接着上文的
6.registerBeanPostProcessors
代码块4：getBean
这个是重点：
@Override
public Object getBean(String name) throws BeansException {
// 获取name对应的bean实例，如果不存在，则创建一个
return doGetBean(name, null, null, false);
}
/**
* 真正实现向IOC容器获取Bean的功能，也是触发依赖注入功能的地方
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
/*
* 通过name获取beanName，这里不用name直接作为beanName的原因：
* 1. name可能是以&开头，表名调用者想获取FactoryBean本身，而非FactoryBean实现类所创建的bean
* 在BeanFactory中，FactoryBean的实现类和其他的Bean存储方式是一致的，所以要讲name中首字符&移除，
* 这样才能从缓存中渠道FactoryBean实例
* 2. 如果是别名，这将别名转成规范的Bean名称
*/
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 先从缓存中取是否已经有被创建的单态类型的Bean
// 对于单例模式的Bean，整个IOC容器只创建一次，不需要重复创建
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// 3.如果beanName的实例存在于缓存中
if (logger.isTraceEnabled()) {
// 校验Bean的创建状态
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 + "'");
}
}
/* 3.1 获取给定Bean的实例对象，主要是完成FactoryBean的相关处理
* 如果sharedInstance是普通的单例Bean，下面的方法直接返回。
* 如果sharedInstance是FactoryBean类型，则需要调用FactoryBean.getObject工厂方法获取真正的Bean实例
* 如果用户想获取FactoryBean本身，这里面也不会做特别处理直接返回即可
* 注意：BeanFactory是管理容器中Bean的工厂，而FactoryBean是创建创建对象的工厂Bean，两者之间有区别
*/
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 4.bean的状态是正在创建中，还没有被实例化，所以如果是原型，直接抛出异常
// scope为prototype的循环依赖校验：如果beanName已经正在创建Bean实例中，
// 而此时我们又要再一次创建beanName的实例，则代表出现了循环依赖，需要抛出异常。
// 例子：如果存在A中有B的属性，B中有A的属性，那么当依赖注入的时候，
// 就会产生当A还未创建完的时候因为对于B的创建再次返回创建A，造成循环依赖
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 5. 检查是否存在默认的父工厂
BeanFactory parentBeanFactory = getParentBeanFactory();
// 5.1 如果parentBeanFactory存在，并且beanName在当前BeanFactory不存在Bean定义，
// 则尝试从parentBeanFactory中获取bean实例
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 5.2 将别名解析成真正的beanName
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
// 5.3 尝试在parentBeanFactory中获取bean对象实例
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.
// 这个方法，以前版本里面没有的，
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
if (!typeCheckOnly) {
// 6.如果不是仅仅做类型检测，而是创建bean实例，这里要将beanName放到alreadyCreated缓存
markBeanAsCreated(beanName);
}
try {
// 7.根据beanName重新获取MergedBeanDefinition（步骤6将MergedBeanDefinition删除了，这边获取一个新的）
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 7.1 检查MergedBeanDefinition
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 8.拿到当前bean依赖的bean名称集合，在实例化自己之前，需要先实例化自己依赖的bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
// 8.1 遍历当前bean依赖的bean名称集合
for (String dep : dependsOn) {
// 8.2 检查dep是否依赖于beanName，即检查是否存在循环依赖
if (isDependent(beanName, dep)) {
// 8.3 如果是循环依赖则抛异常
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 8.4 将dep和beanName的依赖关系注册到缓存中
registerDependentBean(dep, beanName);
try {
// 8.5 获取dep对应的bean实例，如果dep还没有创建bean实例，则创建dep的bean实例
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
// 9.针对不同的scope进行bean的创建
if (mbd.isSingleton()) {
// 实例化bean
// 9.1 scope为singleton的bean创建（新建了一个ObjectFactory，并且重写了getObject方法）
sharedInstance = getSingleton(beanName, () -> {
try {
// 9.1.1 创建Bean实例
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;
}
});
// 9.1.2 返回beanName对应的实例对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 下面检查其他工作，暂时不做细说
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
// 9.2 scope为prototype的bean创建，创建一个新的实例
Object prototypeInstance = null;
try {
// 9.2.1 创建实例前的操作（将beanName保存到prototypesCurrentlyInCreation缓存中）
