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Though the Bean Validation API defines a whole set of standard constraint annotations one can easily think of situations in which these standard annotations won't suffice. For these cases you are able to create custom constraints tailored to your specific validation requirements in a simple manner.
To create a custom constraint, the following three steps are required:
Create a constraint annotation
Implement a validator
Define a default error message
Let's write a constraint annotation, that can be used to express
that a given string shall either be upper case or lower case. We'll
apply it later on to the licensePlate field of the
Car class from Chapter 1, Getting started to ensure, that the field is
always an upper-case string.
First we need a way to express the two case modes. We might use
String constants, but a better way to go is to
use a Java 5 enum for that purpose:
Example 3.1. Enum CaseMode to express upper vs. lower
case
package com.mycompany;
public enum CaseMode {
UPPER,
LOWER;
}
Now we can define the actual constraint annotation. If you've never designed an annotation before, this may look a bit scary, but actually it's not that hard:
Example 3.2. Defining CheckCase constraint annotation
package com.mycompany;
import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;
import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;
import javax.validation.Constraint;
import javax.validation.Payload;
@Target( { METHOD, FIELD, ANNOTATION_TYPE })
@Retention(RUNTIME)
@Constraint(validatedBy = CheckCaseValidator.class)
@Documented
public @interface CheckCase {
String message() default "{com.mycompany.constraints.checkcase}";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
CaseMode value();
}
An annotation type is defined using the @interface
keyword. All attributes of an annotation type are declared in a
method-like manner. The specification of the Bean Validation API
demands, that any constraint annotation defines
an attribute message that returns the default key for creating error messages in case the constraint is violated
an attribute groups that allows the
specification of validation groups, to which this constraint belongs
(see Section 2.3, “Validating groups”).
This must default to an empty array of type
Class<?>.
an attribute payload that can be used
by clients of the Bean Validation API to assign custom payload
objects to a constraint. This attribute is not used by the API
itself.
An example for a custom payload could be the definition of a severity.
public class Severity {
public static class Info extends Payload {};
public static class Error extends Payload {};
}
public class ContactDetails {
@NotNull(message="Name is mandatory", payload=Severity.Error.class)
private String name;
@NotNull(message="Phone number not specified, but not mandatory", payload=Severity.Info.class)
private String phoneNumber;
// ...
}Now a client can after the validation of a
ContactDetails instance access the
severity of a constraint using
ConstraintViolation.getConstraintDescriptor().getPayload()
and adjust its behaviour depending on the severity.
Besides those three mandatory attributes
(message, groups and
payload) we add another one allowing for the
required case mode to be specified. The name value
is a special one, which can be omitted upon using the annotation, if it
is the only attribute specified, as e.g. in
@CheckCase(CaseMode.UPPER).
In addition we annotate the annotation type with a couple of so-called meta annotations:
@Target({ METHOD, FIELD, ANNOTATION_TYPE }):
Says, that methods, fields and annotation declarations may be
annotated with @CheckCase (but not type declarations e.g.)
@Retention(RUNTIME): Specifies, that annotations
of this type will be available at runtime by the means of
reflection
@Constraint(validatedBy =
CheckCaseValidator.class): Specifies the validator to be used
to validate elements annotated with @CheckCase
@Documented: Says, that the use of
@CheckCase will be contained in the JavaDoc of elements
annotated with it
Next, we need to implement a constraint validator, that's able to
validate elements with a @CheckCase annotation.
To do so, we implement the interface ConstraintValidator as shown
below:
Example 3.3. Implementing a constraint validator for the constraint
CheckCase
package com.mycompany;
import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
public class CheckCaseValidator implements ConstraintValidator<CheckCase, String> {
private CaseMode caseMode;
public void initialize(CheckCase constraintAnnotation) {
this.caseMode = constraintAnnotation.value();
}
public boolean isValid(String object, ConstraintValidatorContext constraintContext) {
if (object == null)
return true;
if (caseMode == CaseMode.UPPER)
return object.equals(object.toUpperCase());
else
return object.equals(object.toLowerCase());
}
}
The ConstraintValidator interface defines
two type parameters, which we set in our implementation. The first one
specifies the annotation type to be validated (in our example
CheckCase), the second one the type of elements,
which the validator can handle (here
String).
