Spring Bean生命周期

本文基于图灵课堂周瑜老师的讲解整理，包括spring bean加载的过程，主要是扫描BeanDefinition以及初始化非懒加载单例Bean两部分，源码取自SpringFramework 5.3.22

1. Bean扫描

本小节介绍的是Spring从给定的扫描位置扫描到待加载的Bean，生成BeanDefinitionMap的过程

SpringBoot启动过程中使用的ApplicationContext是AnnotationConfigApplicationContext，而它初始化的时候会顺带初始化两个BeanDefinitionReader：AnnotatedBeanDefinitionReader和ClassPathBeanDefinitionScanner，前者是可以直接通过给定的class注册Bean，后者则可以扫描给定目录下所有的目标Bean，下面着重介绍后者扫描Bean组件的过程。

ClassPathBeanDefinitionScanner扫描的入口是public int scan(String… basePackages)，而真正做事情的是protected Set<BeanDefinitionHolder> doScan(String… basePackages)方法，代码如下:

/**
     * Perform a scan within the specified base packages,
     * returning the registered bean definitions.
     * <p>This method does <i>not</i> register an annotation config processor
     * but rather leaves this up to the caller.
     * @param basePackages the packages to check for annotated classes
     * @return set of beans registered if any for tooling registration purposes (never {@code null})
     */
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basePackage下的所有候选组件，这里只是把classname赋值给了BeanDefinition
            Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
            for (BeanDefinition candidate : candidates) {
                // 获取组件的scope，默认是单例的
                ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
                candidate.setScope(scopeMetadata.getScopeName());
                // 获取beanname
                String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
                if (candidate instanceof AbstractBeanDefinition) {
                    postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
                }
                // 处理通用的注解
                if (candidate instanceof AnnotatedBeanDefinition) {
                    AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
                }
                // 最后再对BeanDefinition做检查，看下是否有重名的
                if (checkCandidate(beanName, candidate)) {
                    BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
                    definitionHolder =
                            AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
                    beanDefinitions.add(definitionHolder);
                    registerBeanDefinition(definitionHolder, this.registry);
                }
            }
        }
        return beanDefinitions;
    }