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• 按住Ctrl点击查看启动类MySpringBootApplication上的注解
  -@SpringBootApplication

@SpringBootApplication
public class MySpringBootApplication {
    public static void main(String[] args) {
        SpringApplication.run(MySpringBootApplication.class);
    }
}

• 注解@SpringBootApplication的源码

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {

	/**
	 * Exclude specific auto-configuration classes such that they will never be applied.
	 * @return the classes to exclude
	 */
	@AliasFor(annotation = EnableAutoConfiguration.class)
	Class<?>[] exclude() default {};

	... ... ...

}

• 其中

  • @SpringBootConfiguration：等同与@Configuration，既标注该类是Spring的一个配置类
  • @EnableAutoConfiguration：SpringBoot自动配置功能开启

• 按住Ctrl点击查看注解@EnableAutoConfiguration

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
	... ... ...
}

• 其中

  • @Import(AutoConfigurationImportSelector.class) 导入了AutoConfigurationImportSelector类

• 按住Ctrl点击查看AutoConfigurationImportSelector源码

public String[] selectImports(AnnotationMetadata annotationMetadata) {
        ... ... ...
        List<String> configurations = getCandidateConfigurations(annotationMetadata,
                                                                   attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return StringUtils.toStringArray(configurations);
}


protected List<String> getCandidateConfigurations(AnnotationMetadata metadata,
			AnnotationAttributes attributes) {
		List<String> configurations = SpringFactoriesLoader.loadFactoryNames(
				getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
		
		return configurations;
}

• 其中
  • SpringFactoriesLoader.loadFactoryNames ()：方法的作用就是从具有META-INF/spring.factories文件的jar包中读取指定类对应的类名称列表
  • selectImports()：方法通过调用SpringFactoriesLoader.loadFactoryNames （）方法获取从具有META-INF/spring.factories文件的jar包中读取指定类对应的类名称列表
• 注意：selectImports（）方法是在 引导类中的SpringApplication.run(MySpringBootApplication.class); 被调用

• spring.factories 文件中有关自动配置的配置信息如下：
• 存储的是一个键值对（key，value）
  • key是EnableAutoConfiguration类的全类名
  • value是一个xxxxAutoConfiguration的类名的列表

... ... ...

org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\

... ... ...

• 上面配置文件存在大量的以Configuration为结尾的类名称，这些类就是存有自动配置信息的类，而SpringApplication在获取这些类名后再加载

总结：

这个@EnableAutoConfiguration注解通过@SpringBootApplication被间接的标记在了Spring Boot的启动类上。在SpringApplication.run(…)的内部就会执行selectImports()方法，找到所有JavaConfig自动配置类的全限定名对应的class，然后将所有自动配置类加载到Spring容器中。

• 我们以ServletWebServerFactoryAutoConfiguration为例来分析源码：

@Configuration
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
@ConditionalOnClass(ServletRequest.class)
@ConditionalOnWebApplication(type = Type.SERVLET)
@EnableConfigurationProperties(ServerProperties.class)
@Import({ ServletWebServerFactoryAutoConfiguration.BeanPostProcessorsRegistrar.class,
		ServletWebServerFactoryConfiguration.EmbeddedTomcat.class,
		ServletWebServerFactoryConfiguration.EmbeddedJetty.class,
		ServletWebServerFactoryConfiguration.EmbeddedUndertow.class })
public class ServletWebServerFactoryAutoConfiguration {
	... ... ...
}

• 其中

  • @EnableConfigurationProperties(ServerProperties.class) 代表加载ServerProperties服务器配置属性类

    进入ServerProperties.class源码如下：

@ConfigurationProperties(prefix = "server", ignoreUnknownFields = true)
public class ServerProperties {

	/**
	 * Server HTTP port.
	 */
	private Integer port;

	/**
	 * Network address to which the server should bind.
	 */
	private InetAddress address;
  
  	... ... ...
  
}

• 其中
  • prefix = “server” 表示SpringBoot配置文件中的前缀，SpringBoot会将配置文件中以server开始的属性映射到该类的字段中。映射关系如下：

原文链接:https://blog.csdn.net/weixin_45112292/article/details/117795036