Custom validator annotation in Spring Boot - Step-by-Step Execution

Concept Flow - Custom validator annotation

Define Annotation Interface

↓

Create Validator Class

↓

Implement isValid() Method

↓

Apply Annotation to Field

↓

Spring Boot Validation Trigger

↓

Validator Checks Field Value

↓

Validation Passes or Fails

↓

Return Result to Framework

This flow shows how a custom annotation is defined, linked to a validator class, applied to a field, and then used by Spring Boot to validate input.

Execution Sample

Spring Boot

@Target({ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = MyValidator.class)
public @interface MyConstraint {
  String message() default "Invalid value";
  Class<?>[] groups() default {};
  Class<? extends Payload>[] payload() default {};
}

public class MyValidator implements ConstraintValidator<MyConstraint, String> {
  @Override
  public boolean isValid(String value, ConstraintValidatorContext context) {
    return value != null && value.matches("\\d+");
  }
}

Defines a custom annotation and validator that checks if a string contains only digits.

Execution Table

Step	Action	Input Value	Validation Check	Result
1	Annotation applied to field	"12345"	Check if value matches digits only	Valid
2	Annotation applied to field	"abc123"	Check if value matches digits only	Invalid
3	Annotation applied to field	null	Check if value is not null and digits only	Invalid
4	Validation completes	-	-	Validation result returned to framework

💡 Validation stops after checking input value and returning pass/fail result

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
value	undefined	"12345"	"abc123"	null	varies per input
isValid	undefined	true	false	false	true or false

Key Moments - 3 Insights

Why does the validator return false when the input is null?

Because the isValid method explicitly checks for null and returns false if the value is null, as shown in step 3 of the execution_table.

How does Spring Boot know to use the custom validator?

The custom annotation is linked to the validator class via @Constraint annotation on the annotation interface, so Spring Boot calls isValid automatically when validating the annotated field.

What happens if the input string contains letters and digits?

The validator returns false because the regex check fails, as shown in step 2 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the validation result when the input is "abc123"?

A Valid

B Invalid

C Null

D Not checked

💡 Hint

Check row 2 in the execution_table where input is "abc123" and result is shown

At which step does the validator return false because the input is null?

A Step 3

B Step 2

C Step 1

D Step 4

💡 Hint

Look at the execution_table row where input value is null

If the regex in isValid changed to allow letters, how would the result for input "abc123" change?

A It would remain Invalid

B It would cause an error

C It would become Valid

D It would be null

💡 Hint

Consider how the validation check result depends on the regex matching the input

Concept Snapshot

Define a custom annotation interface with @Constraint.
Create a validator class implementing ConstraintValidator.
Override isValid() to check the field value.
Apply annotation to fields to validate.
Spring Boot calls isValid during validation.
Returns true if valid, false if invalid.

Full Transcript

This visual execution shows how to create and use a custom validator annotation in Spring Boot. First, you define an annotation interface with @Constraint linking to a validator class. Then, you create a validator class implementing ConstraintValidator and override the isValid method to check the input value. When you apply the annotation to a field, Spring Boot automatically calls the validator during validation. The execution table traces different input values through the validator, showing when validation passes or fails. Key moments clarify why null inputs fail and how Spring Boot connects the annotation to the validator. The quiz tests understanding of validation results and behavior changes. This step-by-step trace helps beginners see how custom validation works in practice.