How to use Java Bean Validators (JSR-303/JSR-349) on elements of an array/list/collection

Question

I'm new to using Java Bean validation (JSR-303/JSR-349/Hibernate Validator), and understand the general concepts. However, I'm not sure how to validate the contents of an composed type vs the type itself.

For example:

@NotNull
private List<String> myString;

will validate that the List myString is not null, but does nothing for validating the contents of the list itself. Or given other types of validators (Min/Max/etc), how do I validate the individual elements of the List? Is there a generic solution for any composed type?

Answer 1

There is no easy generic solution as of Bean Validation 1.0/1.1. You could implement a custom constraint like @NoNullElements:

@NoNullElements
private List<String> myStrings;

The constraint's validator would iterate over the list and check that no element is null. Another approach is to wrap your String into a more domain-specific type:

public class EmailAddress {

    @NotNull
    @Email
    private String value;

    //...
}

And apply cascaded validation to the list via @Valid:

@Valid
private List<EmailAddress> addresses;

Having such a domain-specific data type is often helpful anyways to convey a data element's meaning as it is passed through an application.

In the future a generic solution for the issue may be to use annotations on type parameters as supported by Java 8 but that's only an idea at this point:

private List<@NotNull String> myStrings;

Answer 2

Take a look at validator-collection – it’s very easy to use any Constraint Annotation on a collection of simple types with this library. Also see https://stackoverflow.com/a/16023061/2217862.