Add Freemarker `prefer-file-system-access` support

Add a `spring.freemarker.prefer-file-system-access` property and update
FreeMarkerAutoConfiguration to support it.

Fixes gh-2901

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerAutoConfiguration.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 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.
@@ -91,6 +91,7 @@ public class FreeMarkerAutoConfiguration {
 
 		protected void applyProperties(FreeMarkerConfigurationFactory factory) {
 			factory.setTemplateLoaderPaths(this.properties.getTemplateLoaderPath());
+			factory.setPreferFileSystemAccess(this.properties.isPreferFileSystemAccess());
 			factory.setDefaultEncoding(this.properties.getCharset());
 			Properties settings = new Properties();
 			settings.putAll(this.properties.getSettings());

spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/freemarker/FreeMarkerProperties.java

 /*
- * Copyright 2012-2014 the original author or authors.
+ * Copyright 2012-2015 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.
@@ -48,6 +48,12 @@ public class FreeMarkerProperties extends AbstractTemplateViewResolverProperties
 	 */
 	private String[] templateLoaderPath = new String[] { DEFAULT_TEMPLATE_LOADER_PATH };
 
+	/**
+	 * Prefer file system access for template loading. File system access enables hot
+	 * detection of template changes.
+	 */
+	private boolean preferFileSystemAccess = true;
+
 	public FreeMarkerProperties() {
 		super(DEFAULT_PREFIX, DEFAULT_SUFFIX);
 	}
@@ -64,6 +70,14 @@ public class FreeMarkerProperties extends AbstractTemplateViewResolverProperties
 		return this.templateLoaderPath;
 	}
 
+	public boolean isPreferFileSystemAccess() {
+		return this.preferFileSystemAccess;
+	}
+
+	public void setPreferFileSystemAccess(boolean preferFileSystemAccess) {
+		this.preferFileSystemAccess = preferFileSystemAccess;
+	}
+
 	public void setTemplateLoaderPath(String... templateLoaderPaths) {
 		this.templateLoaderPath = templateLoaderPaths;
 	}

spring-boot-docs/src/main/asciidoc/appendix-application-properties.adoc

@@ -197,6 +197,7 @@ content into your application; rather pick only the properties that you need.
 	spring.freemarker.expose-session-attributes=false
 	spring.freemarker.expose-spring-macro-helpers=false
 	spring.freemarker.prefix=
+	spring.freemarker.prefer-file-system-access=true # prefer file system access for template loading
 	spring.freemarker.request-context-attribute=
 	spring.freemarker.settings.*=
 	spring.freemarker.suffix=.ftl