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9. Validation, Data Binding, and Type Conversion
JSR-303/JSR-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 Section 9.8.3, “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.
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:
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:
supports(Class)
- Can thisValidator
validate instances of the suppliedClass
?validate(Object, org.springframework.validation.Errors)
- validates the given object and in case of validation errors, registers those with the givenErrors
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 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");
}
}
}
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:
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 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.
We’ve talked about databinding and validation. Outputting messages corresponding to validation errors is the last thing we need to discuss. In the example we’ve shown above, we rejected the name
and the age
field. If we’re going to output the error messages by using a MessageSource
, we will do so using the error code we’ve given when rejecting the field ('name' and 'age' in this case). When you call (either directly, or indirectly, using for example the ValidationUtils
class) rejectValue
or one of the other reject
methods from the Errors
interface, the underlying implementation will not only register the code you’ve passed in, but also a number of additional error codes. What error codes it registers is determined by the MessageCodesResolver
that is used. By default, the DefaultMessageCodesResolver
is used, which for example not only registers a message with the code you gave, but also messages that include the field name you passed to the reject method. So in case you reject a field using rejectValue("age", "too.darn.old")
, apart from the too.darn.old
code, Spring will also register too.darn.old.age
and too.darn.old.age.int
(so the first will include the field name and the second will include the type of the field); this is done as a convenience to aid developers in targeting error messages and suchlike.
More information on the MessageCodesResolver
and the default strategy can be found online in the javadocs of MessageCodesResolver
and DefaultMessageCodesResolver
, respectively.
The org.springframework.beans
package adheres to the JavaBeans standard provided by Oracle. A JavaBean is simply a class with a default no-argument constructor, which follows a naming convention where (by way of an example) a property named bingoMadness
would have a setter method setBingoMadness(..)
and a getter method getBingoMadness()
. For more information about JavaBeans and the specification, please refer to Oracle’s website ( javabeans ).
One quite important class in the beans package is the BeanWrapper
interface and its corresponding implementation ( BeanWrapperImpl
). As quoted from the javadocs, the BeanWrapper
offers functionality to set and get property values (individually or in bulk), get property descriptors, and to 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. Then, the BeanWrapper
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 the setting of indexed properties. The BeanWrapper
usually isn’t used by application code directly, but 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, like setting and retrieving properties.
Setting and getting properties is done using the setPropertyValue(s)
and getPropertyValue(s)
methods that both come with a couple of overloaded variants. They’re all described in more detail in the javadocs Spring comes with. What’s important to know is that there are a couple of conventions for indicating properties of an object. A couple of examples:
Table 9.1. Examples of properties
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 key COMPANYNAME of the Map property |
Below you’ll find some examples of working with the BeanWrapper
to get and set properties.
(This next section is not vitally important to you if you’re not planning to work with the BeanWrapper
directly. If you’re just using the DataBinder
and the BeanFactory
and their out-of-the-box implementation, you should skip ahead to the section about PropertyEditors
.)
Consider the following two classes:
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 PropertyEditors
to effect the conversion between an Object
and a String
. If you think about it, it sometimes might be handy to be able 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’re still able to 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 alternately in a specific IoC container as mentioned in the previous chapter, gives it the knowledge of how to convert properties to the desired type. Read more about PropertyEditors
in the javadocs of the java.beans
package provided by Oracle.
A couple of examples where property editing is used in Spring:
- setting properties on beans is done using
PropertyEditors
. When mentioningjava.lang.String
as the value of a property of some bean you’re declaring in XML file, Spring will (if the setter of the corresponding property has aClass
-parameter) use theClassEditor
to try to resolve the parameter to aClass
object. - parsing HTTP request parameters in Spring’s MVC framework is done using all kinds of
PropertyEditors
that you can manually bind in all subclasses of theCommandController
.
Spring has a number of built-in PropertyEditors
to make life easy. Each of those is listed below and they are all located in the org.springframework.beans.propertyeditors
package. Most, but not all (as indicated below), are registered by default by BeanWrapperImpl
. Where the property editor is configurable in some fashion, you can of course still register your own variant to override the default one:
Table 9.2. Built-in PropertyEditors
Class | Explanation |
---|---|
|
Editor for byte arrays. Strings will simply be converted to their corresponding byte representations. Registered by default by |
|
Parses Strings representing classes to actual classes and the other way around. When a class is not found, an |
|
Customizable property editor for |
|
Property editor for Collections, converting any source |
|
Customizable property editor for java.util.Date, supporting a custom DateFormat. NOT registered by default. Must be user registered as needed with appropriate format. |
|
Customizable property editor for any Number subclass like |
|
Capable of resolving Strings to |
|
One-way property editor, capable of taking a text string and producing (via an intermediate |
|
Capable of resolving Strings to |
|
Capable of resolving Strings to |
|
Capable of converting Strings (formatted using the format as defined in the javadocs of the |
|
Property editor that trims Strings. Optionally allows transforming an empty string into a |
|
Capable of resolving 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 will automatically discover 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 FooEditor
class to be recognized and used as the PropertyEditor
for Foo
-typed properties.
