder only for existing files in /resources/react_build/static

/admin/**        => react /resources/react_build/index.html for everything but /admin/static/**

/**              =>  Theamleaf templates for everything else 
In this demo it is IndexController and template will come from /resources/templates/index.thml

Project structure looks like this

frontend
    build
    ... react code
    
src
    main
        java
            ... java code
        resources
            react_build => symlink to /frontend/build
            templates
                index.html

react_build is actually a symlink to /frontend/build created with

cd src/main/resources/
ln -s ../../../frontend/build/ react_build

The main part is in BaseWebMvcConfigurerAdapter inner class AdminResourceResolver.

public void addResourceHandlers(ResourceHandlerRegistry registry) {

    ResourceResolver resolver = new AdminResourceResolver();
    registry.addResourceHandler("/admin/**")
            .resourceChain(false)
            .addResolver(resolver);


    registry.addResourceHandler("/admin/")
            .resourceChain(false)
            .addResolver(resolver);
}


private class AdminResourceResolver implements ResourceResolver {
     ... //custom ResourceResolver code
}

I'm using this approach to forward directory and every subdirectory to a single react/angular/any other js lib file

One thing to mention is that i'm using gradle com.moowork.node plugin to build js before java app starts, so if you want to disable it, just comment the last line in build.gradle processResources.dependsOn 'webpack'

> gradle bootStart

will host app on localhost:8080