本文只对IOC源码(注解实现)进行深度解析，关于IOC的注解用法等读者需查阅其他文档。
下面所有的源码解析将围绕下面测试方法，代码太多无法全部贴出来，只贴出关键代码，读者可自行进入源码一步步进入查看。
public static void main(String[] args) {
AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext(MyConfig.class);
DemoService bean = ctx.getBean(DemoService.class);
System.out.println(ctx.getBean(DemoService.class));
}
首先，将断点打在第一行代码，进入AnnotationConfigApplicationContext构造器
public AnnotationConfigApplicationContext(Class<?>... annotatedClasses) {
this();
register(annotatedClasses);
refresh();
}
1 this()方法
public AnnotationConfigApplicationContext() {
this.reader = new AnnotatedBeanDefinitionReader(this);
this.scanner = new ClassPathBeanDefinitionScanner(this);
}
1.1 AnnotatedBeanDefinitionReader
作用是将spring内部的bean定义信息(内部的后置处理器)注册到容器中,如ConfigurationClassPostProcessor、AutowiredAnnotationBeanPostProcessor的bean定义信息等
此处提一下beanDefinition和bean的区别:
beanDefinition:用于描述bean的信息，比如bean的构造方法、参数、是否懒加载等等
bean:所谓的实体类，已经初始化的对象
public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
Assert.notNull(environment, "Environment must not be null");
//bean定义注册器，用于注册所有的bean定义信息
this.registry = registry;
//condition注解的解析器，用于解析condition注解
this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
//为容器注册spring内部的后置处理器，如ConfigurationClassPostProcessor、AutowiredAnnotationBeanPostProcessor等等
AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
}
1.2 ClassPathBeanDefinitionScanner
作用是定义包扫描策略。
public ClassPathBeanDefinitionScanner(BeanDefinitionRegistry registry, boolean useDefaultFilters,
Environment environment, ResourceLoader resourceLoader) {
Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
this.registry = registry;
//是否使用默认的扫描策略,默认true
if (useDefaultFilters) {
registerDefaultFilters();
}
setEnvironment(environment);
setResourceLoader(resourceLoader);
}
useDefaultFilters:是否使用spring默认配置的扫描策略，默认是true,进入registerDefaultFilters()方法，发现可以扫描到basePackages路径参数配置下所有配置Component注解的bean。
这就是为什么在使用includeFilters扫描过滤配置时，需在后面配置useDefaultFilters=false，否则会被默认的配置覆盖掉。
2 register(annotatedClasses)方法
作用是将自己写的配置类注册到容器中
public void registerBean(Class<?> annotatedClass, String name, Class<? extends Annotation>... qualifiers) {
AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(annotatedClass);
if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
return;
}
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
abd.setScope(scopeMetadata.getScopeName());
String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
if (qualifiers != null) {
for (Class<? extends Annotation> qualifier : qualifiers) {
if (Primary.class == qualifier) {
abd.setPrimary(true);
}
else if (Lazy.class == qualifier) {
abd.setLazyInit(true);
}
else {
abd.addQualifier(new AutowireCandidateQualifier(qualifier));
}
}
}
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
}
3 refresh()方法(最为关键的一步!!!)
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 校验启动容器所需参数并创造一个保存早期事件的集合
prepareRefresh();
// 获取beanFactory容器
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 为容器注册spring底层组件
prepareBeanFactory(beanFactory);
try {
// 1 模板方法，供子类调用，对BeanFactory进行后置处理
postProcessBeanFactory(beanFactory);
// 2 执行BeanFactory后置处理器，将所有配置的bean定义信息注册到容器中
invokeBeanFactoryPostProcessors(beanFactory);
// 3 注册所有的BeanPostProcessor
registerBeanPostProcessors(beanFactory);
// 4 初始化国际化工具类MessageSource
initMessageSource();
// 5 初始化spring的事件多播器
initApplicationEventMulticaster();
// 6 开放式接口，供子类使用
onRefresh();
// 7 向容器中注册事件发布的监听器
registerListeners();
// 8 创建和初始化所有bean
finishBeanFactoryInitialization(beanFactory);
// 9 发布事件
finishRefresh();
}
catch (BeansException ex) {
if (logger.isWarnEnabled()) {
logger.warn("Exception encountered during context initialization - " +
"cancelling refresh attempt: " + ex);
}
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
finally {
// Reset common introspection caches in Spring's core, since we
// might not ever need metadata for singleton beans anymore...
