Configure ResourceUrlProvider in WebFlux

Prior to this commit, no `ResourceUrlProvider` was configured
in WebFlux (no bean was contributed by the WebFlux infrastructure).
Also, several `ResourceTransformer` instances that extend the
`ResourceTransformerSupport` base class need a `ResourceUrlProvider`
to resolve absolute URLs when rewriting resource URLs. At this point,
no `ResourceUrlProvider` was configured and they could only resolve
relative URLs.

This commit contributes a new `ResourceUrlProvider` to the WebFlux
configuration; this bean can be reused by the WebFlux infrastructure and
application code.

This also automatically configure this shared `ResourceUrlProvider`
instance on the resource chain where needed.

Issue: SPR-17433

spring-webflux/src/main/java/org/springframework/web/reactive/config/ResourceHandlerRegistry.java

@@ -28,6 +28,8 @@ import org.springframework.core.io.ResourceLoader;
 import org.springframework.lang.Nullable;
 import org.springframework.web.reactive.handler.AbstractUrlHandlerMapping;
 import org.springframework.web.reactive.handler.SimpleUrlHandlerMapping;
+import org.springframework.web.reactive.resource.ResourceTransformerSupport;
+import org.springframework.web.reactive.resource.ResourceUrlProvider;
 import org.springframework.web.reactive.resource.ResourceWebHandler;
 import org.springframework.web.server.WebHandler;
 
@@ -49,6 +51,7 @@ import org.springframework.web.server.WebHandler;
  * period for served resources.
  *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.0
  */
 public class ResourceHandlerRegistry {
@@ -57,7 +60,10 @@ public class ResourceHandlerRegistry {
 
 	private final List<ResourceHandlerRegistration> registrations = new ArrayList<>();
 
-	private int order = Ordered.LOWEST_PRECEDENCE -1;
+	private int order = Ordered.LOWEST_PRECEDENCE - 1;
+
+	@Nullable
+	private ResourceUrlProvider resourceUrlProvider;
 
 
 	/**
@@ -69,6 +75,17 @@ public class ResourceHandlerRegistry {
 		this.resourceLoader = resourceLoader;
 	}
 
+	/**
+	 * Configure the {@link ResourceUrlProvider} that can be used by
+	 * {@link org.springframework.web.reactive.resource.ResourceTransformer} instances.
+	 * @param resourceUrlProvider the resource URL provider to use
+	 * @since 5.1.2
+	 */
+	public void setResourceUrlProvider(@Nullable ResourceUrlProvider resourceUrlProvider) {
+		this.resourceUrlProvider = resourceUrlProvider;
+	}
+
+
 
 	/**
 	 * Add a resource handler for serving static resources based on the specified
@@ -121,6 +138,11 @@ public class ResourceHandlerRegistry {
 		for (ResourceHandlerRegistration registration : this.registrations) {
 			for (String pathPattern : registration.getPathPatterns()) {
 				ResourceWebHandler handler = registration.getRequestHandler();
+				handler.getResourceTransformers().forEach(transformer -> {
+					if (transformer instanceof ResourceTransformerSupport) {
+						((ResourceTransformerSupport) transformer).setResourceUrlProvider(this.resourceUrlProvider);
+					}
+				});
 				try {
 					handler.afterPropertiesSet();
 				}

spring-webflux/src/main/java/org/springframework/web/reactive/config/WebFluxConfigurationSupport.java

@@ -55,6 +55,7 @@ import org.springframework.web.reactive.function.server.support.RouterFunctionMa
 import org.springframework.web.reactive.function.server.support.ServerResponseResultHandler;
 import org.springframework.web.reactive.handler.AbstractHandlerMapping;
 import org.springframework.web.reactive.handler.WebFluxResponseStatusExceptionHandler;
+import org.springframework.web.reactive.resource.ResourceUrlProvider;
 import org.springframework.web.reactive.result.SimpleHandlerAdapter;
 import org.springframework.web.reactive.result.method.annotation.ArgumentResolverConfigurer;
 import org.springframework.web.reactive.result.method.annotation.RequestMappingHandlerAdapter;
@@ -74,6 +75,7 @@ import org.springframework.web.server.i18n.LocaleContextResolver;
  * <p>Import directly or extend and override protected methods to customize.
  *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.0
  */
 public class WebFluxConfigurationSupport implements ApplicationContextAware {
@@ -229,6 +231,7 @@ public class WebFluxConfigurationSupport implements ApplicationContextAware {
 			resourceLoader = new DefaultResourceLoader();
 		}
 		ResourceHandlerRegistry registry = new ResourceHandlerRegistry(resourceLoader);
+		registry.setResourceUrlProvider(resourceUrlProvider());
 		addResourceHandlers(registry);
 
 		AbstractHandlerMapping handlerMapping = registry.getHandlerMapping();
@@ -249,6 +252,11 @@ public class WebFluxConfigurationSupport implements ApplicationContextAware {
 		return handlerMapping;
 	}
 
+	@Bean
+	public ResourceUrlProvider resourceUrlProvider() {
+		return new ResourceUrlProvider();
+	}
+
 	/**
 	 * Override this method to add resource handlers for serving static resources.
 	 * @see ResourceHandlerRegistry