Validation using Spring’s Validator interface
9. Validation, Data Binding, and Type Conversion
9.1 Introduction
JSR-303JSR-349 Bean Validation Spring Framework 4.0 supports Bean Validation 1.0 JSR-303 and Bean Validation 1.1 JSR-349 in terms of setup support, also adapting it to Spring’s Validator interface. An application can choose to enable Bean Validation once globally, as described in Section 9.8, “Spring Validation”, and use it exclusively for all validation needs. An application can also register additional Spring Validator instances per DataBinder instance, as described in the section called “Configuring a DataBinder”. This may be useful for plugging in validation logic without the use of annotations. 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, should be easy to localize and it should be possible to plug in any validator available. Considering the above, Spring has come up with a Validator interface that is both basic and eminently usable in every layer of an application. Data binding is useful for allowing user input to be dynamically bound to the domain model of an application or whatever objects you use to process user input. Spring provides the so-called 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 MVC framework. The BeanWrapper is a fundamental concept in the Spring Framework and is used in a lot of places. However, you probably will not have the need to use the BeanWrapper directly. Because this is reference documentation however, we felt that some explanation might be in order. We will explain the BeanWrapper in this chapter since, if you were going to use it at all, you would most likely do so when trying to bind data to objects. Spring’s DataBinder and the lower-level BeanWrapper both use PropertyEditors to parse and format property values. The PropertyEditor concept is part of the JavaBeans specification, and is also explained in this chapter. Spring 3 introduces a core.convert package that provides a general type conversion facility, as well as a higher-level format package for formatting UI field values. These new packages may be used as simpler alternatives to PropertyEditors, and will also be discussed in this chapter.9.2 Validation using Spring’s Validator interface
Spring features a Validator interface that you can use to validate objects. The Validator interface works using an Errors object so that while validating, validators can report validation failures to the Errors object. Let’s consider a small data object: 4.3.9.RELEASE Spring Framework 171 public class Person { private String name; private int age; the usual getters and setters... } We’re going to provide validation behavior for the Person class by implementing the following two methods of the org.springframework.validation.Validator interface: • supportsClass - Can this Validator validate instances of the supplied Class ? • validateObject, org.springframework.validation.Errors - validates the given object and in case of validation errors, registers those with the given Errors object Implementing a Validator is fairly straightforward, especially when you know of the ValidationUtils helper class that the Spring Framework also provides. public class PersonValidator implements Validator { This Validator validates just Person instances public boolean supportsClass clazz { return Person. class .equalsclazz; } public void validateObject obj, Errors e { ValidationUtils.rejectIfEmptye, name , name.empty ; Person p = Person obj; if p.getAge 0 { e.rejectValue age , negativevalue ; } else if p.getAge 110 { e.rejectValue age , too.darn.old ; } } } As you can see, the static rejectIfEmpty.. method on the ValidationUtils class is used to reject the name property if it is null or the empty string. Have a look at the ValidationUtils javadocs to see what functionality it provides besides the example shown previously. While it is certainly possible to implement a single Validator class to validate each of the nested objects in a rich object, it may be better to encapsulate the validation logic for each nested class of object in its own Validator implementation. A simple example of a rich object would be a Customer that is composed of two String properties a first and second name and a complex Address object. Address objects may be used independently of Customer objects, and so a distinct AddressValidator has been implemented. If you want your CustomerValidator to reuse the logic contained within the AddressValidator class without resorting to copy-and-paste, you can dependency-inject or instantiate an AddressValidator within your CustomerValidator , and use it like so: 4.3.9.RELEASE Spring Framework 172 public class CustomerValidator implements Validator { private final Validator addressValidator; public CustomerValidatorValidator addressValidator { if addressValidator == null { throw new IllegalArgumentException The supplied [Validator] is + required and must not be null. ; } if addressValidator.supportsAddress. class { throw new IllegalArgumentException The supplied [Validator] must + support the validation of [Address] instances. ; } this .addressValidator = addressValidator; } This Validator validates Customer instances, and any subclasses of Customer too public boolean supportsClass clazz { return Customer. class .isAssignableFromclazz; } public void validateObject target, Errors errors { ValidationUtils.rejectIfEmptyOrWhitespaceerrors, firstName , field.required ; ValidationUtils.rejectIfEmptyOrWhitespaceerrors, surname , field.required ; Customer customer = Customer target; try { errors.pushNestedPath address ; ValidationUtils.invokeValidator this .addressValidator, customer.getAddress, errors; } finally { errors.popNestedPath; } } } Validation errors are reported to the Errors object passed to the validator. In case of Spring Web MVC you can use spring:bind tag to inspect the error messages, but of course you can also inspect the errors object yourself. More information about the methods it offers can be found in the javadocs.9.3 Resolving codes to error messages
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