Add 'module' repacker layout

Add a 'module' layout for the repackager which includes all 'compile'
and 'runtime' scope dependencies and does not require a main class.

Fixes gh-1941

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

@@ -44,4 +44,9 @@ public interface Layout {
 	 */
 	String getClassesLocation();
 
+	/**
+	 * Returns if loader classes should be included to make the archive executable.
+	 */
+	boolean isExecutable();
+
 }

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

@@ -69,6 +69,12 @@ public class Layouts {
 		public String getClassesLocation() {
 			return "";
 		}
+
+		@Override
+		public boolean isExecutable() {
+			return true;
+		}
+
 	}
 
 	/**
@@ -84,7 +90,7 @@ public class Layouts {
 	}
 
 	/**
-	 * Executable expanded archive layout.
+	 * No layout.
 	 */
 	public static class None extends Jar {
 
@@ -92,6 +98,12 @@ public class Layouts {
 		public String getLauncherClassName() {
 			return null;
 		}
+
+		@Override
+		public boolean isExecutable() {
+			return false;
+		}
+
 	}
 
 	/**
@@ -122,6 +134,42 @@ public class Layouts {
 		public String getClassesLocation() {
 			return "WEB-INF/classes/";
 		}
+
+		@Override
+		public boolean isExecutable() {
+			return true;
+		}
+
+	}
+
+	/**
+	 * Module layout (designed to be used as a "plug-in")
+	 */
+	public static class Module implements Layout {
+
+		@Override
+		public String getLauncherClassName() {
+			return null;
+		}
+
+		@Override
+		public String getLibraryDestination(String libraryName, LibraryScope scope) {
+			if (LibraryScope.COMPILE.equals(scope) || LibraryScope.RUNTIME.equals(scope)) {
+				return "lib/";
+			}
+			return null;
+		}
+
+		@Override
+		public String getClassesLocation() {
+			return "";
+		}
+
+		@Override
+		public boolean isExecutable() {
+			return false;
+		}
+
 	}
 
 }

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

@@ -177,7 +177,7 @@ public class Repackager {
 				}
 			});
 
-			if (!(this.layout instanceof Layouts.None)) {
+			if (this.layout.isExecutable()) {
 				writer.writeLoaderClasses();
 			}
 		}

spring-boot-tools/spring-boot-loader-tools/src/test/java/org/springframework/boot/loader/tools/LayoutsTests.java

@@ -24,6 +24,7 @@ import org.junit.rules.ExpectedException;
 
 import static org.hamcrest.Matchers.equalTo;
 import static org.hamcrest.Matchers.instanceOf;
+import static org.hamcrest.Matchers.nullValue;
 import static org.junit.Assert.assertThat;
 
 /**
@@ -81,4 +82,15 @@ public class LayoutsTests {
 				equalTo("WEB-INF/lib/"));
 	}
 
+	@Test
+	public void moduleLayout() throws Exception {
+		Layout layout = new Layouts.Module();
+		assertThat(layout.getLibraryDestination("lib.jar", LibraryScope.COMPILE),
+				equalTo("lib/"));
+		assertThat(layout.getLibraryDestination("lib.jar", LibraryScope.PROVIDED),
+				nullValue());
+		assertThat(layout.getLibraryDestination("lib.jar", LibraryScope.RUNTIME),
+				equalTo("lib/"));
+	}
+
 }