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@Value注解的底层原理（一）

news 2025/9/1 5:58:30

一.概述

@Value注解是如何工作的，大致分为以下几个步骤：

1.获取Value值

2.解析${}

3.解析#{SEL表达式}（对于这种情况本文没有演示）

4.类型转换

二.测试

package com.example.springdemo.demos.a14;import org.springframework.beans.factory.annotation.Value;
import org.springframework.beans.factory.config.DependencyDescriptor;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.ContextAnnotationAutowireCandidateResolver;
import org.springframework.stereotype.Component;import java.lang.reflect.Field;/*** @author zhou* @version 1.0* @description TODO* @date 2025/8/31 20:58*/
@Configuration
public class TestValue {public static void main(String[] args) throws NoSuchFieldException {AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(TestValue.class);ContextAnnotationAutowireCandidateResolver resolver = new ContextAnnotationAutowireCandidateResolver();resolver.setBeanFactory(context.getDefaultListableBeanFactory());Field home = Bean1.class.getDeclaredField("home");extracted(context, resolver, home);Field age = Bean1.class.getDeclaredField("age");extracted2(context,resolver,age);}private static void extracted(AnnotationConfigApplicationContext context, ContextAnnotationAutowireCandidateResolver resolver, Field field) {DependencyDescriptor descriptor = new DependencyDescriptor(field,true);//获取@Value的内容String value = resolver.getSuggestedValue(descriptor).toString();System.out.println(value);//解析${}value = context.getEnvironment().resolvePlaceholders(value);System.out.println(value);}private static void extracted2(AnnotationConfigApplicationContext context, ContextAnnotationAutowireCandidateResolver resolver, Field field) {DependencyDescriptor descriptor = new DependencyDescriptor(field,true);//获取@Value的内容String value = resolver.getSuggestedValue(descriptor).toString();System.out.println(value);//解析${}value = context.getEnvironment().resolvePlaceholders(value);System.out.println(value);//类型转换Object age = context.getBeanFactory().getTypeConverter().convertIfNecessary(value, descriptor.getDependencyType());System.out.println(age.getClass());}public class Bean1{@Value("${JAVA_HOME}")private String home;@Value("18")private int age;}}

文中抽取了两个方法对@Value注解作测试

1.解析带有${}的属性

@Value("${JAVA_HOME}")
private String home;

测试方法一里面详细介绍了步骤，首先需要拿到DependencyDescriptor对象，然后通过ContextAnnotationAutowireCandidateResolver对象到@Value里面的值。原始的值是${JAVA_HOME}，随后通过context.getEnvironment().resolvePlaceholders方法解析里面的值。

2.类型不一致转换

@Value("18")
private int age;

@Value里面的值类型是String与int类型不一致，前面获取值的步骤是一致的，不同的地方在于最后一步，利用下面的类型转换器把String类型转换为Integer类型,后面自动拆箱为int类型。

 //类型转换Object age = context.getBeanFactory().getTypeConverter().convertIfNecessary(value, descriptor.getDependencyType());System.out.println(age.getClass());

测试结果：