在上一篇spring ioc源码解析（bean定义加载）的内容中，我们看了关于beanDefinition的加载过程，下面我们来看bean实例化的内容，bean的实例化就是调用getBean这个方法，看什么时候创建bean，就要看什么时候调用getBean这个方法，我们还是进入源码了解下
我们调用的方法，是在AbstractBeanFactory这个类里边
@Override
public Object getBean(String name) throws BeansException {
//doGetBean这个方法是做事情的
return doGetBean(name, null, null, false);
}
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
//解析bean的别名，因为每个bean可以有多个别名，需要这里解析找到beanName
final String beanName = transformedBeanName(name);
Object bean;
//从缓存中获取bean，保证bean的单例，这个方法很重要，是解决ioc循环依赖的重要方法，里边包含3级缓存
// Eagerly check singleton cache for manually registered singletons.
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 + "'");
}
}
//当从缓存中获取到BeanInstance，我们还不确定，这个bean是factoryBean，还是单例bean，这个方法就是去获取bean的实例化
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//当前bean是不是正在创建，判断多例对象循环依赖的拦截，抛出异常，（多例对象不能进行循环依赖，因为多例对象没有缓存）。
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
//当没有获取到bean的时候，会获取其父类的beanFactory
// Check if bean definition exists in this factory.
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) {
markBeanAsCreated(beanName);
}
try {
//将beanDefinition转换为RootBeanDefinition，如果指定beanName是子bean的话同时会合并父类的相关属性
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
//检查bean是否是abstract类型的，abstract类型的bean不能创建对象
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
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);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
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, () -> {
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 && !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;
}
仅从代码量上看，bean的加载经历了相当复杂的过程，其中涉及到各种的考虑，我们依照源码粗略了解整个sping加载bean的过程可以分为以下几步
1.转换对应beanName
2.尝试从缓存中加载单例
3.bean的实例化
4.原型模式的依赖检查
5.检测parentBeanFactory
6.将beanDefinition转换为RootBeanDefinition
7.寻找依赖
8.针对不同的scope进行bean的创建
9.类型转换
接下来我们对，每一步进行分析
1.转换对应beanName
这里传入的name可能是别名，可能是factoryBean，所以需要进行一系列的解析。
去除FactoryBean的修饰符，也就是如果name="&aa"，那么会首先去除&而使name="aa"。
取指定alias所表示的最终beanName，例如别名A指向名称为B的bean则返回B；若别名A指向别名B，别名B又指向名称为C的bean则返回C。
final String beanName = transformedBeanName(name);
protected String transformedBeanName(String name) {
return canonicalName(BeanFactoryUtils.transformedBeanName(name));
}
public String canonicalName(String name) {
String canonicalName = name;
// Handle aliasing...
String resolvedName;
//这里用循环获取bean的名称，说明bean的别名不止一个
do {
resolvedName = this.aliasMap.get(canonicalName);
if (resolvedName != null) {
canonicalName = resolvedName;
}
}
while (resolvedName != null);
return canonicalName;
}
2.尝试从缓存中加载单例
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//检查缓存中是否存在实例
Object singletonObject = this.singletonObjects.get(beanName);
//如果为空，判断当前bean是不是正在创建
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//如果实例对象为空，并且bean正在加载，锁定全局变量
synchronized (this.singletonObjects) {
//获取早期创建的bean，不为空说明bean正在加载，不做处理
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
//当某些方法需要提前初始化的时候，则会调用addSingletonFactory方法将对应的ObjectFactory初始化
//策略存储到singletonFactories
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//调用预先设置的getObject方法
singletonObject = singletonFactory.getObject();
//记录在缓存中
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
这里我们需要了解这几个缓存里边分别存放bean的信息。
singletonObjects: 用于保存BeanName 和 创建Bean实例的关系
bean name -- bean instance
singletonFactories: 用于保存BeanName和bean工程之间的关系
bean name -- ObjectFactory
earllySingletonObjects: 这个也是保存BeanName 和 创建Bean实例的关系，与singletonObjects的不同是，当一个单例bean被放到这里后，当这个bean正在创建中，就可以通过getBean获取到，其目的是用来检测循环引用，解决循环依赖。
registeredSingletons: 保存当前所有已经注册的bean
3..bean的实例化
整个getBean方法中，这一步是最主要的
在上一步中，如果我们从缓存中获取到bean的实例，可以调用getObjectForBeanInstance方法，获取实例对象。其实就是用于检测当前bean是否是FactoryBean类型的bean，如果是，那么需要调用该bean对应的FactoryBean实例中的getObject()作为返回值。