com
chank
pop
Foo
FooEditor // the PropertyEditor for the Foo class
Note that you can also use the standard BeanInfo
JavaBeans mechanism here as well (described in not-amazing-detail here ). Find below an example of using the BeanInfo
mechanism for explicitly registering one or more PropertyEditor
instances with the properties of an associated class.
com
chank
pop
Foo
FooBeanInfo // the BeanInfo for the Foo class
Here is the Java source code for the referenced FooBeanInfo
class. This would associate a CustomNumberEditor
with the age
property of the Foo
class.
public class FooBeanInfo extends SimpleBeanInfo {
public PropertyDescriptor[] getPropertyDescriptors() {
try {
final PropertyEditor numberPE = new CustomNumberEditor(Integer.class, true);
PropertyDescriptor ageDescriptor = new PropertyDescriptor("age", Foo.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 a string value, a Spring IoC container ultimately uses standard JavaBeans PropertyEditors
to convert these Strings to the complex type of the property. Spring pre-registers a number of custom PropertyEditors
(for example, to convert a classname expressed as a string into a real Class
object). Additionally, Java’s standard JavaBeans PropertyEditor
lookup mechanism allows a PropertyEditor
for a class simply to be named appropriately and placed in the same package as the class it provides support for, to be found automatically.
If there is a need to register other custom PropertyEditors
, there are several mechanisms available. The most manual approach, which is not normally convenient or recommended, is to simply 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 bean factory post-processors can be used with BeanFactory
implementations, the CustomEditorConfigurer
has a nested property setup, so it is strongly recommended that it is used with the ApplicationContext
, where it may be deployed in similar fashion to any other bean, and automatically detected and applied.
Note that all bean factories and application contexts automatically use a number of built-in property editors, through their use of something called 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 an additional number of 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. CustomEditorConfigurer
, a bean factory post-processor, may be used to conveniently add support for additional PropertyEditor
instances to an ApplicationContext
.
Consider a user class ExoticType
, and another class 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
will behind the scenes convert into an actual ExoticType
instance:
<bean id="sample" class="example.DependsOnExoticType">
<property name="type" value="aNameForExoticType"/>
</bean>
The PropertyEditor
implementation could look similar to this:
// converts string representation to ExoticType object
package example;
public class ExoticTypeEditor extends PropertyEditorSupport {
public void setAsText(String text) {
setValue(new ExoticType(text.toUpperCase()));
}
}
Finally, we 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: write a corresponding registrar and reuse that in each case. PropertyEditorRegistrars
work in conjunction with an interface called PropertyEditorRegistry
, an interface that is implemented by the Spring BeanWrapper
(and DataBinder
). PropertyEditorRegistrars
are particularly convenient when used in conjunction with the CustomEditorConfigurer
(introduced here), which exposes a property called setPropertyEditorRegistrars(..)
: PropertyEditorRegistrars
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.
Using a PropertyEditorRegistrar
is perhaps best illustrated with an example. First off, you need 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.
Next we 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. Find below an example of using 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 just one line long!), and allows common PropertyEditor
registration code to be encapsulated in a class and then shared amongst as many Controllers
as needed.
Spring 3 introduces a core.convert
package that provides a general type conversion system. The system defines an SPI to implement type conversion logic, as well as an API to execute type conversions at runtime. Within a Spring container, this system can be used as an alternative to PropertyEditors to convert externalized bean property value strings to required property types. The public API may also be used anywhere in your application where type conversion is needed.
The SPI to implement type conversion logic is simple and strongly typed:
package org.springframework.core.convert.converter;
public interface Converter<S, T> {
T convert(S source);
}
To create your own converter, simply implement the interface above. Parameterize S
as the type you are converting from, and T
as the type you are converting to. Such a converter can also be applied transparently if a collection or array of S
needs to be converted to an array or collection of T
, provided that a delegating array/collection converter has been registered as well (which DefaultConversionService
does by default).
