springboot项目中读取.env文件参数值的两种主流实现方式

<dependency>
    <groupId>io.github.cdimascio</groupId>
    <artifactId>dotenv-java</artifactId>
    <version>3.0.0</version> <!-- 推荐使用最新版本 -->
</dependency>

implementation 'io.github.cdimascio:dotenv-java:3.0.0'

# .env文件内容
DB_URL=jdbc:mysql://localhost:3306/mydb
DB_USERNAME=root
DB_PASSWORD=123456
APP_NAME=my-spring-boot-app

import io.github.cdimascio.dotenv.Dotenv;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class EnvDemoApplication {

    public static void main(String[] args) {
        // 加载.env文件（默认读取项目根目录的.env）
        Dotenv dotenv = Dotenv.load();
        // 将.env中的配置设置到系统环境变量（可选，也可直接通过dotenv.get()获取）
        dotenv.entries().forEach(entry -> System.setProperty(entry.getKey(), entry.getValue()));

        SpringApplication.run(EnvDemoApplication.class, args);
    }
}

import io.github.cdimascio.dotenv.Dotenv;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.env.EnvironmentPostProcessor;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.MapPropertySource;
import org.springframework.core.io.ClassPathResource;

import java.io.IOException;
import java.util.HashMap;
import java.util.Map;

public class DotenvEnvironmentPostProcessor implements EnvironmentPostProcessor {

    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
        // 方式1：读取项目根目录的.env
        Dotenv dotenv = Dotenv.load();

        // 方式2：读取src/main/resources下的.env（推荐，打包后可包含在jar中）
        // Dotenv dotenv = Dotenv.configure().directory(new ClassPathResource("").getPath()).load();

        // 将.env中的配置转换为MapPropertySource，注入到Spring环境中
        Map<String, Object> envMap = new HashMap<>();
        dotenv.entries().forEach(entry -> envMap.put(entry.getKey(), entry.getValue()));
        environment.getPropertySources().addFirst(new MapPropertySource("dotenv", envMap));
    }
}

org.springframework.boot.env.EnvironmentPostProcessor=com.example.demo.DotenvEnvironmentPostProcessor

import org.springframework.beans.factory.annotation.Value;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.core.env.Environment;

import javax.annotation.Resource;

@RestController
public class EnvController {

    // 方式1：使用@Value注解
    @Value("${DB_URL}")
    private String dbUrl;

    @Value("${APP_NAME}")
    private String appName;

    // 方式2：使用Environment对象
    @Resource
    private Environment environment;

    @GetMapping("/env")
    public String getEnv() {
        String dbUsername = environment.getProperty("DB_USERNAME");
        String dbPassword = environment.getProperty("DB_PASSWORD");
        return "dbUrl: " + dbUrl + "<br/>" +
                "appName: " + appName + "<br/>" +
                "dbUsername: " + dbUsername + "<br/>" +
                "dbPassword: " + dbPassword;
    }
}

import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.MapPropertySource;
import org.springframework.util.ResourceUtils;

import java.io.FileInputStream;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;

public class EnvReader {

    public static void loadEnv(ConfigurableEnvironment environment) {
        Properties properties = new Properties();
        Map<String, Object> envMap = new HashMap<>();

        try {
            // 读取src/main/resources下的.env文件（也可读取项目根目录的.env）
            FileInputStream inputStream = new FileInputStream(ResourceUtils.getFile("classpath:.env"));
            properties.load(inputStream);

            // 解析Properties到Map
            properties.forEach((key, value) -> envMap.put(key.toString(), value.toString()));

            // 注入到Spring环境中（优先级最高）
            environment.getPropertySources().addFirst(new MapPropertySource("customEnv", envMap));
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.core.env.ConfigurableEnvironment;

@SpringBootApplication
public class EnvDemoApplication {

    public static void main(String[] args) {
        SpringApplication application = new SpringApplication(EnvDemoApplication.class);
        // 启动前加载.env
        application.addInitializers(context -> {
            ConfigurableEnvironment environment = context.getEnvironment();
            EnvReader.loadEnv(environment);
        });
        application.run(args);
    }
}