private static int getPluginSpiVersionFromManifest(final Attributes manifestAttributes)
            throws InvalidJarPluginException {
        final String spiVersionString = getAttributeFromManifest(manifestAttributes, MANIFEST_PLUGIN_SPI_VERSION);

        if (spiVersionString == null) {
            throw new InvalidJarPluginException("SPI version of JAR plugin is not specified.");
        }

        try {
            return Integer.parseInt(spiVersionString);
        } catch (NumberFormatException ex) {
            throw new InvalidJarPluginException("SPI version of JAR plugin is not an integer: \""
                                                + spiVersionString
                                                + "\"", 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