如果没有获取到bean的实例，就需要从头开始bean的加载过程了，而Spring中使用getSingleton的重载方法实现bean的加载过程。
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
//全局变量需要同步
synchronized (this.singletonObjects) {
//首先检查对应的bean是否已经加载过，因为单例模式就是服用已经创建的bean，这一步必须操作
Object singletonObject = this.singletonObjects.get(beanName);
//如果为空才可以进行singleton的bean初始化操作
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 + "'");
}
//记录加载状态，将当前正要创建的bean记录在缓存中，这样便可以对循环依赖进行检测。
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
//初始化bean 这里调用的方法，是在上个方法中传入的
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;
}
//当bean加载结束后需要移除缓存中对该bean的正在加载状态的记录。
afterSingletonCreation(beanName);
}
if (newSingleton) {
//加入缓存中
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
ObjectFactory的核心部分其实只是调用了createBean的方法，所以我们还需要到createBean方法中追寻真理。
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;
//确保bean类在这一点上得到了实际的解析，并且在动态解析类的情况下克隆bean定义
//不能存储在共享合并bean定义中。
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
//验证及准备覆盖的方法
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
//给BeanPostProcessors一个返回代理来替代真实bean实例的机会。这是aop用到的，这里不做解释
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 {
//常规bean的创建在这里实现，本次主要关注这个方法
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);
}
}
上面代码中我们可以看出以下步骤
（1）根据设置的class属性或者根据className来解析Class。
（2）对override属性进行标记及验证。
（3）应用初始化前的后处理器，解析指定bean是否存在初始化前的短路操作。（4）创建bean。
我们主要看创建bean的方法doCreateBean
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 根据指定bean使用指定的策略创建新的实例，例如 工厂方法，构造函数自动注入，简单的初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final 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 {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
//是否需要提早曝光 单例&允许循环依赖&当前bean正在创建中，检查循环依赖，
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");
}
//为避免后期循环依赖，可以在bean初始化前，将创建实例的ObjectFactory加入到工厂缓存中
//getEarlyBeanReference 回调方法，对bean再一次依赖引用，其中aop就是在这里将advice动态织入bean中
//若没有则直接返回bean, 不做任何处理
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
//对bean进行填充，将各个属性值注入，其中可能存在依赖于其它bean属性，则会递归初始依赖bean
populateBean(beanName, mbd, instanceWrapper);
//调用初始化方法
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);
//只有在存在循环依赖的情况才会不为空
if (earlySingletonReference != null) {
//如果exposedObject没有在初始化方法中被改变，也就是没有被增强
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 " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
//根据scopse注册bean
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
在上面的方法，我们主要看创建bean实例的方法createBeanInstance
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// 确保bean类在这一点上得到了实际的解析。
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);
}
//如果工厂方法不为空，则使用工厂方法初始化策略
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
//一个类有多个构造方法，每个构造方法都有不同的参数，所以调用其需要根据参数锁定构造方法或
//对应的工厂方法
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
//如果已经解析过，则使用解析好的构造函数方法，不需要再次锁定
if (resolved) {
if (autowireNecessary) {
//构造函数自动注入
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用默认构造函数构造
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
//需要根据参数解析构造函数
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
//构造函数自动注入
return autowireConstructor(beanName, mbd, ctors, args);
}
// 默认构造的首选构造函数
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
//构造函数自动注入
return autowireConstructor(beanName, mbd, ctors, null);
}
//无需特殊处理：只需使用无参数构造函数
return instantiateBean(beanName, mbd);
}
对于实例的创建Spring中分成了两种情况，一种是通用的实例化，另一种是带有参数的实例化。带有参数的实例化过程相当复杂，因为存在着不确定性，所以在判断对应参数上做了大量工作。这里不做过多解释，有需要的朋友可以在网上查找相关资料。
我们就返回创建好的实例对象。实例对象返回后，我们就回来doGetBean的方法，对返回的对象，进行相应的处理。
到这里，我们对bean的实例化就分析完了。
总结：本章只介绍的普通bean实例化的一些源码步骤，对于Aop，Spring事务相关的动态代理常见的实例对象，这里没有看到。在后面的源码分析里边会继续乘上，源码分析的路还很长，希望大家共同前进，有问题的朋友可以留言交流。
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