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简明易理解的@SpringBootApplication注解源码分析

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Java SpringBoot 面试

springApplication

一、`@SpringBootApplication` 的作用是什么？

Q：springboot项目的启动类上，都会有个注解@SpringBootApplication，这个注解起到了什么作用？

@SpringBootApplication
public class MicroServiceApplication {

    public static void main(String[] args) {
        SpringApplication.run(MicroServiceApplication.class, args);
    }

}

我们进入@SpringBootApplication注解，发现它等价于三个注解：@SpringBootConfiguration @EnableAutoConfiguration @ComponentScan

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
	...
}

1） @SpringBootConfiguration 就相当于 @Configuration

@Configuration
public @interface SpringBootConfiguration {

}

2） @EnabelAutoConfiguration 相当于将这两个类的实例加入到容器中AutoConfigurationImportSelector.class AutoConfigurationPackages.Registrar.class

@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    ...
}

@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {

}

AutoConfigurationImportSelector.class的作用是，注入spring.factories文件中EnableAutoConfiguration对应的类的实例，当然要经过spring.factories文件中AutoConfigurationImportFilter对应的过滤器（OnBeanCondition、OnClassCondition、OnWebApplicationCondition等等）的过滤。还要排除掉@EnableAutoConfiguration中的exclude和excludeName

具体见ConfigurationClassParser的getImports方法，其中调用了AutoConfigurationImportSelector的process方法和selectImports方法。
```
public Iterable<Group.Entry> getImports() {
			for (DeferredImportSelectorHolder deferredImport : this.deferredImports) {
				this.group.process(deferredImport.getConfigurationClass().getMetadata(),
						deferredImport.getImportSelector());
			}
			return this.group.selectImports();
		}
```
AutoConfigurationImportSelector实现了DeferredImportSelector，所以它是解析@Configuration的最后一步。DeferredImportSelector可以和@Order搭配使用。AutoConfigurationImportSelector的意义在于引入其他包时，可以直接注入其他包的@Configuration，当然需要在其他包的resources文件夹下，新建META-INF目录，在META-INF目录下新建spring.factories文件，加入org.springframework.boot.autoconfigure.EnableAutoConfiguration = @Configuration标注的类的路径

AutoConfigurationPackages.Registrar.class的作用是，注入一个名称为AutoConfigurationPackages的BasePackages.class实例。这个实例的作用在于保存自动扫描的包路径，供以后使用（比如JPA 的entity扫描）

public static void register(BeanDefinitionRegistry registry, String... packageNames) {
		if (registry.containsBeanDefinition(BEAN)) {
			BeanDefinition beanDefinition = registry.getBeanDefinition(BEAN);
			ConstructorArgumentValues constructorArguments = beanDefinition
					.getConstructorArgumentValues();
			constructorArguments.addIndexedArgumentValue(0,
					addBasePackages(constructorArguments, packageNames));
		}
		else {
			GenericBeanDefinition beanDefinition = new GenericBeanDefinition();
			beanDefinition.setBeanClass(BasePackages.class);
			beanDefinition.getConstructorArgumentValues().addIndexedArgumentValue(0,
					packageNames);
			beanDefinition.setRole(BeanDefinition.ROLE_INFRASTRUCTURE);
			registry.registerBeanDefinition(BEAN, beanDefinition);
		}
	}

	private static String[] addBasePackages(
			ConstructorArgumentValues constructorArguments, String[] packageNames) {
		String[] existing = (String[]) constructorArguments
				.getIndexedArgumentValue(0, String[].class).getValue();
		Set<String> merged = new LinkedHashSet<>();
		merged.addAll(Arrays.asList(existing));
		merged.addAll(Arrays.asList(packageNames));
		return StringUtils.toStringArray(merged);
	}

3） @ComponentScan 当然是将路径下合适的类加载到容器中

Q：它为什么要用到TypeExcludeFilter.class 和AutoConfigurationExcludeFilter.class 这两个过滤器

@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(excludeFilters = {
		@Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
public @interface SpringBootApplication {
	...
}

这两个过滤器在ComponentScanAnnotationParser的parse方法中，被加入

public Set<BeanDefinitionHolder> parse(AnnotationAttributes componentScan, final String declaringClass) {
    //此处加入了三个默认的includeFilter，一个用来筛选@Componment标记的类，一个用来筛选javax.annotation.ManagedBean标记的类，一个用来筛选JSR-330中javax.inject.Named标记的类（如果引入了JSR-330依赖注入标准的话，即引入javax.inject包）
    ClassPathBeanDefinitionScanner scanner = new ClassPathBeanDefinitionScanner(this.registry,
				componentScan.getBoolean("useDefaultFilters"), this.environment, this.resourceLoader);
		...
            
		for (AnnotationAttributes filter : componentScan.getAnnotationArray("excludeFilters")) {
			for (TypeFilter typeFilter : typeFiltersFor(filter)) {
				scanner.addExcludeFilter(typeFilter);
			}
		}

		...

		scanner.addExcludeFilter(new AbstractTypeHierarchyTraversingFilter(false, false) {
			@Override
			protected boolean matchClassName(String className) {
				return declaringClass.equals(className);
			}
		});
		return scanner.doScan(StringUtils.toStringArray(basePackages));
	}

