Form validation


Form validation components were designed with following goals in mind:

  • To be compatible with any UI library and CSS framework.
  • To not force users to follow certain guidelines, let them follow their guidelines instead.
  • To be type safe, as well as the whole framework.
  • To be flexible.

Validation components are components for template engine, therefore they can only be used from HTML templates.


To start using validation components, you should first include corresponding component package to template. Additionally, importing Converter interface may be useful:

<?import org.teavm.flavour.validation.Converter?>
<?use v:org.teavm.flavour.validation?>

The first component, called validator, is used to define validation logic. It has the following syntax:

<v:validator as="validatorName">
  <v:validation of="dataField1" as="fieldName1" convert="converter1">
    <v:check rule="validationExpression" as="ruleName"/>
    <!-- etc -->
  <v:validation of="dataField2" as="fieldName2" convert="converter2">
    <v:check rule="validationExpression" as="ruleName"/>
    <!-- etc -->
  <!-- more v:validation entries -->
  <!-- body -->


  • as="validationName"" name of variable that provides validator state object.
  • of="dataFieldN" path to mutable field or property of a view object that stores actual data.
  • as="fieldNameN" name of variable that provides field state object
  • convert="converterN" expression that yields Converter instance that should be used to convert data to and from string.
  • rule="validationExpression" expression that yields boolean value, true if validation passes.
  • as="ruleName" name of variable that provides boolean value which indicates whether the rule passes.

The second component, bind, is used to bind validations to input fields:


where fieldName points to field state object defined by containing v:validator.

Standard converters

Standard converters are available via static methods of Converter interface:

  • Converter<String> stringFormat() does not perform any conversion.
  • Converter<Integer> integerFormat()
  • Converter<Double> doubleFormat(String formatString) where formatString is a string accepted by DecimalFormat class.
  • Converter<Date> dateFormat(String formatString) where formatString is a string accepted by SimpleDateFormat class.
  • Converter<Date> dateFormat() uses default locale-specific date format.
  • Converter<Date> mediumDateFormat() uses local-specific medium date format.
  • Converter<Date> shortDateFormat() uses local-specific medium date format.
  • Converter<Date> shortDateFormat() uses local-specific medium date format.
  • Converter<Date> dateFormat(DateFormat dateFormat) uses custom date format.

Validator state

Validator state is available via ValidatorState class which has the following API:

  • boolean isValid() indicates whether form passed validation, i.e. all fields a valid.
  • void submit(Runnable action) validates form and performs given action if form is valid.

Field state

Field state is available via Validation class which has the following API:

  • boolean isValidFormat() indicates whether converter could successfully recognize value in a text field.
  • boolean isValid() indicates whether all validation rules pass.


<?import org.teavm.flavour.validation.Converter?>
<?use v:org.teavm.flavour.validation?>
<v:validator as="validator">
  <v:validation of="title" as="titleField" convert="Converter.stringFormat()">
    <v:check rule="!it.empty" as="titleSpecified"/>
  <v:validation of="startDate" as="startDateField" convert="Converter.dateFormat('yyyy-MM-dd')">
    <v:check rule="it != null" as="startDateSpecified"/>
    <v:check rule="endDate == null or it.before(endDate)" as="startDateConsistent"/>
  <v:validation of="endDate" as="endDateField" convert="Converter.dateFormat('yyyy-MM-dd')">
    <v:check rule="it != null" as="endDateSpecified"/>
    <v:check rule="startDate == null or startDate.before(it)" as="endDateConsistent"/>

    <div attr:class="titleField.valid ? '' : 'error'"><input type="text" v:bind="titleField"/></div>
    <std:if condition="!titleSpecified"><div>Please, specify title</div></std:if>

    <div><label>Start date</label></div>
    <div attr:class="startDateField.valid ? '' : 'error'"><input type="text" v:bind="startDateField"/></div>
    <std:if condition="!startDateSpecified"><div>Please, specify start date</div></std:if>
    <div><label>End date</label></div>
    <div attr:class="endDateField.valid ? '' : 'error'"><input type="text" v:bind="endDateField"/></div>
    <std:if condition="!endDateSpecified"><div>Please, specify end date</div></std:if>

  <std:if condition="!startDateConsistent"><div>Start date must be later than end date</div></std:if>

  <button type="button" event:click="validator.submit(() -> save())" html:enabled="validator.valid">Submit</button>


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