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// Copyright 2007 The Apache Software Foundation
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//     http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package org.apache.tapestry.services;

import org.apache.tapestry.ioc.AnnotationProvider;
import org.apache.tapestry.ioc.ObjectLocator;
import org.apache.tapestry.ioc.ObjectProvider;

/**
 * Rolls up a number of {@link ObjectProvider}, but allows for the case where no object may be
 * provided.
 */
public interface MasterObjectProvider
{
    /**
     * Provides an object based on an expression. The process of providing objects occurs within a
     * particular context, which will typically be a service builder method, service
     * contributor method, or service decorator method. The locator parameter provides access to the
     * services visible to that context.
     * 
     * When the value is required and no {@link ObjectProvider} provided a non-null value, then
     * {@link ObjectLocator#getService(Class)} is invoked, to provide a uniquely matching service,
     * or throw a failure exception if no single service can be found.
     *
     * @param objectType         the expected object type
     * @param annotationProvider provides access to annotations (typically, the field or parameter to which an
     *                           injection-related annotation is attached); annotations on the field or parameter
     *                           may also be used when resolving the desired object
     * @param locator            locator for the context in which the provider is being used
     * @param required           if true (normal case) a value must be provided; if false then it is allowed for no
     *                           ObjectProvider to provide a value, and this method may return null to indicate the
     *                           failure
     * @param 
     * @return the requested object, or null if this object provider can not supply an object
     * @throws RuntimeException if the expression can not be evaluated, or the type of object identified is not
     *                          assignable to the type specified by the objectType parameter
     */
     T provide(Class objectType, AnnotationProvider annotationProvider,
                  ObjectLocator locator, boolean required);
}