🌟 @Valid vs @Validated in Spring Boot — The Complete Guide with Fun Examples 🎭💡 🌟

🤔 Extra or Missing Fields in JSON? The Untold Spring Boot Story!

Have you ever sent a JSON request from Postman thinking, “Ah, backend will understand me!”… only to later find out half your fields vanished into thin air? 😅

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🎯 Scenario 1 — Missing Fields in Request

public class Employee {

    private String name;

    private String department;

    private int age;

    private String email;

}

Postman JSON:

{

  "name": "Alice",

  "department": "HR"

}

Result:

Field	Value
name	Alice
department	HR
age	0 (default int)
email	null

📝 Why? Jackson only maps what it sees in JSON. The rest remain null (or default for primitives).

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🎯 Scenario 2 — Extra Fields in Request

public class Employee {

    private String name;

    private String department;

}

Postman JSON:

{

  "name": "Bob",

  "department": "Finance",

  "age": 30,

  "location": "London"

}

Result: Only name and department are populated. age and location vanish like that friend who said “I’ll be there in 5 minutes” ⏳😂

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🔍 Who’s Doing the Magic?

• Spring MVC receives the request 📬
• MappingJackson2HttpMessageConverter turns JSON → Java
• Jackson’s ObjectMapper maps matching fields
• Unknown fields are ignored (by default)

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🚫 When Will It Throw an Error?

1. Global Config: spring.jackson.deserialization.fail-on-unknown-properties=true
2. Per Class: @JsonIgnoreProperties(ignoreUnknown = false)
3. Data type mismatch (e.g., sending "thirty" for an int)
4. Using constructor/record without matching parameters
5. Bean Validation failures (@NotNull, @Valid)

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✅ Validating Missing Fields

import jakarta.validation.constraints.NotNull;

public class Employee {

    @NotNull(message = "Name is required")

    private String name;

    @NotNull(message = "Department is required")

    private String department;

}

Controller:

import org.springframework.validation.annotation.Validated;

@RestController

@Validated // Needed for method-level validation

public class EmployeeController {

    @PostMapping("/create")

    public ResponseEntity create(@Valid @RequestBody Employee employee) {

        // validation happens before this line

        return ResponseEntity.ok("Employee created");

    }

}

💡 Important:

• For request body DTO validation, @Valid alone is enough on the method parameter.
• For method-level validation inside the same bean, you must also add @Validated on the class.

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📝 Best Practices

• Use DTOs instead of directly binding to entities
• Validate with @Valid and Bean Validation
• Enable strict mode in dev/test, allow leniency in prod during deployments
• Document your API to avoid “ghost fields” 👻

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🎤 Interview Q&A Corner

Q: What happens if your JSON request has more fields than your DTO?

A: By default, extra fields are ignored by Jackson.

Q: How can you make Spring Boot fail on unknown fields?

A: Use spring.jackson.deserialization.fail-on-unknown-properties=true or @JsonIgnoreProperties(ignoreUnknown = false).

Q: How do you enforce certain fields to always be present?

A: Add @NotNull with @Valid and, if validating inside the same bean, also @Validated at the class level.

Q: Which Spring component handles JSON → Java object mapping?

A: MappingJackson2HttpMessageConverter with Jackson’s ObjectMapper.

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🌀 Wrapping Up

Today we busted a common myth — extra fields in JSON won’t break your Spring Boot app unless you tell them to. Missing fields? They just become null or default values. But remember — just because your app doesn’t break, doesn’t mean your data is correct! 🧐

💬 Have you ever chased a bug for hours, only to find out the frontend was sending “bonus” fields the backend never saved? Share your story in the comments — let's laugh (and cry) together! 😂