resetCommonCaches();
}
}
}
3.1 postProcessBeanFactory(beanFactory)
作用是定义了一个模板方法，供子类实现调用，用来对BeanFactory进行后置处理。
3.2 invokeBeanFactoryPostProcessors(beanFactory)
作用是执行BeanFactory后置处理器(BeanDefinitionRegistryPostProcessor和BeanFactoryPostProcessor)，将自己定义的bean的定义信息注册到容器中。
附上流程图
springioc扫描bean定义源码图.png
public static void invokeBeanFactoryPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
// 将容器转换成bean定义注册器
Set<String> processedBeans = new HashSet<String>();
if (beanFactory instanceof BeanDefinitionRegistry) {
BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<BeanFactoryPostProcessor>();
List<BeanDefinitionRegistryPostProcessor> registryProcessors = new LinkedList<BeanDefinitionRegistryPostProcessor>();
for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
BeanDefinitionRegistryPostProcessor registryProcessor =
(BeanDefinitionRegistryPostProcessor) postProcessor;
registryProcessor.postProcessBeanDefinitionRegistry(registry);
registryProcessors.add(registryProcessor);
}
else {
regularPostProcessors.add(postProcessor);
}
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
// Separate between BeanDefinitionRegistryPostProcessors that implement
// PriorityOrdered, Ordered, and the rest.
List<BeanDefinitionRegistryPostProcessor> currentRegistryProcessors = new ArrayList<BeanDefinitionRegistryPostProcessor>();
// 首先，执行实现 BeanDefinitionRegistryPostProcessors 接口且实现PriorityOrdered接口的后置处理器
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
// 接着,执行实现了BeanDefinitionRegistryPostProcessors 接口且实现Ordered接口的后置处理器
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
// 然后, 执行只实现了BeanDefinitionRegistryPostProcessors的后置处理器
boolean reiterate = true;
while (reiterate) {
reiterate = false;
postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
for (String ppName : postProcessorNames) {
if (!processedBeans.contains(ppName)) {
currentRegistryProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
processedBeans.add(ppName);
reiterate = true;
}
}
sortPostProcessors(currentRegistryProcessors, beanFactory);
registryProcessors.addAll(currentRegistryProcessors);
invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
currentRegistryProcessors.clear();
}
// Now, invoke the postProcessBeanFactory callback of all processors handled so far.
invokeBeanFactoryPostProcessors(registryProcessors, beanFactory);
invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
}
else {
// Invoke factory processors registered with the context instance.
invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// 获取所有实现BeanFactoryPostProcessor接口的后置处理器
String[] postProcessorNames =
beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
List<String> orderedPostProcessorNames = new ArrayList<String>();
List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
for (String ppName : postProcessorNames) {
if (processedBeans.contains(ppName)) {
// skip - already processed in first phase above
}
else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
else {
nonOrderedPostProcessorNames.add(ppName);
}
}
// 首先, 执行实现了 BeanFactoryPostProcessors 接口且实现了PriorityOrdered接口的后置处理器
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
// 然后,执行实现了 BeanFactoryPostProcessors 接口且实现了Ordered接口的后置处理器
List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : orderedPostProcessorNames) {
orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
sortPostProcessors(orderedPostProcessors, beanFactory);
invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
// 最后, 执行只实现了 BeanFactoryPostProcessors 接口的后置处理器
List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : nonOrderedPostProcessorNames) {
nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);
// Clear cached merged bean definitions since the post-processors might have
// modified the original metadata, e.g. replacing placeholders in values...
beanFactory.clearMetadataCache();
}
简单的说就是执行所有beanFactory后置处理器，根据该后置处理器的优先顺序先后执行而已。
先执行实现了BeanDefinitionRegistryPostProcessor和PriorityOrdered接口的
再执行实现了BeanDefinitionRegistryPostProcessor和Ordered接口的
再执行实现了BeanDefinitionRegistryPostProcessor接口的
再执行实现了BeanFactoryPostProcessor和PriorityOrdered接口的
再执行实现了BeanFactoryPostProcessor和Ordered接口的
再执行实现了BeanFactoryPostProcessor接口的
关键看invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry)方法(执行后置处理器的方法)。由于ConfigurationClassPostProcessor在1中已经注册到容器中了，而且实现了BeanDefinitionRegistryPostProcessor和PriorityOrdered接口，所以优先执行此后置处理器。下面就拿ConfigurationClassPostProcessor执行后置处理器的方法源码研究，源码进入ConfigurationClassPostProcessor#processConfigBeanDefinitions
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
List<BeanDefinitionHolder> configCandidates = new ArrayList<BeanDefinitionHolder>();
String[] candidateNames = registry.getBeanDefinitionNames();
//从容器中找到所有BeanDefinitionRegistryPostProcessor，过滤出未执行的后置处理器，即MainConfig配置类的BeanDefinitionRegistryPostProcessor
for (String beanName : candidateNames) {
BeanDefinition beanDef = registry.getBeanDefinition(beanName);
if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
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;
}
// 将配置类根据order注解进行排序
Collections.sort(configCandidates, new Comparator<BeanDefinitionHolder>() {
@Override
public int compare(BeanDefinitionHolder bd1, BeanDefinitionHolder bd2) {
