目录
代码块3:doGetObjectFromFactoryBean
代码块4:postProcessObjectFromFactoryBean
代码块9:beforeSingletonCreation、afterSingletonCreation
代码块11:beforePrototypeCreation、afterPrototypeCreation
Spring IoC源码学习全系列
Spring IoC源码学习:ApplicationContext 刷新前的配置
Spring IoC源码学习:obtainFreshBeanFactory详解
Spring IoC源码学习:parseDefaultElement详解
Spring IoC源码学习:parseCustomElement详解
Spring IoC源码学习:obtainFreshBeanFactory详解
Spring IoC源码学习:invokeBeanFactoryPostProcessors详解
Spring IoC源码学习:registerBeanPostProcessors详解
Spring IoC源码学习:finishBeanFactoryInitialization详解
Spring IoC源码学习:createBean详解(上)
Spring IoC源码学习:createBean详解(下)
Spring IoC源码学习:finishRefresh 详解
前言
接着 Spring IoC:finishBeanFactoryInitialization详解,我们正式开始学习获取 bean 实例方法,该方法是 Spring 最核心的方法。
正文
单击 preInstantiateSingletons 方法里的 getBean(beanName) 代码,进入该方法。
getBean
@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);
}
见 doGetBean 方法详解。
doGetBean
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 1.解析beanName,主要是解析别名、去掉FactoryBean的前缀“&”
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 2.尝试从缓存中获取beanName对应的实例
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// 3.如果beanName的实例存在于缓存中
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
} else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
// 3.1 返回beanName对应的实例对象(主要用于FactoryBean的特殊处理,普通Bean会直接返回sharedInstance本身)
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 4.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.获取parentBeanFactory
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);
// 5.3 尝试在parentBeanFactory中获取bean对象实例
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
} else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
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);
// 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);
}
}
// Create bean instance.
// 9.针对不同的scope进行bean的创建
if (mbd.isSingleton()) {
// 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, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException { //
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);
} else if (mbd.isPrototype()) {
// 9.2 scope为prototype的bean创建
// It's a prototype -> create a new instance.
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, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
// 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 && bean != null && !requiredType.isInstance(bean)) {
try {
// 10.1 类型不对,则尝试转换bean类型
return getTypeConverter().convertIfNecessary(bean, requiredType);
} catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("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;
}
1.解析 beanName,主要是解析别名、去掉 FactoryBean 的修饰符 “&”,在 Spring IoC:finishBeanFactoryInitialization详解 中的代码块4已解析过。
2.尝试从缓存中获取 beanName 对应的实例,在 Spring IoC:finishBeanFactoryInitialization详解 中的代码块7已解析过。
3.1 返回 beanName 对应的实例对象(主要用于 FactoryBean 的特殊处理,普通 bean 会直接返回 sharedInstance 本身),见代码块1详解。
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,见代码块5详解。
7.根据 beanName 重新获取 MergedBeanDefinition,在 Spring IoC:finishBeanFactoryInitialization详解 中的代码块2已解析过。
8.2 检查 dep 是否依赖于 beanName,即检查是否存在循环依赖,见代码块6详解。
8.4 将 dep 和 beanName 的依赖关系注册到缓存中,见代码块7详解。
9.1 scope 为 singleton 的 bean 创建(新建了一个 ObjectFactory,并且重写了 getObject 方法),见代码块8详解。
9.1.1、9.2.2、9.3.4 创建 bean 实例,限于篇幅,在下篇文章单独解析。
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。
9.2.1 scope 为 prototype 时创建实例前的操作、9.2.3 scope 为 prototype 时 创建实例后的操作,相对应的两个方法,见代码块11详解。
代码块1:getObjectForBeanInstance
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
// 1.如果name以“&”为前缀,但是beanInstance不是FactoryBean,则抛异常
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// 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) || BeanFactoryUtils.isFactoryDereference(name)) {
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) {
// 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;
}
如果对 FactoryBean 不熟悉的,可以回头去看 Spring IoC:finishBeanFactoryInitialization详解 中对 FactoryBean 的简单介绍。
6.mbd 为空,但是该 bean 的 BeanDefinition 在缓存中存在,则获取该 bean 的 MergedBeanDefinition,在 Spring IoC:finishBeanFactoryInitialization详解 中的代码块2已经解析过。
8.从 FactoryBean 获取对象实例,见代码块2详解。
代码块2:getObjectFromFactoryBean
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
// 1.如果是单例,并且已经存在于单例对象缓存中
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
