SPA / Spring wildcard resource serve - This is how you configure Spring Boot to work with for example react-router. It will serve the `index.html` file on all routes that aren't used by REST controllers.

Config.java

import java.io.IOException;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.io.ClassPathResource;
import org.springframework.core.io.Resource;
import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
import org.springframework.web.servlet.resource.PathResourceResolver;

@Configuration
public class Config implements WebMvcConfigurer {

    @Override
    public void addResourceHandlers(ResourceHandlerRegistry registry) {

        // By setting this, you instruct Spring to prioritize this handler above the
        // default one (which is order 0), obviously don't do this. But it's good to
        // understand.
        // -- registry.setOrder(-1);

        registry
                // Capture everything (REST controllers get priority over this, see above)
                .addResourceHandler("/**")
                // Add locations where files might be found
                .addResourceLocations("classpath:/static/**")
                // Needed to allow use of `addResolver` below
                .resourceChain(true)
                // This thing is what does all the resolving. This impl. is responsible for
                // resolving ALL files. Meaning nothing gets resolves automatically by pointing
                // out "static" above.
                .addResolver(new PathResourceResolver() {
                    @Override
                    protected Resource getResource(String resourcePath, Resource location) throws IOException {
                        Resource requestedResource = location.createRelative(resourcePath);

                        // If we actually hit a file, serve that. This is stuff like .js and .css files.
                        if (requestedResource.exists() && requestedResource.isReadable()) {
                            return requestedResource;
                        }

                        // Anything else returns the index.
                        return new ClassPathResource("/static/index.html");
                    }
                });

    }
}

pom.xml

<!-- Additionally you will need this in your POM file to properly copy over all the JS files into the JAR -->
<!-- This particular example assumes your JS build output folder is called "build" -->
<plugin>
  <artifactId>maven-resources-plugin</artifactId>
  <executions>
    <execution>
      <id>resources</id>
      <phase>prepare-package</phase>
      <goals>
        <goal>resources</goal>
      </goals>
      <configuration>
        <resources>
          <resource>
            <directory>build/</directory>
            <targetPath>static</targetPath>
            <includes>
              <include>**/*</include>
            </includes>
          </resource>
          <resource>
            <directory>src/main/resources/</directory>
            <includes>
              <include>**/*</include>
            </includes>
          </resource>
        </resources>
      </configuration>
    </execution>
  </executions>
</plugin>

On Spring Boot 3.4, using gradle, something broke and it was not longer identifying the resource path correctly. I had to modify the path to:

    //...
    override fun addResourceHandlers(registry: ResourceHandlerRegistry) {
        registry 
            .addResourceHandler("/**") 
            .addResourceLocations("classpath:/static/")  // No ** at the end.  In debbugger I verified that the location was being parsed as 'static/**/my_static_file.js'
            //...