从零手写一个 Spring Boot Starter：深入理解自动装配原理

<?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 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <modelVersion>4.0.0</modelVersion>

    <groupId>top.dhbxs.demo</groupId>
    <artifactId>banner-spring-boot-starter</artifactId>
    <version>1.0-SNAPSHOT</version>

    <properties>
        <maven.compiler.source>21</maven.compiler.source>
        <maven.compiler.target>21</maven.compiler.target>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    </properties>

    <dependencies>
        <!-- springboot 自动配置 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
            <version>3.5.13</version>
        </dependency>

        <!-- springboot 配置处理器 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-configuration-processor</artifactId>
            <version>2.0.4.RELEASE</version>
            <!-- 添加optional属性，不传递此依赖，此依赖只在编译时期生成配置元数据 -->
            <optional>true</optional>
        </dependency>
    </dependencies>


    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
                <version>3.5.13</version>
            </plugin>
        </plugins>
    </build>

</project>

package top.dhbxs.demo.config;

import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import top.dhbxs.demo.properties.BannerProperties;
import top.dhbxs.demo.service.BannerService;

/**
 * Banner 自动配置类
 *
 * @author dhbxs
 * @since 2026/4/9
 */
@Configuration
// 如果当前类路径下有 BannerService 这个类，就把当前类加入 IoC 容器
@ConditionalOnClass(BannerService.class)
// 注册属性组件，SpringBoot 会读取用户在配置文件中指定的值，注入属性组件
@EnableConfigurationProperties(BannerProperties.class)
public class BannerAutoConfiguration {

    private final BannerProperties bannerProperties;

    public BannerAutoConfiguration(BannerProperties bannerProperties) {
        this.bannerProperties = bannerProperties;
    }

    @Bean
    // 条件注入 当 IoC 容器中没有这个 bean 时，才把返回对象注入到 IoC 容器中，如果有，则不注入
    @ConditionalOnMissingBean
    public BannerService bannerService() {
        // 创建BannerService时，把用户配置的是否启用也传进去
        return new BannerService(bannerProperties.getEnable());
    }
}

package top.dhbxs.demo.service;

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.util.stream.Collectors;

/**
 * 打印 Banner 业务类
 *
 * @author dhbxs
 * @since 2026/4/9
 */
public class BannerService {

    // 是否启用
    private final Boolean enable;

    public BannerService(Boolean enable) {
        this.enable = enable;
        this.print();
    }

    /**
     * 从classpath读取banner.txt并打印到控制台
     *
     * @param bannerPath classpath下的路径，如 "banner.txt" 或 "static/banner.txt"
     */
    public void print(String bannerPath) {
        try (InputStream is = BannerService.class.getClassLoader().getResourceAsStream(bannerPath)) {
            if (is == null) {
                System.out.println("Banner not found: " + bannerPath);
                return;
            }
            String banner = new BufferedReader(new InputStreamReader(is, StandardCharsets.UTF_8))
                    .lines()
                    .collect(Collectors.joining("\n"));
            System.out.println(banner);
        } catch (Exception e) {
            System.out.println("Failed to print banner: " + e.getMessage());
        }
    }

    /**
     * 使用默认路径 banner.txt
     */
    public void print() {
        if (Boolean.TRUE.equals(this.enable)) {
            print("dhbxs/banner.txt");
        }
    }
}