Bean 的实例化
doGetBean
├─finishBeanFactoryInitialization(beanFactory);
│ ├─beanFactory.preInstantiateSingletons();
│ │ ├─getBean(beanName);
│ │ │ ├─doGetBean(name, null, null, false);java
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
// 获取一个beanName，处理两种情况，一个是前面说的 FactoryBean (前面带 '&')，
// 一个是别名问题，因为这个方法是 getBean，获取 Bean 用的，你要是传一个别名进来，是完全可以的
String beanName = transformedBeanName(name);
Object beanInstance;
// Eagerly check singleton cache for manually registered singletons.
/**
* 检查缓存中或者实例工厂中是否有对应的实例
* 因为在创建单例 bean的时候会存在依赖注入，而在创建依赖的时候为了避免循环依赖，
* spring创建bean的原则是：不等 bean 创建完成就将创建bean的 ObjectFactory 提早曝光，
* 换句话说，也就是将 ObjectFactory 加入到缓存中，一旦下一个bean创建的时候需要依赖上
* 一个bean则直接使用 ObjectFactory
* 先从一级缓存中获取，没有再从二级缓存中获取，还没有从三级缓存中获取，拿到后，放入二级缓存，然后从三级缓存中移除，完成从三级缓存到二级缓存的升级。
*/
Object sharedInstance = getSingleton(beanName);
//如果缓存里面能拿到实例
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 + "'");
}
}
// 该方法是 FactoryBean 接口的调用入口
// 如果是普通 Bean 的话，直接返回 sharedInstance（直接返回对象本身）
// 如果是 FactoryBean 的话，返回它创建的那个实例对象(返回指定方法返回的实例)
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
/**
* 处理原型模式下循环依赖的问题
* --触发场景：原型模式下，如果存在A中有B属性，B中有A属性，那么当依赖注入的时候，
* 就会产生当A还没创建完的时候，由于对B的创建再次返回创建A，造成循环依赖
*
* 如果 singletonObjects 缓存里面没有，则走下来
* 如果是 scope 是 Prototype 的，校验是否有出现循环依赖，如果有则直接报错
*/
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 检查一下这个 BeanDefinition 在容器中是否存在
BeanFactory parentBeanFactory = getParentBeanFactory();
// 如果不存在 beanDefinitionMap (一个concurrenthashmap)容器中，也就是所有已经
// 加载的类中不包括beanName，则尝试从 parentBeanFactory 中检测
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 如果当前容器不存在这个 BeanDefinition，试试父容器中有没有
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.
// 使用显式参数委托给父进程。
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) {
markBeanAsCreated(beanName);
}
StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
.tag("beanName", name);
/*
* 总结：
* 到这里的话，要准备创建 Bean 了，对于 singleton 的 Bean 来说，容器中还没创建过此 Bean；
* 对于 prototype 的 Bean 来说，本来就是要创建一个新的 Bean。
*/
try {
if (requiredType != null) {
beanCreation.tag("beanType", requiredType::toString);
}
/**
* 合并父类的相关属性
* -- 将存储的 GenericBeanDefinition 转换为 RootBeanDefinition，如果指定
* beanName 是子类bean的话，同时合并父类的相关属性
*/
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//获取依赖对象属性，依赖对象要先实例化
// 先初始化依赖的所有 Bean，这个很好理解。
// 注意，这里的依赖指的是 depends-on 中定义的依赖
String[] dependsOn = mbd.getDependsOn();
// 如果存在依赖则递归实例化依赖的bean
if (dependsOn != null) {
for (String dep : dependsOn) {
// 检查是不是有循环依赖，这里的循环依赖和我们前面说的循环依赖又不一样，
// 这里肯定是不允许出现的
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 注册一下依赖关系，缓存依赖调用
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
//着重看，大部分是单例的情况
// 实例化完依赖的bean后，便可以实例化 mbd 引用对象本身了
// 如果是 singleton scope 的，创建 singleton 的实例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
// 执行创建 Bean
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.
