Do not expose servlet locations in WebFlux

The `spring.resources.static-locations` key (see `ResourceProperties`)
lists many locations by default, including `"classpath:/static"` and
`"/"`. The latter is about exposing resources located in the root
servlet location.

Reactive web applications don't depend on a Servlet-related
`ApplicationContext`/`ResourceLoader`, so `"/"` is then resolved as
`"classpath:/"`, which exposes the whole classpath.

To avoid such situations, the `GenericReactiveWebApplicationContext`
overrides the `getResourceByPath(String path)` method and always returns
non existent resources if the path starts with `"/"`.

Fixes gh-8485

spring-boot/src/main/java/org/springframework/boot/web/reactive/context/GenericReactiveWebApplicationContext.java

@@ -16,13 +16,20 @@
 
 package org.springframework.boot.web.reactive.context;
 
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.io.InputStream;
+
 import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.core.io.AbstractResource;
+import org.springframework.core.io.Resource;
 
 /**
  * Subclass of {@link AnnotationConfigApplicationContext}, suitable for reactive web
  * environments.
  *
  * @author Stephane Nicoll
+ * @author Brian Clozel
  * @since 2.0.0
  */
 public class GenericReactiveWebApplicationContext extends
@@ -30,6 +37,8 @@ public class GenericReactiveWebApplicationContext extends
 
 	private String namespace;
 
+	private final NonExistentResource nonExistentResource = new NonExistentResource();
+
 	public GenericReactiveWebApplicationContext() {
 		super();
 	}
@@ -48,4 +57,45 @@ public class GenericReactiveWebApplicationContext extends
 		return this.namespace;
 	}
 
+	@Override
+	protected Resource getResourceByPath(String path) {
+		// No ServletContext is available
+		if (path.startsWith("/")) {
+			return this.nonExistentResource;
+		}
+		else {
+			return super.getResourceByPath(path);
+		}
+	}
+
+	/**
+	 * Resource implementation that replaces the
+	 * {@link org.springframework.web.context.support.ServletContextResource}
+	 * in a reactive web application.
+	 *
+	 * <p>{@link #exists()} always returns null in order to avoid exposing
+	 * the whole classpath in a non-servlet environment.
+	 */
+	class NonExistentResource extends AbstractResource {
+
+		@Override
+		public boolean exists() {
+			return false;
+		}
+
+		@Override
+		public Resource createRelative(String relativePath) throws IOException {
+			return this;
+		}
+
+		@Override
+		public String getDescription() {
+			return "NonExistentResource";
+		}
+
+		@Override
+		public InputStream getInputStream() throws IOException {
+			throw new FileNotFoundException(this.getDescription() + " cannot be opened because it does not exist");
+		}
+	}
 }

spring-boot/src/test/java/org/springframework/boot/web/reactive/context/GenericReactiveWebApplicationContextTests.java

@@ -0,0 +1,40 @@
+/*
+ * Copyright 2012-2017 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.boot.web.reactive.context;
+
+import org.junit.Test;
+
+import org.springframework.core.io.Resource;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link GenericReactiveWebApplicationContext}
+ *
+ * @author Brian Clozel
+ */
+public class GenericReactiveWebApplicationContextTests {
+
+	@Test
+	public void getResourceByPath() throws Exception {
+		GenericReactiveWebApplicationContext context = new GenericReactiveWebApplicationContext();
+		Resource rootResource = context.getResourceByPath("/");
+		assertThat(rootResource.exists()).isFalse();
+		assertThat(rootResource.createRelative("application.properties").exists()).isFalse();
+	}
+
+}