    001// Copyright 2007, 2008, 2011 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.services;
    016
    017import org.apache.tapestry5.ioc.AnnotationProvider;
    018import org.apache.tapestry5.ioc.ObjectLocator;
    019import org.apache.tapestry5.ioc.ObjectProvider;
    020import org.apache.tapestry5.ioc.annotations.UsesOrderedConfiguration;
    021
    022/**
    023 * A service that acts as a chain-of-command over a number of {@link org.apache.tapestry5.ioc.ObjectProvider}, but
    024 * allows for the case where no object may be provided.
    025 * <p/>
    026 * This service is itself a key part of Tapestry's general injection mechanism; it is used when instantiating a service
    027 * implementation instance, invoking module methods (service builder, decorator, or contribution methods), when
    028 * {@linkplain ObjectLocator#autobuild(Class) autobuilding} objects of any type.
    029 * <p/>
    030 * As of Tapestry 5.3, the MasterObjectProvider allows injection of {@link org.apache.tapestry5.ioc.OperationTracker} as a
    031 * special case (not based on a contributed ObjectProvider).
    032 */
    033@UsesOrderedConfiguration(ObjectProvider.class)
    034public interface MasterObjectProvider
    035{
    036    /**
    037     * Provides an object based on an expression. The process of providing objects occurs within a particular
    038     * <em>context</em>, which will typically be a service builder method, service contributor method, or service
    039     * decorator method. The locator parameter provides access to the services visible <em>to that context</em>.
    040     * <p/>
    041     * When the value is required and no {@link ObjectProvider} provided a non-null value, then {@link
    042     * ObjectLocator#getService(Class, Class[])}  is invoked (with no marker annotations),
    043     * to provide a uniquely matching service, or throw a failure exception
    044     * if no <em>single</em> service can be found.
    045     *
    046     * @param objectType         the expected object type
    047     * @param annotationProvider provides access to annotations (typically, the field or parameter to which an
    048     *                           injection-related annotation is attached); annotations on the field or parameter may
    049     *                           also be used when resolving the desired object
    050     * @param locator            locator for the <em>context</em> in which the provider is being used
    051     * @param required           if true (normal case) a value must be provided; if false then it is allowed for no
    052     *                           ObjectProvider to provide a value, and this method may return null to indicate the
    053     *                           failure
    054     * @param <T>
    055     * @return the requested object, or null if this object provider can not supply an object
    056     * @throws RuntimeException if the expression can not be evaluated, or the type of object identified is not
    057     *                          assignable to the type specified by the objectType parameter
    058     */
    059    <T> T provide(Class<T> objectType, AnnotationProvider annotationProvider, ObjectLocator locator, boolean required);
    060}