好久没更新了，年末最后一个季度确实会比较忙，也是抽空看完了Spring的源码，这里进行一下回顾总结，现在Spring的源码实在太多，有的分析的也很细致，这里就不打算分析的太细了。还是按照之前的节奏，按照我看源码的几个点进行分析。如果有什么问题，还希望多多指教。下面开始源码分析
Spring相信大家都用的最多的框架了，基本功能也都了解，这里就做过多介绍了（别问，问就是懒～）。我们直切主题。反正我困惑的就几个点
- IoC是怎么获取bean信息，并管理bean的
IoC引以为豪的依赖注入
- IoC是怎么解决循环依赖的（没错完全是因为网上说面试爱问）
下面就针对这几个问题来看。
前期准备
环境准备
jdk:1.8
官方项目地址:https://github.com/spring-pro...
个人gitee地址：https://gitee.com/Nortyr/spri...
分支：self_note 原分支：5.2.x
测试代码
跟踪原始代码是通过传统的xml配置的方式，所以本篇以xml的方式为主，但是现在应该大多数都适用注解配置了，最后会讲解下注解和xml的异同（就是扫描的方式不同）。
传统spring写法
public class Son {
public void say(){
System.out.println("say hello!");
}
}public class Father {
private Son son;
public void say(){
son.say();
}
public void setSon(Son son) {
}
}public class Demo {
public static void main(String[] args) {
AbstractApplicationContext context = new ClassPathXmlApplicationContext("lifecycleTests2.xml",Demo.class);
Father father=(Father) context.getBean("father");
father.say();
}
}
注解版本
@Component
public class Son {
public void say(){
System.out.println("abcdefg");
}
}@Component
public class Father {
@Autowired
private Son son;
public void say(){
son.say();
System.out.println("say hello");
}
}@ComponentScan("org.springframework.test.self.annotation")
public class Demo {
public static void main(String[] args) {
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(Demo.class);
applicationContext.refresh();
Father bean = (Father) applicationContext.getBean("father");
bean.say();
}
}IoC如何获取Bean
根据mybatis的经验，肯定就是到哪个地方解析xml，并生成对应的对象，然后根据配置，生成对应的类。spring解析也差不多这样
public ClassPathXmlApplicationContext(String[] paths, Class clazz, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
Assert.notNull(paths, "Path array must not be null");
Assert.notNull(clazz, "Class argument must not be null");
this.configResources = new Resource[paths.length];
for (int i = 0; i < paths.length; i++) {
this.configResources[i] = new ClassPathResource(paths[i], clazz);
}
//初始化容器
refresh();
}下面就进入到ioc核心，refresh()方法。
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
//准备此上下文以进行刷新，各种初始化配置
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//调用refreshBeanFactory()，通过createBeanFactory构造了一个ApplicationContext(DefaultApplicationContext)
//同时启动了loadBeanDefinitions来载入BeanDefinition
//告诉子类刷新内部 bean 工厂。
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
//准备在此上下文中使用的beanFactory，配置系统默认的一些bean
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
//设置BeanFactory的后置处理
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
//调用在上下文中注册为 beanFactory 的后置处理器。
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
//注册bean的后处理器，在bean创建的过程中调用
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//初始化此上下文的消息源
initMessageSource();
// Initialize event multicaster for this context.
//为此上下文初始化事件机制
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
//初始化特定上下文子类中的其他特殊 bean。
onRefresh();
// Check for listener beans and register them.
//检查侦听bean 并注册它们。
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//实例化所有剩余的（非延迟初始化）单例。
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
//最后一步：发布相应的事件。
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.｜销毁已经创建的单例以避免悬空资源。
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...
//重置 Spring 核心中的常见内省缓存，因为我们可能不再需要单例 bean 的元数据......
resetCommonCaches();
}
}
}本次解析的过程就是在这个方法中完成ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
protected ConfigurableListableBeanFactory obtainFreshBeanFactory() {
refreshBeanFactory();
return getBeanFactory();
}AbstractRefreshableApplicationContext#refreshBeanFactory核心就是这个方法，它创建了BeanFactory，BeanFactory就可以理解为IoC容器，封装了基本的方法。在各种ApplicationContext 例如ClassPathXmlApplicationContext，`
AnnotationConfigApplicationContext`中的具体实现也是通过BeanFactory
/**
* This implementation performs an actual refresh of this context's underlying
* bean factory, shutting down the previous bean factory (if any) and
* initializing a fresh bean factory for the next phase of the context's lifecycle.
