private boolean checkPackageSealing(final String packageName, final Manifest manifest, final URL url) {
        final Package packageInstance = this.getPackage(packageName);

        if (packageInstance == null) {
            return false;
        }

        if (packageInstance.isSealed()) {
            if (!packageInstance.isSealed(url)) {
                throw new SecurityException(String.format(
                        "Package \"%s\" is already loaded, and sealed with a different code source URL.", packageName));
            }
        } else {
            if ((manifest != null) && isManifestToSeal(packageName, manifest)) {
                throw new SecurityException(String.format(
                        "Package \"%s\" is already loaded, and unsealed.", packageName));
            }
        }
        return true;
    }
Finds resources recognized as the given name.

<p>Note that {@link java.net.URLClassLoader#findResources} is public while {@link java.lang.ClassLoader#findResources}
is protected.
embulk-core/src/main/java/org/embulk/cli/SelfContainedJarAwareURLClassLoader.java