see more from https://github.com/openzipkin/brave/issues/867, also see https://github.com/apache/incubator-dubbo/issues/3576
* use internal thread local to improve performance
     */
    
    private static final InternalThreadLocal<RpcContext> LOCAL = new InternalThreadLocal<RpcContext>() {
        @Override
        protected RpcContext initialValue() {
            return new RpcContext();
        }
    };
    
    private static final InternalThreadLocal<RpcContext> SERVER_LOCAL = new InternalThreadLocal<RpcContext>() {
        @Override
        protected RpcContext initialValue() {
            return new RpcContext();
        }
    };
    private final Map<String, String> attachments = new HashMap<String, String>();
    private final Map<String, Object> values = new HashMap<String, Object>();
    private Future<?> future;
    private List<URL> urls;
    private URL url;
    private String methodName;
    private Class<?>[] parameterTypes;
    private Object[] arguments;
    private InetSocketAddress localAddress;
    private InetSocketAddress remoteAddress;
    private String remoteApplicationName;
    @Deprecated
    private List<Invoker<?>> invokers;
    @Deprecated