* 此实现执行此上下文的底层 bean 工厂的实际刷新，关闭先前的 bean 工厂（如果有）并为上下文生命周期的下一个阶段初始化一个新的 bean 工厂。
*/
@Override
protected final void refreshBeanFactory() throws BeansException {
if (hasBeanFactory()) {//如果有就销毁
destroyBeans();
closeBeanFactory();
}
try {
//创建DefaultListableBeanFactory
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//载入BeanDefinition信息，使用BeanDefinitionReader
loadBeanDefinitions(beanFactory);
this.beanFactory = beanFactory;
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}下面就进入bean的解析了，这里你就可以看到和MyBatis解析的差不多，创建一个对象，解析xml，最终结果都差不多，这里就不深入了，喜欢的可以看前面的万字整理MyBatis源码
@Override
protected void loadBeanDefinitions(DefaultListableBeanFactory beanFactory) throws BeansException, IOException {
// Create a new XmlBeanDefinitionReader for the given BeanFactory.｜为给定的 BeanFactory 创建一个新的 XmlBeanDefinitionReader。
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
// Configure the bean definition reader with this context's
// resource loading environment.
beanDefinitionReader.setEnvironment(this.getEnvironment());
beanDefinitionReader.setResourceLoader(this);
beanDefinitionReader.setEntityResolver(new ResourceEntityResolver(this));
// Allow a subclass to provide custom initialization of the reader,
// then proceed with actually loading the bean definitions.
initBeanDefinitionReader(beanDefinitionReader);
loadBeanDefinitions(beanDefinitionReader);
}最后在XmlBeanDefinitionReader#doLoadBeanDefinitions中解析xml
protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {
try {
Document doc = doLoadDocument(inputSource, resource);
int count = registerBeanDefinitions(doc, resource);//对BeanDefinition解析的详细过程，这个解析会使用到Spring的Bean配置规则
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + count + " bean definitions from " + resource);
}
return count;
}
...catch略
}接着就会解析各个节点，然后进入DefaultBeanDefinitionDocumentReader#processBeanDefinition进行解析
/** 处理给定的 bean 元素，解析 bean 定义并将其注册到注册表。
* Process the given bean element, parsing the bean definition
* and registering it with the registry.
*/
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
//具体处理委托给BeanDefinitionParserDelegate完成
//创建GenericBeanDefinition，设置属性，解析xml各个属性封装进BeanDefinitionHolder
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
//解析自定义标签
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// 注册解析得到的BeanDefinition
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// 在BeanDefinition向IOC容器注册完成以后，发送消息
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}这个BeanDefinitionHolder就是封装了BeanDefinition和bean名称与别名
public class BeanDefinitionHolder implements BeanMetadataElement {
private final BeanDefinition beanDefinition;
private final String beanName;
@Nullable
private final String[] aliases;
//...其余略
}BeanDefinition是个啥呢,可以看下接口规定的一些属性，这些属性基本上就是xml定义的bean标签的各种属性，相当于对应了每一个bean标签，解析过程如下（和mybatis很类似，稍微看看就行了）
解析过程也如下所示（BeanDefinitionParserDelegate#parseBeanDefinitionElement(Element, 、String, BeanDefinition)）
@Nullable
public AbstractBeanDefinition parseBeanDefinitionElement(
Element ele, String beanName, @Nullable BeanDefinition containingBean) {
this.parseState.push(new BeanEntry(beanName));
String className = null;
if (ele.hasAttribute(CLASS_ATTRIBUTE)) {
className = ele.getAttribute(CLASS_ATTRIBUTE).trim();
}
String parent = null;
if (ele.hasAttribute(PARENT_ATTRIBUTE)) {
parent = ele.getAttribute(PARENT_ATTRIBUTE);
}
try {
AbstractBeanDefinition bd = createBeanDefinition(className, parent);
//解析各个xml属性，并设置进beanDefinition
parseBeanDefinitionAttributes(ele, beanName, containingBean, bd);
bd.setDescription(DomUtils.getChildElementValueByTagName(ele, DESCRIPTION_ELEMENT));
parseMetaElements(ele, bd);
parseLookupOverrideSubElements(ele, bd.getMethodOverrides());
parseReplacedMethodSubElements(ele, bd.getMethodOverrides());
parseConstructorArgElements(ele, bd);
parsePropertyElements(ele, bd);
parseQualifierElements(ele, bd);
bd.setResource(this.readerContext.getResource());
bd.setSource(extractSource(ele));
return bd;
}
...catch略过
return null;
}
public AbstractBeanDefinition parseBeanDefinitionAttributes(Element ele, String beanName,
@Nullable BeanDefinition containingBean, AbstractBeanDefinition bd) {
if (ele.hasAttribute(SINGLETON_ATTRIBUTE)) {
error("Old 1.x 'singleton' attribute in use - upgrade to 'scope' declaration", ele);
}
else if (ele.hasAttribute(SCOPE_ATTRIBUTE)) {
bd.setScope(ele.getAttribute(SCOPE_ATTRIBUTE));
}
else if (containingBean != null) {
// Take default from containing bean in case of an inner bean definition.
