前言：探究springboot自动装配原理之前需要对spring有一定了解
先放结论：AutoConfigurationImportSelector.class 加载自动装配类，我们的目标是找到这个类，看它是如何加载的，网上很多文章会从启动类上的注解入手，@SpringBootApplication->@EnableAutoConfiguration->@Import(AutoConfigurationImportSelector.class) 不是说这个方法错，但这个不是追踪源码正向推导出来的，而是结论导向得出的结果。难道你没有疑问？springboot是怎么一步步加载这些注解的，怎么找到这个类的，为什么找到AutoConfigurationImportSelector类就能实现自动装配，这些疑问下面会来一一解答！
众所周知，想搞懂springboot自动装配原理最好的方式就是看源码，但看源码需要有主线，否则就会陷入源码的海洋里不能自拔，先放出路线图以便在追踪源码的过程中时刻提醒探究的主流程：
springboot自动装配原理图
- new SpringApplication(primarySources);
public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
this.resourceLoader = resourceLoader;
Assert.notNull(primarySources, "PrimarySources must not be null");
this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
this.webApplicationType = WebApplicationType.deduceFromClasspath();
// 初始化动作,重点关注getSpringFactoriesInstances方法
setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
// 加载监听器相关的类,重点关注getSpringFactoriesInstances方法
setListeners((Collection)
getSpringFactoriesInstances(ApplicationListener.class));
this.mainApplicationClass = deduceMainApplicationClass();
}
2、getSpringFactoriesInstances，很重要多个地方用到，从spring.factories中读取相关类并实例化
private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
ClassLoader classLoader = getClassLoader();
// Use names and ensure unique to protect against duplicates
// SpringFactoriesLoader.loadFactoryNames根据传入的class从spring.factories读取到相关类的全限定名
Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
// 实例化上边读取的类
List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
AnnotationAwareOrderComparator.sort(instances);
return instances;
}
3、run方法
public ConfigurableApplicationContext run(String... args) {
// 计时器 统计启动过程花了多长时间
StopWatch stopWatch = new StopWatch();
stopWatch.start();
// 声明一个空的上下文
ConfigurableApplicationContext context = null;
// 创建一个空的异常报告集合
Collection<SpringBootExceptionReporter> exceptionReporters = new ArrayList<>();
configureHeadlessProperty();
// 获取到EventPublishingRunListener 监听器实例
SpringApplicationRunListeners listeners = getRunListeners(args);
// 启动监听器
listeners.starting();
try {
ApplicationArguments applicationArguments = new DefaultApplicationArguments(args);
// 初始化一些环境信息，如jdk信息
ConfigurableEnvironment environment = prepareEnvironment(listeners, applicationArguments);
configureIgnoreBeanInfo(environment);
// 打印信息，如果需要个性化设置，可以在这里设置
Banner printedBanner = printBanner(environment);
// 创建上下文
context = createApplicationContext();
exceptionReporters = getSpringFactoriesInstances(SpringBootExceptionReporter.class,
new Class[] { ConfigurableApplicationContext.class }, context);
// 初始化上下文信息，包括environment、beanFactor、加载启动类等
prepareContext(context, environment, listeners, applicationArguments, printedBanner);
// 跳转到spring的入口
refreshContext(context);
afterRefresh(context, applicationArguments);
stopWatch.stop();
4、refresh方法，相当重要，后面会有专门的文章分析spring源码，本次不深入说明了
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
// 主要是一些set ignore add方法，所以它是设置beanFactory属性的方法
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// 包含但不限于自动装配的源码
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// 国际化相关设置
// Initialize message source for this context.
initMessageSource();
// 设置多播器，应用于观察者模式，可以理解为对象的变动，会将变动信息通知到这个对象相关的引用
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
// 注册监听器
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
5、invokeBeanFactoryPostProcessors方法，注册由BeanFactoryPostProcessors处理的bean到上下文context中
public static void invokeBeanFactoryPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
// Invoke BeanDefinitionRegistryPostProcessors first, if any.
