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 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 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 MVC framework.
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 felt 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 3 introduced a core.convert package that 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.
As of version 4.0, Spring Framework supports Bean Validation 1.0 (JSR-303) and
Bean Validation 1.1 (JSR-349) for setup support and adapting them to Spring’s
Validator interface.
An application can choose to enable Bean Validation once globally, as described in Spring Validation, and use it exclusively for all validation needs.
An application can also register additional Spring Validator instances for each
DataBinder instance, as described in Configuring a DataBinder. This may be useful for
plugging in validation logic without the use of annotations.
Spring features a Validator interface that you can use to validate objects. The
Validator interface works by using an Errors object so that, while validating,
validators can report validation failures to the Errors object.
Consider the following example of a small data object:
public class Person {
private String name;
private int age;
// the usual getters and setters...
}The next example provides validation behavior for the Person class by implementing the
following two methods of the org.springframework.validation.Validator interface:
-
supports(Class): Can thisValidatorvalidate instances of the suppliedClass? -
validate(Object, org.springframework.validation.Errors): Validates the given object and, in case of validation errors, registers those with the givenErrorsobject.
Implementing a Validator is fairly straightforward, especially when you know of the
ValidationUtils helper class that the Spring Framework also provides. The following
example implements Validator for Person instances:
public class PersonValidator implements Validator {
/**
* This Validator validates *only* Person instances
*/
public boolean supports(Class clazz) {
return Person.class.equals(clazz);
}
public void validate(Object obj, Errors e) {
ValidationUtils.rejectIfEmpty(e, "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");
}
}
}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
{api-spring-framework}/validation/ValidationUtils.html[ValidationUtils] javadoc
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 a second name) and a complex Address object. Address objects
may be used independently of Customer objects, 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,
as the following example shows:
public class CustomerValidator implements Validator {
private final Validator addressValidator;
public CustomerValidator(Validator addressValidator) {
if (addressValidator == null) {
throw new IllegalArgumentException("The supplied [Validator] is " +
"required and must not be null.");
}
if (!addressValidator.supports(Address.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 supports(Class clazz) {
return Customer.class.isAssignableFrom(clazz);
}
public void validate(Object target, Errors errors) {
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "firstName", "field.required");
ValidationUtils.rejectIfEmptyOrWhitespace(errors, "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 the case
of Spring Web MVC, you can use the <spring:bind/> tag to inspect the error messages, but
you can also inspect the Errors object yourself. More information about the
methods it offers can be found in the {api-spring-framework}validation/Errors.html[javadoc].
We covered databinding and validation. This section covers outputting messages that correspond
to validation errors. In the example shown in the preceding section,
we rejected the name and age fields. If we want to output the error messages by using a
MessageSource, we can do so using the error code we provide when rejecting the field
('name' and 'age' in this case). When you call (either directly, or indirectly, by using,
for example, the ValidationUtils class) rejectValue or one of the other reject methods
from the Errors interface, the underlying implementation not only registers the code you
passed in but also registers a number of additional error codes. The MessageCodesResolver
determines which error codes the Errors interface registers. By default, the
DefaultMessageCodesResolver is used, which (for example) not only registers a message
with the code you gave but also registers messages that include the field name you passed
to the reject method. So, if you reject a field by using rejectValue("age", "too.darn.old"),
apart from the too.darn.old code, Spring also registers too.darn.old.age and
too.darn.old.age.int (the first includes the field name and the second includes the type
of the field). This is done as a convenience to aid developers when targeting error messages.
More information on the MessageCodesResolver and the default strategy can be found
in the javadoc of
{api-spring-framework}/validation/MessageCodesResolver.html[MessageCodesResolver] and
{api-spring-framework}/validation/DefaultMessageCodesResolver.html[DefaultMessageCodesResolver],
respectively.
The org.springframework.beans package adheres to the JavaBeans standard.
A JavaBean is a class with a default no-argument constructor and that follows
a naming convention where (for example) a property named bingoMadness would
have a setter method setBingoMadness(..) and a getter method getBingoMadness(). For
more information about JavaBeans and the specification, see
javabeans.
One quite important class in the beans package is the BeanWrapper interface and its
corresponding implementation (BeanWrapperImpl). As quoted from the javadoc, the
BeanWrapper offers functionality to set and get property values (individually or in
bulk), get property descriptors, and query properties to determine if they are
readable or writable. Also, the BeanWrapper offers support for nested properties,
enabling the setting of properties on sub-properties to an unlimited depth. The
BeanWrapper also supports the ability to add standard JavaBeans PropertyChangeListeners
and VetoableChangeListeners, without the need for supporting code in the target class.
Last but not least, the BeanWrapper provides support for setting indexed properties.
The BeanWrapper usually is not used by application code directly but is used by the
DataBinder and the BeanFactory.
The way the BeanWrapper works is partly indicated by its name: it wraps a bean to
perform actions on that bean, such as setting and retrieving properties.
Setting and getting properties is done by using the setPropertyValue,
setPropertyValues, getPropertyValue, and getPropertyValues methods which come
with a couple of overloaded variants. Springs javadoc describes them in more detail.
