Simplify temp directory creation and improve diagnostics

Closes gh-23622

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/mock/web/SpringBootMockServletContext.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.io.File;
 import java.io.IOException;
 import java.net.MalformedURLException;
 import java.net.URL;
+import java.nio.file.Files;
 
 import org.springframework.core.io.FileSystemResourceLoader;
 import org.springframework.core.io.Resource;
@@ -93,9 +94,7 @@ public class SpringBootMockServletContext extends MockServletContext {
 			try {
 				if (this.emptyRootFolder == null) {
 					synchronized (this) {
-						File tempFolder = File.createTempFile("spr", "servlet");
-						tempFolder.delete();
-						tempFolder.mkdirs();
+						File tempFolder = Files.createTempDirectory("spr-servlet").toFile();
 						tempFolder.deleteOnExit();
 						this.emptyRootFolder = tempFolder;
 					}

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/server/AbstractConfigurableWebServerFactory.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * Copyright 2012-2020 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.
@@ -19,6 +19,7 @@ package org.springframework.boot.web.server;
 import java.io.File;
 import java.io.IOException;
 import java.net.InetAddress;
+import java.nio.file.Files;
 import java.util.Arrays;
 import java.util.LinkedHashSet;
 import java.util.Set;
@@ -169,9 +170,7 @@ public abstract class AbstractConfigurableWebServerFactory implements Configurab
 	 */
 	protected final File createTempDir(String prefix) {
 		try {
-			File tempDir = File.createTempFile(prefix + ".", "." + getPort());
-			tempDir.delete();
-			tempDir.mkdir();
+			File tempDir = Files.createTempDirectory(prefix + "." + getPort() + ".").toFile();
 			tempDir.deleteOnExit();
 			return tempDir;
 		}