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Validation, Data Binding, and Type Conversion
There are pros and cons for considering validation as business logic, and Spring offers
a design for validation and data binding that does not exclude either one of them.
Specifically, validation should not be tied to the web tier and should be easy to localize,
and it should be possible to plug in any available validator. Considering these concerns,
Spring provides a Validator
contract that is both basic and eminently usable
in every layer of an application.
Data binding is useful for letting user input be dynamically bound to the domain
model of an application (or whatever objects you use to process user input). Spring
provides the aptly named DataBinder
to do exactly that. The Validator
and the
DataBinder
make up the validation
package, which is primarily used in but not
limited to the web layer.
The BeanWrapper
is a fundamental concept in the Spring Framework and is used in a lot
of places. However, you probably do not need to use the BeanWrapper
directly. Because
this is reference documentation, however, we feel that some explanation might be in
order. We explain the BeanWrapper
in this chapter, since, if you are going to use it at
all, you are most likely do so when trying to bind data to objects.
Spring’s DataBinder
and the lower-level BeanWrapper
both use PropertyEditorSupport
implementations to parse and format property values. The PropertyEditor
and
PropertyEditorSupport
types are part of the JavaBeans specification and are also
explained in this chapter. Spring’s core.convert
package provides a general type
conversion facility, as well as a higher-level format
package for formatting UI field
values. You can use these packages as simpler alternatives to PropertyEditorSupport
implementations. They are also discussed in this chapter.
Spring supports Java Bean Validation through setup infrastructure and an adaptor to
Spring’s own Validator
contract. Applications can enable Bean Validation once globally,
as described in Java Bean Validation, and use it exclusively for all validation
needs. In the web layer, applications can further register controller-local Spring
Validator
instances per DataBinder
, as described in Configuring a DataBinder
, which can
be useful for plugging in custom validation logic.