// 从单例缓存中显式删除实例:它可能已经被放在那里了
// 急切被创建进程，以允许循环引用解析。
// 还要删除接收到该bean的临时引用的所有bean。
destroySingleton(beanName);
throw ex;
}
});
// 该方法是FactoryBean接口的调用入口
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 如果是 prototype scope 的，创建 prototype 的实例
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);
}
beanInstance = 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);
}
});
beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new ScopeNotActiveException(beanName, scopeName, ex);
}
}
}
catch (BeansException ex) {
beanCreation.tag("exception", ex.getClass().toString());
beanCreation.tag("message", String.valueOf(ex.getMessage()));
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
finally {
beanCreation.end();
}
}
return adaptBeanInstance(name, beanInstance, requiredType);
}getSingleton(beanName);
├─finishBeanFactoryInitialization(beanFactory);
│ ├─beanFactory.preInstantiateSingletons();
│ │ ├─getBean(beanName);
│ │ │ ├─doGetBean(name, null, null, false);
│ │ │ │ ├─Object sharedInstance = getSingleton(beanName);java
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
//根据beanName从缓存中拿实例
//先从一级缓存拿
Object singletonObject = this.singletonObjects.get(beanName);
//如果bean还正在创建，还没创建完成，其实就是堆内存有了，只是属性还没有DI依赖注入
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//从二级缓存中拿
singletonObject = this.earlySingletonObjects.get(beanName);
//如果还拿不到，并且允许bean提前暴露（Spring 默认允许）
if (singletonObject == null && allowEarlyReference) {
synchronized (this.singletonObjects) {
// Consistent creation of early reference within full singleton lock
// 再次尝试从一级缓存中获取
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
// 再次尝试从二级缓存中获取
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null) {
//从三级缓存中拿到对象工厂
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//从工厂中拿到对象
singletonObject = singletonFactory.getObject();
//升级到二级缓存
this.earlySingletonObjects.put(beanName, singletonObject);
//删除三级缓存
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}createBean()
├─finishBeanFactoryInitialization(beanFactory);
│ ├─beanFactory.preInstantiateSingletons();
│ │ ├─getBean(beanName);
│ │ │ ├─doGetBean(name, null, null, false);
│ │ │ │ ├─if (mbd.isSingleton()) {
│ │ │ │ │ ├─下面的 getSingleton 方法传入了一个 Lambda 表达式，并在 getSingleton 中调用了传入的 lambda 表达式，创建了 bean 实例。
│ │ │ │ │ ├─sharedInstance = getSingleton(beanName, () -> {
│ │ │ │ │ │ ├─return createBean(beanName, mbd, args);java
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
if (logger.isTraceEnabled()) {
logger.trace("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 {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
/**
* TargetSource接口的运用，可以在用改一个类实现该接口，然后在里面定义实例化对象的方式，然后返回
* 也就是说不需要spring帮助我们实例化对象
* 这里可以直接返回实例本身
* 这个代码不用看，实际开发过程中用不到，我会做为一个甜点分享
*/
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);
}
try {
//主要看这个方法，重要程度 5
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}doCreateBean()
java
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
// 封装bean的实例
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建实例,,重点看，重要程度：5
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//CommonAnnotationBeanPostProcessor 支持了@PostConstruct，@PreDestroy,@Resource注解
//AutowiredAnnotationBeanPostProcessor 支持 @Autowired,@Value注解
//BeanPostProcessor接口的典型运用，这里要理解这个接口
//对类中注解的装配过程
//重要程度5，必须看
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
// 是否单例 bean 提前暴露
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");
}
//这里着重理解，对理解循环依赖帮助非常大，重要程度 5 添加三级缓存
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//ioc di，依赖注入的核心方法，该方法必须看，重要程度：5
populateBean(beanName, mbd, instanceWrapper);
//bean 实例化+ioc依赖注入完以后的调用，非常重要，重要程度：5
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 {
//注册bean销毁时的类DisposableBeanAdapter
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}createBeanInstance()
java
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
//反射拿到Class对象
Class<?> beanClass = resolveBeanClass(mbd, beanName);
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
//如果有FactoryMethodName属性
if (mbd.getFactoryMethodName() != null) {
// 反射的方式调用FactoryMethod
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
// 寻找当前正在实例化的bean中有@Autowired注解的构造函数
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
// 如果ctors不为空，就说明构造函数上有@Autowired注解，
// 接下来解析构造函数里的参数
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
//无参构造函数的实例化,大部分的实例是采用的无参构造函数的方式实例化
return instantiateBean(beanName, mbd);
}instantiateBean(beanName, mbd)
无参构造函数的实例化,大部分的实例是采用的无参构造函数的方式实例化。到这里，就已经完成了 bean 对象的创建。
至于后面的依赖注入参考专门的章节。