customconstraints.xml

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<chapter id="validator-customconstraints">
  <title>Creating custom constraints</title>

  <para>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.</para>

  <section id="validator-customconstraints-simple">
    <title>Creating a simple constraint</title>

    <para>To create a custom constraint, the following three steps are
    required:</para>

    <itemizedlist>
      <listitem>
        <para>Create a constraint annotation</para>
      </listitem>

      <listitem>
        <para>Implement a validator</para>
      </listitem>

      <listitem>
        <para>Define a default error message</para>
      </listitem>
    </itemizedlist>

    <section id="validator-customconstraints-constraintannotation">
      <title>The constraint annotation</title>

      <para>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 <property>licensePlate</property> field of the
      <classname>Car</classname> class from <xref
      linkend="validator-gettingstarted" /> to ensure, that the field is
      always an upper-case string.</para>

      <para>First we need a way to express the two case modes. We might use
      <classname>String</classname> constants, but a better way to go is to
      use a Java 5 enum for that purpose:</para>

      <example>
        <title>Enum <classname>CaseMode</classname> to express upper vs. lower
        case</title>

        <programlisting language="JAVA" role="JAVA">package com.mycompany;

public enum CaseMode {
    UPPER, 
    LOWER;
}</programlisting>
      </example>

      <para>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:</para>

      <example id="example-defining-custom-constraint">
        <title>Defining CheckCase constraint annotation</title>

        <programlisting language="JAVA" role="JAVA">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&lt;?&gt;[] groups() default {};

    Class&lt;? extends Payload&gt;[] payload() default {};
    
    CaseMode value();

}</programlisting>
      </example>

      <para>An annotation type is defined using the <code>@interface</code>
      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</para>

      <itemizedlist>
        <listitem>
          <para>an attribute <property>message</property> that returns the
          default key for creating error messages in case the constraint is
          violated</para>
        </listitem>

        <listitem>
          <para>an attribute <property>groups</property> that allows the
          specification of validation groups, to which this constraint belongs
          (see <xref linkend="validator-usingvalidator-validationgroups" />).
          This must default to an empty array of type
          <classname>Class&lt;?&gt;</classname>.</para>
        </listitem>

        <listitem>
          <para>an attribute <classname>payload</classname> 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. <tip>
              <para>An example for a custom payload could be the definition of
              a severity.</para>

              <programlisting>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;

    // ...
}</programlisting>

              <para>Now a client can after the validation of a
              <classname>ContactDetails</classname> instance access the
              severity of a constraint using
              <methodname>ConstraintViolation.getConstraintDescriptor().getPayload()</methodname>
              and adjust its behaviour depending on the severity.</para>
            </tip></para>
        </listitem>
      </itemizedlist>

      <para>Besides those three mandatory attributes
      (<property>message</property>, <property>groups</property> and
      <property>payload</property>) we add another one allowing for the
      required case mode to be specified. The name <property>value</property>
      is a special one, which can be omitted upon using the annotation, if it
      is the only attribute specified, as e.g. in
      <code>@CheckCase(CaseMode.UPPER)</code>.</para>

      <para>In addition we annotate the annotation type with a couple of
      so-called meta annotations:</para>

      <itemizedlist>
        <listitem>
          <para><code>@Target({ METHOD, FIELD, ANNOTATION_TYPE })</code>:
          Says, that methods, fields and annotation declarations may be
          annotated with @CheckCase (but not type declarations e.g.)</para>
        </listitem>

        <listitem>
          <para><code>@Retention(RUNTIME)</code>: Specifies, that annotations
          of this type will be available at runtime by the means of
          reflection</para>
        </listitem>

        <listitem>
          <para><code>@Constraint(validatedBy =
          CheckCaseValidator.class)</code>: Specifies the validator to be used
          to validate elements annotated with @CheckCase</para>
        </listitem>

        <listitem>
          <para><code>@Documented</code>: Says, that the use of
          <code>@CheckCase</code> will be contained in the JavaDoc of elements
          annotated with it</para>
        </listitem>
      </itemizedlist>

      <tip>
        <para>Hibernate Validator provides support for the validation of
        method parameters using constraint annotations (see <xref
        linkend="validator-customoptions-methodvalidation" />).</para>

