    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.ioc.services;
    014
    015import org.apache.tapestry5.ioc.AnnotationProvider;
    016import org.apache.tapestry5.ioc.ObjectLocator;
    017import org.apache.tapestry5.ioc.ObjectProvider;
    018import org.apache.tapestry5.ioc.annotations.UsesOrderedConfiguration;
    019
    020/**
    021 * A service that acts as a chain-of-command over a number of {@link org.apache.tapestry5.ioc.ObjectProvider}, but
    022 * allows for the case where no object may be provided.
    023 *
    024 * This service is itself a key part of Tapestry's general injection mechanism; it is used when instantiating a service
    025 * implementation instance, invoking module methods (service builder, decorator, or contribution methods), when
    026 * {@linkplain ObjectLocator#autobuild(Class) autobuilding} objects of any type.
    027 *
    028 * As of Tapestry 5.3, the MasterObjectProvider allows injection of {@link org.apache.tapestry5.ioc.OperationTracker} as a
    029 * special case (not based on a contributed ObjectProvider).
    030 */
    031@UsesOrderedConfiguration(ObjectProvider.class)
    032public interface MasterObjectProvider
    033{
    034    /**
    035     * Provides an object based on an expression. The process of providing objects occurs within a particular
    036     * <em>context</em>, which will typically be a service builder method, service contributor method, or service
    037     * decorator method. The locator parameter provides access to the services visible <em>to that context</em>.
    038     *
    039     * When the value is required and no {@link ObjectProvider} provided a non-null value, then {@link
    040     * ObjectLocator#getService(Class, Class[])}  is invoked (with no marker annotations),
    041     * to provide a uniquely matching service, or throw a failure exception
    042     * if no <em>single</em> service can be found.
    043     *
    044     * @param objectType         the expected object type
    045     * @param annotationProvider provides access to annotations (typically, the field or parameter to which an
    046     *                           injection-related annotation is attached); annotations on the field or parameter may
    047     *                           also be used when resolving the desired object
    048     * @param locator            locator for the <em>context</em> in which the provider is being used
    049     * @param required           if true (normal case) a value must be provided; if false then it is allowed for no
    050     *                           ObjectProvider to provide a value, and this method may return null to indicate the
    051     *                           failure
    052     * @param <T>
    053     * @return the requested object, or null if this object provider can not supply an object
    054     * @throws RuntimeException if the expression can not be evaluated, or the type of object identified is not
    055     *                          assignable to the type specified by the objectType parameter
    056     */
    057    <T> T provide(Class<T> objectType, AnnotationProvider annotationProvider, ObjectLocator locator, boolean required);
    058}