简明易理解的@SpringBootApplication注解源码解析（包含面试提问）
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《提升能力，涨薪可待篇》- @SpringBootApplication注解源码解析
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.classAutoConfigurationExcludeFilter.classAbstractTypeHierarchyTraversingFilter.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; }
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