Spring Boot生成项目中System.out.println输出两次问题咨询

我通过start.spring.io生成了一个Spring Boot项目，运行时主类main方法中的System.out.println("Demo")语句会输出两次，且创建的其他类未被调用，特此咨询原因及解决办法。

POM配置 #

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>3.1.2</version>
        <relativePath/> <!-- lookup parent from repository -->
    </parent>
    <groupId>com.example.demo</groupId>
    <artifactId>demo</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <name>demo</name>
    <description>Demo project for Spring Boot</description>
    <properties>
        <java.version>17</java.version>
    </properties>
<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-groovy-templates</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-jdbc</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-thymeleaf</artifactId>
    </dependency>
    <dependency>
        <groupId>org.flywaydb</groupId>
        <artifactId>flyway-core</artifactId>
    </dependency>
    <dependency>
        <groupId>org.mybatis.spring.boot</groupId>
        <artifactId>mybatis-spring-boot-starter</artifactId>
        <version>3.0.2</version>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-devtools</artifactId>
        <scope>runtime</scope>
        <optional>true</optional>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-docker-compose</artifactId>
        <scope>runtime</scope>
        <optional>true</optional>
    </dependency>
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
        <optional>true</optional>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.mybatis.spring.boot</groupId>
        <artifactId>mybatis-spring-boot-starter-test</artifactId>
        <version>3.0.2</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>org.jetbrains</groupId>
        <artifactId>annotations</artifactId>
        <version>RELEASE</version>
        <scope>compile</scope>
    </dependency>
    <dependency>
        <groupId>jakarta.annotation</groupId>
        <artifactId>jakarta.annotation-api</artifactId>
        <version>1.3.5</version>
    </dependency>
    <dependency>
        <groupId>org.junit.jupiter</groupId>
        <artifactId>junit-jupiter-api</artifactId>
        <version>5.9.3</version>
        <scope>test</scope>
    </dependency>
    <dependency>
        <groupId>junit</groupId>
        <artifactId>junit</artifactId>
        <version>4.13.2</version>
        <scope>test</scope>
    </dependency>
</dependencies>

<build>
    <plugins>
        <plugin>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-maven-plugin</artifactId>
            <configuration>
                <excludes>
                    <exclude>
                        <groupId>org.projectlombok</groupId>
                        <artifactId>lombok</artifactId>
                    </exclude>
                </excludes>
            </configuration>
        </plugin>
    </plugins>
</build>

主类代码 #

package com.example.demo.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.ComponentScan;

@SpringBootApplication
@ComponentScan(basePackages = "com.example.demo") // Add the package of your component
public class DemoApplication {

    public static void main(String[] args) {
        System.out.println("Demo");
        SpringApplication.run(DemoApplication.class, args);
    }
}

相关截图 #

原因分析及解决办法 #

1. 日志输出两次的核心原因 #

问题出在spring-boot-devtools依赖的重启类加载器机制：DevTools为实现热部署，会使用两个类加载器——一个加载Spring核心库等不变的基础类，另一个加载应用代码的重启类加载器。启动过程中，由于DevTools的初始化逻辑，main方法会被执行两次。

2. 解决日志重复输出的方案 #

• 禁用DevTools重启功能：在application.properties中添加配置：

  spring.devtools.restart.enabled=false
  

• 移除DevTools依赖：如果不需要热部署，直接删除POM中的spring-boot-devtools依赖项
• 调整启动方式：使用Maven命令mvn spring-boot:run启动项目，避免IDE对DevTools的兼容性问题导致重复执行

3. 自定义类未被调用的解决要点 #

• 自定义类必须添加Spring组件注解（如@Component、@Service、@Repository等），否则无法被Spring容器扫描管理
• 确认自定义类的包路径在com.example.demo或其子包下，与@ComponentScan指定范围匹配
• 若需在启动时执行自定义逻辑，可实现ApplicationRunner/CommandLineRunner接口，或在类方法上添加@PostConstruct注解

内容的提问来源于stack exchange，提问作者guradio