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As of Bean Validation 1.1, constraints can not only be applied to JavaBeans and their properties, but also to the parameters and return values of the methods and constructors of any Java type. That way Bean Validation constraints can be used to specify
For the purpose of this reference guide, the term method constraint refers to both, method and constructor constraints, if not stated otherwise. Occasionally, the term executable is used when referring to methods and constructors.
This approach has several advantages over traditional ways of checking the correctness of parameters and return values:
IllegalArgumentException
or
similar), resulting in less code to write and maintainIn order to make annotations show up in the JavaDoc of annotated elements, the annotation types themselves must be annotated with the meta annotation @Documented. This is the case for all built-in constraints and is considered a best practice for any custom constraints.
In the remainder of this chapter you will learn how to declare parameter and return value
constraints and how to validate them using the ExecutableValidator
API.
You specify the preconditions of a method or constructor by adding constraint annotations to its parameters as demonstrated in Example 3.1, “Declaring method and constructor parameter constraints”.
Example 3.1. Declaring method and constructor parameter constraints
package org.hibernate.validator.referenceguide.chapter03.parameter;
public class RentalStation {
public RentalStation(@NotNull String name) {
//...
}
public void rentCar(
@NotNull Customer customer,
@NotNull @Future Date startDate,
@Min(1) int durationInDays) {
//...
}
}
The following preconditions are declared here:
name
passed to the RentalCar
constructor must not be null
rentCar()
method, the given customer
must not be null
, the rental’s start
date must not be null
as well as be in the future and finally the rental duration must be at least
one dayNote that declaring method or constructor constraints itself does not automatically cause their
validation upon invocation of the executable. Instead, the ExecutableValidator
API (see
Section 3.2, “Validating method constraints”) must be used to perform the validation, which is
often done using a method interception facility such as AOP, proxy objects etc.
Constraints may only be applied to instance methods, i.e. declaring constraints on static methods is not supported. Depending on the interception facility you use for triggering method validation, additional restrictions may apply, e.g. with respect to the visibility of methods supported as target of interception. Refer to the documentation of the interception technology to find out whether any such limitations exist.
Sometimes validation does not only depend on a single parameter but on several or even all parameters of a method or constructor. This kind of requirement can be fulfilled with help of a cross-parameter constraint.
Cross-parameter constraints can be considered as the method validation equivalent to class-level constraints. Both can be used to implement validation requirements which are based on several elements. While class-level constraints apply to several properties of a bean, cross-parameter constraints apply to several parameters of an executable.
In contrast to single-parameter constraints, cross-parameter constraints are declared on the method
or constructor as you can see in Example 3.2, “Declaring a cross-parameter constraint”. Here the cross-
parameter constraint @LuggageCountMatchesPassengerCount
declared on the load()
method is used to
ensure that no passenger has more than two pieces of luggage.
Example 3.2. Declaring a cross-parameter constraint
package org.hibernate.validator.referenceguide.chapter03.crossparameter;
public class Car {
@LuggageCountMatchesPassengerCount(piecesOfLuggagePerPassenger = 2)
public void load(List<Person> passengers, List<PieceOfLuggage> luggage) {
//...
}
}
As you will learn in the next section, return value constraints are also declared on the method
level. In order to distinguish cross-parameter constraints from return value constraints, the
constraint target is configured in the ConstraintValidator
implementation using the
@SupportedValidationTarget
annotation. You can find out about the details in
Section 6.3, “Cross-parameter constraints” which shows how to implement your own cross-parameter constraint.
In some cases a constraint can be applied to an executable’s parameters (i.e. it is a cross- parameter constraint), but also to the return value. One example for this are custom constraints which allow to specify validation rules using expression or script languages.
Such constraints must define a member validationAppliesTo()
which can be used at declaration time to
specify the constraint target. As shown in Example 3.3, “Specifying a constraint’s target” you apply the
constraint to an executable’s parameters by specifying
validationAppliesTo = ConstraintTarget.PARAMETERS
, while ConstraintTarget.RETURN_VALUE
is used
to apply the constraint to the executable return value.
