Errors Definition

In Mule, errors are a way of communicating that something went wrong and providing meaningful information so that a user can take corrective action depending on the kind of error that was thrown.

See About Error Handling for more detail.

Defining Module Errors

The first step is to define all the errors that can be thrown by a Module. To do so, you define an enum that implements the ErrorTypeDefinition interface. Each value defined in this enum is considered to be an Error.

public enum SimpleError implements ErrorTypeDefinition<SimpleError> {

Errors Hierarchy

Module errors can only inherit from errors of the same module (values of the same enum) or Mule errors. Mule errors are defined in org.mule.runtime.extension.api.error.MuleErrors.

Note that any error that does not define a parent error automatically inherits from MULE:ANY.

public enum HierarchalError implements ErrorTypeDefinition<HierarchalError> {

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

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

    HierarchalError() {

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

Registering Errors in the Module

The @Extension annotated class should be annotated with @ErrorTypes to indicate which errors a Module handles. This annotation references an ErrorTypeDefinition enum containing the defined errors.

@Extension(name = "Foo")
@Xml(prefix = "test-connector")
public class FooExtension {
  // Content

Specifying Errors That Can Be Thrown

After declaring all the possible errors in your Module, you need to provide the information that binds the errors with the Operations that can throw them.

To do that, you need to implement an ErrorTypeProvider, which is a class that communicates the errors that can be thrown by an Operation.

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

After defining the ErrorTypeProvider, you must bind it with the proper Operation. You bind it using the @Throws annotation at the Operation level, for example:

public void execute(){
  // operation body

In the Studio or Flow Designer, an operation that is annotated with @Throws annotated Operation will provide a hint to the user about the errors the operation can throw:

studio errors

The XML looks like this:

<flow name="flowName">
    <error-handler >
      <on-error-continue type="TEST-CONNECTOR:ILLEGAL_ACTION">
        <logger level="INFO" message="#[error]"/>

Throwing Errors

Errors are a Mule representation of a Java Exception that is bound to an specific Error.

There is no static binding between Errors and Exceptions. To communicate an error, the Operation should throw org.mule.runtime.extension.api.exception.ModuleException or child exceptions of this class, indicating in the Constructor the desired ErrorTypeDefinition to throw.

public void execute(){
  try {
      throw new IllegalStateException();
  } catch (IllegalStateException e){
      throw new ModuleException(HierarchalErrors.ILLEGAL_ACTION, e);

Also, a recommended practice is to wrap this logic inside new exception classes:

public final class IllegalActionException extends ModuleException {

  public IllegalActionException(Exception cause) {
    super(HierarchalErrors.ILLEGAL_ACTION, cause);
Throwing an Error that is not declared in the ErrorTypeProvider of an Operation will result in an Unexpected Error Exception. Operations are not allowed to throw undeclared Errors.
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