深入理解Spring IOC源码分析

Spring容器初始化

本文使用的是Spring 5.1.7版本

写在前面：我们看源码一般有3种方式。

• 第一种直接用class文件，IDEA会帮我们反编译成看得懂的java代码
• 第二种是用maven的download Sources
• 第三种是直接下载源码编译

如果随便看看，那么第二种就行了。如果想仔细研究，可以选第三种，这样可以给代码加注释，或者修改一些东西等等。

Spring源码构建

• 下载源码（github）
• 安装gradle
• 导入项目
• 编译工程（顺序：core->xom->context->beans->aspects->aop）
• 编译方法：工程->tasks->compileTestJava

• 编译完成后，新建一个测试module，用gradle的方式，build.gradle为：

group 'org.springframework'
version '5.1.21.BUILD-SNAPSHOT'

apply plugin: 'java'

sourceCompatibility = 1.8

repositories {
mavenCentral()
}

dependencies {
compile(project(":spring-context"))
testCompile group: 'junit', name: 'junit', version: '4.12'
}

如果运行报错，需要设置IDEA。 需要将Preferences > Build, Execution, Deployment > Build Tools > Gradle > Runner里的 Delegate IDE build/run actions to gradle勾选上。

Spring IOC容器初始化的关键环节就在AbstractApplicationContext的refresh方法中。

public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
//第一步：刷新前的预处理
prepareRefresh();

// 第二步：获取BeanFactory；加载BeanDefition并注册到BeanDefitionRegistry
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

// 第三步：BeanFactory的预准备工作
prepareBeanFactory(beanFactory);

try {
// 第四步：BeanFactory准备工作完成之后进行的后置处理
postProcessBeanFactory(beanFactory);

// 第五步：实例化并调用实现了BeanFactoryPostProcessor接口的Bean
invokeBeanFactoryPostProcessors(beanFactory);

// 第六步：注册BeanPostProcessor，在创建bean的前后等执行。
registerBeanPostProcessors(beanFactory);

// 第七步：初始化MessageSource组件（做国际化功能；消息绑定，消息解析）
initMessageSource();

// 第八步：初始化事件派发器
initApplicationEventMulticaster();

//第九步：提供给子类重写，容器刷新的适合可以自定义逻辑
onRefresh();

// 第十步：注册应用监听器。就是注册实现了ApplicationListener接口的监听器bean
registerListeners();

// 第十一步：初始化所有剩下的非懒加载的单例bean，
//填充属性
//初始化方法调用
//调用BeanPostProcessor对bean进行后置处理
finishBeanFactoryInitialization(beanFactory);

// 第十二步：完成context的刷新。主要是调用LifecycleProcessor的onRefresh方法，并且发布ContextRefreshedEvent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();
}
}
}

接下来我们分析第二步中的两个子流程：

2.1. 获取BeanFactory子流程

时序图如下：

2.2. BeanDefinition加载解析及注册⼦流程

BeanDefinition加载流程入口在AbstractRefreshableApplicationContext#refreshBeanFactory ⽅法中，会调用到loadBeanDefinitions方法，这个方法最终会由XmlBeanDefinitionReader的loadBeanDefinitions方法来完成。然后会调用它自己的doLoadBeanDefinitions方法。

protected int doLoadBeanDefinitions(InputSource inputSource, Resource resource)
throws BeanDefinitionStoreException {

try {
//读取xml信息，将xml信息保存到Document中
Document doc = doLoadDocument(inputSource, resource);
//解析doc，封装到BeanDefinition对象并进行注册
int count = registerBeanDefinitions(doc, resource);
if (logger.isDebugEnabled()) {
logger.debug("Loaded " + count + " bean definitions from " + resource);
}
return count;
}
//省略部分代码....