The JavaBeans specification has conventions for indicating properties of an
object. The following table shows some examples of these conventions:
| Expression | Explanation |
|---|---|
|
Indicates the property |
|
Indicates the nested property |
|
Indicates the third element of the indexed property |
|
Indicates the value of the map entry indexed by the |
(This next section is not vitally important to you if you do not plan to work with
the BeanWrapper directly. If you use only the DataBinder and the BeanFactory
and their default implementations, you should skip ahead to the
section on PropertyEditors.)
The following two example classes use the BeanWrapper to get and set
properties:
public class Company {
private String name;
private Employee managingDirector;
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public Employee getManagingDirector() {
return this.managingDirector;
}
public void setManagingDirector(Employee managingDirector) {
this.managingDirector = managingDirector;
}
}public class Employee {
private String name;
private float salary;
public String getName() {
return this.name;
}
public void setName(String name) {
this.name = name;
}
public float getSalary() {
return salary;
}
public void setSalary(float salary) {
this.salary = salary;
}
}The following code snippets show some examples of how to retrieve and manipulate some of
the properties of instantiated Companies and Employees:
BeanWrapper company = new BeanWrapperImpl(new Company());
// setting the company name..
company.setPropertyValue("name", "Some Company Inc.");
// ... can also be done like this:
PropertyValue value = new PropertyValue("name", "Some Company Inc.");
company.setPropertyValue(value);
// ok, let's create the director and tie it to the company:
BeanWrapper jim = new BeanWrapperImpl(new Employee());
jim.setPropertyValue("name", "Jim Stravinsky");
company.setPropertyValue("managingDirector", jim.getWrappedInstance());
// retrieving the salary of the managingDirector through the company
Float salary = (Float) company.getPropertyValue("managingDirector.salary");Spring uses the concept of a PropertyEditor to effect the conversion between an
Object and a String. It can be handy
to represent properties in a different way than the object itself. For example, a Date
can be represented in a human readable way (as the String: '2007-14-09'), while
we can still convert the human readable form back to the original date (or, even
better, convert any date entered in a human readable form back to Date objects). This
behavior can be achieved by registering custom editors of type
java.beans.PropertyEditor. Registering custom editors on a BeanWrapper or,
alternatively, in a specific IoC container (as mentioned in the previous chapter), gives it
the knowledge of how to convert properties to the desired type. For more about
PropertyEditor, see the javadoc of the java.beans package from Oracle.
A couple of examples where property editing is used in Spring:
-
Setting properties on beans is done by using
PropertyEditorimplementations. When you useStringas the value of a property of some bean that you declare in an XML file, Spring (if the setter of the corresponding property has aClassparameter) usesClassEditorto try to resolve the parameter to aClassobject. -
Parsing HTTP request parameters in Spring’s MVC framework is done by using all kinds of
PropertyEditorimplementations that you can manually bind in all subclasses of theCommandController.
Spring has a number of built-in PropertyEditor implementations to make life easy.
They are all located in the org.springframework.beans.propertyeditors
package. Most, (but not all, as indicated in the following table) are, by default, registered by
BeanWrapperImpl. Where the property editor is configurable in some fashion, you can
still register your own variant to override the default one. The following table describes
the various PropertyEditor implementations that Spring provides:
PropertyEditor Implementations
| Class | Explanation |
|---|---|
|
Editor for byte arrays. Converts strings to their corresponding byte
representations. Registered by default by |
|
Parses Strings that represent classes to actual classes and vice-versa. When a
class is not found, an |
|
Customizable property editor for |
|
Property editor for collections, converting any source |
|
Customizable property editor for |
|
Customizable property editor for any |
|
Resolves strings to |
|
One-way property editor that can take a string and produce (through an
intermediate |
|
Can resolve strings to |
|
Can resolve strings to |
|
Can convert strings (formatted with the format defined in the javadoc of the
|
|
Property editor that trims strings. Optionally allows transforming an empty string
into a |
|
Can resolve a string representation of a URL to an actual |
Spring uses the java.beans.PropertyEditorManager to set the search path for property
editors that might be needed. The search path also includes sun.bean.editors, which
includes PropertyEditor implementations for types such as Font, Color, and most of
the primitive types. Note also that the standard JavaBeans infrastructure
automatically discovers PropertyEditor classes (without you having to register them
explicitly) if they are in the same package as the class they handle and have the same
name as that class, with Editor appended. For example, one could have the following
class and package structure, which would be sufficient for the SomethingEditor class to be
recognized and used as the PropertyEditor for Something-typed properties.
