springboot2.0 主程序类@SpringBootApplication详解，@SpringBootConfiguration，@EnableAutoConfiguration

浅浅的花香味﹌ 2021-09-28 23:10 237阅读 0赞

## 1.创建一个springboot项目 ##

[《 spring boot2.0 第一个程序《快速开始》》][spring boot2.0]

## 2.项目主类入口 ##

package com.badger.web;
    
    import org.springframework.boot.SpringApplication;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    
    @SpringBootApplication
    public class SpringbootHelloApplication {
    
        public static void main(String[] args) {
            SpringApplication.run(SpringbootHelloApplication.class, args);
        }
    }

我们首先springboot的第一个注解@SpringBootApplication

@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 {
    
    	//自动注入的时候，排序的类
    	@AliasFor(annotation = EnableAutoConfiguration.class)
    	Class<?>[] exclude() default {};
    
    	//自动注入的时候，排序的类的名字
    	@AliasFor(annotation = EnableAutoConfiguration.class)
    	String[] excludeName() default {};
    
    	//自动注入的时候，扫描的包路径	
    	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    	String[] scanBasePackages() default {};
    
    	//自动注入的时候，扫描的类注解（@Service，@Controller等）
    	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
    	Class<?>[] scanBasePackageClasses() default {};
    
    }

我们可以看到，@SpringBootApplication注解，是三个组合注解：@SpringBootConfiguration  @EnableAutoConfiguration @ComponentScan

### 1）@SpringBootConfiguration：标注这个类是一个配置类 ###

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Configuration
    public @interface SpringBootConfiguration {
    
    }

可以看到，这个注解，只是@Configuration注解的派生注解，跟@Configuration注解的功能一致，标注这个类是一个配置类，只不过@SpringBootConfiguration是springboot的注解，而@Configuration是spring的注解，关于@Configuration注解，更详细的，可以参看[《spring注解 @Conditional的使用》][spring_ _Conditional]

[《springboot2.0 @Conditional派生注解@ConditionalOnMissingBean，@ConditionalOnProperty等详解》][springboot2.0 _Conditional_ConditionalOnMissingBean_ConditionalOnProperty]

### 2）.@EnableAutoConfiguration注解：开启自动装配 ###

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import(AutoConfigurationImportSelector.class)
    public @interface EnableAutoConfiguration {
    
    	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
    
    	//排除
    	Class<?>[] exclude() default {};
        //排除
    	String[] excludeName() default {};
    
    }

@EnableAutoConfiguration注解，又是一个组合注解：@AutoConfigurationPackage和@Import(AutoConfigurationImportSelector.class)

**@Import(AutoConfigurationImportSelector.class) 导入AutoConfigurationImportSelector.class类**

关于@Import注解，可以像@Configuration导入类或者组件，导入类，分三种

1.  直接导入普通的类
2.  导入实现了ImportSelector接口的类
3.  导入实现了ImportBeanDefinitionRegistrar接口的类

可以详细参考[《spring注解 @Import的使用，ImportSelector接口，ImportBeanDefinitionRegistrar接口》][spring_ _Import_ImportSelector_ImportBeanDefinitionRegistrar]

AutoConfigurationImportSelector.class的作用

@Override
    	public String[] selectImports(AnnotationMetadata annotationMetadata) {
    		if (!isEnabled(annotationMetadata)) {
    			return NO_IMPORTS;
    		}
    		AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader
    				.loadMetadata(this.beanClassLoader);
    		AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
    				autoConfigurationMetadata, annotationMetadata);
    		return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
    	}

直接看重写的public String\[\] selectImports(AnnotationMetadata annotationMetadata)方法，查看getAutoConfigurationEntry法

AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(
    				autoConfigurationMetadata, annotationMetadata);

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI4NDEwMjgz_size_16_color_FFFFFF_t_70][]

查看getCandidateConfigurations方法，根据条件配置

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI4NDEwMjgz_size_16_color_FFFFFF_t_70 1][]

可以看到，加载文件到list中，查看SpringFactoriesLoader.loadFactoryNames的方法

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI4NDEwMjgz_size_16_color_FFFFFF_t_70 2][]

最终，会拿到所有的类的全路径，然后反射实例化bean，并注入到spring容器中，静态变量就是获取文件的路径

META-INF/spring.factories

/**
    	 * The location to look for factories.
    	 * <p>Can be present in multiple JAR files.
    	 */
    	public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3FxXzI4NDEwMjgz_size_16_color_FFFFFF_t_70 3][]

这个部分，也是springboot自动装配的整个过程

’**@AutoConfigurationPackage注解：自动配置包**

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

@AutoConfigurationPackage注解，自动注入主类下所在包下（com.badger.web.SpringbootHelloApplication），所有的加了注解的类（@Controller，@Service等），以及配置类（@Configuration）

可以看到，就是@Import(AutoConfigurationPackages.Registrar.class)注解，导入（实例化）一个AutoConfigurationPackages.Registrar.class类；

我们直接看这个AutoConfigurationPackages.Registrar.class类的一些简单实现

static class Registrar implements ImportBeanDefinitionRegistrar, DeterminableImports {
    
    		@Override
    		public void registerBeanDefinitions(AnnotationMetadata metadata,
    				BeanDefinitionRegistry registry) {
                //new PackageImport(metadata).getPackageName() 扫描主类包下的所有的类 ,
                //并且注入到spring容器
    			register(registry, new PackageImport(metadata).getPackageName());
    		}
    
    		@Override
    		public Set<Object> determineImports(AnnotationMetadata metadata) {
    			return Collections.singleton(new PackageImport(metadata));
    		}
    
    	}

,感兴趣的，可以继续看下

### 3） @ComponentScan注解：包扫描 ###

大家详细参看[《spring注解 @ComponentScan，@ComponentScans 以及@Filter的使用》][spring_ _ComponentScan_ComponentScans _Filter]就可以了

[spring boot2.0]: https://blog.csdn.net/qq_28410283/article/details/83450272
[spring_ _Conditional]: https://blog.csdn.net/qq_28410283/article/details/91356551
[springboot2.0 _Conditional_ConditionalOnMissingBean_ConditionalOnProperty]: https://blog.csdn.net/qq_28410283/article/details/91355365
[spring_ _Import_ImportSelector_ImportBeanDefinitionRegistrar]: https://blog.csdn.net/qq_28410283/article/details/90136845
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[spring_ _ComponentScan_ComponentScans _Filter]: https://blog.csdn.net/qq_28410283/article/details/90115259