继续跟踪registerBeanDefinitions方法。

public int registerBeanDefinitions(Document doc, Resource resource) throws BeanDefinitionStoreException {
BeanDefinitionDocumentReader documentReader = createBeanDefinitionDocumentReader();
//获取已有的BeanDefinition的数量
int countBefore = getRegistry().getBeanDefinitionCount();
//注册BeanDefinitions
documentReader.registerBeanDefinitions(doc, createReaderContext(resource));
//返回新注册的BeanDefinition的数量
return getRegistry().getBeanDefinitionCount() - countBefore;
}

它又交由documentReader来执行registerBeanDefinitions方法。documentReader接收后调用本类的doRegisterBeanDefinitions来执行。这个方法里我们需要关注parseBeanDefinitions.preProcessXml和postProcessXml都是留给子类实现的。

protected void doRegisterBeanDefinitions(Element root) {
//省略部分代码...
preProcessXml(root);
parseBeanDefinitions(root, this.delegate);
postProcessXml(root);

this.delegate = parent;
}

跟踪parseBeanDefinitions方法，可以看到由DefaultBeanDefinitionDocumentReader#parseDefaultElement在进行解析

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
//import处理
if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
importBeanDefinitionResource(ele);
}
//alias处理
else if (delegate.nodeNameEquals(ele, ALIAS_ELEMENT)) {
processAliasRegistration(ele);
}
//解析bean元素
else if (delegate.nodeNameEquals(ele, BEAN_ELEMENT)) {
processBeanDefinition(ele, delegate);
}
//嵌套bean处理
else if (delegate.nodeNameEquals(ele, NESTED_BEANS_ELEMENT)) {
// recurse
doRegisterBeanDefinitions(ele);
}
}

最后跟踪processBeanDefinition的代码

protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
//解析bean元素为BeanDefinition,但是此时使用BeanDefinitionHolder又包装了一层
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// 完成BeanDefinition的注册
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// Send registration event.
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}

到这里就完成了BeanDefinition的注册。我们发现，所谓的注册就是把 XML 中定义的Bean信息封装为BeanDefinition对象之后放⼊⼀个Map中。

this.beanDefinitionMap.put(beanName, beanDefinition);

简略的时序图如下：

接下来我们分析一下Bean创建的流程。Bean创建是在refresh主流程中的第十一步里面。

11.1 Bean创建子流程

我们进入finishBeanFactoryInitialization方法。

protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
//省略部分代码...
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}

// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);

// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();

// 实例化所有非懒加载的bean
beanFactory.preInstantiateSingletons();
}

跟踪preInstantiateSingletons方法

public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("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<>(this.beanDefinitionNames);

// 遍历所有的beanName
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//如果是工厂bean，即实现了FactoryBean接口
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(
(PrivilegedAction<Boolean>) ((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
//实例化当前bean
getBean(beanName);
}
}
}
//省略...
}

我们发现不管是工厂Bean，还是普通Bean都会调用getBean(beanName);方法。然后调用到doGetBean方法。

protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {

String beanName = transformedBeanName(name);
Object bean;

// 从缓存中拿
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}

else {
//如果已经有创建过程中的同beanName的bean，抛异常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}

// 从parentBeanFactory里找对应的bean
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}

if (!typeCheckOnly) {
//打一个已创建的标记
markBeanAsCreated(beanName);
}

try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);

// 保证当前bean所依赖的bean的初始化
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
// 单例bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
//具体的创建bean逻辑
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);
}
//多例bean
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();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
}
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, () -> {
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;
}
}

// 检查所需类型是否与实际bean实例的类型匹配。
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}

可以看到经过一些判断和校验，会调createBean方法去创建bean，然后调用doCreateBean去完成创建。

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {

// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建bean实例，但是尚未设置属性
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}

//允许后处理器修改合并的bean定义
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;
}
}

//急切地缓存单例，以便能够解析循环引用
//即使由BeanFactoryAware等生命周期接口触发。
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}

// Initialize the bean instance.
Object exposedObject = bean;
try {
//设置属性
populateBean(beanName, mbd, instanceWrapper);
//应用初始化方法，调用BeanPostProcessor后置处理器
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<>(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 " +
"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}

// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}

return exposedObject;
}

在上述的方法中会先实例化一个bean，然后注入property里面配置的属性，最后执行初始化相关方法。

简单时序图：