@responsebody to return string instead of view

I want my string method to return value. The below code returns a view(jsp). In the code "line" stores the product details.I want the method to return this value. Could someone show me how to do it.

@RequestMapping(value="addtocart{id}")
@ResponseBody
    public String addToCart(@PathVariable("id") int id, @ModelAttribute("cart") Cart cart)
    {
        Product product = productService.getProductById(id);
            if (product != null) {
            CartLine line = new CartLine();
            line.setProduct(product);
            line.setQuantity(1);
return "viewcart"; 
    }

xml config

<bean id="jsonConverter" class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
        <property name="prefixJson" value="false"/>
        <property name="supportedMediaTypes" value="application/json"/>
    </bean>