org.springframework.beans.factory.annotation.Value Maven / Gradle / Ivy

/*
 * Copyright 2002-2020 the original author or authors.
 *
 * 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
 *
 *      https://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.springframework.beans.factory.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Annotation used at the field or method/constructor parameter level
 * that indicates a default value expression for the annotated element.
 *
 * Typically used for expression-driven or property-driven dependency injection.
 * Also supported for dynamic resolution of handler method arguments — for
 * example, in Spring MVC.
 *
 * 
A common use case is to inject values using
 * #{systemProperties.myProp} style SpEL (Spring Expression Language)
 * expressions. Alternatively, values may be injected using
 * ${my.app.myProp} style property placeholders.
 *
 * Note that actual processing of the {@code @Value} annotation is performed
 * by a {@link org.springframework.beans.factory.config.BeanPostProcessor
 * BeanPostProcessor} which in turn means that you cannot use
 * {@code @Value} within
 * {@link org.springframework.beans.factory.config.BeanPostProcessor
 * BeanPostProcessor} or
 * {@link org.springframework.beans.factory.config.BeanFactoryPostProcessor BeanFactoryPostProcessor}
 * types. Please consult the javadoc for the {@link AutowiredAnnotationBeanPostProcessor}
 * class (which, by default, checks for the presence of this annotation).
 *
 * @author Juergen Hoeller
 * @since 3.0
 * @see AutowiredAnnotationBeanPostProcessor
 * @see Autowired
 * @see org.springframework.beans.factory.config.BeanExpressionResolver
 * @see org.springframework.beans.factory.support.AutowireCandidateResolver#getSuggestedValue
 */
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Value {

	/**
	 * The actual value expression such as #{systemProperties.myProp}
	 * or property placeholder such as ${my.app.myProp}.
	 */
	String value();

}