int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
return (i1 < i2) ? -1 : (i1 > i2) ? 1 : 0;
}
});
// 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 && sbr.containsSingleton(CONFIGURATION_BEAN_NAME_GENERATOR)) {
BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
//componentScan扫描出来的bean的名字生成器
this.componentScanBeanNameGenerator = generator;
//import导入的bean的名字生成器
this.importBeanNameGenerator = generator;
}
}
// 创建配置类的解析器
ConfigurationClassParser parser = new ConfigurationClassParser(
this.metadataReaderFactory, this.problemReporter, this.environment,
this.resourceLoader, this.componentScanBeanNameGenerator, registry);
Set<BeanDefinitionHolder> candidates = new LinkedHashSet<BeanDefinitionHolder>(configCandidates);
Set<ConfigurationClass> alreadyParsed = new HashSet<ConfigurationClass>(configCandidates.size());
do {
//解析配置类
parser.parse(candidates);
parser.validate();
Set<ConfigurationClass> configClasses = new LinkedHashSet<ConfigurationClass>(parser.getConfigurationClasses());
configClasses.removeAll(alreadyParsed);
// Read the model and create bean definitions based on its content
if (this.reader == null) {
this.reader = new ConfigurationClassBeanDefinitionReader(
registry, this.sourceExtractor, this.resourceLoader, this.environment,
this.importBeanNameGenerator, parser.getImportRegistry());
}
//将除了@Component注解注册的bean外的其他方式注册的bean定义注册到容器中
this.reader.loadBeanDefinitions(configClasses);
alreadyParsed.addAll(configClasses);
candidates.clear();
//校验是否所有的bean定义信息已经注册完毕
if (registry.getBeanDefinitionCount() > candidateNames.length) {
String[] newCandidateNames = registry.getBeanDefinitionNames();
Set<String> oldCandidateNames = new HashSet<String>(Arrays.asList(candidateNames));
Set<String> alreadyParsedClasses = new HashSet<String>();
for (ConfigurationClass configurationClass : alreadyParsed) {
alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
}
for (String candidateName : newCandidateNames) {
if (!oldCandidateNames.contains(candidateName)) {
BeanDefinition bd = registry.getBeanDefinition(candidateName);
if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
!alreadyParsedClasses.contains(bd.getBeanClassName())) {
candidates.add(new BeanDefinitionHolder(bd, candidateName));
}
}
}
candidateNames = newCandidateNames;
}
}
while (!candidates.isEmpty());
// Register the ImportRegistry as a bean in order to support ImportAware @Configuration classes
if (sbr != null) {
if (!sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
}
}
if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();
}
}
从容器中找到自己写的配置类，对其进行parse解析。
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
throws IOException {
// Recursively process any member (nested) classes first
processMemberClasses(configClass, sourceClass);
// 解析@PropertySource 注解
for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), PropertySources.class,
org.springframework.context.annotation.PropertySource.class)) {
if (this.environment instanceof ConfigurableEnvironment) {
processPropertySource(propertySource);
}
else {
logger.warn("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
"]. Reason: Environment must implement ConfigurableEnvironment");
}
}
// 解析@ComponentScan 注解
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) {
// 解析
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());
}
}
}
}
// 解析 @Import 注解
processImports(configClass, sourceClass, getImports(sourceClass), true);
// Process any @ImportResource annotations
if (sourceClass.getMetadata().isAnnotated(ImportResource.class.getName())) {
AnnotationAttributes importResource =
AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
String[] resources = importResource.getStringArray("locations");
Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
for (String resource : resources) {
String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
configClass.addImportedResource(resolvedResource, readerClass);
}
}
// 解析 @Bean 注解
Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}
// 处理接口
processInterfaces(configClass, sourceClass);
// 处理超类
if (sourceClass.getMetadata().hasSuperClass()) {
String superclass = sourceClass.getMetadata().getSuperClassName();
if (!superclass.startsWith("java") && !this.knownSuperclasses.containsKey(superclass)) {
this.knownSuperclasses.put(superclass, configClass);
// Superclass found, return its annotation metadata and recurse
return sourceClass.getSuperClass();
}
}
// No superclass -> processing is complete
return null;
}
依此解析@PropertySource@ComponentScan@Import@ImportResource@Bean五个注解、接口、超类。由于常用@ComponentScan@Import两个注解，下面只对解析@ComponentScan@Import的源码分析
-1 解析@ComponentScan
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
Assert.notEmpty(basePackages, "At least one base package must be specified");
Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<BeanDefinitionHolder>();
//遍历配置的basepackage路径
for (String basePackage : basePackages) {
//找到路径下所有Componet注解的组件
Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
for (BeanDefinition candidate : candidates) {
ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
candidate.setScope(scopeMetadata.getScopeName());
String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