For each call to convert(S)
, the source argument is guaranteed to be NOT null. Your Converter may throw any unchecked exception if conversion fails; specifically, an IllegalArgumentException
should be thrown 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. Consider StringToInteger
as an example for 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 java.lang.Enum objects, implement ConverterFactory
:
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 StringToEnum
ConverterFactory 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 the GenericConverter interface. With a more flexible but less strongly typed signature, a GenericConverter supports converting between multiple source and target types. In addition, a GenericConverter makes available source and target field context you can use when implementing your conversion logic. Such context allows a type conversion to be driven by a field annotation, or generic information declared on a field signature.
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 implement your conversion logic. The source TypeDescriptor provides access to the source field holding the value being converted. The target TypeDescriptor provides access to the target field where the converted value will 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 allows each element in the source array to 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, only use it when you need it. Favor Converter or ConverterFactory for basic type conversion needs. |
Sometimes you only want a Converter
to execute if a specific condition holds true. For example, you might only want to execute a Converter
if a specific annotation is present on the target field. Or you might only want to execute a Converter
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 allows you to 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 an persistent entity identifier and an entity reference. Such a EntityConverter might only match if the target entity type declares a static finder method e.g. findAccount(Long)
. You would perform such a finder method check in the implementation of matches(TypeDescriptor, TypeDescriptor)
.
The ConversionService defines a unified API for executing type conversion logic at runtime. Converters are often executed behind this 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, then shared between multiple threads. In a Spring application, you typically configure a ConversionService instance per Spring container (or ApplicationContext). That ConversionService will be picked up by Spring and then used whenever a type conversion needs to be performed by the framework. You may 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 id 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 converter(s), set the converters
property. Property values may implement either 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 Section 22.16.3, “Conversion and Formatting” in the Spring MVC chapter.
In certain situations you may wish to apply formatting during conversion. See Section 9.6.3, “FormatterRegistry SPI” for details on using FormattingConversionServiceFactoryBean
.
To work with a ConversionService instance programmatically, simply inject a reference to it like you would for any other bean:
@Service
public class MyService {
@Autowired
public MyService(ConversionService conversionService) {
this.conversionService = conversionService;
}
public void doIt() {
this.conversionService.convert(...)
}
}
For most use cases, the convert
method specifying the targetType can be used but it will not work with more complex types such as a collection of a parameterized element. If you want to convert a List
of Integer
to a List
of String
programmatically, for instance, you need to provide a formal definition of the source and target types.
Fortunately, TypeDescriptor
provides various options to make that straightforward:
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
registers converters automatically which are appropriate for most environments. This includes collection converters, scalar converters, and also basic Object
to String
converters. The same converters can be registered with any ConverterRegistry
using the static addDefaultConverters
method on the DefaultConversionService
class.
Converters for value types will be 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 introduces a convenient Formatter SPI that provides a simple and robust alternative to PropertyEditors for client environments.
In general, use the Converter SPI when you need to implement general-purpose type conversion logic; for example, for converting between a java.util.Date and and java.lang.Long. Use the Formatter SPI when you’re working 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:
package org.springframework.format;
public interface Formatter<T> extends Printer<T>, Parser<T> {
}
Where Formatter extends from the Printer and Parser building-block 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, simply implement the Formatter interface above. 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 IllegalArgumentException if a parse attempt fails. Take care to ensure your Formatter implementation is thread-safe.
Several Formatter implementations are provided in format
subpackages as a convenience. The number
package provides a NumberStyleFormatter
, CurrencyStyleFormatter
, and PercentStyleFormatter
to format java.lang.Number
objects using 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 .
Consider DateFormatter
as 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 jira.spring.io to contribute.
As you will see, field formatting can be configured by field type or annotation. To bind an Annotation to a formatter, implement AnnotationFormatterFactory:
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);
}
Parameterize A to be the field annotationType you wish to associate formatting logic with, for example org.springframework.format.annotation.DateTimeFormat
. Have getFieldTypes()
return the types of fields the annotation may be used on. 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 example AnnotationFormatterFactory implementation below binds the @NumberFormat Annotation to a formatter. This annotation allows either a number style or pattern to 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, simply annotate fields with @NumberFormat:
public class MyModel {
@NumberFormat(style=Style.CURRENCY)
private BigDecimal decimal;
}
A portable format annotation API exists in the org.springframework.format.annotation
package. Use @NumberFormat to format java.lang.Number fields. Use @DateTimeFormat to format java.util.Date, java.util.Calendar, java.util.Long, or Joda-Time fields.
