{"slug": "u-replaced-by-u-is-a-big-error-insight-from-my-on-going-spring-boot-project", "title": "'u' Replaced By 'U' Is A Big Error,Insight From My On Going Spring Boot Project", "summary": "A developer building an AI-aided e-commerce project with Spring Boot, OpenAI models, and BLIP-CLIP models spent three days debugging a cart endpoint that returned 'The given id must not be null'. The issue was caused by DTO field names starting with capital letters (e.g., 'UserId' instead of 'userId'), which broke JSON deserialization. The developer emphasizes the importance of following naming conventions to avoid such errors.", "body_md": "For Three days I was struck on running my Cart endpoints.Everything seems correct.\n\nBut the Error on running the endpoint was :\n\n```\n{\n  \"success\": false,\n  \"message\": \"The given id must not be null\",\n  \"data\": null\n}\n```\n\nSomewhere the ID was becoming null before it reaches the repository even when i was passing the Id.\n\nLet me first clear the endpoints and scenario to you.\n\nI am Building an Full Fledged AI-aided E-com project using spring boot,OpenAI Models and BLIP-CLIP models for the Image-to-Text and Text-to-Image processing.\n\nRight now,I am creating the Entities,Repository,Services and Controllers for basic working of the website.\n\nI am also using DTO's for the structured data transfer.\n\nThe problem started with Cart endpoints,my 'addToCart' service was not actually adding the products into the cart even when I was passing the values I was getting the id to be null.\n\n`/cart/addToCart`\n\nRequest body\n\napplication/json\n\nExample Value\n\nSchema\n\n{\n\n\"userId\": 9007199254740991,\n\n\"productId\": 9007199254740991,\n\n\"quantity\": 1073741824\n\n}\n\nResponse\n\n{\n\n\"success\": boolean,\n\n\"message\": \"string\",\n\n\"data\": \"string\"\n\n}\n\nI queried all the sources and I was getting more confused,Nothing Clearly was error there in my code yet the endpoints were not responsing.\n\nchecklist :\n\n`@RequestBody`\n\nfor JSON data sent in a POST or PUT body.`@RequestParam`\n\nfor query parameters in the URL (like ?name=test).`@PathVariable`\n\nfor variables built directly into the URL path (like /users/{id}).Ensure your JSON property names match your Java class variable names exactly.\n\nVerify that your DTO class has getter and setter methods (or use Lombok `@Data`\n\nor `@Getter`\n\n/`@Setter`\n\n), which Jackson needs to read the data.\n\nCheck that the Content-Type header in your client request is set to application/json when sending a JSON body.\n\nLets take one example,we are creating a cart service in an e-com project and we have this controller for add to cart service.\n\n```\nResponseEntity<ApiResponse<String>> addToCart(@RequestBody CartRequest request){\n        String status = service.addToCart(request);\n\n        ApiResponse<String> response =\n                new ApiResponse<>(\n                        true,\n                        \"Added\",\n                        status\n                );\n\n        return ResponseEntity.ok(response);\n    }\n```\n\nWith the response DTO as cartRequest as follows:\n\n```\npublic class CartRequest {\n\n    @NotNull\n    @Positive\n    private Long UserId;\n\n    @NotNull\n    @Positive\n    private Long ProductId;\n\n    @NotNull\n    @Positive\n    @Min(1)\n    private Integer Quantity;\n\n}\n```\n\nEverything is in sync right?\n\nCongrats! you have hit the error.\n\nRemember?\n\nEnsure your JSON property names match your Java class variable names exactly.\n\nThis is the error I was struck into, this the 'u' replaced with 'U'.Notice the Capital letter at the beginning of the the DTO variables.\n\n```\npublic class CartRequest {\n\n    @NotNull\n    @Positive\n    private Long ~~UserId;~~\n\n    @NotNull\n    @Positive\n    private Long ~~ProductId;~~\n\n    @NotNull\n    @Positive\n    @Min(1)\n    private Integer ~~Quantity;~~\n\n}\n```\n\nConventions Matters and not every advice and error handling will be provided to you as such.\n\nkeep Things in mind and follow conventions for devlopment.", "url": "https://wpnews.pro/news/u-replaced-by-u-is-a-big-error-insight-from-my-on-going-spring-boot-project", "canonical_source": "https://dev.to/tanisha_suyal_8efb4a5b201/u-replaced-by-u-is-a-big-errorinsight-from-my-on-going-spring-boot-project-eo", "published_at": "2026-08-22 16:42:25+00:00", "updated_at": "2026-08-22 17:13:44.747776+00:00", "lang": "en", "topics": ["developer-tools"], "entities": ["Spring Boot", "OpenAI", "BLIP-CLIP"], "alternates": {"html": "https://wpnews.pro/news/u-replaced-by-u-is-a-big-error-insight-from-my-on-going-spring-boot-project", "markdown": "https://wpnews.pro/news/u-replaced-by-u-is-a-big-error-insight-from-my-on-going-spring-boot-project.md", "text": "https://wpnews.pro/news/u-replaced-by-u-is-a-big-error-insight-from-my-on-going-spring-boot-project.txt", "jsonld": "https://wpnews.pro/news/u-replaced-by-u-is-a-big-error-insight-from-my-on-going-spring-boot-project.jsonld"}}