Fallback to JVM URL handler on exception

Update the executable JAR `Handler` to fallback to the JVM handler if
the jar cannot be opened. This prevents exceptions when trying to
open URLs in the form "jar:jndi:/localhost...".

Fixes spring-projectsgh-347

Author: Phillip Webb

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

@@ -18,10 +18,13 @@
 
 import java.io.File;
 import java.io.IOException;
+import java.lang.reflect.Method;
 import java.net.MalformedURLException;
 import java.net.URL;
 import java.net.URLConnection;
 import java.net.URLStreamHandler;
+import java.util.logging.Level;
+import java.util.logging.Logger;
 
 /**
  * {@link URLStreamHandler} for Spring Boot loader {@link JarFile}s.
@@ -38,8 +41,26 @@ public class Handler extends URLStreamHandler {
 
 	private static final String SEPARATOR = JarURLConnection.SEPARATOR;
 
+	private static final String[] FALLBACK_HANDLERS = { "sun.net.www.protocol.jar.Handler" };
+
+	private static final Method OPEN_CONNECTION_METHOD;
+	static {
+		Method method = null;
+		try {
+			method = URLStreamHandler.class
+					.getDeclaredMethod("openConnection", URL.class);
+		}
+		catch (Exception ex) {
+		}
+		OPEN_CONNECTION_METHOD = method;
+	}
+
+	private final Logger logger = Logger.getLogger(getClass().getName());
+
 	private final JarFile jarFile;
 
+	private URLStreamHandler fallbackHandler;
+
 	public Handler() {
 		this(null);
 	}
@@ -50,8 +71,60 @@ public Handler(JarFile jarFile) {
 
 	@Override
 	protected URLConnection openConnection(URL url) throws IOException {
-		JarFile jarFile = (this.jarFile != null ? this.jarFile : getJarFileFromUrl(url));
-		return new JarURLConnection(url, jarFile);
+		if (this.jarFile != null) {
+			return new JarURLConnection(url, this.jarFile);
+		}
+		try {
+			return new JarURLConnection(url, getJarFileFromUrl(url));
+		}
+		catch (Exception ex) {
+			return openFallbackConnection(url, ex);
+		}
+	}
+
+	private URLConnection openFallbackConnection(URL url, Exception reason)
+			throws IOException {
+		try {
+			return openConnection(getFallbackHandler(), url);
+		}
+		catch (Exception ex) {
+			this.logger.log(Level.WARNING, "Unable to open fallback handler", ex);
+			if (reason instanceof IOException) {
+				throw (IOException) reason;
+			}
+			if (reason instanceof RuntimeException) {
+				throw (RuntimeException) reason;
+			}
+			throw new IllegalStateException(reason);
+		}
+	}
+
+	private URLStreamHandler getFallbackHandler() {
+		if (this.fallbackHandler != null) {
+			return this.fallbackHandler;
+		}
+
+		for (String handlerClassName : FALLBACK_HANDLERS) {
+			try {
+				Class<?> handlerClass = Class.forName(handlerClassName);
+				this.fallbackHandler = (URLStreamHandler) handlerClass.newInstance();
+				return this.fallbackHandler;
+			}
+			catch (Exception ex) {
+				// Ignore
+			}
+		}
+		throw new IllegalStateException("Unable to find fallback handler");
+	}
+
+	private URLConnection openConnection(URLStreamHandler handler, URL url)
+			throws Exception {
+		if (OPEN_CONNECTION_METHOD == null) {
+			throw new IllegalStateException(
+					"Unable to invoke fallback open connection method");
+		}
+		OPEN_CONNECTION_METHOD.setAccessible(true);
+		return (URLConnection) OPEN_CONNECTION_METHOD.invoke(handler, url);
 	}
 
 	public JarFile getJarFileFromUrl(URL url) throws IOException {