if (candidate instanceof AbstractBeanDefinition) {
postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
}
if (candidate instanceof AnnotatedBeanDefinition) {
AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
}
if (checkCandidate(beanName, candidate)) {
BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
definitionHolder =
AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
beanDefinitions.add(definitionHolder);
//注册该当前组件的bean定义信息到容器中
registerBeanDefinition(definitionHolder, this.registry);
}
}
}
return beanDefinitions;
}
遍历basePackage配置的路径数组，找到路径下的@Component注解注入的bean组件，将该组件的bean定义注册到容器中。注意:这里只是注册bean定义信息，并不是创建bean。
-2 解析@Import
private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,
Collection<SourceClass> importCandidates, 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) {
//判断该组件是否实现了ImportSelector
if (candidate.isAssignable(ImportSelector.class)) {
// Candidate class is an ImportSelector -> delegate to it to determine imports
Class<?> candidateClass = candidate.loadClass();
ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);
ParserStrategyUtils.invokeAwareMethods(
selector, this.environment, this.resourceLoader, this.registry);
if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) {
this.deferredImportSelectors.add(
new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));
}
//调用selectImports方法
else {
String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());
Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);
processImports(configClass, currentSourceClass, importSourceClasses, false);
}
}
////判断该组件是否实现了ImportBeanDefinitionRegistrar
else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {
// Candidate class is an ImportBeanDefinitionRegistrar ->
// delegate to it to register additional bean definitions
Class<?> candidateClass = candidate.loadClass();
ImportBeanDefinitionRegistrar registrar =
BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);
ParserStrategyUtils.invokeAwareMethods(
registrar, this.environment, this.resourceLoader, this.registry);
//将自己定义的bean定义添加到注册器中 configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());
}
else {
// Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->
// 添加普通的import注解配置的bean定义信息到容器中
this.importStack.registerImport(
currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());
processConfigurationClass(candidate.asConfigClass(configClass));
}
}
}
catch (BeanDefinitionStoreException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to process import candidates for configuration class [" +
configClass.getMetadata().getClassName() + "]", ex);
}
finally {
this.importStack.pop();
}
}
}
处理@Import注解时，并没有直接将组件注册到容器中，只是添加进去。
@Import注解配置bean有三种方式。
方法1:@Import(value = {Person.class, Car.class})
方法2:@Import(value = {MyImportSelector.class})，MyImportSelector必须实现ImportSelector接口，重写selectImports方法，返回一个bean类路径的数组。
public class MyImportSelector implements ImportSelector {
@Override
public String[] selectImports(AnnotationMetadata importingClassMetadata) {
return new String[]{"com.testimport.compent.Dog"};
}
}
方法3:@Import(value = {MyBeanDefinitionRegister.class}),MyBeanDefinitionRegister必须实现ImportBeanDefinitionRegistrar接口，重写registerBeanDefinitions方法，在方法中注册bean定义信息
public class MyBeanDefinitionRegister implements ImportBeanDefinitionRegistrar {
@Override
public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
RootBeanDefinition rootBeanDefinition = new RootBeanDefinition(Cat.class);
registry.registerBeanDefinition("cat",rootBeanDefinition);
}
}
-3 解析@Bean注解
// Process individual @Bean methods
Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}
和@Import注解一样，只是把配置类中@Bean注解的方法加进去，并没有将该组件的bean定义注册到容器中。
解析完所有组件，回到ConfigurationClassPostProcessor#processConfigBeanDefinitions方法的parse那行，此时@Component注解注册的bean定义信息已经注册到容器中，但是@Import、@Bean注解等其他方式注册的bean定义信息只是添加保存到各自集合中，并没有注册。
接着，进入下面this.reader.loadBeanDefinitions(configClasses);那行，这里将注册其他方式的bean定义信息，进入源码。
private void loadBeanDefinitionsForConfigurationClass(
ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {
if (trackedConditionEvaluator.shouldSkip(configClass)) {
String beanName = configClass.getBeanName();
if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
this.registry.removeBeanDefinition(beanName);
}
this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
return;
}
//将@import注解注册的bean定义信息注册到容器中
if (configClass.isImported()) {
registerBeanDefinitionForImportedConfigurationClass(configClass);
}
//将@Bean注解注册的bean定义信息注册到容器中
for (BeanMethod beanMethod : configClass.getBeanMethods()) {
loadBeanDefinitionsForBeanMethod(beanMethod);
}
//将@ImportedResources注解注册的bean定义信息注册到容器中 loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
//将实现BeanDefinitionRegistryPostProcessor接口注册的bean定义信息注册到容器中 loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
}
此时，@Import、@Bean注解等注册的bean定义信息已全部注册到容器中
3.3 registerBeanPostProcessors(beanFactory)
作用是将所有的bean后置处理器注册到容器中。
public static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, AbstractApplicationContext applicationContext) {
//获取所有beanPostProcessor（bean定义）
String[] postProcessorNames = beanFactory.getBeanNamesForType(BeanPostProcessor.class, true, false);
// 获取所有beanPostProcessor的数量
