Class Holder

src/org/apache/pig/scripting/js/JsScriptEngine.java:54–81

holds the singleton for UDFs

    /** holds the singleton for UDFs */
    private static final class Holder {
        static JsScriptEngine instance;
        static {
            if (clientInstance!=null) { 
                /** on client side UDFs re-use the same instance */ 
                instance = clientInstance;
            } else {
                /** on the hadoop slave a new instance is created using the UDFContext for initialization */
                instance = new JsScriptEngine();
                String scriptPath = (String) UDFContext.getUDFContext().getUDFProperties(JsFunction.class).get(JsScriptEngine.class.getName()+".scriptFile");
                instance.scriptPath = scriptPath;
                if (scriptPath == null) {
                    throw new IllegalStateException("could not get script path from UDFContext");
                }
                InputStream is = getScriptAsStream(scriptPath);
                try {
                    instance.load(scriptPath, is);
                } finally {
                    try {
                        is.close();
                    } catch (IOException e) {
                        LOG.warn("Could not close stream for file "+scriptPath, e);
                    }
                }

            }
        }
    }

    public static JsScriptEngine getInstance() {
        JsScriptEngine instance = Holder.instance;