Allow PropertyEditors to transform null values to alternative representation (empty Collection, empty array, domain specific NullObject, et cetera) [SPR-1729]

[spring-projects-issues]

Taras Tielkes opened SPR-1729 and commented

The wide variation of domain models includes many flavors of representing null/nil members/relationships.

Among the various approaches are:
-Empty Collection members
-Empty array members
-Domain model specific NullObject pattern applications

We've been heavily using the PropertyEditor support in Spring MVC for binding directly to real domain model classes.
One of the missing (IMHO) pieces of functionality is more control over translating null values.

Scenario:
-Domain object has a member of type Collection (or subtype): for instance: the Roles assigned to a User
-Web form presents all possible selections, and indicates which are currently assigned (select multiple or a set of checkboxes)
-Controller class registers a CustomCollectionEditor for the "roles" property

When the form is submitted with all roles deselected, no HTTP parameters are sent. A common solution for this is to use the "field marker" support in WebDataBinder. This support adds a PropertyValue with value null to the current binding operation.

The current implemetation of BeanWrapperImpl has conditional blocks checking for a null value. In such a case, custom editors are never invoked.

This request comes down to giving registered editors a chance to process null values. Especially for editors like CustomCollectionEditor, being able to return an empty List, an empty Set or a NullObject instance would be a great improvement.

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Affects: 1.2.6

[spring-projects-issues]

Juergen Hoeller commented

Thanks for the suggestion!

I've refined BeanWrapperImpl to pass a null value to a PropertyEditor's "setValue" method, allowing for conversion of null values.

I've also added an explicit "nullAsEmptyCollection" flag to CustomCollectionEditor, allowing to convert a null value to an empty Collection (instead of keeping null as-is).

Juergen