Example 3.3. Specifying a constraint’s target
package org.hibernate.validator.referenceguide.chapter03.crossparameter.constrainttarget;
public class Garage {
@ELAssert(expression = "...", validationAppliesTo = ConstraintTarget.PARAMETERS)
public Car buildCar(List<Part> parts) {
//...
}
@ELAssert(expression = "...", validationAppliesTo = ConstraintTarget.RETURN_VALUE)
public Car paintCar(int color) {
//...
}
}
Although such a constraint is applicable to the parameters and return value of an executable, the target can often be inferred automatically. This is the case, if the constraint is declared on
In these situations you don’t have to specify the constraint target. It is still recommended to do so if it increases readability of the source code. If the constraint target is not specified in situations where it can’t be determined automatically, a ConstraintDeclarationException is raised.
The postconditions of a method or constructor are declared by adding constraint annotations to the executable as shown in Example 3.4, “Declaring method and constructor return value constraints”.
Example 3.4. Declaring method and constructor return value constraints
package org.hibernate.validator.referenceguide.chapter03.returnvalue;
public class RentalStation {
@ValidRentalStation
public RentalStation() {
//...
}
@NotNull
@Size(min = 1)
public List<Customer> getCustomers() {
//...
}
}
The following constraints apply to the executables of RentalStation:
RentalStation
object must satisfy the @ValidRentalStation
constraintgetCustomers()
must not be null
and must contain at least on elementSimilar to the cascaded validation of JavaBeans properties (see
Section 2.1.6, “Object graphs”), the @Valid
annotation can be used to mark executable
parameters and return values for cascaded validation. When validating a parameter or return value
annotated with @Valid
, the constraints declared on the parameter or return value object are
validated as well.
In Example 3.5, “Marking executable parameters and return values for cascaded validation”, the car
parameter of the method Garage#checkCar()
as
well as the return value of the Garage
constructor are marked for cascaded validation.
Example 3.5. Marking executable parameters and return values for cascaded validation
package org.hibernate.validator.referenceguide.chapter03.cascaded;
public class Garage {
@NotNull
private String name;
@Valid
public Garage(String name) {
this.name = name;
}
public boolean checkCar(@Valid @NotNull Car car) {
//...
}
}
package org.hibernate.validator.referenceguide.chapter03.cascaded;
public class Car {
@NotNull
private String manufacturer;
@NotNull
@Size(min = 2, max = 14)
private String licensePlate;
public Car(String manufacturer, String licencePlate) {
this.manufacturer = manufacturer;
this.licensePlate = licencePlate;
}
//getters and setters ...
}
When validating the arguments of the checkCar()
method, the constraints on the properties of the
passed Car
object are evaluated as well. Similarly, the @NotNull
constraint on the name field of
Garage
is checked when validating the return value of the Garage
constructor.
Generally, the cascaded validation works for executables in exactly the same way as it does for JavaBeans properties.
In particular, null
values are ignored during cascaded validation (naturally this can’t happen
during constructor return value validation) and cascaded validation is performed recursively, i.e.
if a parameter or return value object which is marked for cascaded validation itself has properties
marked with @Valid
, the constraints declared on the referenced elements will be validated as well.
Cascaded validation can not only be applied to simple object references but also to collection-typed
parameters and return values. This means when putting the @Valid
annotation to a parameter or return
value which
java.lang.Iterable
java.util.Map
each contained element gets validated. So when validating the arguments of the checkCars()
method in
Example 3.6, “List-typed method parameter marked for cascaded validation”, each element instance of the passed list will
be validated and a ConstraintViolation
created when any of the contained Car
instances is invalid.
Example 3.6. List-typed method parameter marked for cascaded validation
package org.hibernate.validator.referenceguide.chapter03.cascaded.collection;
public class Garage {
public boolean checkCars(@Valid @NotNull List<Car> cars) {
//...
}
}
When declaring method constraints in inheritance hierarchies, it is important to be aware of the following rules:
These rules are motivated by the concept of behavioral subtyping which requires that wherever a
type T
is used, also a subtype S
of T
may be used without altering the program’s behavior.
