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2016
08-03

Spring bean是如何加载的 – 出门向左 –

Spring bean是如何加载的
加载bean的主要逻辑
在AbstractBeanFactory中doGetBean对加载bean的不同情况进行拆分处理,并做了部分准备工作
具体如下

获取原始bean name

根据alia获取原始bean name
去除FactoryBean时的& [如果是需要获取FactoryBean自省,配置时需要在bean name前添加&]

尝试从缓存中获取实例

如果获取到实例,还要委托getObjectForBeanInstance解决FactoryBean的场景,就是调用getObject

判断原型场景的循环依赖问题,如果是原型同时bean又正在创建,说明是循环依赖,那直接抛异常,spring不尝试解决原型的循环依赖
如果在本容器中没有定义该bean,需要去父容器查找

如果有参数,结合参数初始化
如果没有参数,需要结合类型初始化,这边的调用是这个分支(当然这边一样没有类型)

如果不是类型检查,这边需要标记bean正在实例化
bean实例化的准备工作

合并父bean的定义,并转化GenericBeanDefinition为RootBeanDefinition
校验BeanDefinition,如果是抽象类或者非原型带参数抛异常[这边注释说的是只有原型才可以配置构造方法的参数]
解决bean的依赖

注册依赖的bean
递归调用getBean实例化依赖bean

创建单例的实例

为解决循环依赖问题,这边使用ObjectFactory在实例化前先暴露bean
老规矩,需要委托getObejctForBeanInstance解决FactoryBean的问题

创建原型实例

创建前的准备工作,使用prototypesCurrentlyInCreation标记bean正在实例化
委托createBean实例化bean
创建后的善后工作,从prototypesCurrentlyInCreation中删除标记
老规矩,委托getObjectForBeanInstance解决工厂方法的问题

创建其他scope的实例,这边的逻辑结合了单例跟原型的处理逻辑,即使用解决循环依赖的ObjectFactory也使用prototypeCreation的标记

获取作用域scope,并校验是否已配置
使用ObjectFactory提早暴露实例
标记bean正在创建并委托createBean实例化
又是委托getObjectForBeanInstance解决工厂方法问题

最后需要对创建的实例进行类型校验,如果不一致,这边还需要委托TypeConverter进行类型装换

AbstractBeanFactory
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {

// 获取原始的bean name,去除&,解决alias问题
final String beanName = transformedBeanName(name);
Object bean;

// 尝试从缓存中获取bean
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
// …
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
// 如果从缓存中或得bean,还需要判断是否是FactoryBean,并调用getObejct
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}

else {
// 如果是原型scope,这边又是正在创建,说明有循环依赖,而原型的循环依赖Spring是不解决的
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}

// 如果当前容器没有配置bean,那么去父容器查找
// 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 (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);
}
}

// 如果不是类型检查,这边需要标记类正在创建
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}

try {
// 实例化类之前,先去容器中获取配置的bean信息,这边需要将之前的GenericBeanDefinition转化为RootBeanDefinition
// 同时如果父bean的话,需要合并到子bean
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 dependsOnBean : dependsOn) {
if (isDependent(beanName, dependsOnBean)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
}
// 解决依赖
registerDependentBean(dependsOnBean, beanName);
getBean(dependsOnBean);
}
}

// 创建单例的实例
// Create bean instance
if (mbd.isSingleton()) {
// 单例情况下,为解决循环依赖,在实例化之前,先新建一个ObjectFactory实例
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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);
}

// 创建原型实例
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);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}

// 创建其他scope的实例
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
// 还是先创建ObejctFactory,只是这边没有处理
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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;
}
}

// 这边需要对实例进行类型校验,如果与requiredType不一致,需要委托TypeConverter尝试类型转换
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
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());
}
}
return (T) bean;
}
bean实例的缓存分析
上面提到在加载bean的时候,doGetBean首先尝试的是从缓存读取,这边我们来细细分析下缓存具体是如何处理的.
这边逻辑是定义在DefaultSingletonBeanRegistry中,它是AbstractBeanFactory的父类,主要职责是共享实例的注册.
这边虽然定义的是singleton,但是实际使用的时候,处理prototype,其他scope均使用了这边进行缓存.
这边主要是需要理解singletonObjects,singletonFactories,earlySingletonObjects,registeredSingletons这4个变量.
singletonObjects 缓存bean name ->实例
Cache of singleton objects: bean name –> bean instance
这边缓存的是实例
singletonFactories 缓存bean name –>ObjectFactory
Cache of singleton factories: bean name –> ObjectFactory
这边缓存的是为解决循环依赖而准备的ObjectFactory
earlySingletonObjects 缓存提早暴露的实例 bean name –>bean instance
Cache of early singleton objects: bean name –> bean instance
这边缓存的也是实例,只是这边的是为解决循环依赖而提早暴露出来的实例,其实是ObjectFactory
registeredSingletons 已经注册的单例bean name
Set of registered singletons, containing the bean names in registration order
上面三个变量,任意一个添加了,这边都会添加bean name,标记已经注册
4个变量的关系如下:

singletonObjects与singletonFactories,earlySingletonObjects,是互斥的.就是一个bean如果在其中任意一个变量中就,不会存在在另一变量中.这三个变量用于记录一个bean的不同状态.
如果bean已经添加到singletonObjects中,那么singltonFactories和earlySinletonObjects都不会考虑
singltonFactories中的bean 通过 ObjectFactory的getObject实例化后,添加到earlySingletonObjects

我们从下面几个方法,可以清楚看懂上面4个变量的使用:
DefaultSingletonBeanRegistry
/**
* 添加实例化的bean
* Add the given singleton object to the singleton cache of this factory.
* <p>To be called for eager registration of singletons.
* @param beanName the name of the bean
* @param singletonObject the singleton object
*/
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}

/**
* 为解决单例的循环依赖,这边注册ObjectFactory
* Add the given singleton factory for building the specified singleton
* if necessary.
* <p>To be called for eager registration of singletons, e.g. to be able to
* resolve circular references.
* @param beanName the name of the bean
* @param singletonFactory the factory for the singleton object
*/
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
/**
* 清除实例
* Remove the bean with the given name from the singleton cache of this factory,
* to be able to clean up eager registration of a singleton if creation failed.
* @param beanName the name of the bean
* @see #getSingletonMutex()
*/
protected void removeSingleton(String beanName) {
synchronized (this.singletonObjects) {
this.singletonObjects.remove(beanName);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.remove(beanName);
}
}
/**
* 获取实例时,调用ObejctFactory的getObject 获取实例
* Return the (raw) singleton object registered under the given name.
* <p>Checks already instantiated singletons and also allows for an early
* reference to a currently created singleton (resolving a circular reference).
* @param beanName the name of the bean to look for
* @param allowEarlyReference whether early references should be created or not
* @return the registered singleton object, or {@code null} if none found
*/
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
未完待续

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