bd.setScope(containingBean.getScope());
}
if (ele.hasAttribute(ABSTRACT_ATTRIBUTE)) {
bd.setAbstract(TRUE_VALUE.equals(ele.getAttribute(ABSTRACT_ATTRIBUTE)));
}
String lazyInit = ele.getAttribute(LAZY_INIT_ATTRIBUTE);
if (isDefaultValue(lazyInit)) {
lazyInit = this.defaults.getLazyInit();
}
bd.setLazyInit(TRUE_VALUE.equals(lazyInit));
String autowire = ele.getAttribute(AUTOWIRE_ATTRIBUTE);
bd.setAutowireMode(getAutowireMode(autowire));
if (ele.hasAttribute(DEPENDS_ON_ATTRIBUTE)) {
String dependsOn = ele.getAttribute(DEPENDS_ON_ATTRIBUTE);
bd.setDependsOn(StringUtils.tokenizeToStringArray(dependsOn, MULTI_VALUE_ATTRIBUTE_DELIMITERS));
}
String autowireCandidate = ele.getAttribute(AUTOWIRE_CANDIDATE_ATTRIBUTE);
if (isDefaultValue(autowireCandidate)) {
String candidatePattern = this.defaults.getAutowireCandidates();
if (candidatePattern != null) {
String[] patterns = StringUtils.commaDelimitedListToStringArray(candidatePattern);
bd.setAutowireCandidate(PatternMatchUtils.simpleMatch(patterns, beanName));
}
}
else {
bd.setAutowireCandidate(TRUE_VALUE.equals(autowireCandidate));
}
...ifelse 属性填充掠过
return bd;
}解析完成之后呢，就进入了
// 注册解析得到的BeanDefinition
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());最后会在DefaultListableBeanFactory#registerBeanDefinition注册bean。也就是把Bean和BeanDefinition对应关系添加进`
DefaultListableBeanFactory::beanDefinitionMap，beanName添加进DefaultListableBeanFactory::beanDefinitionNames `
别名和beanName的对应关系添加进SimpleAliasRegistry::aliasMap
@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
//检查是否有同名的bean
BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
if (existingDefinition != null) {
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
}
else if (existingDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (logger.isInfoEnabled()) {
logger.info("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
existingDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(existingDefinition)) {
if (logger.isDebugEnabled()) {
logger.debug("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (logger.isTraceEnabled()) {
logger.trace("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + existingDefinition +
"] with [" + beanDefinition + "]");
}
}
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// Cannot modify startup-time collection elements anymore (for stable iteration)
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
removeManualSingletonName(beanName);
}
}
else {//将bean添加进this.beanDefinitionMap，beanName添加进beanDefinitionNames
// Still in startup registration phase｜仍处于启动注册阶段
this.beanDefinitionMap.put(beanName, beanDefinition);
this.beanDefinitionNames.add(beanName);
removeManualSingletonName(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (existingDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
else if (isConfigurationFrozen()) {
clearByTypeCache();
}
} 至此，bean解析就完了（解析过程还真是千篇一律）
下面我们来看看bean解析的方式
在类上定义了@ComponentScan("org.springframework.test.self.annotation")那大概就是扫描包下的所有文件了
@ComponentScan("org.springframework.test.self.annotation")
public class Demo {
public static void main(String[] args) {
AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
applicationContext.register(Demo.class);
applicationContext.refresh();
Father bean = (Father) applicationContext.getBean("father");
bean.say();
}
}解析也不是在ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();中完了，而是在invokeBeanFactoryPostProcessors中完成
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
...略
// Invoke factory processors registered as beans in the context.
