001/*
002 * Copyright 2002-2020 the original author or authors.
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 *      https://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package org.springframework.beans.factory.annotation;
018
019import java.lang.annotation.Documented;
020import java.lang.annotation.ElementType;
021import java.lang.annotation.Retention;
022import java.lang.annotation.RetentionPolicy;
023import java.lang.annotation.Target;
024
025/**
026 * An annotation that indicates 'lookup' methods, to be overridden by the container
027 * to redirect them back to the {@link org.springframework.beans.factory.BeanFactory}
028 * for a {@code getBean} call. This is essentially an annotation-based version of the
029 * XML {@code lookup-method} attribute, resulting in the same runtime arrangement.
030 *
031 * <p>The resolution of the target bean can either be based on the return type
032 * ({@code getBean(Class)}) or on a suggested bean name ({@code getBean(String)}),
033 * in both cases passing the method's arguments to the {@code getBean} call
034 * for applying them as target factory method arguments or constructor arguments.
035 *
036 * <p>Such lookup methods can have default (stub) implementations that will simply
037 * get replaced by the container, or they can be declared as abstract - for the
038 * container to fill them in at runtime. In both cases, the container will generate
039 * runtime subclasses of the method's containing class via CGLIB, which is why such
040 * lookup methods can only work on beans that the container instantiates through
041 * regular constructors: i.e. lookup methods cannot get replaced on beans returned
042 * from factory methods where we cannot dynamically provide a subclass for them.
043 *
044 * <p><b>Recommendations for typical Spring configuration scenarios:</b>
045 * When a concrete class may be needed in certain scenarios, consider providing stub
046 * implementations of your lookup methods. And please remember that lookup methods
047 * won't work on beans returned from {@code @Bean} methods in configuration classes;
048 * you'll have to resort to {@code @Inject Provider<TargetBean>} or the like instead.
049 *
050 * @author Juergen Hoeller
051 * @since 4.1
052 * @see org.springframework.beans.factory.BeanFactory#getBean(Class, Object...)
053 * @see org.springframework.beans.factory.BeanFactory#getBean(String, Object...)
054 */
055@Target(ElementType.METHOD)
056@Retention(RetentionPolicy.RUNTIME)
057@Documented
058public @interface Lookup {
059
060        /**
061         * This annotation attribute may suggest a target bean name to look up.
062         * If not specified, the target bean will be resolved based on the
063         * annotated method's return type declaration.
064         */
065        String value() default "";
066
067}