private static Manifest loadJarPluginManifest(final JarURLConnection connection, final Path jarPath)
            throws InvalidJarPluginException {
        try {
            return connection.getManifest();
        } catch (IOException ex) {
            throw new InvalidJarPluginException("Manifest in JAR plugin specified is invalid: " + jarPath.toString(),
                                                ex);
        }
    }
Opens JarURLConnection for the given JAR file path.

Note that JarURLConnection does not have {@code disconnect} nor {@code close}.
embulk-core/src/main/java/org/embulk/plugin/jar/JarPluginLoader.java