        <para>In order to use a custom constraint for parameter validation the
        <classname>ElementType.PARAMETER</classname> must be specified within
        the <classname>@Target</classname> annotation. This is already the
        case for all constraints defined by the Bean Validation API and also
        the custom constraints provided by Hibernate Validator.</para>
      </tip>
    </section>

    <section id="validator-customconstraints-validator">
      <title id="section-constraint-validator">The constraint
      validator</title>

      <para>Next, we need to implement a constraint validator, that's able to
      validate elements with a <classname>@CheckCase</classname> annotation.
      To do so, we implement the interface
      <classname>ConstraintValidator</classname> as shown below:</para>

      <example id="example-constraint-validator">
        <title>Implementing a constraint validator for the constraint
        <classname>CheckCase</classname></title>

        <programlisting language="JAVA" role="JAVA">package com.mycompany;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class CheckCaseValidator implements ConstraintValidator&lt;CheckCase, String&gt; {

    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());
    }

}</programlisting>
      </example>

      <para>The <classname>ConstraintValidator</classname> interface defines
      two type parameters, which we set in our implementation. The first one
      specifies the annotation type to be validated (in our example
      <classname>CheckCase</classname>), the second one the type of elements,
      which the validator can handle (here
      <classname>String</classname>).</para>

      <para>In case a constraint annotation is allowed at elements of
      different types, a <classname>ConstraintValidator</classname> for each
      allowed type has to be implemented and registered at the constraint
      annotation as shown above.</para>

      <para>The implementation of the validator is straightforward. The
      <methodname>initialize()</methodname> method gives us access to the
      attribute values of the annotation to be validated. In the example we
      store the <classname>CaseMode</classname> in a field of the validator
      for further usage.</para>

      <para>In the <methodname>isValid()</methodname> method we implement the
      logic, that determines, whether a <classname>String</classname> is valid
      according to a given <classname>@CheckCase</classname> annotation or
      not. This decision depends on the case mode retrieved in
      <classname>initialize()</classname>. As the Bean Validation
      specification recommends, we consider <code>null</code> values as being
      valid. If <code>null</code> is not a valid value for an element, it
      should be annotated with <code>@NotNull</code> explicitly.</para>

      <section>
        <title>The ConstraintValidatorContext</title>

        <para><xref linkend="example-constraint-validator" /> relies on the
        default error message generation by just returning
        <constant>true</constant> or <constant>false</constant> from the
        <methodname>isValid</methodname> call. Using the passed
        <classname>ConstraintValidatorContext</classname> 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 <classname>ConstraintValidatorContext</classname> API is
        modeled as fluent interface and is best demonstrated with an
        example:</para>

        <example id="example-constraint-validator-context">
          <title>Use of ConstraintValidatorContext to define custom error
          messages</title>

          <programlisting language="JAVA" role="JAVA">package com.mycompany;

import javax.validation.ConstraintValidator;
import javax.validation.ConstraintValidatorContext;

public class CheckCaseValidator implements ConstraintValidator&lt;CheckCase, String&gt; {

    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;
    }

}</programlisting>
        </example>

        <para><xref linkend="example-constraint-validator-context" os="" />
        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
        <classname>ConstraintValidatorContext</classname> results in the same
        error message as the default error message generation. <tip>
            <para>It is important to end each new constraint violation with
            <methodname>addConstraintViolation</methodname>. Only after that
            the new constraint violation will be created.</para>
          </tip></para>

        <para>In case you are implementing a
        <classname>ConstraintValidator</classname> 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 <xref
        linkend="example-custom-error" />.</para>

        <example id="example-custom-error">
          <title>Adding new <classname>ConstraintViolation</classname> with
          custom property path</title>

          <programlisting language="JAVA" role="JAVA">public boolean isValid(Group group, ConstraintValidatorContext constraintValidatorContext) {
    boolean isValid = false;
    ...

    if(!isValid) {
        constraintValidatorContext
            .buildConstraintViolationWithTemplate( "{my.custom.template}" )
            .addNode( "myProperty" ).addConstraintViolation();
    }
    return isValid;
}

</programlisting>
        </example>
      </section>
    </section>

    <section id="validator-customconstraints-errormessage">
      <title>The error message</title>