//调用在上下文中注册为 beanFactory 的后置处理器。
invokeBeanFactoryPostProcessors(beanFactory);
...略
}
} ConfigurationClassParser#doProcessConfigurationClass
@Nullable
protected final SourceClass doProcessConfigurationClass(
ConfigurationClass configClass, SourceClass sourceClass, Predicate filter)
throws IOException {
if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
// Recursively process any member (nested) classes first|首先递归处理任何成员（嵌套）类
processMemberClasses(configClass, sourceClass, filter);
}
// Process any @PropertySource annotations
for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), PropertySources.class,
org.springframework.context.annotation.PropertySource.class)) {
if (this.environment instanceof ConfigurableEnvironment) {
processPropertySource(propertySource);
}
else {
logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
"]. Reason: Environment must implement ConfigurableEnvironment");
}
}
// Process any @ComponentScan annotations
Set componentScans = AnnotationConfigUtils.attributesForRepeatable(
sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
if (!componentScans.isEmpty() &&
!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
for (AnnotationAttributes componentScan : componentScans) {
// The config class is annotated with @ComponentScan -> perform the scan immediately配置类使用 @ComponentScan 注解 -> 立即执行扫描,在这一步进行扫描
Set 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());
}
}
}
}
// Process any @Import annotations
processImports(configClass, sourceClass, getImports(sourceClass), filter, true);
// Process any @ImportResource annotations
AnnotationAttributes importResource =
AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
if (importResource != null) {
String[] resources = importResource.getStringArray("locations");
Class readerClass = importResource.getClass("reader");
for (String resource : resources) {
String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
configClass.addImportedResource(resolvedResource, readerClass);
}
}
// Process individual @Bean methods
Set beanMethods = retrieveBeanMethodMetadata(sourceClass);
for (MethodMetadata methodMetadata : beanMethods) {
configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
}
// Process default methods on interfaces
processInterfaces(configClass, sourceClass);
// Process superclass, if any
if (sourceClass.getMetadata().hasSuperClass()) {
String superclass = sourceClass.getMetadata().getSuperClassName();
if (superclass != null && !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;
} 然后就是层层解析，找到符合的class文件
private Set scanCandidateComponents(String basePackage) {
Set candidates = new LinkedHashSet<>();
try {
String packageSearchPath = ResourcePatternResolver.CLASSPATH_ALL_URL_PREFIX +
resolveBasePackage(basePackage) + '/' + this.resourcePattern;
//TODO 重要注解扫描某个包下的所有文件
Resource[] resources = getResourcePatternResolver().getResources(packageSearchPath);
boolean traceEnabled = logger.isTraceEnabled();
boolean debugEnabled = logger.isDebugEnabled();
for (Resource resource : resources) {
if (traceEnabled) {
logger.trace("Scanning " + resource);
}
try {
MetadataReader metadataReader = getMetadataReaderFactory().getMetadataReader(resource);
if (isCandidateComponent(metadataReader)) {
ScannedGenericBeanDefinition sbd = new ScannedGenericBeanDefinition(metadataReader);
sbd.setSource(resource);
if (isCandidateComponent(sbd)) {
if (debugEnabled) {
logger.debug("Identified candidate component class: " + resource);
}
candidates.add(sbd);
}
else {
if (debugEnabled) {
logger.debug("Ignored because not a concrete top-level class: " + resource);
}
}
}
else {
if (traceEnabled) {
logger.trace("Ignored because not matching any filter: " + resource);
}
}
}
catch (FileNotFoundException ex) {
if (traceEnabled) {
logger.trace("Ignored non-readable " + resource + ": " + ex.getMessage());
}
}
catch (Throwable ex) {
throw new BeanDefinitionStoreException(
"Failed to read candidate component class: " + resource, ex);
}
}
}
catch (IOException ex) {
throw new BeanDefinitionStoreException("I/O failure during classpath scanning", ex);
}
return candidates;
} 生成ScannedGenericBeanDefinition
后面就差不多一样了，把Bean和BeanDefinition对应关系添加进`
DefaultListableBeanFactory::beanDefinitionMap，beanName添加进DefaultListableBeanFactory::beanDefinitionNames `
总结
总结一下spring解析的大体流程：
寻找：
- xml：解析配置，生成对应的对象。
- 注解，找到前期注册进上下文中的类，循环找到对应的
@ComponentScan注解找到包下的所有类
- bean解析：获取到对应的配置信息，封装进BeanDefinition,(xml:
GenericBeanDefinition,注解:ScannedGenericBeanDefinition) 注册:
- Bean和BeanDefinition对应关系添加进
DefaultListableBeanFactory::beanDefinitionMap - beanName添加进
DefaultListableBeanFactory::beanDefinitionNames
- Bean和BeanDefinition对应关系添加进