com
chank
pop
Something
SomethingEditor // the PropertyEditor for the Something class
Note that you can also use the standard BeanInfo JavaBeans mechanism here as well
(described to some extent
here). The following example use the BeanInfo mechanism to
explicitly register one or more PropertyEditor instances with the properties of an
associated class:
com
chank
pop
Something
SomethingBeanInfo // the BeanInfo for the Something class
The following Java source code for the referenced SomethingBeanInfo class
associates a CustomNumberEditor with the age property of the Something class:
public class SomethingBeanInfo extends SimpleBeanInfo {
public PropertyDescriptor[] getPropertyDescriptors() {
try {
final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true);
PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Something.class) {
public PropertyEditor createPropertyEditor(Object bean) {
return numberPE;
};
};
return new PropertyDescriptor[] { ageDescriptor };
}
catch (IntrospectionException ex) {
throw new Error(ex.toString());
}
}
}When setting bean properties as string values, a Spring IoC container ultimately uses
standard JavaBeans PropertyEditor implementations to convert these strings to the complex type of the
property. Spring pre-registers a number of custom PropertyEditor implementations (for example, to
convert a class name expressed as a string into a Class object). Additionally,
Java’s standard JavaBeans PropertyEditor lookup mechanism lets a PropertyEditor
for a class be named appropriately and placed in the same package as the class
for which it provides support, so that it can be found automatically.
If there is a need to register other custom PropertyEditors, several mechanisms are
available. The most manual approach, which is not normally convenient or
recommended, is to use the registerCustomEditor() method of the
ConfigurableBeanFactory interface, assuming you have a BeanFactory reference.
Another (slightly more convenient) mechanism is to use a special bean factory
post-processor called CustomEditorConfigurer. Although you can use bean factory post-processors
with BeanFactory implementations, the CustomEditorConfigurer has a
nested property setup, so we strongly recommend that you use it with the
ApplicationContext, where you can deploy it in similar fashion to any other bean and
where it can be automatically detected and applied.
Note that all bean factories and application contexts automatically use a number of
built-in property editors, through their use a BeanWrapper to
handle property conversions. The standard property editors that the BeanWrapper
registers are listed in the previous section.
Additionally, ApplicationContexts also override or add additional editors to handle
resource lookups in a manner appropriate to the specific application context type.
Standard JavaBeans PropertyEditor instances are used to convert property values
expressed as strings to the actual complex type of the property. You can use
CustomEditorConfigurer, a bean factory post-processor, to conveniently add
support for additional PropertyEditor instances to an ApplicationContext.
Consider the following example, which defines a user class called ExoticType and
another class called DependsOnExoticType, which needs ExoticType set as a property:
package example;
public class ExoticType {
private String name;
public ExoticType(String name) {
this.name = name;
}
}
public class DependsOnExoticType {
private ExoticType type;
public void setType(ExoticType type) {
this.type = type;
}
}When things are properly set up, we want to be able to assign the type property as a
string, which a PropertyEditor converts into an actual
ExoticType instance. The following bean definition shows how to set up this relationship:
<bean id="sample" class="example.DependsOnExoticType">
<property name="type" value="aNameForExoticType"/>
</bean>The PropertyEditor implementation could look similar to the following:
// converts string representation to ExoticType object
package example;
public class ExoticTypeEditor extends PropertyEditorSupport {
public void setAsText(String text) {
setValue(new ExoticType(text.toUpperCase()));
}
}Finally, the following example shows how to use CustomEditorConfigurer to register the new PropertyEditor with the
ApplicationContext, which will then be able to use it as needed:
<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
<property name="customEditors">
<map>
<entry key="example.ExoticType" value="example.ExoticTypeEditor"/>
</map>
</property>
</bean>Another mechanism for registering property editors with the Spring container is to
create and use a PropertyEditorRegistrar. This interface is particularly useful when
you need to use the same set of property editors in several different situations.
You can write a corresponding registrar and reuse it in each case.
PropertyEditorRegistrar instances work in conjunction with an interface called
PropertyEditorRegistry, an interface that is implemented by the Spring BeanWrapper
(and DataBinder). PropertyEditorRegistrar instances are particularly convenient
when used in conjunction with CustomEditorConfigurer (described
here), which exposes a property
called setPropertyEditorRegistrars(..). PropertyEditorRegistrar instances added
to a CustomEditorConfigurer in this fashion can easily be shared with DataBinder and
Spring MVC controllers. Furthermore, it avoids the need for synchronization on custom
editors: A PropertyEditorRegistrar is expected to create fresh PropertyEditor
instances for each bean creation attempt.
The following example shows how to create your own PropertyEditorRegistrar implementation:
package com.foo.editors.spring;
public final class CustomPropertyEditorRegistrar implements PropertyEditorRegistrar {
public void registerCustomEditors(PropertyEditorRegistry registry) {
// it is expected that new PropertyEditor instances are created
registry.registerCustomEditor(ExoticType.class, new ExoticTypeEditor());
// you could register as many custom property editors as are required here...
}
}See also the org.springframework.beans.support.ResourceEditorRegistrar for an example
PropertyEditorRegistrar implementation. Notice how in its implementation of the
registerCustomEditors(..) method ,it creates new instances of each property editor.