beforePrototypeCreation(beanName);
// 9.2.2 创建Bean实例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
// 9.2.3 创建实例后的操作（将创建完的beanName从prototypesCurrentlyInCreation缓存中移除）
afterPrototypeCreation(beanName);
}
// 9.2.4 返回beanName对应的实例对象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
// 9.3 其他scope的bean创建，可能是request之类的
// 9.3.1 根据scopeName，从缓存拿到scope实例
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 {
// 9.3.2 其他scope的bean创建（新建了一个ObjectFactory，并且重写了getObject方法）
Object scopedInstance = scope.get(beanName, () -> {
// 9.3.3 创建实例前的操作（将beanName保存到prototypesCurrentlyInCreation缓存中）
beforePrototypeCreation(beanName);
try {
// 9.3.4 创建bean实例
return createBean(beanName, mbd, args);
}
finally {
// 9.3.5 创建实例后的操作（将创建完的beanName从prototypesCurrentlyInCreation缓存中移除）
afterPrototypeCreation(beanName);
}
});
// 9.3.6 返回beanName对应的实例对象
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) {
// 如果创建bean实例过程中出现异常，则将beanName从alreadyCreated缓存中移除
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
// 10.检查所需类型是否与实际的bean对象的类型匹配
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
// 10.1 类型不对，则尝试转换bean类型
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());
}
}
// 11.返回创建出来的bean实例对象
return (T) bean;
}
9.1.2、9.2.4、9.3.6 返回 beanName 对应的实例对象，见代码块1详解。
代码块4.1：getObjectForBeanInstance
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
// 1.判断他是不是FactoryBean,只要看他是不是&开头,
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, beanInstance.getClass());
}
if (mbd != null) {
mbd.isFactoryBean = true;
}
return beanInstance;
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
// 2.1 如果beanInstance不是FactoryBean（也就是普通bean），则直接返回beanInstance
// 2.2 如果beanInstance是FactoryBean，并且name以“&”为前缀，则直接返回beanInstance（以“&”为前缀代表想获取的是FactoryBean本身）
if (!(beanInstance instanceof FactoryBean)) {
return beanInstance;
}
// 3.走到这边，代表beanInstance是FactoryBean，但name不带有“&”前缀，表示想要获取的是FactoryBean创建的对象实例
Object object = null;
if (mbd != null) {
// 标记是否是FactoryBean
mbd.isFactoryBean = true;
}
else {
// 4.如果mbd为空，则尝试从factoryBeanObjectCache缓存中获取该FactoryBean创建的对象实例
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
// 5.只有beanInstance是FactoryBean才能走到这边，因此直接强转
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
// 6.mbd为空，但是该bean的BeanDefinition在缓存中存在，则获取该bean的MergedBeanDefinition
mbd = getMergedLocalBeanDefinition(beanName);
}
// 7.mbd是否是合成的（这个字段比较复杂，mbd正常情况都不是合成的，也就是false，有兴趣的可以自己查阅资料看看）
boolean synthetic = (mbd != null && mbd.isSynthetic());
// 8.从FactoryBean获取对象实例
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
// 9.返回对象实例
return object;
}
代码块4.2：postProcessObjectFromFactoryBean
/**
* 对所有已注册的BeanPostProcessors应用postProcessAfterInitialization回调，
* 使它们有机会对从FactoryBeans获得的对象进行后处理(例如，自动代理它们)。
*/
@Override
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
return applyBeanPostProcessorsAfterInitialization(object, beanName);
}
代码块4.3：markBeanAsCreated
protected void markBeanAsCreated(String beanName) {
if (!this.alreadyCreated.contains(beanName)) {
synchronized (this.mergedBeanDefinitions) {
// 1.如果alreadyCreated缓存中不包含beanName,
// 这边代码为啥判断两次contains，知道的留个言告知一下。
if (!this.alreadyCreated.contains(beanName)) {
// Let the bean definition get re-merged now that we're actually creating
// the bean... just in case some of its metadata changed in the meantime.
