本文介绍 Spring Boot 2 外部化配置实现方案中 @Value 的使用。
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
开发环境
- Oracle JDK 1.8.0_201
- Apache Maven 3.6.0
- IntelliJ IDEA (Version 2018.3.3)
基础示例
创建 Spring Boot 工程，参考：IntelliJ IDEA 创建 Spring Boot 工程。
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
- 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!
- 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("${tutorial.spring.boot.configuration.welcome}") String welcome) {
System.out.println(welcome);
}
}
- 启动运行。
. ____ _ __ _ _
/\\ / ___'_ __ _ _(_)_ __ __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
\\/ ___)| |_)| | | | | || (_| | ) ) ) )
' |____| .__|_| |_|_| |_\__, | / / / /
=========|_|==============|___/=/_/_/_/
:: Spring Boot :: (v2.1.3.RELEASE)
2019-03-16 10:15:59.500 INFO 9888 --- [ main] s.b.c.SpringBootConfigurationApplication : Starting SpringBootConfigurationApplication on ... with PID 9888 (D:\Tutorial\spring-boot-configuration\target\classes started by ... in D:\Tutorial\spring-boot-configuration)
2019-03-16 10:15:59.504 INFO 9888 --- [ main] s.b.c.SpringBootConfigurationApplication : No active profile set, falling back to default profiles: default
Welcome to Spring Boot Configuration Tutorial!
2019-03-16 10:16:00.167 INFO 9888 --- [ main] s.b.c.SpringBootConfigurationApplication : Started SpringBootConfigurationApplication in 1.083 seconds (JVM running for 1.889)
@Value 详解
@Value 注解不仅可以修饰参数，还可以修饰属性和方法，参考 @Value 源码。
package org.springframework.beans.factory.annotation;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
@Target({ElementType.FIELD, ElementType.METHOD, ElementType.PARAMETER, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Value {
String value();
}
示例：@Value 修饰属性
- 修改
WelcomeConfig类。
package tutorial.spring.boot.configuration.config;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.context.annotation.Configuration;
import javax.annotation.PostConstruct;
@Configuration
public class WelcomeConfig {
@Value("${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。
- 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)
示例：@Value 修饰方法
- 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;
}
}
- 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)
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!