The next example shows how to configure a CustomEditorConfigurer and inject an instance of our
CustomPropertyEditorRegistrar into it:
<bean class="org.springframework.beans.factory.config.CustomEditorConfigurer">
<property name="propertyEditorRegistrars">
<list>
<ref bean="customPropertyEditorRegistrar"/>
</list>
</property>
</bean>
<bean id="customPropertyEditorRegistrar"
class="com.foo.editors.spring.CustomPropertyEditorRegistrar"/>Finally (and in a bit of a departure from the focus of this chapter for those of you
using Spring’s MVC web framework), using PropertyEditorRegistrars in
conjunction with data-binding Controllers (such as SimpleFormController) can be very
convenient. The following example uses a PropertyEditorRegistrar in the
implementation of an initBinder(..) method:
public final class RegisterUserController extends SimpleFormController {
private final PropertyEditorRegistrar customPropertyEditorRegistrar;
public RegisterUserController(PropertyEditorRegistrar propertyEditorRegistrar) {
this.customPropertyEditorRegistrar = propertyEditorRegistrar;
}
protected void initBinder(HttpServletRequest request,
ServletRequestDataBinder binder) throws Exception {
this.customPropertyEditorRegistrar.registerCustomEditors(binder);
}
// other methods to do with registering a User
}This style of PropertyEditor registration can lead to concise code (the implementation
of initBinder(..) is only one line long) and lets common PropertyEditor
registration code be encapsulated in a class and then shared amongst as many
Controllers as needed.
Spring 3 introduced a core.convert package that provides a general type conversion
system. The system defines an SPI to implement type conversion logic and an API
to perform type conversions at runtime. Within a Spring container, you can use this system
as an alternative to PropertyEditor implementations to convert externalized bean property value
strings to the required property types. You can also use the public API anywhere in your
application where type conversion is needed.
The SPI to implement type conversion logic is simple and strongly typed, as the following interface definition shows:
package org.springframework.core.convert.converter;
public interface Converter<S, T> {
T convert(S source);
}To create your own converter, implement the Converter interface and parameterize S
as the type you are converting from and T as the type you are converting to. You can also transparently apply such a
converter if a collection or array of S needs to be
converted to an array or collection of T, provided that a delegating array or collection
converter has been registered as well (which DefaultConversionService does by default).
For each call to convert(S), the source argument is guaranteed to not be null. Your
Converter may throw any unchecked exception if conversion fails. Specifically, it should throw an
IllegalArgumentException to report an invalid source value.
Take care to ensure that your Converter implementation is thread-safe.
Several converter implementations are provided in the core.convert.support package as
a convenience. These include converters from strings to numbers and other common types.
The following listing shows the StringToInteger class, which is a typical Converter implementation:
package org.springframework.core.convert.support;
final class StringToInteger implements Converter<String, Integer> {
public Integer convert(String source) {
return Integer.valueOf(source);
}
}When you need to centralize the conversion logic for an entire class hierarchy
(for example, when converting from String to Enum objects), you can implement
ConverterFactory, as the following example shows:
package org.springframework.core.convert.converter;
public interface ConverterFactory<S, R> {
<T extends R> Converter<S, T> getConverter(Class<T> targetType);
}Parameterize S to be the type you are converting from and R to be the base type defining
the range of classes you can convert to. Then implement getConverter(Class<T>),
where T is a subclass of R.
Consider the StringToEnumConverterFactory as an example:
package org.springframework.core.convert.support;
final class StringToEnumConverterFactory implements ConverterFactory<String, Enum> {
public <T extends Enum> Converter<String, T> getConverter(Class<T> targetType) {
return new StringToEnumConverter(targetType);
}
private final class StringToEnumConverter<T extends Enum> implements Converter<String, T> {
private Class<T> enumType;
public StringToEnumConverter(Class<T> enumType) {
this.enumType = enumType;
}
public T convert(String source) {
return (T) Enum.valueOf(this.enumType, source.trim());
}
}
}When you require a sophisticated Converter implementation, consider using the
GenericConverter interface. With a more flexible but less strongly typed signature
than Converter, a GenericConverter supports converting between multiple source and
target types. In addition, a GenericConverter makes available source and target field
context that you can use when you implement your conversion logic. Such context lets a
type conversion be driven by a field annotation or by generic information declared on a
field signature. The following listing shows the interface definition of GenericConverter:
package org.springframework.core.convert.converter;
public interface GenericConverter {
public Set<ConvertiblePair> getConvertibleTypes();
Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
}To implement a GenericConverter, have getConvertibleTypes() return the supported
source→target type pairs. Then implement convert(Object, TypeDescriptor,
TypeDescriptor) to contain your conversion logic. The source TypeDescriptor provides
access to the source field that holds the value being converted. The target TypeDescriptor
provides access to the target field where the converted value is to be set.
A good example of a GenericConverter is a converter that converts between a Java array
and a collection. Such an ArrayToCollectionConverter introspects the field that declares
the target collection type to resolve the collection’s element type. This lets each
element in the source array be converted to the collection element type before the
collection is set on the target field.
|
Note
|
Because GenericConverter is a more complex SPI interface, you should use
it only when you need it. Favor Converter or ConverterFactory for basic type
conversion needs.
|
Sometimes, you want a Converter to run only if a specific condition holds true. For
example, you might want to run a Converter only if a specific annotation is present
on the target field, or you might want to run a Converter only if a specific method
(such as a static valueOf method) is defined on the target class.
