Spring中FactoryBean的作用和实现原理
BeanFactory与FactoryBean，相信很多刚翻看Spring源码的同学跟我一样很好奇这俩货怎么长得这么像，分别都是干啥用的。
- BeanFactory是Spring中Bean工厂的顶层接口，也是我们常说的SpringIOC容器，它定下了IOC容器的一些规范和常用方法并管理着Spring中所有的Bean；
- FactoryBean首先它是一个Bean，但又不仅仅是一个Bean。它是一个能生产或修饰对象生成的工厂Bean，类似于设计模式中的工厂模式和装饰器模式。它能在需要的时候生产一个对象，且不仅仅限于它自身，它能返回任何Bean的实例。
我们都知道在Spring中我们的Bean都会被Spring的IOC容器所管理，在AbstractApplicationContext中有一个很重要的方法：refresh()，项目启动或重启的时候refresh()会调用getBean()初始化所有的Bean，这个getBean()最终会指向AbstractBeanFactory中的getBean()方法。
在AbstractBeanFactory中，不管是根据名称还是根据类型，getBean()最终都会调用doGetBean()，在doGetBean()方法中一开始就获取了名称beanName和实例sharedInstance，这个方法太长，这里就贴前面两行。
String beanName = this.transformedBeanName(name);
Object sharedInstance = this.getSingleton(beanName);
transformedBeanName(name)是为了获取Bean真正的名称，它会去掉name前面的’&'，而getSingleton(beanName)是从父类容器singletonObjects中取的这个Bean的实例。在Spring中还有很多这样的容器，比如DefaultListableBeanFactory中的beanDefinitionMap，它就是的IOC容器真正保存Bean的地方，它是一个HashMap。类似的还有FactoryBeanRegistrySupport中的factoryBeanObjectCache等。
回到doGetBean()方法中，拿到sharedInstance后，后面的一大堆操作做了单例、多例等判断，最终会走到this.getObjectForBeanInstance()，关键部分就在这个方法中，进入方法代码。
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.
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) {
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)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
在上面的代码中有两个判断分别是beanInstance instanceof FactoryBean和BeanFactoryUtils.isFactoryDereference(name)，前面判断的是beanInstance是否属于FactoryBean或其子类的实例，后面这个方法判断name是否不为空且以&开头。
public static boolean isFactoryDereference(@Nullable String name) {
return (name != null && name.startsWith(BeanFactory.FACTORY_BEAN_PREFIX));
}
如果beanInstance不属于FactoryBean或其子类的实例，或者name是以&开头就直接返回实例对象beanInstance，
否则继续往下走。各种if … ==null判断是为了提高性能，咱们只挑关键部分看object = getObjectFromFactoryBean(factory, beanName, !synthetic);
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) {
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 {
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;
}
}
看源码的时候如果我们一直追究所有的细节那会让我们会越陷越深，掉入细节的无底洞，稍不留神脑回路跟不上就会蒙圈。我们要学会找源码中的关键部分看，弄懂主要流程和本次看源码的目的的那部分就行。等我们对Spring整体有了一个很好的理解之后，再回头看之前不懂的代码就会豁然开朗。
在上面这个方法中不管是走上面的if分支还是到下边的else中，关键部分就是object = this.doGetObjectFromFactoryBean(factory, beanName)这段代码调用，继续点进去
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged((PrivilegedExceptionAction<Object>) factory::getObject, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
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.
if (object == null) {
if (isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
object = new NullBean();
}
return object;
}
在这个方法中有一行关键代码：object = factory.getObject(); 这个factory就是我们传入的beanInstance实例。绕了这么一大圈，getBean方法返回的居然是我们实现FactoryBean接口定义的getObject方法。那么跟一开始对FactoryBean的描述印证了，FactoryBean是一个能生产或修饰对象生成的工厂Bean。一个Bean如果实现了FactoryBean接口，那么根据该Bean的名称获取到的实际上是getObject()返回的对象，而不是这个Bean自身实例，如果要获取这个Bean自身实例，那么需要在名称前面加上’&'符号。
一般情况下，Spring通过反射机制利用的class属性指定实现类实例化Bean，在某些情况下，实例化Bean过程比较复杂，如果按照传统的方式，则需要在中提供大量的配置信息。配置方式的灵活性是受限的，这时采用编码的方式可能会得到一个简单的方案。Spring为此提供了一个org.springframework.bean.factory.FactoryBean的工厂类接口，用户可以通过实现该接口定制实例化Bean的逻辑。FactoryBean接口对于Spring框架来说占用重要的地位，Spring自身就提供了70多个FactoryBean的实现。它们隐藏了实例化一些复杂Bean的细节，给上层应用带来了便利。从Spring3.0开始，FactoryBean开始支持泛型，即接口声明改为FactoryBean的形式
package com.lvyuanj.core.service;
//接口
public interface FactoryBeanService {
/**
* 测试FactoryBean
*/
void testFactoryBean();
}
package com.lvyuanj.core.service.Impl;
import com.lvyuanj.core.service.FactoryBeanService;
//实现类
public class FactoryBeanServiceImpl implements FactoryBeanService {
@Override
public void testFactoryBean() {
System.out.println("我是FactoryBean的一个测试类。。。。");
}
}
package com.lvyuanj.core.service.Impl;
import com.lvyuanj.core.service.FactoryBeanService;
import org.springframework.beans.factory.FactoryBean;
import org.springframework.stereotype.Component;
@Component
public class MyFactoryBean implements FactoryBean<FactoryBeanService> {
@Override
public FactoryBeanService getObject() throws Exception {
//这个Bean是我们自己new的，这里我们就可以控制Bean的创建过程了
return new FactoryBeanServiceImpl();
}
@Override
public Class<?> getObjectType() {
return FactoryBeanService.class;
}
@Override
public boolean isSingleton() {
return true;
}
}
@Test
public void test2() throws Exception {
ApplicationContext applicationContext = new AnnotationConfigApplicationContext(MyConfig.class);
String[] beanDefinitionNames = applicationContext.getBeanDefinitionNames();
for (String beanName : beanDefinitionNames) {
System.out.println("beanName: " + beanName);
}
FactoryBeanService factoryBeanService = (FactoryBeanService) applicationContext.getBean("MyFactoryBean");
System.out.println(factoryBeanService);
MyFactoryBean factoryBeanService3 = (MyFactoryBean) applicationContext.getBean("&myFactoryBean");
System.out.println(factoryBeanService3);
}
