Mastering Custom Validation in Spring Boot: A Step-by-Step Guide
Tags: java, interview, development, spring
Published: 2024-01-24 13:16:57


#### Introduction:
- Brief overview of validation in Spring Boot.

![Image description](https://dev-to-uploads.s3.amazonaws.com/uploads/articles/6c661q5j3z968b91e877.jpg)

- Importance of custom validation for specific business requirements.
- Introduction to the example scenario involving `FinancialProjectDto` and the need for custom validation.

#### Section 1: Setting the Stage
- Explanation of the example scenario: Financial Project Budgeting with the `FinancialProjectDto`.
- Overview of the DTO structure and the significance of accurate data entry.
- Discuss the limitations of standard validation annotations.

#### Section 2: Creating Custom Validation Annotation
- In-depth walkthrough of creating a custom validation annotation (`BudgetCodeConstraint`).
- Explanation of the motivation behind a custom annotation for `budgetCode`.
- Code snippet:

```java
// BudgetCodeConstraint.java
@Target({ElementType.FIELD, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = BudgetCodeValidator.class)
public @interface BudgetCodeConstraint {
    String message() default "Budget code must have less than 5 digits";
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};
    Class<?>[] customValidationGroups() default {};
}
```

#### Section 3: Crafting a Custom Validator
- Step-by-step guide on implementing a custom validator (`BudgetCodeValidator`).
- Explanation of the `ConstraintValidator` interface and its role.
- Code snippet:

```java
// BudgetCodeValidator.java
public class BudgetCodeValidator implements ConstraintValidator<BudgetCodeConstraint, String> {
    @Override
    public void initialize(BudgetCodeConstraint constraintAnnotation) {
    }

    @Override
    public boolean isValid(String budgetCode, ConstraintValidatorContext context) {
        return budgetCode == null || budgetCode.length() < 5;
    }
}
```

#### Section 4: Using Validation Groups
- Introduction to validation groups and their significance.
- Creation of a custom validation group (`BudgetCustomValidationGroup`).
- Application of the custom validation group to the DTO (`FinancialProjectDto`).
- Code snippet:

```java
// BudgetCustomValidationGroup.java
public interface BudgetCustomValidationGroup {
}

// FinancialProjectDto.java
@BudgetCodeConstraint(customValidationGroups = BudgetCustomValidationGroup.class)
public class FinancialProjectDto {
    // Fields and methods...
}
```

#### Section 5: Integrating with Spring Boot
- Explanation of how Spring Boot integrates with validation.
- Annotation of the controller method with `@Validated`.
- Handling validation errors with `BindingResult`.
- Code snippet:

```java
// FinancialProjectController.java
@PostMapping("/financialProjectEndpoint")
public ResponseEntity<?> financialProjectControllerMethod(@Validated(BudgetCustomValidationGroup.class) @RequestBody FinancialProjectDto financialProjectDto, BindingResult bindingResult) {
    if (bindingResult.hasErrors()) {
        // Handle validation errors
        return ResponseEntity.badRequest().body("Validation failed");
    }

    // Your controller logic for a valid DTO
    return ResponseEntity.ok("DTO is valid");
}
```

#### Section 6: Troubleshooting and Best Practices
- Common issues faced during custom validation and their solutions.
- Best practices for organizing custom validation code and packages.
- Tips for handling complex validation scenarios.

#### Section 7: Real-World Analogies
- Drawing analogies between custom validation in software and real-world scenarios.
- Making comparisons with everyday situations to enhance understanding.
- Example analogies to illustrate the importance and impact of validation in various contexts.

#### Section 8: Postman Response and Sample Body
- Include a section with a sample Postman-generated response and a sample request body.
- Provide a step-by-step guide on how to use Postman to test the custom validation.
- Example Response:

```json
{
    "status": "Bad Request",
    "message": "Validation failed",
    "errors": [
        {
            "field": "budgetCode",
            "message": "Budget code must have less than 5 digits"
        },
        // Other validation errors...
    ]
}
```

- Example Request Body:

```json
{
    "budgetCode": "123456", // This should trigger validation error
    // Other fields...
}
```

#### Revise:
- Recap of the importance of custom validation in Spring Boot.
- Encouragement for developers to tailor validation to their specific needs.
- Acknowledgment of the enhanced readability and maintainability achieved through custom validation.

#### Additional Resources:
1. - https://www.google.com/amp/s/blog.tericcabrel.com/write-custom-validator-for-body-request-in-spring-boot/amp/
2. -
https://docs.jboss.org/hibernate/validator/5.1/reference/en-US/html/validator-customconstraints.html