001    // Copyright 2006, 2007 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    
015    package org.apache.tapestry5.ioc;
016    
017    /**
018     * Object providers represent an alternate way to locate an object provided somewhere in the {@link
019     * org.apache.tapestry5.ioc.Registry}. Instead of using a just the service id to gain access to a service within the
020     * Registry, object providers in different flavors are capable of vending, or even creating, objects of disparate types
021     * from disparate sources.
022     * <p/>
023     * Object providers are consulted in a strict order, and the first non-null result is taken.
024     * <p/>
025     * In many cases, an object provider searches for additional annotations on the element (usually a parameter, or perhaps
026     * a field) for which a value is required.
027     */
028    public interface ObjectProvider
029    {
030        /**
031         * Provides an object based on an expression. The process of providing objects occurs within a particular
032         * <em>context</em>, which will typically be a service builder method, service contributor method, or service
033         * decorator method. The locator parameter provides access to the services visible <em>to that context</em>.
034         *
035         * @param objectType         the expected object type
036         * @param annotationProvider provides access to annotations (typically, the field or parameter to which an
037         *                           injection-related annotation is attached); annotations on the field or parameter may
038         *                           also be used when resolving the desired object
039         * @param locator            locator for the <em>context</em> in which the provider is being used
040         * @param <T>
041         * @return the requested object, or null if this object provider can not supply an object
042         * @throws RuntimeException if the expression can not be evaluated, or the type of object identified is not
043         *                          assignable to the type specified by the objectType parameter
044         */
045        <T> T provide(Class<T> objectType, AnnotationProvider annotationProvider, ObjectLocator locator);
046    }