As an example, consider a class invoking a method on an object with the static type T
. If the
runtime type of that object was S
and S
imposed additional preconditions, the client class might
fail to satisfy these preconditions as is not aware of them. The rules of behavioral subtyping are
also known as the Liskov
substitution principle.
The Bean Validation specification implements the first rule by disallowing parameter constraints on methods which override or implement a method declared in a supertype (superclass or interface). Example 3.7, “Illegal method parameter constraint in subtype” shows a violation of this rule.
Example 3.7. Illegal method parameter constraint in subtype
package org.hibernate.validator.referenceguide.chapter03.inheritance.parameter;
public interface Vehicle {
void drive(@Max(75) int speedInMph);
}
package org.hibernate.validator.referenceguide.chapter03.inheritance.parameter;
public class Car implements Vehicle {
@Override
public void drive(@Max(55) int speedInMph) {
//...
}
}
The @Max
constraint on Car#drive()
is illegal since this method implements the interface method
Vehicle#drive()
. Note that parameter constraints on overriding methods are also disallowed, if the
supertype method itself doesn’t declare any parameter constraints.
Furthermore, if a method overrides or implements a method declared in several parallel supertypes
(e.g. two interfaces not extending each other or a class and an interface not implemented by that
class), no parameter constraints may be specified for the method in any of the involved types. The
types in Example 3.8, “Illegal method parameter constraint in parallel types of a hierarchy” demonstrate a violation of that
rule. The method RacingCar#drive()
overrides Vehicle#drive()
as well as Car#drive()
.
Therefore the constraint on Vehicle#drive()
is illegal.
Example 3.8. Illegal method parameter constraint in parallel types of a hierarchy
package org.hibernate.validator.referenceguide.chapter03.inheritance.parallel;
public interface Vehicle {
void drive(@Max(75) int speedInMph);
}
package org.hibernate.validator.referenceguide.chapter03.inheritance.parallel;
public interface Car {
public void drive(int speedInMph);
}
package org.hibernate.validator.referenceguide.chapter03.inheritance.parallel;
public class RacingCar implements Car, Vehicle {
@Override
public void drive(int speedInMph) {
//...
}
}
The previously described restrictions only apply to parameter constraints. In contrast, return value constraints may be added in methods overriding or implementing any supertype methods.
In this case, all the method’s return value constraints apply for the subtype method, i.e. the constraints declared on the subtype method itself as well as any return value constraints on overridden/implemented supertype methods. This is legal as putting additional return value constraints in place may never represent a weakening of the postconditions guaranteed to the caller of a method.
So when validating the return value of the method Car#getPassengers()
shown in
Example 3.9, “Return value constraints on supertype and subtype method”, the @Size
constraint on the method itself as well
as the @NotNull
constraint on the implemented interface method Vehicle#getPassengers()
apply.
Example 3.9. Return value constraints on supertype and subtype method
package org.hibernate.validator.referenceguide.chapter03.inheritance.returnvalue;
public interface Vehicle {
@NotNull
List<Person> getPassengers();
}
package org.hibernate.validator.referenceguide.chapter03.inheritance.returnvalue;
public class Car implements Vehicle {
@Override
@Size(min = 1)
public List<Person> getPassengers() {
//...
}
}
If the validation engine detects a violation of any of the aforementioned rules, a
ConstraintDeclarationException
will be raised.
The rules described in this section only apply to methods but not constructors. By definition, constructors never override supertype constructors. Therefore, when validating the parameters or the return value of a constructor invocation only the constraints declared on the constructor itself apply, but never any constraints declared on supertype constructors.
The validation of method constraints is done using the ExecutableValidator
interface.
In Section 3.2.1, “Obtaining an ExecutableValidator
instance” you will learn how to obtain an ExecutableValidator
instance while Section 3.2.2, “ExecutableValidator
methods” shows how to use the different methods
offered by this interface.
Instead of calling the ExecutableValidator
methods directly from within application code, they are
usually invoked via a method interception technology such as AOP, proxy objects, etc. This causes
executable constraints to be validated automatically and transparently upon method or constructor
invocation. Typically a ConstraintViolationException
is raised by the integration layer in case any
of the constraints is violated.
