Spring之单例Bean的创建
本文基于Spring 5.2.7
Spring最基础的功能是IoC容器，能够容纳bean，就需要实例化bean并放入容器内。在Spring中，bean是有范围属性的，这个范围是时间、空间维度的，叫做scope，一个bean如果存在与应用整个生命期内，就是一个shared bean instance，他的scope值是singleton，如果一个bean只存在于应用生命器的一段时间内，是一个prototype bean instance，他的scope值就是prototype。这里的prototype意思和设计模式中的原型模式类似，就是直接创建对象的意思。
一、Spring的设计
对于bean的创建，用户是不需要知道的，用户需要知道的是获取bean，所以Spring将创建bean的入口放在获取bean中。
二、创建bean的关键点
创建一个bean有一些关键步骤：
1、实例化，生成引用值，这个动作由jdk实现，Spring不用干涉
2、对象需要一次或多次代理，Spring需要完成代理对象创建
3、完成依赖注入
4、当依赖注入时发生环形依赖，且同时有代理对象，Spring要处理好
5、只有scope=singleton的bean才会添加到缓存中
6、只有scope=singleton且允许环形依赖才会添加到第三级缓存singletonFactories中，也就是说基本只要是单例的bean，都会到三级缓存中
7、添加到第三级缓存中的是一个ObjectFactory类型的函数接口，取出的时候会执行SmartInstantiationAwareBeanPostProcessor#getEarlyBeanReference()方法(如果存在的话)
8、
三、单例bean的创建步骤
1、通过反射实例化bean，生成原生对象引用
2、如果是scope=singleton且允许环形依赖，添加一个getEarlyBeanReference()到第三级缓存中(处理一下对象)
3、populateBean()，填充bean引用的对象，也就是依赖注入，这里就是可能触发环形依赖的地方
在给引用赋值时，需要创建引用对象，如果依赖的应用对象引用的当前对象，那么就会触发从三级缓存中获取当前对象，执行早期引用的逻辑，然后升级到二级缓存，并删除三级缓存中当前对象，此时二级缓存中的对象依然是未初始化完成的对象，注意，在升级到二级缓存之后，如果还有动态代理逻辑要处理，这是不行的，因为此时引用已经给出去了，引用的那些对象是拿不到最终的代理对象的，这样会使某些功能失效。所以如果想要在循环依赖时将代理对象返回给其他对象，必须在将代理的创建逻辑放在SmartInstantiationAwareBeanPostProcessor#getEarlyBeanReference()中。
4、initializeBean()，在完成所有的依赖注入后，进行bean的各种后置处理，可以生成代理对象
5、如果scope=singleton且允许环形依赖，检测一下bean的类型和被其他类型引用的类型是否一致，不一致就要报错
6、初始化完毕后，调用addSingleton()方法，将bean从二级缓存中提升到一级缓存
所以，为什么需要三级缓存，
三、BeanFactory获取bean
Spring中获取bean的入口都在AbstractBeanFactory中，创建bean的入口也在这里。
org.springframework.beans.factory.support.AbstractBeanFactory
这里getBean()方法一共重载了4次，最终都调用了doGetBean()方法
public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
@Override
public T getBean(String name, Class requiredType) throws BeansException {
return doGetBean(name, requiredType, null, false);
}
@Override
public Object getBean(String name, Object... args) throws BeansException {
return doGetBean(name, null, args, false);
}
/**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
public T getBean(String name, @Nullable Class requiredType, @Nullable Object... args)
throws BeansException {
return doGetBean(name, requiredType, args, false);
}
......
} 其中涉及的流程可以从
org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean(...)
这个方法开始，这个方法是个关键的方法，因为里面包含了创建Bean的流程，Spring在设计时，将createBean放到了getBean的流程里
第一步我们看到第8行代码，
急切的检查手动注册单例的单例缓存，eager我们通常解释为饿汉式，也有点乐观的意思，这里就直接到单例缓存中去取，如果取到那肯定就是单例的，没取到就需要创建。这种套路我们进行实际编程时应该学到，是个很常见的编程模式。
这里的逻辑是直接从单例池中获取，如果获取到了就可以直接返回bean，如果没有获取到就需要创建，在创建的逻辑里再去判断是singleton还是prototype。
doGetBean()整个逻辑分为2个分支：
分支一：是从单例缓存中获取到了bean，此时处理后可以直接返回。
分支二：是没有从单例缓存中获取到bean，原因可能是还没有创建单例bean，也可能是因为scope是prototype的。
分支二的逻辑如下：
1、先判断是否是环形依赖导致的获取bean，这里可以看到26行抛出了一个异常，原因是这个bean是一个prototype的，而且发现这个bean已经被加入到了正在创建的bean队列中，只有一种可能就是，在创建他的时候，他被加入到正在创建队列中，他引用了别人，在创建别人时，别人引用了他，这种prototype的循环引用，Spring不提供解决方案，直接抛出异常，Spring希望这种情况下的循环引用由应用自己修改逻辑。
2、
下面我们可以看到82、100、118行就是真正的实例化bean的地方，org.springframework.beans.factory.support.AbstractBeanFactory#createBean()，这个方法在AbstractBeanFactory中是抽象的空方法，交给了子类去实现，由于createBean中涉及了三级缓存相关的逻辑所以我们不得不在看看相关的源码。
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
// 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 + "'");
}
}
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 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 {
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
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;
} 如下就是createBean()方法的源码，老长了
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#createBean(java.lang.String, org.springframework.beans.factory.support.RootBeanDefinition, java.lang.Object[])
很不幸，方法虽长，核心却被封装到了另一个方法doCreateBean()中，看看41行，我们又要去粘贴源码了.......
@Override
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.
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 {
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);
}
}
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean()
看第10行，是实例化对象，可以理解为给对象分配内存
看第41行，就是加入第三级缓存，终于找到这个关键点了，注意这里有个非常重要的点，就是传递的是一个lambda表达式，也就是一个函数，也就是说三级缓存里面直接存储的是并不一个bean，想要从三级缓存里拿出bean，需要调用一下这个函数，调用时机是在getSingleton()函数触发的，也就是说这里只是关联上了方法，不会去调用。
看第47行，是填充属性的方法，这个方法里会触发引用对象的创建，包括循环依赖的对象的创建
看第48行，是后置处理逻辑的地方
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) {
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;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
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 {
populateBean(beanName, mbd, instanceWrapper);
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 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;
} 这个就是属性装配的方法，里面涉及了引用对象的创建
org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#populateBean()
看org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#applyPropertyValues()这个方法里面就知道。
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
if (bw == null) {
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}
}
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
int resolvedAutowireMode = mbd.getResolvedAutowireMode();
if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
return;
}
}
pvs = pvsToUse;
}
}
}
if (needsDepCheck) {
if (filteredPds == null) {
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
}
checkDependencies(beanName, mbd, filteredPds, pvs);
}
if (pvs != null) {
applyPropertyValues(beanName, mbd, bw, pvs);
}
}