// 2.从FactoryBean创建的单例对象的缓存中获取该bean实例
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
// 3.调用FactoryBean的getObject方法获取对象实例
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
// 4.如果该beanName已经在缓存中存在,则将object替换成缓存中的
if (alreadyThere != null) {
object = alreadyThere;
} else {
if (object != null && shouldPostProcess) {
try {
// 5.对bean实例进行后置处理,执行所有已注册的BeanPostProcessor的postProcessAfterInitialization方法
object = postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
}
// 6.将beanName和object放到factoryBeanObjectCache缓存中
this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
}
// 7.返回object对象实例
return (object != NULL_OBJECT ? object : null);
}
} else {
// 8.调用FactoryBean的getObject方法获取对象实例
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (object != null && shouldPostProcess) {
try {
// 9.对bean实例进行后置处理,执行所有已注册的BeanPostProcessor的postProcessAfterInitialization方法
object = postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
// 10.返回object对象实例
return object;
}
}
3.调用 FactoryBean 的 getObject 方法获取对象实例,见代码块3详解。
5.对 bean 实例进行后续处理,执行所有已注册的 BeanPostProcessor 的 postProcessAfterInitialization 方法,见代码块4详解。
代码块3:doGetObjectFromFactoryBean
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
// 1.调用FactoryBean的getObject方法获取bean对象实例
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
@Override
public Object run() throws Exception {
// 1.1 带有权限验证的
return factory.getObject();
}
}, acc);
} catch (PrivilegedActionException pae) {
throw pae.getException();
}
} else {
// 1.2 不带权限
object = factory.getObject();
}
} catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
} catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
// 2.getObject返回的是空值,并且该FactoryBean正在初始化中,则直接抛异常,不接受一个尚未完全初始化的FactoryBean的getObject返回的空值
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
// 3.返回创建好的bean对象实例
return object;
}
很简单的方法,就是直接调用 FactoryBean 的 getObject 方法来获取到对象实例。
细心的同学可以发现,该方法是以 do 开头,看过 Spring IoC:源码学习总览 的同学知道,我在总览里就特别提到以 do 开头的方法是最终进行实际操作的方法,例如本方法就是 FactoryBean 最终实际进行创建 bean 对象实例的方法。
代码块4:postProcessObjectFromFactoryBean
@Override
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
return applyBeanPostProcessorsAfterInitialization(object, beanName);
}
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
// 1.遍历所有注册的BeanPostProcessor实现类,调用postProcessAfterInitialization方法
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
// 2.在bean初始化后,调用postProcessAfterInitialization方法
result = beanProcessor.postProcessAfterInitialization(result, beanName);
if (result == null) {
// 3.如果返回null,则不会调用后续的BeanPostProcessors
return result;
}
}
return result;
}
这边走的是 AbstractAutowireCapableBeanFactory 里的方法。通过前面的介绍,我们知道创建的 BeanFactory 为 DefaultListableBeanFactory,而 DefaultListableBeanFactory 继承了 AbstractAutowireCapableBeanFactory,因此这边会走 AbstractAutowireCapableBeanFactory 的重写方法。
在 Spring IoC:registerBeanPostProcessors详解 中已经学过 BeanPostProcessor,在创建完 bean 实例后,会执行 BeanPostProcessor 的 postProcessAfterInitialization 方法。
代码块5: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
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.将beanName的MergedBeanDefinition从mergedBeanDefinitions缓存中移除,
// 在之后重新获取MergedBeanDefinition,避免BeanDefinition在创建过程中发生变化
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);
}
}
}
}
protected void clearMergedBeanDefinition(String beanName) {
this.mergedBeanDefinitions.remove(beanName);
}
2.这边会将 beanName 对应的 MergedBeanDefinition 移除,然后在之后的代码重新获取,主要是为了使用最新的 MergedBeanDefinition 来进行创建操作。
代码块6:isDependent
private boolean isDependent(String beanName, String dependentBeanName, 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(即隔层依赖)
for (String transitiveDependency : dependentBeans) {
if (alreadySeen == null) {
alreadySeen = new HashSet<String>();
}
// 6.已经检查过的添加到alreadySeen,避免重复检查
alreadySeen.add(beanName);
if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
return true;
}
}
return false;
}
这边引入了一个缓存 dependentBeanMap:beanName -> 所有依赖 beanName 对应的 bean 的 beanName 集合。内容比较简单,就是检查依赖 beanName 的集合中是否包含 dependentBeanName,隔层依赖也算。例如:A 依赖了 B,B 依赖了 C,则 A 也算依赖了 C。
代码块7:registerDependentBean
public void registerDependentBean(String beanName, String dependentBeanName) {
// A quick check for an existing entry upfront, avoiding synchronization...
