Bean Validation

Jakarta Bean Validation (formerly JSR-380) provides a declarative way to validate objects using annotations. This article covers built-in constraints, custom validators, groups, and integration with Spring Boot.

📋 At a Glance

Aspect	Details
Specification	Jakarta Bean Validation 3.0
Implementation	Hibernate Validator
Dependency	spring-boot-starter-validation
Scope	DTOs, entities, method parameters

🎯 What You'll Learn

Built-in constraints and when to use them
Custom validators for business rules
Validation groups for context-specific validation
Cross-field validation for complex rules
Spring integration with @Valid and @Validated

Production Story: The Manual Validation Nightmare

A registration service had validation scattered everywhere:

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Problems:

Validation logic duplicated across endpoints
Inconsistent error messages
Easy to forget validation in new code
Hard to test
Business logic mixed with validation

The fix: Bean Validation annotations:

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🔬 Deep Dive

Pattern 1: Built-in Constraints

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Pattern 2: Nested Validation

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Pattern 3: Custom Validators

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Pattern 4: Cross-Field Validation

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Pattern 5: Validation Groups

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Pattern 6: Method Parameter Validation

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Pattern 7: Conditional Validation

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⚠️ Common Mistakes

Mistake 1: Forgetting @Valid for Nested Objects

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Mistake 2: @Validated vs @Valid

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🐛 Debug This: The Silent Validation

A developer reports: "Our nested object validation doesn't work! Invalid addresses are saved to the database."

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Why doesn't the address validation work?

✅ Solution:

The bug is a missing @Valid annotation on the nested shippingAddress field. Bean Validation does NOT automatically cascade to nested objects - you must explicitly tell it to validate nested objects with @Valid.

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The fix:

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Same issue applies to collections:

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The lesson: Always add @Valid to fields that contain nested objects or collections that need validation. Bean Validation only checks the immediate field by default.

💻 Exercises

Exercise 1: Basic Constraints

⭐ Difficulty: Easy | ⏱️ Time: 10 minutes

Task: Add validation constraints to this DTO.

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✅ Solution:

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Exercise 2: Custom Password Validator

⭐⭐ Difficulty: Medium | ⏱️ Time: 20 minutes

Task: Create a custom password validator with configurable requirements.

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✅ Solution:

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Exercise 3: Cross-Field Validation

⭐⭐ Difficulty: Medium | ⏱️ Time: 20 minutes

Task: Create a class-level validator for password confirmation.

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✅ Solution:

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Exercise 4: Validation Groups

⭐⭐⭐ Difficulty: Medium-Hard | ⏱️ Time: 20 minutes

Task: Implement validation groups for create vs update operations.

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✅ Solution:

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Exercise 5: Method Parameter Validation

⭐⭐⭐⭐ Difficulty: Hard | ⏱️ Time: 25 minutes

Task: Enable validation on service method parameters.

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✅ Solution:

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🎤 Interview Questions

Q1: Difference between @NotNull, @NotEmpty, @NotBlank?

Answer:

@NotNull: Not null, but can be empty string or empty collection
@NotEmpty: Not null AND not empty (size > 0), for String/Collection/Map/Array
@NotBlank: Not null AND not empty AND not just whitespace, only for String

Q2: How to validate method parameters in a service?

Answer:

Add @Validated to the class
Add constraints to method parameters
Spring AOP intercepts and validates
ConstraintViolationException thrown on violation

📝 Summary

Constraint	Use For
@NotNull/@NotBlank/@NotEmpty	Required fields
@Size	Length limits
@Min/@Max/@Positive	Numeric bounds
@Email/@Pattern	Format validation
@Past/@Future	Date constraints
@Valid	Nested validation
Custom validators	Business rules
Groups	Context-specific validation

📅 Review Schedule for This Article

Day	Task	Time
Day 1	Review built-in constraints table	5 min
Day 3	Redo Exercise 1 (Basic Constraints)	10 min
Day 7	Answer interview questions without looking	10 min
Day 14	Redo Debug This (Silent Validation)	15 min
Day 30	Audit validation in one DTO in your codebase	20 min

Next: [Part 18: Fluent Validation]

📋 At a Glance

🎯 What You'll Learn

Production Story: The Manual Validation Nightmare

🔬 Deep Dive

Pattern 1: Built-in Constraints

Pattern 2: Nested Validation

Pattern 3: Custom Validators

Pattern 4: Cross-Field Validation

Pattern 5: Validation Groups

Pattern 6: Method Parameter Validation

Pattern 7: Conditional Validation

⚠️ Common Mistakes

Mistake 1: Forgetting @Valid for Nested Objects

Mistake 2: @Validated vs @Valid

🐛 Debug This: The Silent Validation

💻 Exercises

Exercise 1: Basic Constraints

Exercise 2: Custom Password Validator

Exercise 3: Cross-Field Validation

Exercise 4: Validation Groups

Exercise 5: Method Parameter Validation

🎤 Interview Questions

Q1: Difference between @NotNull, @NotEmpty, @NotBlank?

Q2: How to validate method parameters in a service?

📝 Summary

📅 Review Schedule for This Article

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