  **也是在parse这个方法里调用了 typeFiltersFor方法，对`TypeFilter.class`的实现类进行了实例化**。（也就是说`TypeExcludeFilter.class`  `AutoConfigurationExcludeFilter.class` `AbstractTypeHierarchyTraversingFilter.class`都在这里进行了实例化，但是没有加入spring的bean池）

private List<TypeFilter> typeFiltersFor(AnnotationAttributes filterAttributes) {
		List<TypeFilter> typeFilters = new ArrayList<>();
		FilterType filterType = filterAttributes.getEnum("type");

		for (Class<?> filterClass : filterAttributes.getClassArray("classes")) {
			switch (filterType) {
				case ANNOTATION:
					Assert.isAssignable(Annotation.class, filterClass,
							"@ComponentScan ANNOTATION type filter requires an annotation type");
					@SuppressWarnings("unchecked")
					Class<Annotation> annotationType = (Class<Annotation>) filterClass;
					typeFilters.add(new AnnotationTypeFilter(annotationType));
					break;
				case ASSIGNABLE_TYPE:
					typeFilters.add(new AssignableTypeFilter(filterClass));
					break;
				case CUSTOM:
					Assert.isAssignable(TypeFilter.class, filterClass,
							"@ComponentScan CUSTOM type filter requires a TypeFilter implementation");
					TypeFilter filter = BeanUtils.instantiateClass(filterClass, TypeFilter.class);
					ParserStrategyUtils.invokeAwareMethods(
							filter, this.environment, this.resourceLoader, this.registry);
					typeFilters.add(filter);
					break;
				default:
					throw new IllegalArgumentException("Filter type not supported with Class value: " + filterType);
			}
		}

		for (String expression : filterAttributes.getStringArray("pattern")) {
			switch (filterType) {
				case ASPECTJ:
					typeFilters.add(new AspectJTypeFilter(expression, this.resourceLoader.getClassLoader()));
					break;
				case REGEX:
					typeFilters.add(new RegexPatternTypeFilter(Pattern.compile(expression)));
					break;
				default:
					throw new IllegalArgumentException("Filter type not supported with String pattern: " + filterType);
			}
		}

		return typeFilters;
	}

ComponentScanAnnotationParser的parse方法中，scanner最后为什么又加了一个AbstractTypeHierarchyTraversingFilter呢？我们看一下它的match方法，发现是过滤掉启动类，不让它作为@configuration标记的候选类，避免再一次解析启动类上的各种注解（因为它的两个参数considerInherited和considerInterfaces在scanner.addExcludeFilter这条语句中，设置成了false，导致下面的判断语句不生效）。

public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory)
			throws IOException {

		...
		ClassMetadata metadata = metadataReader.getClassMetadata();
		if (matchClassName(metadata.getClassName())) {
			return true;
		}

		if (this.considerInherited) {
			...
		}

		if (this.considerInterfaces) {
			...
		}

		return false;
	}

注：启动类本身还是会注入到spring的bean池中的，具体见SpringApplication的load方法

ExcludeFilter在ClassPathScanningCandidateComponentProvider的isCandidateComponent方法中被使用。

protected boolean isCandidateComponent(MetadataReader metadataReader) throws IOException {
		for (TypeFilter tf : this.excludeFilters) {
			if (tf.match(metadataReader, getMetadataReaderFactory())) {
				return false;
			}
		}
		for (TypeFilter tf : this.includeFilters) {
			if (tf.match(metadataReader, getMetadataReaderFactory())) {
				return isConditionMatch(metadataReader);
			}
		}
		return false;
	}

AutoConfigurationExcludeFilter 的作用是过滤掉会自动配置的配置类，避免重复

@Override
	public boolean match(MetadataReader metadataReader,
			MetadataReaderFactory metadataReaderFactory) throws IOException {
		//如果这个类被@Configuration标注，且属于自动加载的配置，那么过滤它，避免重复
        return isConfiguration(metadataReader) && isAutoConfiguration(metadataReader);
	}

	private boolean isConfiguration(MetadataReader metadataReader) {
		return metadataReader.getAnnotationMetadata()
				.isAnnotated(Configuration.class.getName());
	}

	private boolean isAutoConfiguration(MetadataReader metadataReader) {
		return getAutoConfigurations()
				.contains(metadataReader.getClassMetadata().getClassName());
	}

	protected List<String> getAutoConfigurations() {
		if (this.autoConfigurations == null) {
            /**
            从META-INF/spring.factories文件中，找出EnableAutoConfiguration.class
            多个jar包中，都存在spring.factories文件
            其中包含EnableAutoConfiguration.class的spring.factories文件，位于spring-boot-autoconfigure
            **/
			this.autoConfigurations = SpringFactoriesLoader.loadFactoryNames(
					EnableAutoConfiguration.class, this.beanClassLoader);
		}
		return this.autoConfigurations;
	}

TypeExcludeFilter 的作用是加载spring bean池中所有针对TypeExcludeFilter的扩展，并循环遍历这些扩展类调用其match方法

public boolean match(MetadataReader metadataReader,
			MetadataReaderFactory metadataReaderFactory) throws IOException {
		if (this.beanFactory instanceof ListableBeanFactory
				&& getClass() == TypeExcludeFilter.class) {
            //加载spring bean池中所有针对TypeExcludeFilter的拓展
			Collection<TypeExcludeFilter> delegates = ((ListableBeanFactory) this.beanFactory)
					.getBeansOfType(TypeExcludeFilter.class).values();
            // 循环遍历，调用其match方法
			for (TypeExcludeFilter delegate : delegates) {
				if (delegate.match(metadataReader, metadataReaderFactory)) {
					return true;
				}
			}
		}
		return false;
	}