ConditionalGenericConverter is the union of the GenericConverter and
ConditionalConverter interfaces that lets you define such custom matching criteria:
public interface ConditionalConverter {
boolean matches(TypeDescriptor sourceType, TypeDescriptor targetType);
}
public interface ConditionalGenericConverter extends GenericConverter, ConditionalConverter {
}A good example of a ConditionalGenericConverter is an EntityConverter that converts
between a persistent entity identifier and an entity reference. Such an EntityConverter
might match only if the target entity type declares a static finder method (for example,
findAccount(Long)). You might perform such a finder method check in the implementation of
matches(TypeDescriptor, TypeDescriptor).
ConversionService defines a unified API for executing type conversion logic at
runtime. Converters are often executed behind the following facade interface:
package org.springframework.core.convert;
public interface ConversionService {
boolean canConvert(Class<?> sourceType, Class<?> targetType);
<T> T convert(Object source, Class<T> targetType);
boolean canConvert(TypeDescriptor sourceType, TypeDescriptor targetType);
Object convert(Object source, TypeDescriptor sourceType, TypeDescriptor targetType);
}Most ConversionService implementations also implement ConverterRegistry, which
provides an SPI for registering converters. Internally, a ConversionService
implementation delegates to its registered converters to carry out type conversion logic.
A robust ConversionService implementation is provided in the core.convert.support
package. GenericConversionService is the general-purpose implementation suitable for
use in most environments. ConversionServiceFactory provides a convenient factory for
creating common ConversionService configurations.
A ConversionService is a stateless object designed to be instantiated at application
startup and then shared between multiple threads. In a Spring application, you typically
configure a ConversionService instance for each Spring container (or ApplicationContext).
Spring picks up that ConversionService and uses it whenever a type
conversion needs to be performed by the framework. You can also inject this
ConversionService into any of your beans and invoke it directly.
|
Note
|
If no ConversionService is registered with Spring, the original PropertyEditor-based
system is used.
|
To register a default ConversionService with Spring, add the following bean definition
with an id of conversionService:
<bean id="conversionService"
class="org.springframework.context.support.ConversionServiceFactoryBean"/>A default ConversionService can convert between strings, numbers, enums, collections,
maps, and other common types. To supplement or override the default converters with your
own custom converters, set the converters property. Property values can implement
any of the Converter, ConverterFactory, or GenericConverter interfaces.
<bean id="conversionService"
class="org.springframework.context.support.ConversionServiceFactoryBean">
<property name="converters">
<set>
<bean class="example.MyCustomConverter"/>
</set>
</property>
</bean>It is also common to use a ConversionService within a Spring MVC application. See
Conversion and Formatting in the Spring MVC chapter.
In certain situations, you may wish to apply formatting during conversion. See
The FormatterRegistry SPI for details on using FormattingConversionServiceFactoryBean.
To work with a ConversionService instance programmatically, you can inject a reference to
it like you would for any other bean. The following example shows how to do so:
@Service
public class MyService {
@Autowired
public MyService(ConversionService conversionService) {
this.conversionService = conversionService;
}
public void doIt() {
this.conversionService.convert(...)
}
}For most use cases, you can use the convert method that specifies the targetType, but it
does not work with more complex types, such as a collection of a parameterized element.
For example, if you want to convert a List of Integer to a List of String programmatically,
you need to provide a formal definition of the source and target types.
Fortunately, TypeDescriptor provides various options to make doing so straightforward,
as the following example shows:
DefaultConversionService cs = new DefaultConversionService();
List<Integer> input = ....
cs.convert(input,
TypeDescriptor.forObject(input), // List<Integer> type descriptor
TypeDescriptor.collection(List.class, TypeDescriptor.valueOf(String.class)));Note that DefaultConversionService automatically registers converters that are
appropriate for most environments. This includes collection converters, scalar
converters, and basic Object-to-String converters. You can register the same converters
with any ConverterRegistry by using the static addDefaultConverters
method on the DefaultConversionService class.
Converters for value types are reused for arrays and collections, so there is
no need to create a specific converter to convert from a Collection of S to a
Collection of T, assuming that standard collection handling is appropriate.
As discussed in the previous section, core.convert is a
general-purpose type conversion system. It provides a unified ConversionService API as
well as a strongly typed Converter SPI for implementing conversion logic from one type
to another. A Spring container uses this system to bind bean property values. In
addition, both the Spring Expression Language (SpEL) and DataBinder use this system to
bind field values. For example, when SpEL needs to coerce a Short to a Long to
complete an expression.setValue(Object bean, Object value) attempt, the core.convert
system performs the coercion.
Now consider the type conversion requirements of a typical client environment, such as a
web or desktop application. In such environments, you typically convert from String
to support the client postback process, as well as back to String to support the
view rendering process. In addition, you often need to localize String values. The more
general core.convert Converter SPI does not address such formatting requirements
directly. To directly address them, Spring 3 introduced a convenient Formatter SPI that
provides a simple and robust alternative to PropertyEditor implementations for client environments.
