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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
the preconditions that must be satisfied by the caller before a method or constructor may be invoked (by applying constraints to the parameters of an executable)
the postconditions that are guaranteed to the caller after a method or constructor invocation returns (by applying constraints to the return value of an executable)
For the purpose of this reference guide, the term method constraint refers to both, method and constructor constraints, if not stated otherwise. Ocassionally, the term executable is used when referering to methods and constructors.
This approach has several advantages over traditional ways of checking the correctness of parameters and return values:
the checks don't have to be performed manually (e.g. by throwing IllegalArgumentExceptions or similar), resulting in less code to write and maintain
an executable's pre- and postconditions don't have to be expressed again in its documentation, since the constraint annotations will automatically be included in the generated JavaDoc. This avoids redundancies and reduces the chance of inconsistencies between implementation and documentation
In order to make annotations show up in the JavaDoc of annoted 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 例 3.1 “Declaring method and constructor parameter constraints”.
例 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:
The name passed to the RentalCar constructor must not be null
When invoking the rentCar() method, the given customer must not be null, the rental's start date must not be null and must be in the future and the rental duration must be at least one day
Note that declaring method or constructor constraints itself does not automatically cause their validation upon invocation of the executable. Instead, the ExecutableValidator API (see 第 3.2 节 “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 例 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.
例 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 第 6.3 节 “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 例 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.
例 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
a void method with parameters (the constraint applies to the parameters)
an executable with return value but no parameters (the constraint applies to the return value)
neither a method nor a constructor, but a field, parameter etc. (the constraint applies to the annotated element)
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 例 3.4 “Declaring method and constructor return value constraints”.
例 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:
Any newly created RentalStation object must satisfy the @ValidRentalStation constraint
The customer list returned by getCustomers() must not be null and must contain at least on element
Similar to the cascaded validation of JavaBeans properties (see 第 2.1.5 节 “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 例 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.
例 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
is an array
implements java.lang.Iterable
or implements java.util.Map
each contained element gets validated. So when validating the arguments of the checkCars() method in 例 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 objects is invalid.
例 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:
The preconditions to be satisified by the caller of a method may not be strengthened in subtypes
The postconditions guaranteed to the caller of a method may not be weakened in subtypes
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). 例 3.7 “Illegal method parameter constraint in subtype” shows a violation of this rule.
例 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 例 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.
例 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 例 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.
例 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 第 3.2.1 节 “Obtaining an ExecutableValidator instance” you will learn how to obtain an ExecutableValidator instance while 第 3.2.2 节 “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 例 3.10 “Obtaining an ExecutableValidator”.
例 3.10. Obtaining an ExecutableValidator
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 第 8 章 Bootstrapping, for instance in order to use a specific parameter name provider (see 第 8.2.4 节 “ParameterNameProvider”).
The ExecutableValidator interface offers altogether four methods:
validateParameters() and validateReturnValue() for method validation
validateConstructorParameters() and validateConstructorReturnValue() for constructor validation
Just 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 例 3.11 “Class Car with constrained methods and constructors”.
例 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. 例 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.
例 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 例 3.13 “Using ExecutableValidator#validateReturnValue()” yields one constraint violation since the getPassengers() method is expect to return at least one Passenger object.
例 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 例 3.14 “Using ExecutableValidator#validateConstructorParameters()”. Due to the @NotNull constraint on the manufacturer parameter, the validation call returns one constraint violation.
例 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 valide a constructor's return value. In 例 3.15 “Using ExecutableValidator#validateConstructorReturnValue()”, validateConstructorReturnValue() returns one constraint violation, since the Car object returned by the constructor doesn't satisfy the @ValidRacingCar constraint (not shown).
例 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 第 2.2.3 节 “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 例 3.16 “Retrieving method and parameter information”.
例 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 第 8.2.4 节 “ParameterNameProvider”) and defaults to arg0, arg1 etc.
In addition to the built-in bean and property-level constraints discussed in 第 2.3 节 “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 第 8.2.4 节 “ParameterNameProvider”). 例 3.17 “Using @ParameterScriptAssert” shows how the validation logic of the @LuggageCountMatchesPassengerCount constraint from 例 3.2 “Declaring a cross-parameter constraint” could be expressed with the help of @ParameterScriptAssert.
例 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) {
//...
}
}
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