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Validators with Mule SDK

Validators are operations validate the Mule Message without changing the message. Validators can produce these effects:

  • If the condition the validator imposes is fulfilled, the flow continues, and the Mule Message remains unchanged.

  • If the condition the validator imposes is not fulfilled, an error is thrown.

What are the Requirements for a Validator?

To be a validator, an operation should meet these requirements:

  • It should be reached by the @Validator annotation. This can be assigned at the class level or at the method level. When you use the @Validator annotation at a class level, all public methods are marked as validators.

  • It should return void.

  • It should throw at least one error that is (or extends from) MULE:VALIDATION. Upon a validation failure, validators must throw a Mule validation error or a child of one.

Methods with the @Validator annotation belong to the Stereotype ValidatorStereotype. Components cannot have two stereotypes, so you cannot assign more than one stereotype to a method.

Validators share the same UX as operations. Once your module is added for use in Studio, the validators will appear in the palette. When a validator is added to a flow, its parameters are assigned just as if it were an operation.

How to Make a Validator

This example shows how to make a simple validator. It creates a simple extension named balance:

@Extension(name = "balance")
public class BalanceExtension {


The BalanceOperations class below adds a validation method that is annotated with @Validator. The method throws an error type that comes from the generic validation error type. Note that you can also annotate the class BalanceOperations with @Validator, but if you do, any other operations in it will be flagged as validators.

public class BalanceOperations {
  public void hasSufficientFunds(Integer balance, Integer price) throws Exception {
    if (price > balance){
      throw new ModuleException(BalanceError.INSUFFICIENT_FUNDS, new IllegalArgumentException("There is not enough money to make the transaction"));

This example creates the error to throw if the validation fails. Notice that it is named according to the validations failure.

public enum BalanceError implements ErrorTypeDefinition<BalanceError> {

  private ErrorTypeDefinition<? extends Enum<?>> parent;

  BalanceError(ErrorTypeDefinition<? extends Enum<?>> parent) {
    this.parent = parent;

  public Optional<ErrorTypeDefinition<? extends Enum<?>>> getParent() {
    return Optional.ofNullable(parent);

The validator method needs an ErrorTypeProvider that knows all the error types that the validation can throw. This example creates an ErrorTypeProvider that says that the only error the method can throw is of type BALANCE:INSUFFICIENT_FUNDS.

public class BalanceErrorsProvider implements ErrorTypeProvider {
  public Set<ErrorTypeDefinition> getErrorTypes() {
    HashSet<ErrorTypeDefinition> errors = new HashSet<>();
    return errors;

How To Use Validators

Assume that you want to use the validator created above on a flow. The syntax will be the same as if it were an operation. Because balance and price are required parameters, it is necessary to assign those. In this case, the validation is made with the values in the variables price and balance, but you could use any expression there:

<balance:has-sufficient-funds balance="#[vars.balance]" price="#[vars.price]"/>

Validators and the "all" Scope

Because validators are of the Stereotype ValidatorStereotype, they can be used in the all scope from the Validation module.

The next example uses a validator from the Validation module. It also assumes that there is a number extension that has a validator named isPositive that validates whether a value is in fact positive.

  <validation:is-not-blank-string value="#[vars.buyerName]"/>
  <number:is-positive value="#[vars.price]"/>
  <number:is-positive value="#[vars.balance]"/>
  <balance:has-sufficient-funds balance="#[vars.balance]" price="#[vars.price]"/>

This example checks that there is a buyerName variable that is not an empty string. After that, it ensures that both variables, price and balance, have positive values. Finally, it checks whether there is a sufficient balance on the account with the validator.

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