Spring Boot Bug: How a Single Capital Letter Broke Cart Endpoints for Three Days

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A developer spent three days debugging a Spring Boot e-commerce cart feature that kept returning null IDs despite correct input. The root cause was a Java naming convention violation — DTO fields like 'UserId' and 'ProductId' started with uppercase letters, which prevented Jackson from properly deserializing the incoming JSON. In Java, field names should follow camelCase convention starting with a lowercase letter, so 'userId' and 'productId' are correct. Jackson relies on getter and setter naming patterns tied to field names, so a mismatched case breaks data binding silently without an obvious error. The incident highlights how ignoring standard naming conventions in Java can cause subtle, hard-to-trace bugs in Spring Boot applications.

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