You can retrieve an ExecutableValidator
instance via Validator#forExecutables()
as shown in
Example 3.10, “Obtaining an ExecutableValidator
instance”.
Example 3.10. Obtaining an ExecutableValidator
instance
ValidatorFactory factory = Validation.buildDefaultValidatorFactory();
executableValidator = factory.getValidator().forExecutables();
In the example the executable validator is retrieved from the default validator factory, but if
required you could also bootstrap a specifically configured factory as described in
Chapter 8, Bootstrapping, for instance in order to use a specific parameter name provider
(see Section 8.2.4, “ParameterNameProvider
”).
The ExecutableValidator
interface offers altogether four methods:
validateParameters()
and validateReturnValue()
for method validationvalidateConstructorParameters()
and validateConstructorReturnValue()
for constructor validationJust as the methods on Validator
, all these methods return a Set<ConstraintViolation>
which contains
a ConstraintViolation
instance for each violated constraint and which is empty if the validation
succeeds. Also all the methods have a var-args groups parameter by which you can pass the validation
groups to be considered for validation.
The examples in the following sections are based on the methods on constructors of the Car
class
shown in Example 3.11, “Class Car
with constrained methods and constructors”.
Example 3.11. Class Car
with constrained methods and constructors
package org.hibernate.validator.referenceguide.chapter03.validation;
public class Car {
public Car(@NotNull String manufacturer) {
//...
}
@ValidRacingCar
public Car(String manufacturer, String team) {
//...
}
public void drive(@Max(75) int speedInMph) {
//...
}
@Size(min = 1)
public List<Passenger> getPassengers() {
//...
}
}
The method validateParameters()
is used to validate the arguments of a method invocation.
Example 3.12, “Using ExecutableValidator#validateParameters()
” shows an example. The validation results in a
violation of the @Max
constraint on the parameter of the drive()
method.
Example 3.12. Using ExecutableValidator#validateParameters()
Car object = new Car( "Morris" );
Method method = Car.class.getMethod( "drive", int.class );
Object[] parameterValues = { 80 };
Set<ConstraintViolation<Car>> violations = executableValidator.validateParameters(
object,
method,
parameterValues
);
assertEquals( 1, violations.size() );
Class<? extends Annotation> constraintType = violations.iterator()
.next()
.getConstraintDescriptor()
.getAnnotation()
.annotationType();
assertEquals( Max.class, constraintType );
Note that validateParameters()
validates all the parameter constraints of a method, i.e. constraints
on individual parameters as well as cross-parameter constraints.
Using validateReturnValue()
the return value of a method can can be validated. The validation in
Example 3.13, “Using ExecutableValidator#validateReturnValue()
” yields one constraint violation since the
getPassengers()
method is expect to return at least one Passenger
instance.
Example 3.13. Using ExecutableValidator#validateReturnValue()
Car object = new Car( "Morris" );
Method method = Car.class.getMethod( "getPassengers" );
Object returnValue = Collections.<Passenger>emptyList();
Set<ConstraintViolation<Car>> violations = executableValidator.validateReturnValue(
object,
method,
returnValue
);
assertEquals( 1, violations.size() );
Class<? extends Annotation> constraintType = violations.iterator()
.next()
.getConstraintDescriptor()
.getAnnotation()
.annotationType();
assertEquals( Size.class, constraintType );
The arguments of constructor invocations can be validated with validateConstructorParameters()
as
shown in method Example 3.14, “Using ExecutableValidator#validateConstructorParameters()
”. Due to the
@NotNull
constraint on the manufacturer parameter, the validation call returns one constraint
violation.
