    001// Licensed under the Apache License, Version 2.0 (the "License");
    002// you may not use this file except in compliance with the License.
    003// You may obtain a copy of the License at
    004//
    005//     http://www.apache.org/licenses/LICENSE-2.0
    006//
    007// Unless required by applicable law or agreed to in writing, software
    008// distributed under the License is distributed on an "AS IS" BASIS,
    009// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    010// See the License for the specific language governing permissions and
    011// limitations under the License.
    012
    013package org.apache.tapestry5.commons;
    014
    015/**
    016 * Object providers represent an alternate way to locate an object provided somewhere in the {@link
    017 * org.apache.tapestry5.ioc.Registry}. Instead of using a just the service id to gain access to a service within the
    018 * Registry, object providers in different flavors are capable of vending, or even creating, objects of disparate types
    019 * from disparate sources.
    020 *
    021 * Object providers are consulted in a strict order, and the first non-null result is taken.
    022 *
    023 * In many cases, an object provider searches for additional annotations on the element (usually a parameter, or perhaps
    024 * a field) for which a value is required.
    025 */
    026public interface ObjectProvider
    027{
    028    /**
    029     * Provides an object based on an expression. The process of providing objects occurs within a particular
    030     * <em>context</em>, which will typically be a service builder method, service contributor method, or service
    031     * decorator method. The locator parameter provides access to the services visible <em>to that context</em>.
    032     *
    033     * @param objectType         the expected object type
    034     * @param annotationProvider provides access to annotations (typically, the field or parameter to which an
    035     *                           injection-related annotation is attached); annotations on the field or parameter may
    036     *                           also be used when resolving the desired object
    037     * @param locator            locator for the <em>context</em> in which the provider is being used
    038     * @param <T>
    039     * @return the requested object, or null if this object provider can not supply an object
    040     * @throws RuntimeException if the expression can not be evaluated, or the type of object identified is not
    041     *                          assignable to the type specified by the objectType parameter
    042     */
    043    <T> T provide(Class<T> objectType, AnnotationProvider annotationProvider, ObjectLocator locator);
    044}