int beanProcessorTargetCount = beanFactory.getBeanPostProcessorCount() + 1 + postProcessorNames.length;
beanFactory.addBeanPostProcessor(new BeanPostProcessorChecker(beanFactory, beanProcessorTargetCount));
// 简单的划分，将后置处理器划分到实现PriorityOrdered接口、Ordered接口、其他普通的集合中
List<BeanPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
List<BeanPostProcessor> internalPostProcessors = new ArrayList<BeanPostProcessor>();
List<String> orderedPostProcessorNames = new ArrayList<String>();
List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
for (String ppName : postProcessorNames) {
if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
priorityOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
orderedPostProcessorNames.add(ppName);
}
else {
nonOrderedPostProcessorNames.add(ppName);
}
}
// 注册实现PriorityOrdered接口的bean后置处理器
sortPostProcessors(priorityOrderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, priorityOrderedPostProcessors);
// 注册实现Ordered接口的bean后置处理器
List<BeanPostProcessor> orderedPostProcessors = new ArrayList<BeanPostProcessor>();
for (String ppName : orderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
orderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
sortPostProcessors(orderedPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, orderedPostProcessors);
// 注册其他没有实现排序的后置处理器
List<BeanPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanPostProcessor>();
for (String ppName : nonOrderedPostProcessorNames) {
BeanPostProcessor pp = beanFactory.getBean(ppName, BeanPostProcessor.class);
nonOrderedPostProcessors.add(pp);
if (pp instanceof MergedBeanDefinitionPostProcessor) {
internalPostProcessors.add(pp);
}
}
registerBeanPostProcessors(beanFactory, nonOrderedPostProcessors);
// 注册spring内部的后置处理器
sortPostProcessors(internalPostProcessors, beanFactory);
registerBeanPostProcessors(beanFactory, internalPostProcessors);
// Re-register post-processor for detecting inner beans as ApplicationListeners,
// moving it to the end of the processor chain (for picking up proxies etc).
beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(applicationContext));
}
依此注册实现PriorityOrdered接口、实现Ordered接口、未实现任何的、spring内部的bean后置处理器。
3.4 initMessageSource()
作用是初始化一个工具类，该工具类用来处理国际化。
3.5 initApplicationEventMulticaster()
作用是初始化spring事件多播器，此多播器用于向监听器广播事件，使所有监听器(spring内部的监听器和自己定义的实现ApplicationListener的监听器)，执行onApplicationEvent(ApplicationEvent event)方法。
protected void initApplicationEventMulticaster() {
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
//APPLICATION_EVENT_MULTICASTER_BEAN_NAME值是applicationEventMulticaster
if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
this.applicationEventMulticaster =
beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
if (logger.isDebugEnabled()) {
logger.debug("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
}
}
else {
this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
if (logger.isDebugEnabled()) {
logger.debug("Unable to locate ApplicationEventMulticaster with name '" +
APPLICATION_EVENT_MULTICASTER_BEAN_NAME +
"': using default [" + this.applicationEventMulticaster + "]");
}
}
}
判断容器中是否有beanName是applicationEventMulticaster的多播器，如果有就获取，如果没有就向容器中注册一个beanName为applicationEventMulticaster的多播器。
3.6 onRefresh()
这是一个空方法，一个开放式接口供子类调用。在springBoot启动tomcat时会调用这个方法。这里不做详述。
3.7 registerListeners()
作用是向容器中注册监听器。此处只是注册监听器，并向监听器广播早期事件；在后面3.9 finishRefresh()中调用publishEvent(new ContextRefreshedEvent(this))，向所有监听器广播所有事件，包括自己定义发布的事件。
protected void registerListeners() {
// 获取3.5注册的多播器，向多播器中注册spring内部的监听器
for (ApplicationListener<?> listener : getApplicationListeners()) {
getApplicationEventMulticaster().addApplicationListener(listener);
}
// 向多播器中注册自己定义的实现ApplicationListener的监听器
String[] listenerBeanNames = getBeanNamesForType(ApplicationListener.class, true, false);
for (String listenerBeanName : listenerBeanNames) {
getApplicationEventMulticaster().addApplicationListenerBean(listenerBeanName);
}
// 广播早期事件
Set<ApplicationEvent> earlyEventsToProcess = this.earlyApplicationEvents;
this.earlyApplicationEvents = null;
if (earlyEventsToProcess != null) {
for (ApplicationEvent earlyEvent : earlyEventsToProcess) {
getApplicationEventMulticaster().multicastEvent(earlyEvent);
}
}
}
public void multicastEvent(final ApplicationEvent event, ResolvableType eventType) {
ResolvableType type = (eventType != null ? eventType : resolveDefaultEventType(event));
for (final ApplicationListener<?> listener : getApplicationListeners(event, type)) {
//获取executor，如果支持异步调用，异步广播事件，不支持就同步广播事件
Executor executor = getTaskExecutor();
if (executor != null) {
executor.execute(new Runnable() {
@Override
public void run() {
invokeListener(listener, event);
}
});
}
else {
invokeListener(listener, event);
}
}
}
private void doInvokeListener(ApplicationListener listener, ApplicationEvent event) {
try {
//调用onApplicationEvent方法，执行方法的业务代码
listener.onApplicationEvent(event);
}
catch (ClassCastException ex) {
String msg = ex.getMessage();
if (msg == null || matchesClassCastMessage(msg, event.getClass())) {
// Possibly a lambda-defined listener which we could not resolve the generic event type for
// -> let's suppress the exception and just log a debug message.
