Fix expansion of static-locations array

This commit fixes a NPE when the static-locations array of
`ResourceProperties` has to be expanded as the setter is cleaning the
values of the array and is affected by a non-intuitive behaviour of the
binder.

When the binder needs to set an element of an array and the size of the
array isn't large enough, the binder proceeds as follows:

* An array of the required size is created
* The content of the original array is copied over
* The setter of the property is invoked with the new array
* The setter of the property is invoked and the returned array is
mutated to set the requested value

While one would expect the array to contain the requested value when the
setter is invoked, this is not the case. Also, the array might contain
null values if a value at index 8 should be set and the array has a size
of 3.

All in all, `ResourceProperties#appendSlashIfNecessary` has to account
for `null` and an additional round of cleaning has to happen once
binding has completed.

Closes gh-12360

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/ResourceProperties.java

 /*
- * Copyright 2012-2017 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -20,6 +20,7 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 
+import org.springframework.beans.factory.InitializingBean;
 import org.springframework.boot.context.properties.ConfigurationProperties;
 import org.springframework.boot.context.properties.NestedConfigurationProperty;
 import org.springframework.context.ResourceLoaderAware;
@@ -37,7 +38,7 @@ import org.springframework.core.io.ResourceLoader;
  * @since 1.1.0
  */
 @ConfigurationProperties(prefix = "spring.resources", ignoreUnknownFields = false)
-public class ResourceProperties implements ResourceLoaderAware {
+public class ResourceProperties implements ResourceLoaderAware, InitializingBean {
 
 	private static final String[] SERVLET_RESOURCE_LOCATIONS = { "/" };
 
@@ -81,6 +82,11 @@ public class ResourceProperties implements ResourceLoaderAware {
 		this.resourceLoader = resourceLoader;
 	}
 
+	@Override
+	public void afterPropertiesSet() {
+		this.staticLocations = appendSlashIfNecessary(this.staticLocations);
+	}
+
 	public String[] getStaticLocations() {
 		return this.staticLocations;
 	}
@@ -93,7 +99,9 @@ public class ResourceProperties implements ResourceLoaderAware {
 		String[] normalized = new String[staticLocations.length];
 		for (int i = 0; i < staticLocations.length; i++) {
 			String location = staticLocations[i];
-			normalized[i] = (location.endsWith("/") ? location : location + "/");
+			if (location != null) {
+				normalized[i] = (location.endsWith("/") ? location : location + "/");
+			}
 		}
 		return normalized;
 	}

spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/ResourcePropertiesBindingTests.java

+/*
+ * Copyright 2012-2018 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.autoconfigure.web;
+
+import org.junit.After;
+import org.junit.Test;
+
+import org.springframework.boot.context.properties.EnableConfigurationProperties;
+import org.springframework.boot.test.util.EnvironmentTestUtils;
+import org.springframework.context.annotation.AnnotationConfigApplicationContext;
+import org.springframework.context.annotation.Configuration;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Binding tests for {@link ResourceProperties}.
+ *
+ * @author Stephane Nicoll
+ */
+public class ResourcePropertiesBindingTests {
+
+	private AnnotationConfigApplicationContext context;
+
+	@After
+	public void close() {
+		if (this.context != null) {
+			this.context.close();
+		}
+	}
+
+	@Test
+	public void staticLocationsExpandArray() {
+		ResourceProperties properties = load(
+				"spring.resources.static-locations[0]=classpath:/one/",
+				"spring.resources.static-locations[1]=classpath:/two",
+				"spring.resources.static-locations[2]=classpath:/three/",
+				"spring.resources.static-locations[3]=classpath:/four",
+				"spring.resources.static-locations[4]=classpath:/five/",
+				"spring.resources.static-locations[5]=classpath:/six");
+		assertThat(properties.getStaticLocations()).contains("classpath:/one/",
+				"classpath:/two/", "classpath:/three/", "classpath:/four/",
+				"classpath:/five/", "classpath:/six/");
+	}
+
+	private ResourceProperties load(String... environment) {
+		AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
+		EnvironmentTestUtils.addEnvironment(ctx, environment);
+		ctx.register(TestConfiguration.class);
+		ctx.refresh();
+		this.context = ctx;
+		return this.context.getBean(ResourceProperties.class);
+	}
+
+	@Configuration
+	@EnableConfigurationProperties(ResourceProperties.class)
+	static class TestConfiguration {
+
+	}
+
+}