      <para>Finally we need to specify the error message, that shall be used,
      in case a <classname>@CheckCase</classname> constraint is violated. To
      do so, we add the following to our custom
      <filename>ValidationMessages.properties</filename> (see also <xref
      linkend="section-message-interpolation" />)</para>

      <example>
        <title>Defining a custom error message for the
        <classname>CheckCase</classname> constraint</title>

        <programlisting>com.mycompany.constraints.CheckCase.message=Case mode must be {value}.</programlisting>
      </example>

      <para>If a validation error occurs, the validation runtime will use the
      default value, that we specified for the message attribute of the
      <classname>@CheckCase</classname> annotation to look up the error
      message in this file.</para>
    </section>

    <section id="validator-customconstraints-using">
      <title>Using the constraint</title>

      <para>Now that our first custom constraint is completed, we can use it
      in the <classname>Car</classname> class from the <xref
      linkend="validator-gettingstarted" /> chapter to specify that the
      <property>licensePlate</property> field shall only contain upper-case
      strings:</para>

      <example id="example-car-with-checkcase">
        <title>Applying the <classname>CheckCase</classname>
        constraint</title>

        <programlisting language="JAVA" role="JAVA">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 ...

}</programlisting>
      </example>

      <para>Finally let's demonstrate in a little test that the
      <classname>@CheckCase</classname> constraint is properly
      validated:</para>

      <example>
        <title>Testcase demonstrating the <classname>CheckCase</classname>
        validation</title>

        <programlisting language="JAVA" role="JAVA">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&lt;ConstraintViolation&lt;Car&gt;&gt; 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&lt;ConstraintViolation&lt;Car&gt;&gt; constraintViolations =
            validator.validate(car);

        assertEquals(0, constraintViolations.size());
    }
}</programlisting>
      </example>
    </section>
  </section>

  <section id="validator-customconstraints-compound">
    <title id="section-constraint-composition">Constraint composition</title>

    <para>Looking at the <property>licensePlate</property> field of the
    <classname>Car</classname> class in <xref
    linkend="example-car-with-checkcase" />, 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 <property>licensePlate</property> field in
    another class, we would have to copy all constraint declarations to the
    other class as well, violating the DRY principle.</para>

    <para>This problem can be tackled using compound constraints. In the
    following we create a new constraint annotation
    <classname>@ValidLicensePlate</classname>, that comprises the constraints
    <classname>@NotNull</classname>, <classname>@Size</classname> and
    <classname>@CheckCase</classname>:</para>

    <example>
      <title>Creating a composing constraint
      <classname>ValidLicensePlate</classname></title>

      <programlisting language="JAVA" role="JAVA">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&lt;?&gt;[] groups() default {};

    Class&lt;? extends Payload&gt;[] payload() default {};

}</programlisting>
    </example>

    <para>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 <classname>@CheckCase</classname> annotation). As
    no additional validation is required for the
    <classname>@ValidLicensePlate</classname> annotation itself, we don't
    declare a validator within the <classname>@Constraint </classname>meta
    annotation.</para>

    <para>Using the new compound constraint at the
    <property>licensePlate</property> field now is fully equivalent to the
    previous version, where we declared the three constraints directly at the
    field itself:</para>

    <example>
      <title>Application of composing constraint
      <classname>ValidLicensePlate</classname></title>

      <programlisting language="JAVA" role="JAVA">package com.mycompany;

public class Car {

    @ValidLicensePlate
    private String licensePlate;

    //...

}</programlisting>
    </example>

    <para>The set of <classname>ConstraintViolations</classname> retrieved
    when validating a <classname>Car</classname> instance will contain an
    entry for each violated composing constraint of the
    <classname>@ValidLicensePlate</classname> constraint. If you rather prefer
    a single <classname>ConstraintViolation</classname> in case any of the
    composing constraints is violated, the
    <classname>@ReportAsSingleViolation</classname> meta constraint can be
    used as follows:</para>

    <example>
      <title>Usage of <classname>@ReportAsSingleViolation</classname></title>

      <programlisting language="JAVA" role="JAVA">//...
@ReportAsSingleViolation
public @interface ValidLicensePlate {

    String message() default "{com.mycompany.constraints.validlicenseplate}";

    Class&lt;?&gt;[] groups() default {};

    Class&lt;? extends Payload&gt;[] payload() default {};

}</programlisting>
    </example>
  </section>
</chapter>