Spring如何将配置文件中的简单值注入Bean中

# 字符串
app.name=My Awesome Application
# 数字
app.thread-pool.size=10
# 布尔值
app.feature.new-ui.enabled=true
# 逗号分隔的列表
app.admin.emails=admin@example.com,support@example.com
# 带默认值的属性 (我们在代码中处理)
# app.description=... (假设这个属性没有被定义)

import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;
import jakarta.annotation.PostConstruct; // 使用 jakarta.* 如果是 Spring Boot 3+
import java.util.List;

@Component
public class AppConfig {

    // 注入字符串
    @Value("${app.name}")
    private String appName;

    // Spring 会自动进行类型转换，将 "10" 转换为 int
    @Value("${app.thread-pool.size}")
    private int threadPoolSize;

    // 同样，"true" 会被转换为 boolean
    @Value("${app.feature.new-ui.enabled}")
    private boolean isNewUiEnabled;

    // 注入一个列表 (Spring 会自动按逗号分割)
    @Value("${app.admin.emails}")
    private List<String> adminEmails;

    // 提供默认值：如果属性不存在，则使用冒号后面的默认值
    @Value("${app.description:This is a default description}")
    private String appDescription;
    
    // 打印出来看看结果
    @PostConstruct
    public void printConfig() {
        System.out.println("Application Name: " + appName);
        System.out.println("Thread Pool Size: " + threadPoolSize);
        System.out.println("Is New UI Enabled: " + isNewUiEnabled);
        System.out.println("Admin Emails: " + adminEmails);
        System.out.println("Application Description: " + appDescription);
    }
}

app.name=My Awesome Application
app.thread-pool.size=10
app.feature.new-ui.enabled=true
app.admin.emails=admin@example.com,support@example.com

import org.springframework.boot.context.properties.ConfigurationProperties;
import java.util.List;

// 这个注解告诉 Spring，将 "app" 前缀的属性绑定到这个类的字段上
@ConfigurationProperties(prefix = "app")
public class AppProperties {

    private String name;
    private List<String> adminEmails;
    // 注意：这里我们创建一个嵌套类来映射嵌套的属性，结构更清晰
    private final ThreadPool threadPool = new ThreadPool();
    private final Feature feature = new Feature();

    // 嵌套类，用于映射 app.thread-pool.*
    public static class ThreadPool {
        private int size;
        // Getter and Setter...
        public int getSize() { return size; }
        public void setSize(int size) { this.size = size; }
    }

    // 嵌套类，用于映射 app.feature.*
    public static class Feature {
        private final NewUi newUi = new NewUi();
        public static class NewUi {
            private boolean enabled;
            // Getter and Setter...
            public boolean isEnabled() { return enabled; }
            public void setEnabled(boolean enabled) { this.enabled = enabled; }
        }
        public NewUi getNewUi() { return newUi; }
    }

    // 主类的 Getters and Setters...
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public List<String> getAdminEmails() { return adminEmails; }
    public void setAdminEmails(List<String> adminEmails) { this.adminEmails = adminEmails; }
    public ThreadPool getThreadPool() { return threadPool; }
    public Feature getFeature() { return feature; }
}

import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
// 告诉 Spring Boot 启用 AppProperties，并将其注册为一个 Bean
@EnableConfigurationProperties(AppProperties.class)
public class MyApplication {
    public static void main(String[] args) {
        SpringApplication.run(MyApplication.class, args);
    }
}

import org.springframework.stereotype.Service;

@Service
public class MyBusinessService {

    private final AppProperties appProperties;

    // 通过构造函数注入，非常清晰
    public MyBusinessService(AppProperties appProperties) {
        this.appProperties = appProperties;
    }

    public void doSomething() {
        System.out.println("Running service for app: " + appProperties.getName());
        if (appProperties.getFeature().getNewUi().isEnabled()) {
            System.out.println("Using the new UI with thread pool size: " + appProperties.getThreadPool().getSize());
        }
    }
}