前言
在SpringBoot中,只需要一个简单的启动类,就能自动完成很多复杂的工作, 其中就有自动扫描classpath,将带有@Component的类生成bean.
@SpringBootApplication
public class SpringLifecycleApplication {
public static void main(String[] args) {
ApplicationContext applicationContext = SpringApplication.run(SpringLifecycleApplication.class, args);
}
}
//自动注册为bean
@Component
public class SimpleBean{
}
本文将探究SpringBoot中@Component的生效逻辑.
正文
@SpringBootApplication简介
@SpringBootApplication注解是SpringBoot的关键,它组合了@Configuration,@ComponentScan等注解,有必要提前了解一下,源码如下
@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 {
@AliasFor(annotation = EnableAutoConfiguration.class)
Class<?>[] exclude() default {};
@AliasFor(annotation = EnableAutoConfiguration.class)
String[] excludeName() default {};
@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
String[] scanBasePackages() default {};
@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
Class<?>[] scanBasePackageClasses() default {};
}
这里要提一下,Spring对注解的处理通常是递归的,例如查找SpringLifecycleApplication这个类是否被@Configuration注解,会经历以下几步
查找SpringLifecycleApplication的注解, 找到
@SpringBootApplication,不是需要的@Configuration,继续查找修饰
@SpringBootApplication的注解, 找到@SpringBootConfiguration,@ComponentScan等注解还是没有符合的,再查看修饰这些注解的注解~ 如此递归查找,在@SpringBootConfiguration中找到
@Configuration,成功
类似的还有@Service,@Controller.它们都被@Component修饰,因此查找包含@Component注解的类时它们也是符合的.
@Component的生效逻辑
下面将按照SpringBoot的启动流程讲解@Component,参见下图
SpringBoot-bean.png
1. createApplicationContext阶段: 注册ConfigurationClassPostProcessor
SpringBoot默认创建AnnotationConfigApplicationContext,它在构造时会创建AnnotatedBeanDefinitionReader,后者构造时会调用AnnotationConfigUtils.registerAnnotationConfigProcessors()注册ConfigurationClassPostProcessor的BeanDefinition.
ConfigurationClassPostProcessor会处理带@Configuartion修饰的bean,下面会用到
SpringBoot-bean-0.png
2. prepareContext阶段: 注册启动类SpringLifeCycleApplication的BeanDefinition
在Spring启动的早期阶段,就会将SpringLifeCycleApplication注册为bean,后续的很多逻辑都会根据它身上的@SpringBootApplication(以及其他注解)去做
SpringBoot-bean-1.png
3. refreshContext阶段的invokeBeanFactoryPostProcessor(): 调用ConfigurationClassPostProcessor.postProcessBeanDefinitionRegistry()
SpringBoot-bean-2.png
插一段,先来理一下invokeBeanFactoryPostProcessor()的具体逻辑,分为两块: postProcessBeanDefinitionRegistry()和postProcessBeanFactory()
-
postProcessBeanDefinitionRegistry()允许添加自定义的BeanDefinition(对于通过xml注册的BeanDefinition或代码注册的称之为正常,其他的如通过注解@Component添加的都称为自定义),经过这一步,Spring不再允许添加BeanDefinition,由于新注册的BeanDefinition可能代表一个BeanFactoryPostProcessor,因此这个过程是迭代进行的,具体逻辑是:
执行所有BeanFactoryPostProcessor,如果执行后有新的BeanFactoryPostProcessor生成,执行它们,循环进行直到没有新的BeanFactoryPostProcessor生成
-
postProcessBeanFactory()允许对现有的BeanDefinition做修改,这个是一次性全部调用
这里有一个涉及到bean生命周期的问题, 如果一个BeanFactoryPostProcessor通过@Autowired依赖其他bean, 会生效吗?
