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 * Annotation used at the field or method/constructor parameter level 027 * that indicates a default value expression for the annotated element. 028 * 029 * <p>Typically used for expression-driven or property-driven dependency injection. 030 * Also supported for dynamic resolution of handler method arguments — for 031 * example, in Spring MVC. 032 * 033 * <p>A common use case is to inject values using 034 * <code>#{systemProperties.myProp}</code> style SpEL (Spring Expression Language) 035 * expressions. Alternatively, values may be injected using 036 * <code>${my.app.myProp}</code> style property placeholders. 037 * 038 * <p>Note that actual processing of the {@code @Value} annotation is performed 039 * by a {@link org.springframework.beans.factory.config.BeanPostProcessor 040 * BeanPostProcessor} which in turn means that you <em>cannot</em> use 041 * {@code @Value} within 042 * {@link org.springframework.beans.factory.config.BeanPostProcessor 043 * BeanPostProcessor} or 044 * {@link org.springframework.beans.factory.config.BeanFactoryPostProcessor BeanFactoryPostProcessor} 045 * types. Please consult the javadoc for the {@link AutowiredAnnotationBeanPostProcessor} 046 * class (which, by default, checks for the presence of this annotation). 047 * 048 * @author Juergen Hoeller 049 * @since 3.0 050 * @see AutowiredAnnotationBeanPostProcessor 051 * @see Autowired 052 * @see org.springframework.beans.factory.config.BeanExpressionResolver 053 * @see org.springframework.beans.factory.support.AutowireCandidateResolver#getSuggestedValue 054 */ 055@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE}) 056@Retention(RetentionPolicy.RUNTIME) 057@Documented 058public @interface Value { 059 060 /** 061 * The actual value expression such as <code>#{systemProperties.myProp}</code> 062 * or property placeholder such as <code>${my.app.myProp}</code>. 063 */ 064 String value(); 065 066}