enable new Angular application

Remove the controller method that returned the VueJS index page.
Add resource handlers that redirect to the Angular application.
I added two implementations, but activated only one. At the moment I am
not sure which solution is the better. We keep both so that we can
easily switch if need arises.

pdb-ui/src/main/java/org/lucares/pdbui/WebConfiguration.java

@@ -1,11 +1,16 @@
 package org.lucares.pdbui;
 
+import java.io.IOException;
 import java.nio.file.Paths;
 
 import org.springframework.beans.factory.annotation.Value;
 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.ViewControllerRegistry;
 import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
+import org.springframework.web.servlet.resource.PathResourceResolver;
 
 @Configuration
 public class WebConfiguration implements WebMvcConfigurer, HardcodedValues, PropertyKeys {
@@ -19,9 +24,65 @@ public class WebConfiguration implements WebMvcConfigurer, HardcodedValues, Prop
     @Override
     public void addResourceHandlers(final ResourceHandlerRegistry registry) {
 
+        addResourceHandlerForPlottedImages(registry);
+
+        // addResourceHandlerForAngular(registry);
+    }
+
+    private void addResourceHandlerForPlottedImages(final ResourceHandlerRegistry registry) {
         final String pathPattern = "/" + WEB_IMAGE_OUTPUT_PATH + "/**";
         final String resourceLocation = "file:" + Paths.get(outputDir).toAbsolutePath() + "/";
 
         registry.addResourceHandler(pathPattern).addResourceLocations(resourceLocation);
     }
+
+    /**
+     * This is one way to make Angular and SpringBoot play together. The other
+     * described in {@link #addViewControllers(ViewControllerRegistry)}.
+     * 
+     * @param registry
+     */
+    @SuppressWarnings("unused")
+    private void addResourceHandlerForAngular(final ResourceHandlerRegistry registry) {
+        // Redirect all to Angular for all non existing resources.
+        // This way Angular's routing & navigation (https://angular.io/guide/router) can
+        // work. Without it SpringBoot would only show 404 pages for all of Angular's
+        // pages, because the request would not even be send to Angular.
+        //
+        // Technically we are answering all requests for which SpringBoot does not find
+        // a resource with Angular's index.html. The Angular will then use its routing
+        // to determine which sub-page to show.
+        //
+        // This makes Angular also responsible for all 404 pages.
+        registry.addResourceHandler("/**").addResourceLocations("classpath:/resources/").resourceChain(true)
+                .addResolver(new PathResourceResolver() {
+                    @Override
+                    protected Resource getResource(final String resourcePath, final Resource location)
+                            throws IOException {
+                        final Resource requestedResource = location.createRelative(resourcePath);
+
+                        return requestedResource.exists() && requestedResource.isReadable() ? requestedResource
+                                : new ClassPathResource("/resources/index.html");
+                    }
+                });
+    }
+
+    /**
+     * This is one way to make Angular and SpringBoot play together. The other
+     * described in {@link #addResourceHandlerForAngular(ResourceHandlerRegistry)}
+     */
+    @Override
+    public void addViewControllers(final ViewControllerRegistry registry) {
+
+        // Redirect requests to Angular.
+        // This works by returning Angular's index.html for requests that should be
+        // handled by Angular.
+        //
+        // Using this solution SpringBoot is responsible for 404s.
+
+        registry.addViewController("/").setViewName("forward:/index.html");
+        registry.addViewController("/vis").setViewName("forward:/index.html");
+        registry.addViewController("/upload").setViewName("forward:/index.html");
+        registry.addViewController("/help").setViewName("forward:/index.html");
+    }
 }