因此，当我处理RestApi并尝试获取请求表单POSTMAN时，这会抛出一个错误，该错误指出
没有用于预设内容类型为"null"的[class java. util. ArrayList]的转换器
我期待requestmapping将返回一个json数据格式的数组列表。
下面是我代码。

@RestController
@RequestMapping("/postman")
public class PostManController {
    @Autowired
    private PostmanProductDao postmanProductDao;
    @ResponseBody
    @GetMapping(value = "/hi")
    public String showPostman(){
        return "Hey, I am the postman. the new Buddy.";
    }

    @ResponseBody
    @GetMapping (value = "/Products",
            produces = MediaType.APPLICATION_JSON_VALUE)
    public List<PostmanProduct> getProducts(){
        return this.postmanProductDao.getProducts();
    }
}

@Component
public class PostmanProductDao {

    List<PostmanProduct> list;

    public postmanProductDao(){
        list = new ArrayList<>();
        list.add(new postmanProduct(100,"Ipod","Apple"));
        list.add(new postmanProduct(102,"Samsung Buds","Samsung"));
    }

    public List<PostmanProduct> getProducts(){
        return list;
    }
}