A null ID in a Spring Boot cart request may come from a mismatch between the incoming JSON names and the DTO properties—not from the repository. In Tanisha Suyal’s account, /cart/addToCart received an ID as null even though the request included productId. The post attributes the problem to DTO fields named ProductId, UserId, and Quantity, and recommends conventional lower-camel-case names. That is the author’s diagnosis, not a demonstrated rule for every Jackson setup.
What went wrong in the reported cart request
Suyal’s DEV Community post describes an e-commerce project built with Spring Boot and reports that a request to /cart/addToCart reached service work without the expected ID. The resulting error was “The given id must not be null”. The JSON example used keys such as userId, productId, and quantity, while the shown CartRequest fields began with uppercase letters: UserId, ProductId, and Quantity.
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The author says that capitalization mismatch caused the null value and changed the fields to userId, productId, and quantity. The post does not include the complete DTO, its accessors, Jackson configuration, dependency versions, or a reproducible test, so the account is best treated as a useful lead to investigate rather than proof of a universal binding rule. Read Suyal’s account on DEV Community.
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Why capitalization can matter—but does not prove the cause by itself
JavaBeans property discovery uses naming conventions. The Java SE 11 Introspector documentation describes decapitalizing property names and notes that “FooBah” becomes “fooBah,” while “URL” stays “URL”. Jackson has related but distinct behavior: its 2.14 documentation describes MapperFeature.USE_STD_BEAN_NAMING, which changes handling of leading uppercase letters. Under the documented default behavior, getURL() is treated as url; with standard bean naming it yields URL.
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These details explain why a field name, getter, and JSON key should be checked together. They do not establish exactly what happened in Suyal’s application: the post does not identify its Jackson version or mapper settings. See Oracle’s Java SE 11 Introspector API and FasterXML’s Jackson Databind 2.14 MapperFeature API.
Trace the value from the HTTP request to the repository
Follow the ID across each boundary. The first point where it becomes null narrows the cause: if it is absent immediately after request binding, investigate the HTTP body and deserialization; if it is present in the DTO and service but absent at repository use, inspect the intervening application logic.
- Inspect the actual request. Confirm the body sent over the wire contains the expected JSON key and value, for example
productId, rather than relying only on the request builder or a sample in documentation. Check that the request hasContent-Type: application/jsonwhen sending a JSON body. - Confirm how the controller binds input. For a JSON request body, inspect whether the controller parameter is intended to use
@RequestBody. If the value is designed to arrive as a query parameter or path segment, check the corresponding@RequestParamor@PathVariablebinding instead. Suyal’s checklist calls out these distinctions; the correct choice depends on the endpoint design. - Inspect the DTO’s actual binding surface. Check field names alongside getters and setters, or the constructor used for deserialization. If Lombok generates accessors, verify the generated naming and the project’s Lombok configuration rather than assuming the field declaration alone determines the property name.
- Check Jackson customization. Look for property annotations, visibility settings, a naming strategy, and any configured
ObjectMapperor mapper features. These can alter which properties are visible and how names map; compare the effective configuration in the running application with the conventions the DTO assumes. - Log or inspect the value at successive points. Check the bound DTO at controller entry, the argument passed into the service, and the ID immediately before the repository call. Avoid logging sensitive request data; inspect only the relevant field where appropriate. This identifies whether the null begins during deserialization or later in application code.
Apply a focused correction and verify it
If inspection confirms that the incoming JSON uses lower-camel-case keys and the DTO exposes incompatible property names, use consistent lower-camel-case Java fields and accessors, such as userId, productId, and quantity, or explicitly map the JSON property names with the project’s chosen Jackson annotations. Then verify the result at the controller boundary and in a request-binding test using the same payload shape as the client.
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Outbyte PC Repair FREERepair Windows errors before they cause bigger problemsFix Now →Outbyte Driver Updater FREEScan for outdated or missing drivers - takes under a minuteDriver Scan →Do not change persistence code merely because the final exception mentions a null ID. First establish whether the ID was bound correctly and where it is lost. If the DTO already contains the expected value after deserialization, the capitalization theory does not explain the later null; continue tracing service transformations and the repository call instead.
Suyal says the issue took three days to debug, a personal account rather than a measure of how often this occurs. The post closes: “Conventions Matters and not every advice and error handling will be provided to you as such.”
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