In general, you can use the Converter SPI when you need to implement general-purpose type
conversion logic — for example, for converting between a java.util.Date and a Long.
You can use the Formatter SPI when you work in a client environment (such as a web
application) and need to parse and print localized field values. The ConversionService
provides a unified type conversion API for both SPIs.
The Formatter SPI to implement field formatting logic is simple and strongly typed. The
following listing shows the Formatter interface definition:
package org.springframework.format;
public interface Formatter<T> extends Printer<T>, Parser<T> {
}Formatter extends from the Printer and Parser building-block interfaces. The
following listing shows the definitions of those two interfaces:
public interface Printer<T> {
String print(T fieldValue, Locale locale);
}import java.text.ParseException;
public interface Parser<T> {
T parse(String clientValue, Locale locale) throws ParseException;
}To create your own Formatter, implement the Formatter interface shown earlier.
Parameterize T to be the type of object you wish to format — for example,
java.util.Date. Implement the print() operation to print an instance of T for
display in the client locale. Implement the parse() operation to parse an instance of
T from the formatted representation returned from the client locale. Your Formatter
should throw a ParseException or an IllegalArgumentException if a parse attempt fails. Take
care to ensure that your Formatter implementation is thread-safe.
The format subpackages provide several Formatter implementations as a convenience.
The number package provides NumberStyleFormatter, CurrencyStyleFormatter, and
PercentStyleFormatter to format Number objects that use a java.text.NumberFormat.
The datetime package provides a DateFormatter to format java.util.Date objects with
a java.text.DateFormat. The datetime.joda package provides comprehensive datetime
formatting support based on the Joda-Time library.
The following DateFormatter is an example Formatter implementation:
package org.springframework.format.datetime;
public final class DateFormatter implements Formatter<Date> {
private String pattern;
public DateFormatter(String pattern) {
this.pattern = pattern;
}
public String print(Date date, Locale locale) {
if (date == null) {
return "";
}
return getDateFormat(locale).format(date);
}
public Date parse(String formatted, Locale locale) throws ParseException {
if (formatted.length() == 0) {
return null;
}
return getDateFormat(locale).parse(formatted);
}
protected DateFormat getDateFormat(Locale locale) {
DateFormat dateFormat = new SimpleDateFormat(this.pattern, locale);
dateFormat.setLenient(false);
return dateFormat;
}
}The Spring team welcomes community-driven Formatter contributions. See
GitHub Issues to contribute.
Field formatting can be configured by field type or annotation. To bind
an annotation to a Formatter, implement AnnotationFormatterFactory. The following
listing shows the definition of the AnnotationFormatterFactory interface:
package org.springframework.format;
public interface AnnotationFormatterFactory<A extends Annotation> {
Set<Class<?>> getFieldTypes();
Printer<?> getPrinter(A annotation, Class<?> fieldType);
Parser<?> getParser(A annotation, Class<?> fieldType);
}To create an implementation:
. Parameterize A to be the field annotationType with which you wish to associate
formatting logic — for example org.springframework.format.annotation.DateTimeFormat.
. Have getFieldTypes() return the types of fields on which the annotation can be used.
. Have getPrinter() return a Printer to print the value of an annotated field.
. Have getParser() return a Parser to parse a clientValue for an annotated field.
The following example AnnotationFormatterFactory implementation binds the @NumberFormat
annotation to a formatter to let a number style or pattern be
specified:
public final class NumberFormatAnnotationFormatterFactory
implements AnnotationFormatterFactory<NumberFormat> {
public Set<Class<?>> getFieldTypes() {
return new HashSet<Class<?>>(asList(new Class<?>[] {
Short.class, Integer.class, Long.class, Float.class,
Double.class, BigDecimal.class, BigInteger.class }));
}
public Printer<Number> getPrinter(NumberFormat annotation, Class<?> fieldType) {
return configureFormatterFrom(annotation, fieldType);
}
public Parser<Number> getParser(NumberFormat annotation, Class<?> fieldType) {
return configureFormatterFrom(annotation, fieldType);
}
private Formatter<Number> configureFormatterFrom(NumberFormat annotation, Class<?> fieldType) {
if (!annotation.pattern().isEmpty()) {
return new NumberStyleFormatter(annotation.pattern());
} else {
Style style = annotation.style();
if (style == Style.PERCENT) {
return new PercentStyleFormatter();
} else if (style == Style.CURRENCY) {
return new CurrencyStyleFormatter();
} else {
return new NumberStyleFormatter();
}
}
}
}To trigger formatting, you can annotate fields with @NumberFormat, as the following example shows:
public class MyModel {
@NumberFormat(style=Style.CURRENCY)
private BigDecimal decimal;
}A portable format annotation API exists in the org.springframework.format.annotation
package. You can use @NumberFormat to format Number fields such as Double and
Long, and @DateTimeFormat to format java.util.Date, java.util.Calendar, Long
(for millisecond timestamps) as well as JSR-310 java.time and Joda-Time value types.
