Use the Vaadin MCP tools (search_vaadin_docs, get_component_java_api) to look up the latest documentation whenever uncertain about a specific API detail. Always set vaadin_version to "25" and ui_language to "java".
For an exact Java API signature or to read source, use the javadoc MCP (mcp__javadoc__* — find via ToolSearch javadoc if not loaded) to read Javadoc and sources from Maven Central instead of unpacking jars from ~/.m2.
Binder Fundamentals
Binder connects UI fields to a Form Data Object (FDO) — a Java bean, record, or DTO. It handles reading values from the FDO into fields, writing field values back, converting between field types and model types, and validating at every level.
Binder can only bind components that implement HasValue (TextField, ComboBox, DatePicker, Checkbox, etc.).
Two Modes of Operation
Buffered mode — changes are held in the Binder until explicitly written:
Binder<Person> binder = new Binder<>(Person.class);
binder.readBean(person); // populate fields from bean
// ... user edits fields ...
if (binder.writeBeanIfValid(person)) {
service.save(person); // only writes if all validation passes
}
Write-through mode — changes are written to the bean immediately on field change:
binder.setBean(person); // binds directly; changes write through
Use buffered mode for forms with Save/Cancel buttons. Use write-through mode for settings panels or simple filters where every change should apply immediately.
Binding Fields
Explicit binding (recommended)
binder.forField(nameField)
.asRequired("Name is required")
.withValidator(new StringLengthValidator("1-100 characters", 1, 100))
.bind(Person::getName, Person::setName);
binder.forField(emailField)
.asRequired()
.withValidator(new EmailValidator("Invalid email"))
.bind(Person::getEmail, Person::setEmail);
The chain order matters: forField() → asRequired() → withValidator() → withConverter() → withValidator() → bind(). Validators and converters execute in the order they appear.
Always prefer binding fields using getter/setter method references. Don't use string property names unless the FDO is a Java record.
Shorthand binding
binder.bind(nameField, Person::getName, Person::setName);
No validation configuration — only useful for simple cases.
BeanValidationBinder with Jakarta annotations
If your bean uses Jakarta Bean Validation annotations (@NotEmpty, @Max, @Email, etc.), use BeanValidationBinder to pick them up automatically:
BeanValidationBinder<Person> binder = new BeanValidationBinder<>(Person.class);
binder.bindInstanceFields(this); // binds fields matching property names
bindInstanceFields() scans the view for fields whose names match bean properties. This is convenient but less explicit — prefer forField().bind() for complex forms.
Converters
When the field's value type doesn't match the bean property type, add a converter:
binder.forField(yearOfBirthField)
.withConverter(new StringToIntegerConverter("Enter a number"))
.bind(Person::getYearOfBirth, Person::setYearOfBirth);
Converters implicitly validate — if conversion fails, the error message is shown as a validation error.
Domain Primitives pattern
Create type-safe value objects with converters:
binder.forField(emailField)
.withConverter(new EmailAddressConverter()) // String → EmailAddress
.withValidator(emailService::notAlreadyInUse, "Email already in use")
.bind(Person::getEmail, Person::setEmail);
Validators can be added after converters — they then validate the converted type.
Validation Layers
1. Required fields
binder.forField(titleField)
.asRequired() // visual indicator, empty check
.bind(Proposal::getTitle, Proposal::setTitle);
binder.forField(typeComboBox)
.asRequired("Please select a type") // custom error message
.bind(Proposal::getType, Proposal::setType);
2. Binding-level validators (per-field)
Run whenever the field value changes. Use built-in validators when possible:
StringLengthValidator,EmailValidator,RegexpValidatorIntegerRangeValidator,DoubleRangeValidator,LongRangeValidatorDateRangeValidator,DateTimeRangeValidatorRangeValidator(generic, with Comparator)
Custom validator with lambda:
binder.forField(ageField)
.withValidator(age -> age >= 0, "Age must be positive")
.bind(Person::getAge, Person::setAge);
Custom validator class:
public class PositiveIntegerValidator implements Validator<Integer> {
@Override
public ValidationResult apply(Integer value, ValueContext context) {
return value >= 0
? ValidationResult.ok()
: ValidationResult.error("Must be positive");
}
}
3. Default validators (component built-in)
Some components have built-in validation (e.g., DatePicker min/max, EmailField). These work alongside Binder validators. Disable them if needed:
binder.forField(datePicker)
.withDefaultValidator(false)
.bind(Bean::getDate, Bean::setDate);
Default validators take precedence over binding-level validators. To customize the error messages of default validators, use the field's setI18n() method.