Log logger = LogFactory.getLog(getClass());
if (logger.isDebugEnabled()) {
logger.debug("Non-matching event type for listener: " + listener, ex);
}
}
else {
throw ex;
}
}
}
3.5和3.7的spring事件驱动模型，用到了观察者模式，观察者模式可以简单的理解为发布-订阅的模式，详细内容可查阅其他资料。
如果想支持异步广播事件，可以手动写个类继承SimpleApplicationEventMulticaster，并取名为容器内默认的多播器名称applicationEventMulticaster，此时容器在多播器初始化的时候就能找到我们自己建的这个多播器，并将多线程设置进去，实现异步方式广播。
@Component(value = "applicationEventMulticaster")
public class MyMulticaster extends SimpleApplicationEventMulticaster{
public MyMulticaster () {
setTaskExecutor(Executors.newSingleThreadExecutor());
}
}
3.8 inishBeanFactoryInitialization(beanFactory)
创建以及初始化bean
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(new StringValueResolver() {
@Override
public String resolveStringValue(String strVal) {
return getEnvironment().resolvePlaceholders(strVal);
}
});
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// 设置类加载器到spring容器中
beanFactory.setTempClassLoader(null);
// 冻结配置，到这边为止，之后不能再修改bean定义信息
beanFactory.freezeConfiguration();
// 开始创建和初始化bean
beanFactory.preInstantiateSingletons();
}
public void preInstantiateSingletons() throws BeansException {
if (logger.isDebugEnabled()) {
logger.debug("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<String>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//判断是否是FactoryBean
if (isFactoryBean(beanName)) {
final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
@Override
public Boolean run() {
return ((SmartFactoryBean<?>) factory).isEagerInit();
}
}, getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
else {
//开始创建bean
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
smartSingleton.afterSingletonsInstantiated();
return null;
}
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
调用AbstractBeanFactory的getBean方法开始创建bean，具体创建流程在4中会说
3.9 finishRefresh()
作用是完成容器刷新后的后续工作。清除缓存、发布事件、初始化并刷新生命周期处理器。
protected void finishRefresh() {
// 初始化上下文生命周期处理器
initLifecycleProcessor();
// 刷新上下文生命周期处理器
getLifecycleProcessor().onRefresh();
// 发布事件
publishEvent(new ContextRefreshedEvent(this));
// 清除上下文资源缓存
LiveBeansView.registerApplicationContext(this);
}
4 getBean()解析
4.1 bean的生命周期
下面我们解析bean的生命周期，直接进入关键方法doGetBean
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
//获取别名，
final String beanName = transformedBeanName(name);
Object bean;
// 从单例缓存池里获取bean
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 判断是否是多例的bean在创建，如果是就抛错
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 检查父工厂里有没有bean
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//根据别名获取bean定义
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
//检查是否是抽象bean，抽象bean不能被实例化
checkMergedBeanDefinition(mbd, beanName, args);
// 检查是否依赖其他的bean，如果存在，就先创建依赖对象
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
//创建bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
总的来说就是先从单例缓存池里获取，如果有就直接获取到bean返回。如果没有，再从父工厂里获取。如果再没有，就自己创建一个，创建完后放入单例缓存池。
注:多例不走缓存池。
进入创建bean源码
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
//设置当前bean正在创建。在单例缓存池的判断中会用到
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
//回调singletonFactory参数的getObject方法
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
//将当前对象移除正在创建的状态
afterSingletonCreation(beanName);
}
if (newSingleton) {
//将创建的bean对象放入单例缓存池中
addSingleton(beanName, singletonObject);
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
进入回调的getObject方法，进入createBean
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 创建一个bean的代理对象,aop中用到，这里只是将切面加到缓存中
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
//创建bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//利用反射创建bean实例
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// 将早期对象放入单例缓存池
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
//初始化bean
Object exposedObject = bean;
try {
//给bean赋值
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
//初始化bean
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// 注册销毁bean的后置处理器
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
将早期对象加入到三级缓存中，以便循环依赖获取此早期对象
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
//调用Aware接口
invokeAwareMethods(beanName, bean);
return null;
}
}, getAccessControlContext());
}
else {
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
//调用实现了BeanPostProcessor接口的postProcessBeforeInitialization方法
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
//调用初始化方法
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
//调用实现了BeanPostProcessor接口的postProcessAfterInitialization方法(aop中使用)，这里会创建代理对象
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}
初始化bean流程就是先调用(如果实现了Aware接口)实现Aware接口的bean的方法，然后调用(如果实现了BeanPostProcessor接口)实现BeanPostProcessor接口的postProcessBeforeInitialization方法，然后调用初始化方法，然后调用(如果实现了BeanPostProcessor接口)实现BeanPostProcessor接口的postProcessAfterInitialization方法。
这里说下bean初始化的多种方法。
方法1:自己定义初始化方法，在注入容器的时候指定初始化方法
@Bean(initMethod = "init",destroyMethod = "destroy")
public Car car() {
return new Car();
}
方法2:通过 InitializingBean和DisposableBean 的二个接口实现bean的初始化化以及销毁方法。
@Component
public class Person implements InitializingBean,DisposableBean {
