private void fieldValidation() { ExecutionPath fieldPath = ExecutionPath.parse("/user"); FieldValidation fieldValidation = new SimpleFieldValidation() .addRule(fieldPath, new BiFunction<FieldAndArguments, FieldValidationEnvironment, Optional<GraphQLError>>() { @Override public Optional<GraphQLError> apply(FieldAndArguments fieldAndArguments, FieldValidationEnvironment environment) { String nameArg = fieldAndArguments.getArgumentValue("name"); if (nameArg.length() > 255) { return Optional.of(environment.mkError("Invalid user name", fieldAndArguments)); } return Optional.empty(); } }); FieldValidationInstrumentation instrumentation = new FieldValidationInstrumentation( fieldValidation ); GraphQL.newGraphQL(schema) .instrumentation(instrumentation) .build(); }