// 2.这边跟以前版本有变化，如果mergedBeanDefinitions中存在当前的bean
// 将当前的RootBeanDefinition的stale设置成true
clearMergedBeanDefinition(beanName);
// 3.将beanName添加到alreadyCreated缓存中，代表该beanName的bean实例已经创建（或即将创建）
this.alreadyCreated.add(beanName);
}
}
}
}
/**
* 删除指定bean的合并bean定义，在下一次访问时重新创建它。
*/
protected void clearMergedBeanDefinition(String beanName) {
RootBeanDefinition bd = this.mergedBeanDefinitions.get(beanName);
if (bd != null) {
bd.stale = true;
}
}
代码块4.4：isDependent
private boolean isDependent(String beanName, String dependentBeanName, @Nullable Set<String> alreadySeen) {
// 已经检查过的直接跳过
if (alreadySeen != null && alreadySeen.contains(beanName)) {
return false;
}
// 1.将别名解析为真正的名称
String canonicalName = canonicalName(beanName);
// 2.拿到依赖canonicalName的beanName集合
Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
// 3.如果dependentBeans为空，则两者必然还未确定依赖关系，返回false
if (dependentBeans == null) {
return false;
}
// 4.如果dependentBeans包含dependentBeanName，则表示两者已确定依赖关系，返回true
if (dependentBeans.contains(dependentBeanName)) {
return true;
}
// 5.循环检查，即检查依赖canonicalName的所有beanName是否存在被dependentBeanName依赖的（即传递依赖）
// 传递依赖:例如：A 依赖了B，B依赖了C，则A也算依赖了C
for (String transitiveDependency : dependentBeans) {
if (alreadySeen == null) {
alreadySeen = new HashSet<>();
}
// 6.已经检查过的添加到alreadySeen，避免重复检查
alreadySeen.add(beanName);
// 通过迭代的方式，处理传递依赖
if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
return true;
}
}
return false;
}
代码块4.5：registerDependentBean
这边又引入了一个跟 dependentBeanMap 类似的缓存，dependenciesForBeanMap：beanName -> beanName 对应的 bean 依赖的所有 bean 的 beanName 集合。
这两个缓存很容易搞混，举个简单例子：例如B依赖了A，则dependentBeanMap缓存中应该存放一对映射：其中 key为A，value为含有B的Set；而dependenciesForBeanMap缓存中也应该存放一对映射：其中key为：B，value为含有A的Set。
public void registerDependentBean(String beanName, String dependentBeanName) {
// 处理Bean名称，将别名转换为规范的Bean名称
String canonicalName = canonicalName(beanName);
// 4.如果依赖关系还没有注册，则将两者的关系注册到dependentBeanMap和dependenciesForBeanMap缓存
synchronized (this.dependentBeanMap) {
// 4.1 将dependentBeanName添加到依赖canonicalName的beanName集合中
Set<String> dependentBeans =
this.dependentBeanMap.computeIfAbsent(canonicalName,
k -> new LinkedHashSet<>(8));
// 向容器中：bean名称-->全部依赖Bean名称集合添加Bean的依赖信息
// 即，将Bean所依赖的Bean添加到容器的集合中
if (!dependentBeans.add(dependentBeanName)) {
return;
}
}
synchronized (this.dependenciesForBeanMap) {
// 4.2 将canonicalName添加到dependentBeanName依赖的beanName集合中
Set<String> dependenciesForBean =
this.dependenciesForBeanMap.computeIfAbsent(dependentBeanName,
k -> new LinkedHashSet<>(8));
dependenciesForBean.add(canonicalName);
}
}
代码块4.6：getSingleton
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
// 1.加锁，避免重复创建单例对象
synchronized (this.singletonObjects) {
// 2.首先检查beanName对应的bean实例是否在缓存中存在，如果已经存在，则直接返回
Object singletonObject = this.singletonObjects.get(beanName);
// 3.beanName对应的bean实例不存在于缓存中，则进行Bean的创建
if (singletonObject == null) {
// 4.当bean工厂的单例处于destruction状态时，不允许进行单例bean创建，抛出异常
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 + "'");
}
// 5.创建单例前的操作
