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, like the rest of the framework.
  • To be flexible.

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


To start using validation components, you should first include the corresponding component package in the template. Additionally, importing the 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 the variable that provides validator state object.
  • of="dataFieldN" path to a mutable field or property of a view object that stores actual data.
  • as="fieldNameN" name of the variable that provides field state object
  • convert="converterN" expression that yields a Converter instance that should be used to convert data to and from string.
  • rule="validationExpression" expression that yields a boolean value, true if validation passes.
  • as="ruleName" name of the variable that provides a 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 the containing v:validator.

Standard converters

Standard converters are available via static methods of the 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 the default locale-specific date format.
  • Converter<Date> mediumDateFormat() uses the locale-specific medium date format.
  • Converter<Date> shortDateFormat() uses the locale-specific medium date format.
  • Converter<Date> shortDateFormat() uses the locale-specific medium date format.
  • Converter<Date> dateFormat(DateFormat dateFormat) uses a custom date format.

Validator state

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

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

Field state

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

  • boolean isValidFormat() indicates whether the 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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