Handle relative URLs in jar's Class-Path when getting changeable URLs

5e0ba6ea added support for reading a jar manifest's Class-Path
attribute when resolving changeable URLs from a URLClassLoader,
however it did not handle relative URLs, i.e. URLs without a protocol,
correctly.

This commit updates ChangeableUrls so that it uses the URL of the
JAR that contains the manifest as the base for any new URLs that
are created. When the Class-Path entry is relative, this base will
be used. When the Class-Path entry is absolutee, URL's constructor
will ignore the supplied base.

Closes gh-5665

spring-boot-devtools/src/main/java/org/springframework/boot/devtools/restart/ChangeableUrls.java

@@ -94,7 +94,7 @@ final class ChangeableUrls implements Iterable<URL> {
 		JarFile jarFile = getJarFileIfPossible(url);
 		if (jarFile != null) {
 			try {
-				return getUrlsFromClassPathAttribute(jarFile.getManifest());
+				return getUrlsFromClassPathAttribute(url, jarFile.getManifest());
 			}
 			catch (IOException ex) {
 				throw new IllegalStateException(
@@ -118,7 +118,7 @@ final class ChangeableUrls implements Iterable<URL> {
 		return null;
 	}
 
-	private static List<URL> getUrlsFromClassPathAttribute(Manifest manifest) {
+	private static List<URL> getUrlsFromClassPathAttribute(URL base, Manifest manifest) {
 		List<URL> urls = new ArrayList<URL>();
 		String classPathAttribute = manifest.getMainAttributes()
 				.getValue(Attributes.Name.CLASS_PATH);
@@ -126,7 +126,7 @@ final class ChangeableUrls implements Iterable<URL> {
 			for (String entry : StringUtils.delimitedListToStringArray(classPathAttribute,
 					" ")) {
 				try {
-					urls.add(new URL(entry));
+					urls.add(new URL(base, entry));
 				}
 				catch (MalformedURLException ex) {
 					throw new IllegalStateException(

spring-boot-devtools/src/test/java/org/springframework/boot/devtools/restart/ChangeableUrlsTests.java

@@ -77,10 +77,12 @@ public class ChangeableUrlsTests {
 	public void urlsFromJarClassPathAreConsidered() throws Exception {
 		URL projectCore = makeUrl("project-core");
 		URL projectWeb = makeUrl("project-web");
+		File relative = this.temporaryFolder.newFolder();
 		ChangeableUrls urls = ChangeableUrls.fromUrlClassLoader(new URLClassLoader(
 				new URL[] { makeJarFileWithUrlsInManifestClassPath(projectCore,
-						projectWeb) }));
-		assertThat(urls.toList(), contains(projectCore, projectWeb));
+						projectWeb, relative.getName() + "/") }));
+		assertThat(urls.toList(),
+				contains(projectCore, projectWeb, relative.toURI().toURL()));
 	}
 
 	private URL makeUrl(String name) throws IOException {
@@ -92,7 +94,7 @@ public class ChangeableUrlsTests {
 		return file.toURI().toURL();
 	}
 
-	private URL makeJarFileWithUrlsInManifestClassPath(URL... urls) throws Exception {
+	private URL makeJarFileWithUrlsInManifestClassPath(Object... urls) throws Exception {
 		File classpathJar = this.temporaryFolder.newFile("classpath.jar");
 		Manifest manifest = new Manifest();
 		manifest.getMainAttributes().putValue(Attributes.Name.MANIFEST_VERSION.toString(),