void init(GltfLoader gltfLoader) {
        this.gltfLoader = gltfLoader;

        if (gltfLoader.getInfo().getKey() instanceof GltfModelKey) {
            this.key = (GltfModelKey) gltfLoader.getInfo().getKey();
        }

        JsonArray extensionUsed = gltfLoader.getDocRoot().getAsJsonArray("extensionsUsed");
        if (extensionUsed != null) {
            for (JsonElement extElem : extensionUsed) {
                String ext = extElem.getAsString();
                if (ext != null) {
                    if (defaultExtensionLoaders.get(ext) == null && (this.key != null && this.key.getExtensionLoader(ext) == null)) {
                        logger.log(Level.WARNING, "Extension " + ext + " is not supported, please provide your own implementation in the GltfModelKey");
                    }
                }
            }
        }
        JsonArray extensionRequired = gltfLoader.getDocRoot().getAsJsonArray("extensionsRequired");
        if (extensionRequired != null) {
            for (JsonElement extElem : extensionRequired) {
                String ext = extElem.getAsString();
                if (ext != null) {
                    if (defaultExtensionLoaders.get(ext) == null && (this.key != null && this.key.getExtensionLoader(ext) == null)) {
                        logger.log(Level.SEVERE, "Extension " + ext + " is mandatory for this file, the loaded scene result will be unexpected.");
                    }
                }
            }
        }
    }
Returns the default extras loader.
@return the default extras loader.
jme3-plugins/src/gltf/java/com/jme3/scene/plugins/gltf/CustomContentManager.java