In case a constraint annotation is allowed at elements of
different types, a ConstraintValidator for each
allowed type has to be implemented and registered at the constraint
annotation as shown above.
The implementation of the validator is straightforward. The
initialize() method gives us access to the
attribute values of the annotation to be validated. In the example we
store the CaseMode in a field of the validator
for further usage.
In the isValid() method we implement the
logic, that determines, whether a String is valid
according to a given @CheckCase annotation or
not. This decision depends on the case mode retrieved in
initialize(). As the Bean Validation
specification recommends, we consider null values as being
valid. If null is not a valid value for an element, it
should be annotated with @NotNull explicitly.
Example 3.3, “Implementing a constraint validator for the constraint
CheckCase” relies on the
default error message generation by just returning
true or false from the
isValid call. Using the passed
ConstraintValidatorContext object it is
possible to either add additional error messages or completely disable
the default error message generation and solely define custom error
messages. The ConstraintValidatorContext API is
modeled as fluent interface and is best demonstrated with an
example:
Example 3.4. Use of ConstraintValidatorContext to define custom error messages
package com.mycompany;
import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;
public class CheckCaseValidator implements ConstraintValidator<CheckCase, String> {
private CaseMode caseMode;
public void initialize(CheckCase constraintAnnotation) {
this.caseMode = constraintAnnotation.value();
}
public boolean isValid(String object, ConstraintValidatorContext constraintContext) {
if (object == null)
return true;
boolean isValid;
if (caseMode == CaseMode.UPPER) {
isValid = object.equals(object.toUpperCase());
}
else {
isValid = object.equals(object.toLowerCase());
}
if(!isValid) {
constraintContext.disableDefaultConstraintViolation();
constraintContext.buildConstraintViolationWithTemplate( "{com.mycompany.constraints.CheckCase.message}" ).addConstraintViolation();
}
return result;
}
}
Example 3.4, “Use of ConstraintValidatorContext to define custom error
messages”
shows how you can disable the default error message generation and add
a custom error message using a specified message template. In this
example the use of the
ConstraintValidatorContext results in the same
error message as the default error message generation.
It is important to end each new constraint violation with
addConstraintViolation. Only after that
the new constraint violation will be created.
In case you are implementing a
ConstraintValidator a class level constraint it
is also possible to adjust set the property path for the created
constraint violations. This is important for the case where you
validate multiple properties of the class or even traverse the object
graph. A custom property path creation could look like Example 3.5, “Adding new ConstraintViolation with
custom property path”.
Example 3.5. Adding new ConstraintViolation with
custom property path
public boolean isValid(Group group, ConstraintValidatorContext constraintValidatorContext) {
boolean isValid = false;
...
if(!isValid) {
constraintValidatorContext
.buildConstraintViolationWithTemplate( "{my.custom.template}" )
.addNode( "myProperty" ).addConstraintViolation();
}
return isValid;
}
Finally we need to specify the error message, that shall be used,
in case a @CheckCase constraint is violated. To
do so, we add the following to our custom
ValidationMessages.properties (see also Section 2.2.4, “Message interpolation”)
Example 3.6. Defining a custom error message for the
CheckCase constraint
com.mycompany.constraints.CheckCase.message=Case mode must be {value}.If a validation error occurs, the validation runtime will use the
default value, that we specified for the message attribute of the
@CheckCase annotation to look up the error
message in this file.
Now that our first custom constraint is completed, we can use it
in the Car class from the Chapter 1, Getting started chapter to specify that the
licensePlate field shall only contain upper-case
strings:
Example 3.7. Applying the CheckCase
constraint
package com.mycompany;
import javax.validation.constraints.Min;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
public class Car {
@NotNull
private String manufacturer;
@NotNull
@Size(min = 2, max = 14)
@CheckCase(CaseMode.UPPER)
private String licensePlate;
@Min(2)
private int seatCount;
public Car(String manufacturer, String licencePlate, int seatCount) {
this.manufacturer = manufacturer;
this.licensePlate = licencePlate;
this.seatCount = seatCount;
}
//getters and setters ...