Set<String> processedBeans = new HashSet<>();
// spring中beanFactory为DefaultListableBeanFactory的实例，而DefaultListableBeanFactory又实现了BeanDefinitionRegistry
if (beanFactory instanceof BeanDefinitionRegistry) {
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
List<BeanFactoryPostProcessor> regularPostProcessors = new ArrayList<>();
List<BeanDefinitionRegistryPostProcessor> registryProcessors = new ArrayList<>();
// 前面的步骤会拿到所有的BeanFactoryPostProcessors
for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
// ConfigurationClassPostProcessor 实现了 BeanDefinitionRegistryPostProcessor 接口，ConfigurationClassPostProcessor会处理所有BeanDefinition中的符合注解条件的BeanDefinition，（@Configuration、@Component、@ComponentScan、@Import、@ImportResource或者@Bean等主解
if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
BeanDefinitionRegistryPostProcessor registryProcessor =
(BeanDefinitionRegistryPostProcessor) postProcessor;
// BeanDefinitionRegistryPostProcessor中定义了postProcessBeanDefinitionRegistry方法
registryProcessor.postProcessBeanDefinitionRegistry(registry);
registryProcessors.add(registryProcessor);
}
else {
regularPostProcessors.add(postProcessor);
}
}
6、processConfigBeanDefinitions 方法，启动类是一个特殊的配置类，所以由ConfigurationClassParser去解析该类
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
String[] candidateNames = registry.getBeanDefinitionNames();
for (String beanName : candidateNames) {
BeanDefinition beanDef = registry.getBeanDefinition(beanName);
if (beanDef.getAttribute(ConfigurationClassUtils.CONFIGURATION_CLASS_ATTRIBUTE) != null) {
if (logger.isDebugEnabled()) {
logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
}
}
else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
}
}
// Return immediately if no @Configuration classes were found
if (configCandidates.isEmpty()) {
return;
}
// Sort by previously determined @Order value, if applicable
configCandidates.sort((bd1, bd2) -> {
int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
return Integer.compare(i1, i2);
});
// Detect any custom bean name generation strategy supplied through the enclosing application context
SingletonBeanRegistry sbr = null;
if (registry instanceof SingletonBeanRegistry) {
sbr = (SingletonBeanRegistry) registry;
if (!this.localBeanNameGeneratorSet) {
BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(
AnnotationConfigUtils.CONFIGURATION_BEAN_NAME_GENERATOR);
if (generator != null) {
this.componentScanBeanNameGenerator = generator;
this.importBeanNameGenerator = generator;
}
}
}
if (this.environment == null) {
this.environment = new StandardEnvironment();
}
// Parse each @Configuration class
//由官方注释可知，ConfigurationClassParser会解析由@Configuration注解的类，我们知道启动类上是有这个注解，这里特别说明下，我们追踪的是springboot自动装配的源码，所以只需要关心跟启动类相关的代码就行
// ConfigurationClassParser提供类parse方法，查看源码可知具体执行的逻辑在doProcessConfigurationClass里，所以分析doProcessConfigurationClass即可
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
do {
parser.parse(candidates);
parser.validate();
7、 processImports(configClass, sourceClass, getImports(sourceClass), filter, true); 该方法完成了获取AutoConfigurationImportSelector类和调用selectImport方法
private void collectImports(SourceClass sourceClass, Set<SourceClass> imports, Set<SourceClass> visited)
throws IOException {
if (visited.add(sourceClass)) {
for (SourceClass annotation : sourceClass.getAnnotations()) {
String annName = annotation.getMetadata().getClassName();
// 这里就是获取@Import的类，也就是文章开头说的AutoConfigurationImportSelector.class
if (!annName.equals(Import.class.getName())) {
collectImports(annotation, imports, visited);
}
}
imports.addAll(sourceClass.getAnnotationAttributes(Import.class.getName(), "value"));
}
}
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
Collection<SourceClass> importCandidates, Predicate<String> exclusionFilter,
boolean checkForCircularImports) {
if (importCandidates.isEmpty()) {
return;
}
if (checkForCircularImports && isChainedImportOnStack(configClass)) {
this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));
}
else {
this.importStack.push(configClass);
try {
for (SourceClass candidate : importCandidates) {
if (candidate.isAssignable(ImportSelector.class)) {
// Candidate class is an ImportSelector -> delegate to it to determine imports
Class<?> candidateClass = candidate.loadClass();
ImportSelector selector = ParserStrategyUtils.instantiateClass(candidateClass, ImportSelector.class,
this.environment, this.resourceLoader, this.registry);
Predicate<String> selectorFilter = selector.getExclusionFilter();
if (selectorFilter != null) {
exclusionFilter = exclusionFilter.or(selectorFilter);
}
if (selector instanceof DeferredImportSelector) {
this.deferredImportSelectorHandler.handle(configClass, (DeferredImportSelector) selector);
}
else {
// 调用了AutoConfigurationImportSelector类的selectImports方法
String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames, exclusionFilter);
processImports(configClass, currentSourceClass, importSourceClasses, exclusionFilter, false);
}
}
8、分析AutoConfigurationImportSelector，根据该类可以从spring.factories加载出相关类对象
public String[] selectImports(AnnotationMetadata annotationMetadata) {
if (!isEnabled(annotationMetadata)) {
return NO_IMPORTS;
}
// getAutoConfigurationEntry方法最终会调用SpringFactoriesLoader.loadFactoryNames方法，至此自动装配完成
AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(annotationMetadata);
return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}
总结( 启动类是特殊的配置类也是由@Configuration注释)：
1、prepareContext()先找到启动类加载到上下文中
2、ConfigurationClassParser.doProcessConfigurationClass方法解析由@Configuration 注解的类
3、ConfigurationClassParser.getImports方法用来获取由@Import注解的类就是我们要找的AutoConfigurationImportSelector类
4、执行AutoConfigurationImportSelector的selectImports方法将spring.factories中类加载进来
我们发现有个频繁调用的方法SpringFactoriesLoader.loadFactoryNames()，该方法就是从spring.factories读取全限定类名并实例化的方法
由于篇幅有限，不能面面俱到，spring经过这么多年的发展，源码体量已经很大了，所以阅读起来可以先捋主线再扣细节，有不对地方，请留言!