The following example uses @DateTimeFormat to format a java.util.Date as an ISO Date
(yyyy-MM-dd):
public class MyModel {
@DateTimeFormat(iso=ISO.DATE)
private Date date;
}The FormatterRegistry is an SPI for registering formatters and converters.
FormattingConversionService is an implementation of FormatterRegistry suitable for
most environments. You can programmatically or declaratively configure this variant
as a Spring bean, e.g. by using FormattingConversionServiceFactoryBean. Because this
implementation also implements ConversionService, you can directly configure it
for use with Spring’s DataBinder and the Spring Expression Language (SpEL).
The following listing shows the FormatterRegistry SPI:
package org.springframework.format;
public interface FormatterRegistry extends ConverterRegistry {
void addFormatterForFieldType(Class<?> fieldType, Printer<?> printer, Parser<?> parser);
void addFormatterForFieldType(Class<?> fieldType, Formatter<?> formatter);
void addFormatterForFieldType(Formatter<?> formatter);
void addFormatterForAnnotation(AnnotationFormatterFactory<?, ?> factory);
}As shown in the preceding listing, you can register formatters by field type or by annotation.
The FormatterRegistry SPI lets you configure formatting rules centrally, instead of
duplicating such configuration across your controllers. For example, you might want to
enforce that all date fields are formatted a certain way or that fields with a specific
annotation are formatted in a certain way. With a shared FormatterRegistry, you define
these rules once, and they are applied whenever formatting is needed.
FormatterRegistrar is an SPI for registering formatters and converters through the
FormatterRegistry. The following listing shows its interface definition:
package org.springframework.format;
public interface FormatterRegistrar {
void registerFormatters(FormatterRegistry registry);
}A FormatterRegistrar is useful when registering multiple related converters and
formatters for a given formatting category, such as date formatting. It can also be
useful where declarative registration is insufficient — for example, when a formatter
needs to be indexed under a specific field type different from its own <T> or when
registering a Printer/Parser pair. The next section provides more information on
converter and formatter registration.
See Conversion and Formatting in the Spring MVC chapter.
By default, date and time fields that are not annotated with @DateTimeFormat are
converted from strings by using the DateFormat.SHORT style. If you prefer, you can
change this by defining your own global format.
To do so, you need to ensure that Spring does not register default formatters. Instead,
you should register all formatters manually. Use the
org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar or
org.springframework.format.datetime.DateFormatterRegistrar class, depending on whether
you use the Joda-Time library.
For example, the following Java configuration registers a global yyyyMMdd
format (this example does not depend on the Joda-Time library):
@Configuration
public class AppConfig {
@Bean
public FormattingConversionService conversionService() {
// Use the DefaultFormattingConversionService but do not register defaults
DefaultFormattingConversionService conversionService = new DefaultFormattingConversionService(false);
// Ensure @NumberFormat is still supported
conversionService.addFormatterForFieldAnnotation(new NumberFormatAnnotationFormatterFactory());
// Register date conversion with a specific global format
DateFormatterRegistrar registrar = new DateFormatterRegistrar();
registrar.setFormatter(new DateFormatter("yyyyMMdd"));
registrar.registerFormatters(conversionService);
return conversionService;
}
}If you prefer XML-based configuration, you can use a
FormattingConversionServiceFactoryBean. The following example shows how to do so (this time using Joda
Time):
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd>
<bean id="conversionService" class="org.springframework.format.support.FormattingConversionServiceFactoryBean">
<property name="registerDefaultFormatters" value="false" />
<property name="formatters">
<set>
<bean class="org.springframework.format.number.NumberFormatAnnotationFormatterFactory" />
</set>
</property>
<property name="formatterRegistrars">
<set>
<bean class="org.springframework.format.datetime.joda.JodaTimeFormatterRegistrar">
<property name="dateFormatter">
<bean class="org.springframework.format.datetime.joda.DateTimeFormatterFactoryBean">
<property name="pattern" value="yyyyMMdd"/>
</bean>
</property>
</bean>
</set>
</property>
</bean>
</beans>|
Note
|
Joda-Time provides separate distinct types to represent date, time, and date-time
values. The dateFormatter, timeFormatter, and dateTimeFormatter properties of the
JodaTimeFormatterRegistrar should be used to configure the different formats for each
type. The DateTimeFormatterFactoryBean provides a convenient way to create formatters.
|
|
Note
|
If you use Spring MVC, remember to explicitly configure the conversion service that
is used. For Java-based @Configuration, this means extending the
WebMvcConfigurationSupport class and overriding the mvcConversionService() method.
For XML, you should use the conversion-service attribute of the
mvc:annotation-driven element.
See Conversion and Formatting for details.
|
Spring 3 introduced several enhancements to its validation support. First, the JSR-303
Bean Validation API is fully supported. Second, when used programmatically, Spring’s
DataBinder can validate objects as well as bind to them. Third, Spring MVC has
support for declaratively validating @Controller inputs.