@Component
public class SimpleBeanFactoryPostProcessor implements BeanDefinitionRegistryPostProcessor {
@Autowired
private SimpleBean simpleBean;
}
不会,BeanFactoryPostProcessor初始化时还没有 BeanpostProcessor,而@Autowired又是依赖AutowiredAnnotationBeanPostProcessor进行注入,这就导致BeanFactoryPostProcessor中的依赖不会被注入,simpleBean一直为null
回归正文,这一步生成并调用前面注册的ConfigurationClassPostProcessor,它逻辑是: 查找现有的BeanDefinition(是包含SpringLifecycleApplication的BeanDefinition),对带有@Configuration注解的,使用ConfigurationClassParser进行解析处理,判断是否为满足@Configuration注解要求的逻辑如下
public static boolean isFullConfigurationCandidate(AnnotationMetadata metadata) {
return metadata.isAnnotated(Configuration.class.getName());
}
//StandardAnnotationMetadata
public boolean isAnnotated(String annotationName) {
return (this.annotations.length > 0 &&
AnnotatedElementUtils.isAnnotated(getIntrospectedClass(), annotationName));
}
//这个方法会递归寻找注解. 如SpringLifeCycleApplication只有注解@SpringBootApplication,没找到@Configuration
//就会去寻找@SpringBootApplication这个注解包含的注解,按此规律递归寻找直至找到或者递归终止,对于@SpringBootApplication
//它拥有的注解@SpringBootConfiguration拥有@Configuration注解,因此最终是符合条件的
public static boolean isAnnotated(AnnotatedElement element, String annotationName) {
return Boolean.TRUE.equals(searchWithGetSemantics(element, null, annotationName, alwaysTrueAnnotationProcessor));
}
4. ConfigurationClassParser解析启动类,获取@ComponScan注解,将解析工作委托给ComponentScanAnnotationParser
由于@SpringBootApplication是被@ComponentScan注解的,这块逻辑会解析到.实际的处理操作会委托给ComponentScanAnnotationParser,由它扫描BeanDefinition
// Process any @ComponentScan annotations
Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
if (!componentScans.isEmpty() &&
!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
for (AnnotationAttributes componentScan : componentScans) {
// ### 扫描生成一些新的BeanDefinition ###
// The config class is annotated with @ComponentScan -> perform the scan immediately
Set<BeanDefinitionHolder> scannedBeanDefinitions =
this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
// Check the set of scanned definitions for any further config classes and parse recursively if needed
for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
if (bdCand == null) {
bdCand = holder.getBeanDefinition();
}
if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
parse(bdCand.getBeanClassName(), holder.getBeanName());
}
}
}
}
5. ComponentScanAnnotationParser解析@ComponentScan的属性,获取到BasePackage再使用ClassPathBeanDefinitionScanner进行扫描
这里主要解析出BasePackage,然后将工作委托给ClassPathBeanDefinitionScanner,默认BasePackage就是被@SpringBootApplication注解的这个类所在的包位置,我们这里就是com.github.alonwang.springlifecycle
SpringBoot-bean-3.png
6. ClassPathBeanDefinitionScanner扫描BasePackage路径,获取到符合条件的Bean
SpringBoot-bean-4.png
这里会扫描BasePackage下的所有class,将所有符合条件的class封装起来,注册到容器中
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
for (String basePackage : basePackages) {
//找到包下所有符合条件的类,生成其BeanDefinition
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) {
//...
if (checkCandidate(beanName, candidate)) {
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
definitionHolder =
AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
beanDefinitions.add(definitionHolder);
// 注册到容器中
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
这里的条件由excludeFilters和includeFilters共同限定,默认includeFilter在构造ClassPathBeanDefinitionScanner时添加,包含@Component的AnnotationTypeFilter. 它也遵循Spring注解递归查找的原则, 因为@Controller,@Service也是被@Component注解修饰的,因此他们也能被扫描到
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;
}
至此@Component生效逻辑讲解完毕.
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