}
Finally let's demonstrate in a little test that the
@CheckCase constraint is properly
validated:
Example 3.8. Testcase demonstrating the CheckCase
validation
package com.mycompany;
import static org.junit.Assert.*;
import java.util.Set;
import javax.validation.ConstraintViolation;
import javax.validation.Validation;
import javax.validation.Validator;
import javax.validation.ValidatorFactory;
import org.junit.BeforeClass;
import org.junit.Test;
public class CarTest {
private static Validator validator;
@BeforeClass
public static void setUp() {
ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
validator = factory.getValidator();
}
@Test
public void testLicensePlateNotUpperCase() {
Car car = new Car("Morris", "dd-ab-123", 4);
Set<ConstraintViolation<Car>> constraintViolations =
validator.validate(car);
assertEquals(1, constraintViolations.size());
assertEquals(
"Case mode must be UPPER.",
constraintViolations.iterator().next().getMessage());
}
@Test
public void carIsValid() {
Car car = new Car("Morris", "DD-AB-123", 4);
Set<ConstraintViolation<Car>> constraintViolations =
validator.validate(car);
assertEquals(0, constraintViolations.size());
}
}
Looking at the licensePlate field of the
Car class in Example 3.7, “Applying the CheckCase
constraint”, we see three constraint
annotations already. In complexer scenarios, where even more constraints
could be applied to one element, this might become a bit confusing easily.
Furthermore, if we had a licensePlate field in
another class, we would have to copy all constraint declarations to the
other class as well, violating the DRY principle.
This problem can be tackled using compound constraints. In the
following we create a new constraint annotation
@ValidLicensePlate, that comprises the constraints
@NotNull, @Size and
@CheckCase:
Example 3.9. Creating a composing constraint
ValidLicensePlate
package com.mycompany;
import static java.lang.annotation.ElementType.*;
import static java.lang.annotation.RetentionPolicy.*;
import java.lang.annotation.Documented;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;
import javax.validation.Constraint;
import javax.validation.Payload;
import javax.validation.constraints.NotNull;
import javax.validation.constraints.Size;
@NotNull
@Size(min = 2, max = 14)
@CheckCase(CaseMode.UPPER)
@Target( { METHOD, FIELD, ANNOTATION_TYPE })
@Retention(RUNTIME)
@Constraint(validatedBy = {})
@Documented
public @interface ValidLicensePlate {
String message() default "{com.mycompany.constraints.validlicenseplate}";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
}
To do so, we just have to annotate the constraint declaration with
its comprising constraints (btw. that's exactly why we allowed annotation
types as target for the @CheckCase annotation). As
no additional validation is required for the
@ValidLicensePlate annotation itself, we don't
declare a validator within the @Constraint meta
annotation.
Using the new compound constraint at the licensePlate field now is fully equivalent to the previous version, where we declared the three constraints directly at the field itself:
Example 3.10. Application of composing constraint
ValidLicensePlate
package com.mycompany;
public class Car {
@ValidLicensePlate
private String licensePlate;
//...
}
The set of ConstraintViolations retrieved
when validating a Car instance will contain an
entry for each violated composing constraint of the
@ValidLicensePlate constraint. If you rather prefer
a single ConstraintViolation in case any of the
composing constraints is violated, the
@ReportAsSingleViolation meta constraint can be
used as follows:
Example 3.11. Usage of @ReportAsSingleViolation
//...
@ReportAsSingleViolation
public @interface ValidLicensePlate {
String message() default "{com.mycompany.constraints.validlicenseplate}";
Class<?>[] groups() default {};
Class<? extends Payload>[] payload() default {};
}
Copyright © 2009, 2010 Red Hat, Inc. & Gunnar Morling