如何在外部对象上使用beanvalidation？

Question

I'd like to use BeanValidation mechanisms like @NotNull, @Size etc on a big object. I only want to validate certain properties that I make use of.

The object comes from an external library, so I cannot add annotations to it's getters. Neither would I like to manually create my own object and make a deep copy of the project.

Is there any change I could use a BeanValidation framework on a class that I cannot change? Similar to:

class MyValidator {
    @NotNull
    static String getUsername(ExternalClass ext) {
         return ext.getUser();
    }
}

Answer 1

As answered by mvb13, you can use the XML based configuration for this. If you're working with Hibernate Validator as your BV provider you also can leverage the API for programmatic constraint declaration :

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

ConstraintMapping constraintMapping = configuration.createConstraintMapping();

constraintMapping
    .type( ExternalClass.class )
        .property( "someProperty", FIELD )
            .constraint( new NotNullDef() )
            .constraint( new SizeDef().min( 2 ).max( 14 ) )
    .type( AnotherExternalClass.class )
        .property( "anotherProperty", METHOD )
            .constraint( new NotNullDef() );

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

Answer 2

Exists variant of using xml config to describe validation rules. This is how it is implemented in Hibernate https://docs.jboss.org/hibernate/validator/4.0.1/reference/en/html/validator-xmlconfiguration.html In this case you don't need to change source code.