    001// Copyright 2011, 2012 The Apache Software Foundation
    002//
    003// Licensed under the Apache License, Version 2.0 (the "License");
    004// you may not use this file except in compliance with the License.
    005// You may obtain a copy of the License at
    006//
    007// http://www.apache.org/licenses/LICENSE-2.0
    008//
    009// Unless required by applicable law or agreed to in writing, software
    010// distributed under the License is distributed on an "AS IS" BASIS,
    011// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    012// See the License for the specific language governing permissions and
    013// limitations under the License.
    014
    015package org.apache.tapestry5.ioc.annotations;
    016
    017import java.lang.annotation.Documented;
    018import java.lang.annotation.Retention;
    019import java.lang.annotation.Target;
    020
    021import static java.lang.annotation.ElementType.METHOD;
    022import static java.lang.annotation.RetentionPolicy.RUNTIME;
    023
    024/**
    025 * Marks a service contribution method within a module as being optional: it is not an error if the contribution does not match against an actual service. In that case, the
    026 * method will simply never be invoked. This is occasionally useful when a module is designed to work with another module <em>if the second module is present</em>. Without
    027 * optional contributions, you would see hard errors when registry is created, and have to create a layer cake of small modules to prevent such errors.
    028 *
    029 * @see Contribute
    030 * @see org.apache.tapestry5.ioc.def.ContributionDef3#isOptional()
    031 * @since 5.3
    032 */
    033@Target(METHOD)
    034@Retention(RUNTIME)
    035@Documented
    036@UseWith(AnnotationUseContext.MODULE)
    037public @interface Optional
    038{
    039}