JSR-303 standardizes validation constraint declaration and metadata for the Java platform. By using this API, you annotate domain model properties with declarative validation constraints and the runtime enforces them. You can use a number of built-in constraints. You can also define your own custom constraints.
Consider the following example, which shows a simple PersonForm model with two properties:
public class PersonForm {
private String name;
private int age;
}JSR-303 lets you define declarative validation constraints against such properties, as the following example shows:
public class PersonForm {
@NotNull
@Size(max=64)
private String name;
@Min(0)
private int age;
}When a JSR-303 Validator validates an instance of this class, these constraints are enforced.
For general information on JSR-303 and JSR-349, see the Bean Validation website. For information on the specific capabilities of the default reference implementation, see the Hibernate Validator documentation. To learn how to set up a bean validation provider as a Spring bean, keep reading.
Spring provides full support for the Bean Validation API. This includes convenient
support for bootstrapping a JSR-303 or JSR-349 Bean Validation provider as a Spring bean.
This lets you inject a javax.validation.ValidatorFactory or javax.validation.Validator
wherever validation is needed in your application.
You can use the LocalValidatorFactoryBean to configure a default Validator as a Spring bean,
as the following example shows:
<bean id="validator"
class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>The basic configuration in the preceding example triggers bean validation to initialize by using its default bootstrap mechanism. A JSR-303 or JSR-349 provider, such as the Hibernate Validator, is expected to be present in the classpath and is automatically detected.
LocalValidatorFactoryBean implements both javax.validation.ValidatorFactory and
javax.validation.Validator, as well as Spring’s
org.springframework.validation.Validator. You can inject a reference to either of
these interfaces into beans that need to invoke validation logic.
You can inject a reference to javax.validation.Validator if you prefer to work with the Bean
Validation API directly, as the following example shows:
import javax.validation.Validator;
@Service
public class MyService {
@Autowired
private Validator validator;You can inject a reference to org.springframework.validation.Validator if your bean requires
the Spring Validation API, as the following example shows:
import org.springframework.validation.Validator;
@Service
public class MyService {
@Autowired
private Validator validator;
}Each bean validation constraint consists of two parts:
* A @Constraint annotation
that declares the constraint and its configurable properties.
* An implementation
of the javax.validation.ConstraintValidator interface that implements the constraint’s
behavior.
To associate a declaration with an implementation, each @Constraint annotation
references a corresponding ConstraintValidator implementation class. At runtime, a
ConstraintValidatorFactory instantiates the referenced implementation when the
constraint annotation is encountered in your domain model.
By default, the LocalValidatorFactoryBean configures a SpringConstraintValidatorFactory
that uses Spring to create ConstraintValidator instances. This lets your custom
ConstraintValidators benefit from dependency injection like any other Spring bean.
The following example shows a custom @Constraint declaration followed by an associated
ConstraintValidator implementation that uses Spring for dependency injection:
@Target({ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy=MyConstraintValidator.class)
public @interface MyConstraint {
}import javax.validation.ConstraintValidator;
public class MyConstraintValidator implements ConstraintValidator {
@Autowired;
private Foo aDependency;
...
}As the preceding example shows, a ConstraintValidator implementation can have its dependencies
@Autowired as any other Spring bean.
You can integrate the method validation feature supported by Bean Validation 1.1 (and, as a custom
extension, also by Hibernate Validator 4.3) into a Spring context
through a MethodValidationPostProcessor bean definition, as follows:
<bean class="org.springframework.validation.beanvalidation.MethodValidationPostProcessor"/>To be eligible for Spring-driven method validation, all target classes need to be annotated with
Spring’s @Validated annotation. (Optionally, you can also declare the validation groups to use.)
See the {api-spring-framework}/validation/beanvalidation/MethodValidationPostProcessor.html[MethodValidationPostProcessor]
javadoc for setup details with the Hibernate Validator and Bean Validation 1.1 providers.
The default LocalValidatorFactoryBean configuration suffices for most
cases. There are a number of configuration options for various Bean Validation
constructs, from message interpolation to traversal resolution. See the
{api-spring-framework}/validation/beanvalidation/LocalValidatorFactoryBean.html[LocalValidatorFactoryBean]
javadoc for more information on these options.
Since Spring 3, you can configure a DataBinder instance with a Validator. Once
configured, you can invoke the Validator by calling binder.validate(). Any validation
Errors are automatically added to the binder’s BindingResult.
The following example shows how to use a DataBinder programmatically to invoke validation
logic after binding to a target object:
Foo target = new Foo();
DataBinder binder = new DataBinder(target);
binder.setValidator(new FooValidator());
// bind to the target object
binder.bind(propertyValues);
// validate the target object
binder.validate();
// get BindingResult that includes any validation errors
BindingResult results = binder.getBindingResult();You can also configure a DataBinder with multiple Validator instances through
dataBinder.addValidators and dataBinder.replaceValidators. This is useful when
combining globally configured bean validation with a Spring Validator configured
locally on a DataBinder instance. See [validation-mvc-configuring].
See Validation in the Spring MVC chapter.