BeanFactory and FactoryBean
2024-09-05 16:33:49
BeanFactory，这是Spring容器的基础实现类，它负责生产和管理Bean的一个工厂。当然BeanFactory只是一个接口，它的常用实现有XmlBeanFactory、DefaultListableBeanFactory、ApplicationContext等。
FactoryBean
FactoryBean是一个接口
public interface FactoryBean<T> {
T getObject() throws Exception;
Class<?> getObjectType();
boolean isSingleton();
}
Spring官方文档有关FactoryBean解释:If you have complex initialization code that is better expressed in Java as opposed to a (potentially) verbose amount of XML, you can create your own FactoryBean, write the complex initialization inside that class, and then plug your custom FactoryBean into the container.
org.springframework.beans.factory.support.DefaultListableBeanFactory
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isDebugEnabled()) {
logger.debug("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
/**
* 判断该bean是否实现FactoryBean接口
* 若需要FactoryBean提前初始化，根据提示可以实现 SmartFactoryBean的isEagerInit的方法
* 若返回true则会在初始化的时候调用getbean方法，在getbean方法中调用对象的getObject方法
*/
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
//doGetBean方法最终会调到org.springframework.beans.factory.support.AbstractBeanFactory#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.
/**
* (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX))
* 判断beanName是否以&开头
*/
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(beanName, 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.
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
//首先尝试从factoryBeanObjectCache缓存中获取
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
//获取描述FactoryBean的BeanDefinition对象
mbd = getMergedLocalBeanDefinition(beanName);
}
//这个判断可以引出一个知识点，暂时还没理解
boolean synthetic = (mbd != null && mbd.isSynthetic());
//对象在这个方法中获取到
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
//org.springframework.beans.factory.support.FactoryBeanRegistrySupport#getObjectFromFactoryBean
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
//具体在这方法中调用FactoryBean中的GetObject方法
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);
if (alreadyThere != null) {
object = alreadyThere;
}
else {
if (shouldPostProcess) {
if (isSingletonCurrentlyInCreation(beanName)) {
// Temporarily return non-post-processed object, not storing it yet..
return object;
}
beforeSingletonCreation(beanName);
try {
//默认不做任何事情，但是它的子类org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory重写了该方法
/*
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
return applyBeanPostProcessorsAfterInitialization(object, beanName);
}
*/
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
finally {
afterSingletonCreation(beanName);
}
}
if (containsSingleton(beanName)) {
this.factoryBeanObjectCache.put(beanName, object);
}
}
}
return object;
}
}
else {
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
return object;
}
}
3.总结FactoryBean和BeanFactory
BeanFactory：工厂类接口，Spring容器的核心接口，实例化bean、配置bean之间的依赖关系
FactoryBean：实例化bean过程比较复杂时可以考虑使用
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