public Person() {
System.out.println("Person的构造方法");
}
@Override
public void destroy() throws Exception {
System.out.println("DisposableBean的destroy()方法 ");
}
@Override
public void afterPropertiesSet() throws Exception {
System.out.println("InitializingBean的 afterPropertiesSet方法");
}
}
方法3:通过JSR250规范提供的注解@PostConstruct 和@ProDestory标注的方法
@Component
public class Book {
public Book() {
System.out.println("book 的构造方法");
}
@PostConstruct
public void init() {
System.out.println("book 的PostConstruct标志的方法");
}
@PreDestroy
public void destory() {
System.out.println("book 的PreDestory标注的方法");
}
}
方法4:通过Spring的BeanPostProcessor的 bean的后置处理器会拦截所有bean创建过程。源码中可见postProcessBeforeInitialization在init前调用，postProcessAfterInitialization在初始化后调用。
附上bean生命周期的图
spring-getbean草图.png
4.2 popularBean方法与循环依赖解决
protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
PropertyValues pvs = mbd.getPropertyValues();
if (bw == null) {
if (!pvs.isEmpty()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// 定义了一个开放接口InstantiationAwareBeanPostProcessor供我们调用，如果实现了该接口，此处会调用该bean的before和after方法
boolean continueWithPropertyPopulation = true;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
//获取注入模式。
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// 根据name注入
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// 根据type注入
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
//给属性赋值
applyPropertyValues(beanName, mbd, bw, pvs);
}
populateBean方法主要就是给bean所有属性赋值。
protected void autowireByName(
String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
//获取非简单属性，即bean属性
String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
for (String propertyName : propertyNames) {
if (containsBean(propertyName)) {
Object bean = getBean(propertyName);
pvs.add(propertyName, bean);
registerDependentBean(propertyName, beanName);
if (logger.isDebugEnabled()) {
logger.debug("Added autowiring by name from bean name '" + beanName +
"' via property '" + propertyName + "' to bean named '" + propertyName + "'");
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
"' by name: no matching bean found");
}
}
}
}
autowireByName方式注入。这里只是将属性设置到缓存中，在后面进行赋值
protected void autowireByType(
String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
Set<String> autowiredBeanNames = new LinkedHashSet<String>(4);
String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
for (String propertyName : propertyNames) {
try {
PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName);
// Don't try autowiring by type for type Object: never makes sense,
// even if it technically is a unsatisfied, non-simple property.
if (Object.class != pd.getPropertyType()) {
MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd);
// Do not allow eager init for type matching in case of a prioritized post-processor.
boolean eager = !PriorityOrdered.class.isAssignableFrom(bw.getWrappedClass());
DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager);
//解析非简单属性
Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter);
if (autowiredArgument != null) {
pvs.add(propertyName, autowiredArgument);
}
for (String autowiredBeanName : autowiredBeanNames) {
registerDependentBean(autowiredBeanName, beanName);
if (logger.isDebugEnabled()) {
logger.debug("Autowiring by type from bean name '" + beanName + "' via property '" +
propertyName + "' to bean named '" + autowiredBeanName + "'");
}
}
autowiredBeanNames.clear();
}
}
catch (BeansException ex) {
throw new UnsatisfiedDependencyException(mbd.getResourceDescription(), beanName, propertyName, ex);
}
}
}
public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}
Class<?> type = descriptor.getDependencyType();
//解析@Value注解，获取值
Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
if (value != null) {
if (value instanceof String) {
String strVal = resolveEmbeddedValue((String) value);
BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
value = evaluateBeanDefinitionString(strVal, bd);
}
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
return (descriptor.getField() != null ?
converter.convertIfNecessary(value, type, descriptor.getField()) :
converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
}
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
//获取容器中有几个bean
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
//判断数量是否大于一个
if (matchingBeans.size() > 1) {
//决定使用哪个，根据哪个标注了@Primary或@Priority注解就用哪个
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
//如果不能决定用哪个，则判断是否是必要的，如果是必要的就抛错，如果不是必要的就返回空
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(type, matchingBeans);
}
else {
// In case of an optional Collection/Map, silently ignore a non-unique case:
// possibly it was meant to be an empty collection of multiple regular beans
// (before 4.3 in particular when we didn't even look for collection beans).
return null;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}
if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
return (instanceCandidate instanceof Class ?