4. Binder-level validators (cross-field)
Validate the entire FDO after all fields are processed. Essential for rules that span multiple fields:
binder.withValidator((bean, context) -> {
if (bean.getStartDate() != null && bean.getEndDate() != null
&& bean.getStartDate().isAfter(bean.getEndDate())) {
return ValidationResult.error("Start date must be before end date");
}
return ValidationResult.ok();
});
In buffered mode, binder-level validators only run when writeBean() or writeBeanIfValid() is called. In write-through mode, they run on every field change.
Triggering Validation
- Automatic: binding-level validators run on every field value change
- Programmatic:
binder.validate()— runs all validators and updates UI - Check only:
binder.isValid()— checks without updating UI - Write with validation:
binder.writeBeanIfValid(bean)— returns false if invalid
Handling Validation Errors
Binding-level errors display next to the field automatically.
Binder-level errors need a status label:
Div errorDisplay = new Div();
errorDisplay.addClassName(LumoUtility.TextColor.ERROR); // Lumo theme only; for Aura, use a custom CSS class
binder.setStatusLabel(errorDisplay);
Form Layout
Lay the fields out in a FormLayout for automatic responsive column adjustment. For choosing fields and components, FormLayout sectioning, auto-responsive vs responsive steps, and column spans — especially when building from a design or spec — use the vaadin-form-layout skill, which covers the layout in depth.
Separating the Form into Its Own Class
Encapsulate the form in a dedicated class that extends Composite<FormLayout>. This keeps the Binder, fields, and form data object together, and exposes a small setFormDataObject / getFormDataObject API to the surrounding view. The constructor builds the fields, adds them to getContent(), and wires up the Binder.
Buffered mode — the form owns the form data object and writes to it on demand:
public class ProposalForm extends Composite<FormLayout> {
private final Binder<Proposal> binder;
private Proposal formDataObject;
public void setFormDataObject(@Nullable Proposal formDataObject) {
this.formDataObject = formDataObject;
if (formDataObject != null) {
binder.readBean(formDataObject);
} else {
binder.refreshFields();
}
}
public Optional<Proposal> getFormDataObject() {
if (formDataObject == null) {
formDataObject = new Proposal();
}
return binder.writeBeanIfValid(formDataObject)
? Optional.of(formDataObject)
: Optional.empty();
}
}
Write-through mode — the bean is set on the Binder and edits flow through immediately; the getter only validates before handing it back:
public class ProposalForm extends Composite<FormLayout> {
private final Binder<Proposal> binder;
public void setFormDataObject(@Nullable Proposal formDataObject) {
binder.setBean(formDataObject);
}
public Optional<Proposal> getFormDataObject() {
if (binder.getBean() == null) {
throw new IllegalStateException("No form data object");
}
return binder.validate().isOk()
? Optional.of(binder.getBean())
: Optional.empty();
}
}
Best Practices
- Use buffered mode for most forms — it gives you control over when data is written and lets you implement Cancel without manual state tracking.
- Prefer explicit binding over
bindInstanceFields— it's more readable, easier to maintain, and doesn't rely on field naming conventions. - Validate at the right level — field format/range → binding-level. Cross-field rules → binder-level. Business rules → service layer.
- Use converters for type safety — domain primitives with converters catch invalid data at the type system level.
- Set
asRequired()on mandatory fields — it provides both the visual indicator and the empty-value check in one call. - Show binder-level errors prominently — they don't attach to a specific field, so users need a clear error display area.
- Encapsulate the form in its own
Composite<FormLayout>class — keep the Binder, fields, and form data object together, and expose a smallsetFormDataObject/getFormDataObjectAPI to the surrounding view.
Detailed Reference
For the complete list of built-in validators, converter patterns, and form templates, see references/form-patterns.md.