Mastering Custom Validation in Spring Boot: A Step-by-Step Guide

Introduction:

• Brief overview of validation in Spring Boot.

• 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:

// 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:

// 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:

// 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:

// 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:

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

• Example Request Body:

{
    "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