Custom Validation Annotations in Spring

1. Define the Custom Validation AnnotationCustom Validation Annotations in Spring

First, you need to define a custom annotation. This annotation should be marked with @Constraint and specify a class that implements the ConstraintValidator interface as its validator.

import javax.validation.Constraint;
import javax.validation.Payload;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

@Constraint(validatedBy = CourseCodeConstraintValidator.class)
@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface CourseCode {
    public String value() default "LUV";
    public String message() default "must start with LUV";
    public Class<?>[] groups() default {};
    public Class<? extends Payload>[] payload() default {};
}

• @Constraint(validatedBy = CourseCodeConstraintValidator.class): Specifies that CourseCode is a constraint annotation and its validation logic is implemented by CourseCodeConstraintValidator.
• @Target({ElementType.METHOD, ElementType.FIELD}): Indicates that the annotation can be applied to methods and fields.
• @Retention(RetentionPolicy.RUNTIME): Ensures that the annotation is available at runtime.
• public Class<?>[] groups() default {};: Used to specify validation groups.
• public Class<? extends Payload>[] payload() default {};: Used to provide custom payload objects for the validation.

2. Create the Custom Validator

Next, create a class that implements the ConstraintValidator interface. This class contains the actual validation logic.

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class CourseCodeConstraintValidator implements ConstraintValidator<CourseCode, String> {
    private String coursePrefix;

    @Override
    public void initialize(CourseCode theCourseCode) {
        coursePrefix = theCourseCode.value();  // Read the value attribute from the annotation
    }

    @Override
    public boolean isValid(String theCode, ConstraintValidatorContext constraintValidatorContext) {
        if (theCode != null) {
            return theCode.startsWith(coursePrefix);
        } else {
            return true;  // Assume null values are valid
        }
    }
}

• CourseCodeConstraintValidator implements ConstraintValidator<CourseCode, String>.
• initialize method is used to read the value attribute from the annotation.
• isValid method contains the validation logic.

3. Apply the Custom Annotation

Apply the custom annotation to the fields that need to be validated.

import javax.validation.constraints.NotNull;

public class Customer {

    @CourseCode(value="TOPS", message="must start with TOPS")
    private String courseCode;

    @NotNull(message = "is required")
    private String firstName;

    // Getters and setters
    public String getCourseCode() {
        return courseCode;
    }

    public void setCourseCode(String courseCode) {
        this.courseCode = courseCode;
    }

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }
}

4. Integrate with Spring MVC

In the Spring MVC controller, use the @Valid annotation to trigger the validation process.

import org.springframework.stereotype.Controller;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PostMapping;

import javax.validation.Valid;

@Controller
public class CustomerController {

    @PostMapping("/processForm")
    public String processForm(@Valid @ModelAttribute("customer") Customer theCustomer, BindingResult theBindingResult) {
        if (theBindingResult.hasErrors()) {
            // Handle validation errors
            return "customer-form";
        }
        // Proceed with normal business logic
        return "customer-confirmation";
    }
}

• @Valid: Enables automatic validation of the Customer object.
• @ModelAttribute("customer") Customer theCustomer: Binds form data to the Customer object and validates it.
• BindingResult theBindingResult: Captures the validation errors.

By following these steps, you can create and use custom validation annotations in a Spring application to ensure that your data meets specific validation criteria.