Locate a default LayoutFactory in spring.factories if possible

As a courtesy to the user, if there is only one LayoutFactory
defined in spring.factories then there is no need to explicitly
configure it.

Author: Dave Syer

spring-boot-docs/src/main/asciidoc/build-tool-plugins.adoc

@@ -563,6 +563,9 @@ springBoot {
 }
 ----
 
+If there is only one custom `LayoutFactory` on the build classpath and
+it is listed in `META-INF/spring.factories` then it is unnecessary to
+explicitly set it in the `springBoot` configuration.
 
 [[build-tool-plugins-understanding-the-gradle-plugin]]
 === Understanding how the Gradle plugin works

spring-boot-tools/spring-boot-gradle-plugin/src/main/java/org/springframework/boot/gradle/SpringBootPluginExtension.java

@@ -26,10 +26,10 @@
 import org.gradle.api.plugins.JavaPlugin;
 
 import org.springframework.boot.gradle.buildinfo.BuildInfo;
-import org.springframework.boot.loader.tools.DefaultLayoutFactory;
 import org.springframework.boot.loader.tools.Layout;
 import org.springframework.boot.loader.tools.LayoutFactory;
 import org.springframework.boot.loader.tools.LayoutType;
+import org.springframework.boot.loader.tools.Layouts;
 
 /**
  * Gradle DSL Extension for 'Spring Boot'. Most of the time Spring Boot can guess the
@@ -96,7 +96,7 @@ public class SpringBootPluginExtension {
 	/**
 	 * The layout factory to use to convert a layout type into an actual layout.
 	 */
-	LayoutFactory layoutFactory = new DefaultLayoutFactory();
+	LayoutFactory layoutFactory = Layouts.getDefaultLayoutFactory();
 
 	/**
 	 * Libraries that must be unpacked from fat jars in order to run. Use Strings in the

spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Layouts.java

@@ -21,9 +21,12 @@
 import java.util.Collections;
 import java.util.HashMap;
 import java.util.HashSet;
+import java.util.List;
 import java.util.Map;
 import java.util.Set;
 
+import org.springframework.core.io.support.SpringFactoriesLoader;
+
 /**
  * Common {@link Layout}s.
  *
@@ -47,7 +50,22 @@ private Layouts() {
 	 * @return a {@link Layout}
 	 */
 	public static Layout forFile(File file) {
-		return new DefaultLayoutFactory().getLayout(LayoutType.forFile(file));
+		return Layouts.getDefaultLayoutFactory().getLayout(LayoutType.forFile(file));
+	}
+
+	/**
+	 * Gets a default layout factory, trying first to find a unique one in spring
+	 * factories, and then falling back to {@link DefaultLayoutFactory} if there isn't
+	 * one.
+	 * @return the default layout factory
+	 */
+	public static LayoutFactory getDefaultLayoutFactory() {
+		List<LayoutFactory> factories = SpringFactoriesLoader
+				.loadFactories(LayoutFactory.class, null);
+		if (factories.size() == 1) {
+			return factories.get(0);
+		}
+		return new DefaultLayoutFactory();
 	}
 
 	/**

spring-boot-tools/spring-boot-loader-tools/src/test/resources/META-INF/spring.factories

@@ -41,10 +41,10 @@
 import org.apache.maven.shared.artifact.filter.collection.ScopeFilter;
 
 import org.springframework.boot.loader.tools.DefaultLaunchScript;
-import org.springframework.boot.loader.tools.DefaultLayoutFactory;
 import org.springframework.boot.loader.tools.LaunchScript;
 import org.springframework.boot.loader.tools.LayoutFactory;
 import org.springframework.boot.loader.tools.LayoutType;
+import org.springframework.boot.loader.tools.Layouts;
 import org.springframework.boot.loader.tools.Libraries;
 import org.springframework.boot.loader.tools.Repackager;
 
@@ -139,7 +139,7 @@ public class RepackageMojo extends AbstractDependencyFilterMojo {
 	 * @since 1.0
 	 */
 	@Parameter
-	private LayoutFactory layoutFactory = new DefaultLayoutFactory();
+	private LayoutFactory layoutFactory = Layouts.getDefaultLayoutFactory();
 
 	/**
 	 * A list of the libraries that must be unpacked from fat jars in order to run.

spring-boot-tools/spring-boot-maven-plugin/src/main/java/org/springframework/boot/maven/RepackageMojo.java

@@ -49,8 +49,10 @@
 ---
 
   The layout is provided as an implementation of <<LayoutFactory>>
-  (from spring-boot-loader-tools) explicitly specified in the pom.
-
+  (from spring-boot-loader-tools) explicitly specified in the pom.  If
+  there is only one custom `LayoutFactory` on the plugin classpath and
+  it is listed in `META-INF/spring.factories` then it is unnecessary
+  to explicitly set it in the plugin configuration.