descriptor.resolveCandidate(autowiredBeanName, type, this) : instanceCandidate);
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}
autowireByType需要判断是否注入多个相同的bean，如果存在多个，先判断哪个上面标了@Primary或@Priority注解，就使用哪个bean。如果没有标注，则判断是否是必要的，在注入属性的时候标注@Autowired(required = false)就不是必要的，返回null，没有标注required,默认是true，会抛异常
下面进入赋值阶段
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
if (pvs == null || pvs.isEmpty()) {
return;
}
if (System.getSecurityManager() != null && bw instanceof BeanWrapperImpl) {
((BeanWrapperImpl) bw).setSecurityContext(getAccessControlContext());
}
MutablePropertyValues mpvs = null;
List<PropertyValue> original;
if (pvs instanceof MutablePropertyValues) {
mpvs = (MutablePropertyValues) pvs;
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
try {
bw.setPropertyValues(mpvs);
return;
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
original = mpvs.getPropertyValueList();
}
else {
original = Arrays.asList(pvs.getPropertyValues());
}
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
List<PropertyValue> deepCopy = new ArrayList<PropertyValue>(original.size());
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
String propertyName = pv.getName();
Object originalValue = pv.getValue();
//解析依赖
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
if (resolvedValue == originalValue) {
if (convertible) {
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
else if (convertible && originalValue instanceof TypedStringValue &&
!((TypedStringValue) originalValue).isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
mpvs.setConverted();
}
// 这里才是给属性赋值
try {
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
下面是解析依赖的代码，如果是非简单对象，会去获取该对象的bean实例
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
String refName = ref.getBeanName();
refName = String.valueOf(doEvaluate(refName));
if (ref.isToParent()) {
if (this.beanFactory.getParentBeanFactory() == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Can't resolve reference to bean '" + refName +
"' in parent factory: no parent factory available");
}
return this.beanFactory.getParentBeanFactory().getBean(refName);
}
else {
//获取依赖对象
Object bean = this.beanFactory.getBean(refName);
this.beanFactory.registerDependentBean(refName, this.beanName);
return bean;
}
}
catch (BeansException ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
}
}
这里会存在一个循环依赖的场景。
<bean id="instanceA" class="com.circulardependencies.InstanceA" >
<property name="instanceB" ref="instanceB"></property>
</bean>
<bean id="instanceB" class="com.circulardependencies.InstanceB" >
<property name="instanceA" ref="instanceA"></property>
</bean>
spring通过三级缓存解决了bean循环依赖的问题。下面就是原理。
-1 首先创建instanceA对象。根据getBean流程，中间会有一块将早期对象设置到三级缓存中。在AbstractAutowireCapableBeanFactory的doCreateBean方法中。
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
-2 接着就是调用populateBean方法，给instanceA属性赋值，即给instanceB赋值，发现instanceB还未创建，此时需先创建instanceB对象，同样的调用getBean方法，一直到创建完instanceB的早期对象，然后加到三级缓存中。
-3 接着调用instanceB的populateBean方法，给属性赋值，即给instanceA赋值，此时又需要重新调用getBean方法获取或创建instanceA。
-4 又进入instanceA的getBean。我们在4.1中知道getBean都会先到单例缓存池缓存中拿。在AbstractBeanFactory的doGetBean的方法中。
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
进入getSIngleton。
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//从一级缓存中获取bean，一般一级缓存存放的都是初始化后的完整的bean
Object singletonObject = this.singletonObjects.get(beanName);
// isSingletonCurrentlyInCreation,在创建的时候会将其设置为true，表示对象正在创建中
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//从三级缓存中取出之前暴露的早期对象。三级缓存放早期对象
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
这里解释下三级缓存。
一级缓存:singletonObject
二级缓存:earlySingletonObjects
三级缓存:singletonFactories
由于一开始在创建instanceA的时候，已经将早期对象放到三级缓存中，所以这里可以直接获取到，返回instanceA。
-5 此时instanceB的属性instanceA已经赋值完成，返回，按照4.1中创建bean流程，将instanceB创建完成。
-6 接着返回到第二步，将创建好的instanceB赋值到instanceA中依赖的instanceB属性。再走完剩下的初始化流程，完成instanceA的创建。
我们可以发现，如果没有缓存，那么A和B将无限循环创建，报错。所以我们可以得出以下两个结论。
-1 多例的bean无法解决循环依赖的问题。因为多例不走缓存。
-2 构造器方式注入bean无法解决循环依赖问题。因为从4.1中看出，构造器方式注入的bean，在暴露早期对象之前就已经通过构造器方式创建bean实例，发现需要依赖instanceB对象，并没有进入到下面的将instanceA的早期对象实例放入缓存的那一步。
@Bean
public InstanceA instanceA(InstanceB instanceB){
return new InstanceA(instanceB);
}
@Bean
public InstanceB instanceB(InstanceA instanceA) {
return new InstanceB(instanceA);
}
上面就是构造器方式注入bean，无法解决循环依赖。
IOC的源码解析介绍到此结束，如有分析不到位，敬请谅解!!!