beforeSingletonCreation(beanName);
boolean newSingleton = false;
// suppressedExceptions用于记录异常相关信息
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 6.执行singletonFactory的getObject方法获取bean实例
singletonObject = singletonFactory.getObject();
// 标记为新的单例对象
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;
}
// 7.创建单例后的操作
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 8.如果是新的单例对象，将beanName和对应的bean实例添加到缓存中（singletonObjects、registeredSingletons）
addSingleton(beanName, singletonObject);
}
}
// 9.返回创建出来的单例对象
return singletonObject;
}
}
代码块4.7：beforeSingletonCreation、afterSingletonCreation
protected void beforeSingletonCreation(String beanName) {
// 先校验beanName是否为要在创建检查排除掉的（inCreationCheckExclusions缓存），如果不是，
// 则将beanName加入到正在创建bean的缓存中（Set），
// 如果beanName已经存在于该缓存，会返回false抛出异常（这种情况出现在构造器的循环依赖）
if (!this.inCreationCheckExclusions.contains(beanName)
&& !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
protected void afterSingletonCreation(String beanName) {
// 先校验beanName是否为要在创建检查排除掉的（inCreationCheckExclusions缓存），如果不是，
// 则将beanName从正在创建bean的缓存中（Set）移除，
// 如果beanName不存在于该缓存，会返回false抛出异常
if (!this.inCreationCheckExclusions.contains(beanName)
&& !this.singletonsCurrentlyInCreation.remove(beanName)) {
throw new IllegalStateException("Singleton '" +
beanName + "' isn't currently in creation");
}
}
代码块4.8：beforePrototypeCreation、afterPrototypeCreation
该方法上面两个方法类似。主要是在进行 bean 实例的创建前，将 beanName 添加到 prototypesCurrentlyInCreation 缓存；bean 实例创建后，将 beanName 从 prototypesCurrentlyInCreation 缓存中移除。这边 prototypesCurrentlyInCreation 存放的类型为 Object，在只有一个 beanName 的时候，直接存该 beanName，也就是 String 类型；当有多个 beanName 时，转成 Set 来存放。
protected void beforePrototypeCreation(String beanName) {
// 1.拿到当前线程中正在创建的prototype的bean的beanName集合
Object curVal = this.prototypesCurrentlyInCreation.get();
if (curVal == null) {
// 2.如果为空，则将ThreadLocal设置成当前的beanName
this.prototypesCurrentlyInCreation.set(beanName);
}
// 3.如果不为空，并且是String类型，则代表目前只有一个beanName，
// 将之前和当前的一起封装成Set<String>，设置到ThreadLocal中
else if (curVal instanceof String) {
Set<String> beanNameSet = new HashSet<>(2);
beanNameSet.add((String) curVal);
beanNameSet.add(beanName);
this.prototypesCurrentlyInCreation.set(beanNameSet);
}
// 4.如果不为空，并且不是String，则必然是Set<String>类型，将当前的beanName加到Set中去
else {
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.add(beanName);
}
}
protected void afterPrototypeCreation(String beanName) {
// 1.拿到当前线程中正在创建的prototype的bean的beanName集合
Object curVal = this.prototypesCurrentlyInCreation.get();
// 2.如果是String类型，则代表目前只有一个beanName，则直接移除
if (curVal instanceof String) {
this.prototypesCurrentlyInCreation.remove();
}
else if (curVal instanceof Set) {
// 3.如果是Set类型，则从Set从移除beanName
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.remove(beanName);
if (beanNameSet.isEmpty()) {
this.prototypesCurrentlyInCreation.remove();
}
}
}
单元测试
留个坑。。
总结
留个坑。。
参考
https://blog.youkuaiyun.com/v123411739/article/details/87954818
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