private static String getPluginMainClassNameFromManifest(final Attributes manifestAttributes)
            throws InvalidJarPluginException {
        final String pluginMainClassName = getAttributeFromManifest(manifestAttributes, MANIFEST_PLUGIN_MAIN_CLASS);

        if (pluginMainClassName == null) {
            throw new InvalidJarPluginException("Main class name of JAR plugin is not specified.");
        }

        return pluginMainClassName;
    }
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