001/* 002 * Copyright 2002-2019 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 * Annotation at the field or method/constructor parameter level 027 * that indicates a default value expression for the affected argument. 028 * 029 * <p>Typically used for expression-driven dependency injection. Also supported 030 * for dynamic resolution of handler method parameters, e.g. in Spring MVC. 031 * 032 * <p>A common use case is to assign default field values using 033 * {@code #{systemProperties.myProp}} style expressions. 034 * 035 * <p>Note that actual processing of the {@code @Value} annotation is performed 036 * by a {@link org.springframework.beans.factory.config.BeanPostProcessor 037 * BeanPostProcessor} which in turn means that you <em>cannot</em> use 038 * {@code @Value} within 039 * {@link org.springframework.beans.factory.config.BeanPostProcessor 040 * BeanPostProcessor} or 041 * {@link org.springframework.beans.factory.config.BeanFactoryPostProcessor BeanFactoryPostProcessor} 042 * types. Please consult the javadoc for the {@link AutowiredAnnotationBeanPostProcessor} 043 * class (which, by default, checks for the presence of this annotation). 044 * 045 * @author Juergen Hoeller 046 * @since 3.0 047 * @see AutowiredAnnotationBeanPostProcessor 048 * @see Autowired 049 * @see org.springframework.beans.factory.config.BeanExpressionResolver 050 * @see org.springframework.beans.factory.support.AutowireCandidateResolver#getSuggestedValue 051 */ 052@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE}) 053@Retention(RetentionPolicy.RUNTIME) 054@Documented 055public @interface Value { 056 057 /** 058 * The actual value expression: for example {@code #{systemProperties.myProp}}. 059 */ 060 String value(); 061 062}