@Override
    public <T extends EventListener> void addListener(T t) {
        if (!context.getState().equals(LifecycleState.STARTING_PREP)) {
            throw new IllegalStateException(
                    sm.getString("applicationContext.addListener.ise",
                            getContextPath()));
        }

        
 
boolean match = false;
        if (t instanceof ServletContextAttributeListener ||
                t instanceof ServletRequestListener ||
                t instanceof ServletRequestAttributeListener ||
                t instanceof HttpSessionIdListener ||
                t instanceof HttpSessionAttributeListener) {
            context.addApplicationEventListener(t);
            match 
 
= true;
        }

        if (t instanceof HttpSessionListener
                || (t instanceof ServletContextListener &&
                        newServletContextListenerAllowed)) {
            // Add listener directly to the list of instances rather than to
            // the list of class names.
 

            context.addApplicationLifecycleListener(t);
            match = true;
        }

        if (match) return;

        if (t instanceof ServletContextListener) {
            throw new IllegalArgumentException(sm.getString(
                    "applicationContext.addListener.iae.sclNotAllowed",
                    t.getClass().getName()));
        } else {
            throw new IllegalArgumentException(sm.getString(
                    "applicationContext.addListener.iae.wrongType",
                    t.getClass().getName()));
        }
    }

這段原始碼，可以看到Listener一定要繼承EventListener，然後conext，到底什麼是上下文，監聽器是其一部分。