Spring Boot @Value

本文介绍 Spring Boot 2 外部化配置实现方案中 @Value 的使用。

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目录

• 开发环境
• 基础示例
• @Value 详解
• 示例：@Value 修饰属性
• 示例：@Value 修饰方法
• 备注

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开发环境

• Oracle JDK 1.8.0_201
• Apache Maven 3.6.0
• IntelliJ IDEA (Version 2018.3.3)

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基础示例

1. 创建 Spring Boot 工程，参考：IntelliJ IDEA 创建 Spring Boot 工程。

2. 生成的 pom 文件如下，不做任何修改

    4.0.0
    
        org.springframework.boot
        spring-boot-starter-parent
        2.1.3.RELEASE
        
    
    tutorial.spring.boot
    spring-boot-configuration
    0.0.1-SNAPSHOT
    spring-boot-configuration
    Demo project for Spring Boot

    
        1.8
    

    
        
            org.springframework.boot
            spring-boot-starter
        

        
            org.springframework.boot
            spring-boot-starter-test
            test
        
    

    
        
            
                org.springframework.boot
                spring-boot-maven-plugin
            
        
    

3. 工程 src/main/resources 目录下自动生成了一个配置文件 application.properties，向此配置文件中添加以下内容。

tutorial.spring.boot.configuration.welcome=Welcome to Spring Boot Configuration Tutorial!

4. 新建一个类 WelcomeConfig，使用 @Configuration 注解标识这是个配置类，在构造函数中使用注解 @Value 将 application.properties 配置文件中 tutorial.spring.boot.configuration.welcome 的值注入到参数 welcome 中。

package tutorial.spring.boot.configuration.config;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;

@Configuration
public class WelcomeConfig {

    public WelcomeConfig(@Value("${tutorial.spring.boot.configuration.welcome}") String welcome) {
        System.out.println(welcome);
    }
}

5. 启动运行。

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/
 :: Spring Boot ::        (v2.1.3.RELEASE)

2019-03-16 10:15:59.500  INFO 9888 --- [           main] s.b.c.SpringBootConfigurationApplication : Starting SpringBootConfigurationApplication on ... with PID 9888 (D:\Tutorial\spring-boot-configuration\target\classes started by ... in D:\Tutorial\spring-boot-configuration)
2019-03-16 10:15:59.504  INFO 9888 --- [           main] s.b.c.SpringBootConfigurationApplication : No active profile set, falling back to default profiles: default
Welcome to Spring Boot Configuration Tutorial!
2019-03-16 10:16:00.167  INFO 9888 --- [           main] s.b.c.SpringBootConfigurationApplication : Started SpringBootConfigurationApplication in 1.083 seconds (JVM running for 1.889)

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@Value 详解

@Value 注解不仅可以修饰参数，还可以修饰属性和方法，参考 @Value 源码。

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;

@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Value {
    String value();
}

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示例：@Value 修饰属性

1. 修改 WelcomeConfig 类。

package tutorial.spring.boot.configuration.config;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;

import javax.annotation.PostConstruct;

@Configuration
public class WelcomeConfig {

    @Value("${tutorial.spring.boot.configuration.welcome}")
    private String welcome;

    @PostConstruct
    public void init() {
        System.out.println(welcome);
    }
}

注意：
(1) 因为 @Value 未直接修饰构造器参数，所以构造器执行过程中 welcome 属性值仍为 null，实际在对象构造完成后执行的属性注入；
(2) @PostConstruct 注解作用是 WelcomeConfig 对象构造完成后执行，此处使用 @PostConstruct 注解只是为了演示属性可正常注入，更多有关 @PostConstruct 注解的用法在此不做详细讨论。

2. 启动运行。

......
2019-03-16 10:35:42.964  INFO 9988 --- [           main] s.b.c.SpringBootConfigurationApplication : Starting SpringBootConfigurationApplication on ... with PID 9988 (D:\Tutorial\spring-boot-configuration\target\classes started by ... in D:\Tutorial\spring-boot-configuration)
2019-03-16 10:35:42.968  INFO 9988 --- [           main] s.b.c.SpringBootConfigurationApplication : No active profile set, falling back to default profiles: default
Welcome to Spring Boot Configuration Tutorial!
2019-03-16 10:35:43.629  INFO 9988 --- [           main] s.b.c.SpringBootConfigurationApplication : Started SpringBootConfigurationApplication in 1.071 seconds (JVM running for 1.891)

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示例：@Value 修饰方法

1. 修改 WelcomeConfig 类。

package tutorial.spring.boot.configuration.config;

import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;

import javax.annotation.PostConstruct;

@Configuration
public class WelcomeConfig {

    private String welcome;

    @PostConstruct
    public void init() {
        System.out.println(welcome);
    }

    @Value("${tutorial.spring.boot.configuration.welcome}")
    public void setWelcome(String welcome) {
        this.welcome = welcome;
    }
}

2. 启动运行。

......
2019-03-16 10:40:16.930  INFO 9268 --- [           main] s.b.c.SpringBootConfigurationApplication : Starting SpringBootConfigurationApplication on ... with PID 9268 (D:\Tutorial\spring-boot-configuration\target\classes started by ... in D:\Tutorial\spring-boot-configuration)
2019-03-16 10:40:16.935  INFO 9268 --- [           main] s.b.c.SpringBootConfigurationApplication : No active profile set, falling back to default profiles: default
Welcome to Spring Boot Configuration Tutorial!
2019-03-16 10:40:17.587  INFO 9268 --- [           main] s.b.c.SpringBootConfigurationApplication : Started SpringBootConfigurationApplication in 1.057 seconds (JVM running for 1.842)

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备注

与本例中 application.properties 等效的 YAML 配置文件内容如下。

tutorial:
  spring:
    boot:
      configuration:
        welcome: Welcome to Spring Boot Configuration Tutorial!