Example 3.14. Using ExecutableValidator#validateConstructorParameters()
Constructor<Car> constructor = Car.class.getConstructor( String.class );
Object[] parameterValues = { null };
Set<ConstraintViolation<Car>> violations = executableValidator.validateConstructorParameters(
constructor,
parameterValues
);
assertEquals( 1, violations.size() );
Class<? extends Annotation> constraintType = violations.iterator()
.next()
.getConstraintDescriptor()
.getAnnotation()
.annotationType();
assertEquals( NotNull.class, constraintType );
Finally, by using validateConstructorReturnValue()
you can validate a constructor’s return value. In
Example 3.15, “Using ExecutableValidator#validateConstructorReturnValue()
”, validateConstructorReturnValue()
returns one constraint violation, since the Car
instance returned by the constructor doesn’t satisfy
the @ValidRacingCar
constraint (not shown).
Example 3.15. Using ExecutableValidator#validateConstructorReturnValue()
//constructor for creating racing cars
Constructor<Car> constructor = Car.class.getConstructor( String.class, String.class );
Car createdObject = new Car( "Morris", null );
Set<ConstraintViolation<Car>> violations = executableValidator.validateConstructorReturnValue(
constructor,
createdObject
);
assertEquals( 1, violations.size() );
Class<? extends Annotation> constraintType = violations.iterator()
.next()
.getConstraintDescriptor()
.getAnnotation()
.annotationType();
assertEquals( ValidRacingCar.class, constraintType );
In addition to the methods introduced in Section 2.2.3, “ConstraintViolation
methods”,
ConstraintViolation
provides two more methods specific to the validation of executable parameters
and return values.
ConstraintViolation#getExecutableParameters()
returns the validated parameter array in case of
method or constructor parameter validation, while ConstraintViolation#getExecutableReturnValue()
provides access to the validated object in case of return value validation.
All the other ConstraintViolation
methods generally work for method validation in the same way as
for validation of beans. Refer to the JavaDoc to learn more about the
behavior of the individual methods and their return values during bean and method validation.
Note that getPropertyPath()
can be very useful in order to obtain detailed information about the
validated parameter or return value, e.g. for logging purposes. In particular, you can retrieve name
and argument types of the concerned method as well as the index of the concerned parameter from the
path nodes. How this can be done is shown in Example 3.16, “Retrieving method and parameter information”.
Example 3.16. Retrieving method and parameter information
Car object = new Car( "Morris" );
Method method = Car.class.getMethod( "drive", int.class );
Object[] parameterValues = { 80 };
Set<ConstraintViolation<Car>> violations = executableValidator.validateParameters(
object,
method,
parameterValues
);
assertEquals( 1, violations.size() );
Iterator<Node> propertyPath = violations.iterator()
.next()
.getPropertyPath()
.iterator();
MethodNode methodNode = propertyPath.next().as( MethodNode.class );
assertEquals( "drive", methodNode.getName() );
assertEquals( Arrays.<Class<?>>asList( int.class ), methodNode.getParameterTypes() );
ParameterNode parameterNode = propertyPath.next().as( ParameterNode.class );
assertEquals( "arg0", parameterNode.getName() );
assertEquals( 0, parameterNode.getParameterIndex() );
The parameter name is determined using the current ParameterNameProvider
(see
Section 8.2.4, “ParameterNameProvider
”) and defaults to arg0
, arg1
etc.
In addition to the built-in bean and property-level constraints discussed in
Section 2.3, “Built-in constraints”, Hibernate Validator currently provides one method-level constraint,
@ParameterScriptAssert
. This is a generic cross-parameter constraint which allows to implement
validation routines using any JSR 223 compatible ("Scripting for the JavaTM Platform") scripting
language, provided an engine for this language is available on the classpath.
To refer to the executable’s parameters from within the expression, use their name as obtained from
the active parameter name provider (see Section 8.2.4, “ParameterNameProvider
”).
Example 3.17, “Using @ParameterScriptAssert
” shows how the validation logic of the @LuggageCountMatchesPassengerCount
constraint from Example 3.2, “Declaring a cross-parameter constraint” could be expressed with the help of
@ParameterScriptAssert
.
Example 3.17. Using @ParameterScriptAssert
package org.hibernate.validator.referenceguide.chapter03.parametersscriptassert;
public class Car {
@ParameterScriptAssert(lang = "javascript", script = "arg1.size() <= arg0.size() * 2")
public void load(List<Person> passengers, List<PieceOfLuggage> luggage) {
//...
}
}