Cheat Sheet

This cheat sheet contains important takeaways from section three.

Field Validation

@Size(min = 2, message = "Username is too short")

• @Valid: validates the fields of the object.
• BindingResult: carries the result of the validation
• th:errors: displays the error message for a violated field.

    @PostMapping(value="path")
    public String postMethodName(@Valid Object object, BindingResult result)

Attention: You must define the BindingResult after Valid.

Custom Constraint Annotation

• @interface: defines a custom annotation.
• @Target(FIELD): target of your annotation is a field.
• @Retention(RUNTIME): retains annotation during the runtime.
• @Constraint(validatedBy = Validator.class): connects the constraint to a validator.

@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = Validator.class)
public @interface AnnotationName {

    //default message if constraint is violated
    String message() default "Invalid Data";
    //boilerplate parameters.
    Class<?>[] groups() default {};
    Class<? extends Payload>[] payload() default {};

}

Validator: implements the ConstraintValidator interface.

public class Validator implements ConstraintValidator<Annotation Type, ValidationType> {
    
    @Override
    public boolean isValid(ValidationType value, ConstraintValidatorContext context) {
        return true OR false;
    }

}

Spring Boot uses your definition of isValid to validate the custom annotated field.

Cross-Field Validation

BindingResult.reject(String field, String errorCode, String errorMessage): rejects a field value and adds an error to the BindingResult.