private static String getAttributeFromManifest(final Attributes manifestAttributes, final String attributeName)
            throws InvalidJarPluginException {
        try {
            return manifestAttributes.getValue(attributeName);
        } catch (IllegalArgumentException ex) {
            throw new InvalidJarPluginException("[FATAL/INTERNAL] "
                                                + attributeName
                                                + " is considered invalid as a manifest attribute.",
                                                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