Spring Boot validation: get max size from property file

Question

I have got a spring boot server and would like to validate my values by spring. Using the @Size validation I can set the max size. But i would like to get this max size from my application.property file.

I have already tried to load this value by "@Value(...)" but I can not use this value in the "@Size" field.

 @Value("${max.size.in.properties}")
 private int MAX_SIZE;

@Size(max = 10)
private String description;

Answer 1

The bad news: there's no way to do what you want with standard annotations from Java Validation API.

The good news: you can easily create a custom annotation that does exactly what you want.

You need to create a custom validation annotation (let's call it @ConfigurableSize) that takes as parameters two strings, one for the name of the property holding the min size and one for the name of the property holding the max size.

@Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER, TYPE_USE })
@Retention(RUNTIME)
@Repeatable(ConfigurableSize.List.class)
@Constraint(validatedBy = {ConfigurableSizeCharSequenceValidator.class})
public @interface ConfigurableSize {

    String message() default "size is not valid";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};

    String minProperty() default "";

    String maxProperty() default "";

    @Target({ METHOD, FIELD, ANNOTATION_TYPE, CONSTRUCTOR, PARAMETER, TYPE_USE })
    @Retention(RUNTIME)
    @Documented
    @interface List {
        ConfigurableSize[] value();
    }

}

The validator will retrieve the property values upon initialization, then it will perform the exact same validation checks as the @Size constraint. Even the constraint violation will have the exact same message. Please notice that if the property name is omitted the min and max will default respectively to 0 and Integer.MAX_VALUE, i.e. the same defaults for @Size.

public class ConfigurableSizeCharSequenceValidator implements ConstraintValidator<ConfigurableSize, CharSequence> {

    private final PropertyResolver propertyResolver;
    private int min;
    private int max;

    @Autowired
    public ConfigurableSizeCharSequenceValidator(PropertyResolver propertyResolver) {
        this.propertyResolver = propertyResolver;
    }

    @Override
    public void initialize(ConfigurableSize configurableSize) {
        String minProperty = configurableSize.minProperty();
        String maxProperty = configurableSize.maxProperty();
        this.min = "".equals(minProperty) ? 0 :
                propertyResolver.getRequiredProperty(minProperty, Integer.class);
        this.max = "".equals(maxProperty) ? Integer.MAX_VALUE :
                propertyResolver.getRequiredProperty(maxProperty, Integer.class);
        validateParameters();
    }

    private void validateParameters() {
        if (this.min < 0) {
            throw new IllegalArgumentException("The min parameter cannot be negative.");
        } else if (this.max < 0) {
            throw new IllegalArgumentException("The max parameter cannot be negative.");
        } else if (this.max < this.min) {
            throw new IllegalArgumentException("The length cannot be negative.");
        }
    }

    @Override
    public boolean isValid(CharSequence value, ConstraintValidatorContext context) {
        if (value == null) {
            return true;
        } else {
            int length = value.length();
            boolean retVal = length >= this.min && length <= this.max;
            if (!retVal) {
                HibernateConstraintValidatorContext hibernateContext =
                        context.unwrap(HibernateConstraintValidatorContext.class);
                hibernateContext.addMessageParameter("min", this.min)
                        .addMessageParameter("max", this.max);
                hibernateContext.disableDefaultConstraintViolation();
                hibernateContext
                        .buildConstraintViolationWithTemplate("{javax.validation.constraints.Size.message}")
                        .addConstraintViolation();
            }
            return retVal;
        }
    }

}

You apply the custom annotation in your bean

public class SomeBean {

    @ConfigurableSize(maxProperty = "max.size.in.properties")
    private String description;

}

Then finally in your application.properties you'll define the property

max.size.in.properties=10

And that's it. You can find more details and a full example in this blog post: https://codemadeclear.com/index.php/2021/03/22/easily-configure-validators-via-properties-in-a-spring-boot-project/

Answer 2

We can programmatically specify constraints using Hibernate Validator, which is already available in the classpath when using spring-boot-starter-web.

Given:

 class MyObject {
     private String description;
     ...
 }

We can setup constraints like this:

@Value("${max.size.in.properties}")
private int MAX_SIZE;

HibernateValidatorConfiguration configuration = Validation
                .byProvider( HibernateValidator.class )
                .configure();
ConstraintMapping constraintMapping = configuration.createConstraintMapping();

constraintMapping.type( MyObject.class )
                 .property( "description", FIELD )
                 .constraint( new SizeDef().min( 1 ).max( MAX_SIZE ) );

and validate an instance of the object with:

Validator validator = configuration.addMapping( constraintMapping )
                      .buildValidatorFactory()
                      .getValidator();

Set<ConstraintViolation<MyObject>> constraintViolations =
    validator.validate( myObjectInstance );

if (constraintViolations.size() > 0) {
   ... // handle constraint violations
}