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To set up Swagger Codegen to generate an error response for an enum field that does not match, you can follow these steps:

1. Specify the valid enum values in the OpenAPI/Swagger specification using the enum keyword in the schema definition for the field.

Example:

properties:
  status:
    type: string
    enum: [draft, published, archived]

1. Add the swagger-codegen-maven-plugin to your project's pom.xml file.

Example:

<build>
  <plugins>
    <plugin>
      <groupId>io.swagger</groupId>
      <artifactId>swagger-codegen-maven-plugin</artifactId>
      <version>2.3.1</version>
      <executions>
        <execution>
          <id>generate-sources</id>
          <goals>
            <goal>generate</goal>
          </goals>
          <configuration>
            <!-- plugin configuration options -->
          </configuration>
        </execution>
      </executions>
    </plugin>
  </plugins>
</build>

1. Configure the plugin to use a custom template for the error response.

Example:

<configuration>
  <templateDirectory>/path/to/custom/templates</templateDirectory>
  <apiPackage>com.example.api</apiPackage>
  <modelPackage>com.example.model</modelPackage>
  <generateApis>false</generateApis>
  <generateModels>true</generateModels>
  <generateSupportingFiles>false</generateSupportingFiles>
  <language>java</language>
  <additionalProperties>
    <useGenericResponse>false</useGenericResponse>
  </additionalProperties>
  <configOptions>
    <errorModel>CustomError</errorModel>
  </configOptions>
</configuration>

1. Create a custom template for the error response using the errorModel.mustache file.

Example:

{
  "message": "Invalid value for {{fieldName}}. Allowed values: {{allowedValues}}",
  "code": 400
}

1. Run the generate-sources goal of the swagger-codegen-maven-plugin to generate the Java code and the custom error response template.

Example:

mvn swagger-codegen:generate

1. Use the generated Java code to implement your API endpoint and handle errors for fields that do not match the enum.

Example:

@POST
@Path("/example")
@Consumes(MediaType.APPLICATION_JSON)
@Produces(MediaType.APPLICATION_JSON)
public Response exampleApiMethod(RequestBody requestBody) {
  try {
    // Validate the request body
    validateRequestBody(requestBody);

    // Process the request
    processRequest(requestBody);

    // Return a successful response
    return Response.ok().build();
  } catch (ValidationException e) {
    // Return a custom error response for the invalid enum value
    return Response
      .status(Response.Status.BAD_REQUEST)
      .entity(renderErrorResponse(e.getFieldName(), e.getAllowedValues()))
      .build();
  }
}

private void validateRequestBody(RequestBody requestBody) throws ValidationException {
  // Validate the enum field values
  validateEnumField("status", requestBody.getStatus(), "draft", "published", "archived");
}

private void validateEnumField(String fieldName, String fieldValue, String... allowedValues) throws ValidationException {
  if (!Arrays.asList(allowedValues).contains(fieldValue)) {
    throw new ValidationException(fieldName, Arrays.asList(allowedValues));
  }
}

private String renderErrorResponse(String fieldName, List<String> allowedValues) {
  Map<String, Object> errorModel = new HashMap<>();
  errorModel.put("fieldName", fieldName);
  errorModel.put("allowedValues", String.join(",", allowedValues));
  return MustacheFactory
    .compiler()
    .compile("custom-error.mustache")
    .render(errorModel);
}

Note: This example assumes that you have already implemented a RequestBody class to represent the request body of your API endpoint. The code for the ValidationException class and the processRequest() method is omitted for brevity.