SpringBoot项目为何引入大量的starter?如何自定义starter?

<dependencies>
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
            <version>1.18.22</version>
        </dependency>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <version>2.5.6</version>
        </dependency>
    </dependencies>

@Data
@ConfigurationProperties(prefix = "com.zhj.vo.student")
public class Student {
​
    private Long id;
​
    private String name;
​
    private Integer age;
}

/**
 * 自动装配类
 */
@Configuration // 需要进行Bean注册的
@EnableConfigurationProperties({Student.class}) //Bean注册
public class AutoConfiguration {
}

<dependency>
    <groupId>junit</groupId>
    <artifactId>junit</artifactId>
    <version>4.12</version>
    <scope>test</scope>
</dependency>
<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-test</artifactId>
    <version>2.5.6</version>
</dependency>

/**
 * 自动注入测试类
 */
@RunWith(SpringRunner.class) // junit4 测试环境
@WebAppConfiguration // 启动web运行环境
@SpringBootTest(classes = AutoConfigApplication.class) // 指定启动类
public class AutoConfigTest {
​
    @Autowired
    @Qualifier("com.zhj.vo.student-com.zhj.vo.Student") // 前缀-类名 注入
    private Student student;
​
    @Test
    public void test01() {
        System.out.println(student);
    }
}

/**
 * 自动装配类
 */
@Configuration // 需要进行Bean注册的
@Import({Student.class}) //Bean注册
public class AutoConfiguration {
}

/**
 * 自动注入测试类
 */
@RunWith(SpringRunner.class) // junit4 测试环境
@WebAppConfiguration // 启动web运行环境
@SpringBootTest(classes = AutoConfigApplication.class) // 指定启动类
public class AutoConfigTest {
​
    @Autowired
    private Student student;
​
    @Test
    public void test01() {
        System.out.println(student);
    }
}

public class DefaultImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata importingClassMetadata) {
        return new String[] {"com.zhj.vo.Student"};
    }
}

/**
 * 自动装配类
 */
@Configuration // 需要进行Bean注册的
@Import({DefaultImportSelector.class})
public class AutoConfiguration {
}

public class DefaultImportBeanDefinitionRegister implements ImportBeanDefinitionRegistrar {
​
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        RootBeanDefinition rootBeanDefinition = new RootBeanDefinition(Student.class); // 配置bean
        registry.registerBeanDefinition("studentInstance", rootBeanDefinition); // Bean 注册
    }
}

/**
 * 自动装配类
 */
@Configuration // 需要进行Bean注册的
@Import({DefaultImportBeanDefinitionRegister.class})
public class AutoConfiguration {
}

<dependency>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-configuration-processor</artifactId>
    <version>2.5.6</version>
</dependency>

{
  "groups": [
    {
      "name": "com.zhj.vo.student",
      "type": "com.zhj.vo.Student",
      "sourceType": "com.zhj.vo.Student"
    }
  ],
  "properties": [
    {
      "name": "com.zhj.vo.student.age",
      "type": "java.lang.Integer",
      "sourceType": "com.zhj.vo.Student"
    },
    {
      "name": "com.zhj.vo.student.id",
      "type": "java.lang.Long",
      "sourceType": "com.zhj.vo.Student"
    },
    {
      "name": "com.zhj.vo.student.name",
      "type": "java.lang.String",
      "sourceType": "com.zhj.vo.Student"
    }
  ],
  "hints": []
}

/**
 * 自动装配类
 */
@Configuration // 需要进行Bean注册的
@Import({DefaultImportBeanDefinitionRegister.class})
public class AutoConfiguration {
​
    // 自动注册Bean
    @Bean(name = "Students")
    public List<String> getNameList() {
        List list = new ArrayList();
        list.add("小明");
        list.add("小红");
        list.add("小李");
        return list;
    }
}

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.zhj.config.AutoConfiguration

<dependencies>
    <dependency>
        <groupId>com.zhj</groupId>
        <artifactId>spring-boot-auto-config-starter</artifactId>
        <version>1.0-SNAPSHOT</version>
    </dependency>
</dependencies>

@Data
@ConfigurationProperties(prefix = "com.zhj.vo.student")
public class Student {
​
    private Long id;
​
    private String name;
​
    private Integer age;
}

com:
  zhj:
    vo:
      student:
        id: 1
        name: '小明'
        age: 12

@RestController
public class HelloController {
​
    @Autowired
    private Student student;
​
    @GetMapping("/hello")
    public String hello() {
        return "hello "+ student;
    }
}