Update JarURLConnection to support jar:file://… URLs

Previously, JarURLConnection would fail when created with a URL that
began with jar:file:// as the double-slash is not included in jarFile.getUrl().getFile().

This commit updates JarURLConnection to normalise the value return from
url.getFile() to remove a double-slash when present.

Fixes gh-5287
Closes gh-5289

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

@@ -73,7 +73,11 @@ class JarURLConnection extends java.net.JarURLConnection {
 		// What we pass to super is ultimately ignored
 		super(EMPTY_JAR_URL);
 		this.url = url;
-		String spec = url.getFile().substring(jarFile.getUrl().getFile().length());
+		String urlFile = url.getFile();
+		if (urlFile.startsWith("file://")) {
+			urlFile = "file:" + urlFile.substring("file://".length());
+		}
+		String spec = urlFile.substring(jarFile.getUrl().getFile().length());
 		int separator;
 		while ((separator = spec.indexOf(SEPARATOR)) > 0) {
 			jarFile = getNestedJarFile(jarFile, spec.substring(0, separator));

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

@@ -334,6 +334,22 @@ public class JarFileTests {
 		assertThat(connection.getEntryName(), equalTo("2.dat"));
 	}
 
+	@Test
+	public void createNonNestedUrlFromStringCreatedUsingPath() throws Exception {
+		// gh-5287
+		JarFile.registerUrlProtocolHandler();
+		String spec = "jar:" + this.rootJarFile.toPath().toUri() + "!/2.dat";
+		URL url = new URL(spec);
+		assertThat(url.toString(), equalTo(spec));
+		InputStream inputStream = url.openStream();
+		assertThat(inputStream, notNullValue());
+		assertThat(inputStream.read(), equalTo(2));
+		JarURLConnection connection = (JarURLConnection) url.openConnection();
+		assertThat(connection.getURL().toString(), equalTo(spec));
+		assertThat(connection.getJarFileURL().toURI(), equalTo(this.rootJarFile.toURI()));
+		assertThat(connection.getEntryName(), equalTo("2.dat"));
+	}
+
 	@Test
 	public void getDirectoryInputStream() throws Exception {
 		InputStream inputStream = this.jarFile