// 1.解析别名
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包含dependentBeanName,则表示依赖关系已经存在,直接返回
if (dependentBeans != null && dependentBeans.contains(dependentBeanName)) {
return;
}
// No entry yet -> fully synchronized manipulation of the dependentBeans Set
// 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
dependentBeans = this.dependentBeanMap.get(canonicalName);
if (dependentBeans == null) {
dependentBeans = new LinkedHashSet<String>(8);
this.dependentBeanMap.put(canonicalName, dependentBeans);
}
dependentBeans.add(dependentBeanName);
}
synchronized (this.dependenciesForBeanMap) {
// 4.2 将canonicalName添加到dependentBeanName依赖的beanName集合中
Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName);
if (dependenciesForBean == null) {
dependenciesForBean = new LinkedHashSet<String>(8);
this.dependenciesForBeanMap.put(dependentBeanName, dependenciesForBean);
}
dependenciesForBean.add(canonicalName);
}
}
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。
代码块8:getSingleton
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' 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);
if (singletonObject == null) {
// 3.beanName对应的bean实例不存在于缓存中,则进行Bean的创建
if (this.singletonsCurrentlyInDestruction) {
// 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
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<Exception>();
}
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 != NULL_OBJECT ? singletonObject : null);
}
}
5.创建单例前的操作,7.创建单例后的操作,这两个方法是对应的,见代码块9详解。
6.执行 singletonFactory 的 getObject 方法获取 bean 实例,该方法会走文章开头 doGetBean 方法的注释 9.1.1。
8.如果是新的单例对象,将 beanName 和对应的单例对象添加到缓存中,见代码块10详解。
代码块9: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");
}
}
inCreationCheckExclusions 是要在创建检查排除掉的 beanName 集合,正常为空,可以不管。这边主要是引入了 singletonsCurrentlyInCreation 缓存:当前正在创建的 bean 的 beanName 集合。在 beforeSingletonCreation 方法中,通过添加 beanName 到该缓存,可以预防出现构造器循环依赖的情况。
为什么无法解决构造器循环依赖?
我们之前在 Spring IoC:finishBeanFactoryInitialization详解 中的代码块7提过,getSingleton 方法是解决循环引用的核心代码。解决逻辑的第一句话:“我们先用构造函数创建一个 “不完整” 的 bean 实例”,从这句话可以看出,构造器循环依赖是无法解决的,因为当构造器出现循环依赖,我们连 “不完整” 的 bean 实例都构建不出来。Spring 能解决的循环依赖有:通过 setter 注入的循环依赖、通过属性注入的循环依赖。
代码块10:addSingleton
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
// 1.添加到单例对象缓存
this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
// 2.将单例工厂缓存移除(已经不需要)
this.singletonFactories.remove(beanName);
// 3.将早期单例对象缓存移除(已经不需要)
this.earlySingletonObjects.remove(beanName);
// 4.添加到已经注册的单例对象缓存
this.registeredSingletons.add(beanName);
}
}
代码块11:beforePrototypeCreation、afterPrototypeCreation
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();
// 2.如果为空,则将ThreadLocal设置成当前的beanName
if (curVal == null) {
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<String>(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();
}
}
}
该方法和代码块9的两个方法类似。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。
总结
本文介绍了获取 bean 实例的大部分内容,包括先从缓存中检查、 FactoryBean 的 bean 创建、实例化自己的依赖(depend-on 属性)、创建 bean 实例的前后一些标记等,在下篇文章中,将解析创建 bean 的内容。