The example below uses @DateTimeFormat to format a java.util.Date as a 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. This implementation may be configured programmatically or declaratively as a Spring bean using FormattingConversionServiceFactoryBean
. Because this implementation also implements ConversionService
, it can be directly configured for use with Spring’s DataBinder and the Spring Expression Language (SpEL).
Review the FormatterRegistry SPI below:
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 above, Formatters can be registered by fieldType or annotation.
The FormatterRegistry SPI allows you to 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 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.
The FormatterRegistrar is an SPI for registering formatters and converters through the FormatterRegistry:
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 Section 22.16.3, “Conversion and Formatting” in the Spring MVC chapter.
By default, date and time fields that are not annotated with @DateTimeFormat
are converted from strings using the DateFormat.SHORT
style. If you prefer, you can change this by defining your own global format.
You will need to ensure that Spring does not register default formatters, and 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 will register 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
. Here is the same example, 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
http://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 |
If you are using 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 Section 22.16.3, “Conversion and Formatting” for details.
Spring 3 introduces several enhancements to its validation support. First, the JSR-303 Bean Validation API is now fully supported. Second, when used programmatically, Spring’s DataBinder can now validate objects as well as bind to them. Third, Spring MVC now has support for declaratively validating @Controller
inputs.
JSR-303 standardizes validation constraint declaration and metadata for the Java platform. Using this API, you annotate domain model properties with declarative validation constraints and the runtime enforces them. There are a number of built-in constraints you can take advantage of. You may also define your own custom constraints.
To illustrate, consider a simple PersonForm model with two properties:
public class PersonForm {
private String name;
private int age;
}
JSR-303 allows you to define declarative validation constraints against such properties:
public class PersonForm {
@NotNull
@Size(max=64)
private String name;
@Min(0)
private int age;
}
When an instance of this class is validated by a JSR-303 Validator, these constraints will be enforced.
For general information on JSR-303/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 setup 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/JSR-349 Bean Validation provider as a Spring bean. This allows for a javax.validation.ValidatorFactory
or javax.validation.Validator
to be injected wherever validation is needed in your application.
Use the LocalValidatorFactoryBean
to configure a default Validator as a Spring bean:
<bean id="validator"
class="org.springframework.validation.beanvalidation.LocalValidatorFactoryBean"/>
The basic configuration above will trigger Bean Validation to initialize using its default bootstrap mechanism. A JSR-303/JSR-349 provider, such as Hibernate Validator, is expected to be present in the classpath and will be detected automatically.
LocalValidatorFactoryBean
implements both javax.validation.ValidatorFactory
and javax.validation.Validator
, as well as Spring’s org.springframework.validation.Validator
. You may inject a reference to either of these interfaces into beans that need to invoke validation logic.
Inject a reference to javax.validation.Validator
if you prefer to work with the Bean Validation API directly:
import javax.validation.Validator;
@Service
public class MyService {
@Autowired
private Validator validator;
Inject a reference to org.springframework.validation.Validator
if your bean requires the Spring Validation API:
import org.springframework.validation.Validator;
@Service
public class MyService {
@Autowired
private Validator validator;
}
Each Bean Validation constraint consists of two parts. First, a @Constraint
annotation that declares the constraint and its configurable properties. Second, 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 allows your custom ConstraintValidators to benefit from dependency injection like any other Spring bean.
Shown below is an example of 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 you can see, a ConstraintValidator implementation may have its dependencies @Autowired like any other Spring bean.
The method validation feature supported by Bean Validation 1.1, and as a custom extension also by Hibernate Validator 4.3, can be integrated into a Spring context through a MethodValidationPostProcessor
bean definition:
<bean class="org.springframework.validation.beanvalidation.MethodValidationPostProcessor"/>
In order to be eligible for Spring-driven method validation, all target classes need to be annotated with Spring’s @Validated
annotation, optionally declaring the validation groups to use. Check out the MethodValidationPostProcessor
javadocs for setup details with Hibernate Validator and Bean Validation 1.1 providers.
The default LocalValidatorFactoryBean
configuration should prove sufficient for most cases. There are a number of configuration options for various Bean Validation constructs, from message interpolation to traversal resolution. See the LocalValidatorFactoryBean
javadocs for more information on these options.
Since Spring 3, a DataBinder instance can be configured with a Validator. Once configured, the Validator may be invoked by calling binder.validate()
. Any validation Errors are automatically added to the binder’s BindingResult.
When working with the DataBinder programmatically, this can be used 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();
A DataBinder can also be configured with multiple Validator
instances via 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 ???.
See Section 22.16.4, “Validation” in the Spring MVC chapter.